# Parallel Lithium packs?



## Coulomb (Apr 22, 2009)

EVfun said:


> It would seem that they would automatically share the load. Would be a good idea to only have the strings in parallel when charging or driving, or could they could stay together all the time?


I think what will happen is that as soon as you parallel the packs, they will try to head for about the same SOC. Well, mainly if they are at one or other end of the discharge curve. Any time that they are from say 20 - 80 % SOC, the two strings would be a very similar voltages, so little current would flow immediately on charging.

So I'd leave them paralleled, even though it means that current will be running from one pack to the other near the end of charge, and at low SOCs. I'd make sure that there was at most a volt difference when making the last connection to put them in parallel, though.



> Has anyone paralleled a 1/2 discharged and fully charged Lithium cell and found out what happens when you discharge the pair?


[Edit: I think there would be an almighty splat when you connected the two packs, and the full pack would attempt to charge the empty pack in a second, resulting in two packs of medium SOC, acting as a combined capacity pack of medium SOC. But the paralleling would be very hard on the contactor and cells. Hence I'd keep them in parallel. ]

I haven't, but here is my guess. I'm assuming that the two packs have rather different capacities, and start out at about the same SOC (not what you wrote, I know). Both packs will contribute current inversely proportional to their internal resistance, so the Headway pack will provide more current for a while. Between bursts of power, the Thunder Skys will charge the Headways. Eventually, the Headways will get low enough SOC that the Thunder Skys will take more of the load. Eventually, the Thunder Skys will take virtually all of the load, so you'll be limited to what the TS can put out. [ Edit: assuming you let the packs get that low in SOC; probably it's a good idea not to let them get that low, at least not often. ]

On charging, assuming you leave the packs in parallel, the current will again go to the cells with the lowest IR, I think. I don't know how that will sort itself out, but eventually the Headways will get full before the TSs. At that point, the current will go to the TSs, and all will be well. After charging, there may be some current from one pack to the other, but this should not be a big issue.

There are a lot of assumptions in this, and I could be very wrong.



> Am I crazy?


Probably  But I don't think that this idea proves it


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## GizmoEV (Nov 28, 2009)

EVfun said:


> Has anyone paralleled a 1/2 discharged and fully charged Lithium cell and found out what happens when you discharge the pair? It would seem that the lower cell couldn't be hurt because if its voltage wanted to fall off that cell would just deliver less current and make its "buddy" take up the slack.


I didn't get a chance to discharge but I did take a cell at 0%SOC and hook it up to a cell that was probably over 80% SOC. This was with two TS-LFP40AHA cells. One had been sitting for a month or so and was at 3.339V. The discharged cell had been discharged at ~8A to 2.500V. After a 19 hour rest the voltage of this cell was at 3.021V. I paralleled the two cells by hooking one set of terminals together with the factory strap. The other two terminals I ran through some 1AWG wire through a 50A 50mV shunt with a digital meter attached. The initial current was ~28A which quickly diminished to 22A and slowly declined after that. When the current was ~15A the two cells' voltages were different by ~0.15V. When the current was 10A the cell voltages were 3.245V and 3.228V. After about 5 minutes the current was down below 9A. At this point I had to return the cells to the owner do I didn't get to finish the test. I just connected the two in parallel with the factory straps and took them back.

I don't know how this would scale to higher pack voltages or larger pack capacities but I wouldn't worry too much about relatively small differences if your final connection point can handle the brief arc but then I would leave the packs hooked up for discharge and charge. The packs will go dead together and reach full together as long as their characteristics are the same since the string voltages will be forced to be the same.


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## Coulomb (Apr 22, 2009)

GizmoEV said:


> The other two terminals I ran through some 1AWG wire through a 50A 50mV shunt with a digital meter attached. The initial current was ~28A which quickly diminished to 22A and slowly declined after that.


Interesting, thanks for the real-world data. The shunt would have introduced an extra 1 milli-ohm (plus a bit from the cables), but without the shunt, the current would maybe have been doubled. So without the shunt it would presumably start at around 1.4C, quickly reducing to 1C, and down to ~ 0.5 C after 5 minutes. I think that these figures would roughly scale to any EV pack size, though there may be some effect that happens at higher voltages (paralleling longer strings of cells).


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## GizmoEV (Nov 28, 2009)

So maybe if someone were to be connecting two packs together in parallel it would be worth first using something like a long extension cord or through something with a little resistance like the shunt and then make the final connection after the current went down. Sort of like a "precharge" procedure.


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## EVfun (Mar 14, 2010)

Coulomb said:


> [snip]
> Both packs will contribute current inversely proportional to their internal resistance, so the Headway pack will provide more current for a while. Between bursts of power, the Thunder Skys will charge the Headways. Eventually, the Headways will get low enough SOC that the Thunder Skys will take more of the load. Eventually, the Thunder Skys will take virtually all of the load, so you'll be limited to what the TS can put out.
> [snip]


That's kinda a downer, but I suspect you are right. The lower resistance Headway cells may end up taking to much load and getting low first. Since the difference between 25% SOC and 50% SOC is just a couple hundredths of a volt no load I suspect the needed amps won't be pushed back into the Headways when I let up on the throttle. After a few good blasts my TS cells would be putting out most of the current, at some unknown (but excessive even by my standards) C rate. 

Using some data from Manzanita Micro it looks like my TS cells would deliver 450 amps with the same voltage where the Headways would be delivering 200 amps. In a strait run to discharge the Headways 75% (after which they should start sagging more under heavy loads) it would take 1.8 minutes to pull 6 amp hours from the HW cells while that would only pull 13.5 amp hours from the TS cells. Of course it wouldn't happen in a strait run, but the ability of the TS cells to charge the HW cells would likely be to slow to make the results much different. 

One possibility is a different cell count and having the Headway pack only connected while driving. There would need to be a precharge circuit and second contactor. Perhaps 30 HW cells so when I took my foot off the throttle the 32 TS cells would try and take them to about 3.5 volts each. That would fairly quickly charge them. They would not be connected when charging but that would be fine since there is no need to fully charge LiFePO4 cells. 

I would likely want to back off a little because the C rate will share differently depending on how I drive. I should be able to 550 amps out of this, in the approximate form of 420 from the TS cells and 130 from the Headway cells. That would be about $700 for an extra 22% more power and an interesting experiment. 

Or, I'm only getting crazier. 

I'm open to suggesting, comments, ideas and experiences here. Thanks!


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## Ziggythewiz (May 16, 2010)

Maybe you could get a few and do a lower voltage test. If you have a lower voltage motor...or with a load tester you could make a 4s2p pack with separate shunts and ammeters and see how the load really splits at 2, 4, 600 amps.

I may do something similar myself, as I've toyed with the idea of using a lithium booster pack for my lead. The main concern is to not exceed the rating on the smaller ah pack.


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## Guest (Oct 16, 2011)

Your small pack will run out fast and then the large pack will push full amperage through the small ones effectively killing them. No different than having a top balanced pack and running it down until your lowest capacity cell is just empty. Then drive the pack amperage through that. 

Don't do it unless you limit your run to the low capacity pack and stop. Your lowest capacity is always your limiting factor. Always. Whether you are charging or discharging.


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## EVfun (Mar 14, 2010)

I'm not planning to put them in series Pete. These would be in parallel so there is no way for one pack to have a lower voltage than the other (other than the voltage drop in 1/0 cable) when both are connected. 

That is a good idea Ziggy. I don't have a way to do 100+ amp hour discharge tests other than in my car so I would have to come up with a suitable load. I do have some TS cells hanging around, with an eye to have the most expensive battery pack in a cordless electric mower. Any suggestions for an inexpensive big load?

I want to point out that this would be a quick drop in for extra driving fun, it would not be in the car for shows. My Buggy is built around the concept of showing how simple an electric car really is.


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## Guest (Oct 16, 2011)

If you do this you might consider going with buddy pairs rather than a single separate pack. I considered a split paralleled pack myself but only of the same AH size. If I were to use differing AH cells I'd suspect that buddy pairing would be better. It is a speculation but I think a good one. The different cells do have different charge acceptances from each other. I'd be careful.


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## major (Apr 4, 2008)

EVfun said:


> Any suggestions for an inexpensive big load?


I use these things in series parallel combination. http://www.meci.com/braking-resistors-157-ohm.html Like this: 










Then to switch on and off and to hold constant current this: 









For single or just a few cells and a constant resistance discharge you can just use a contactor or knife switch. And of course there is always the coat hanger in a bucket of water


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## dougingraham (Jul 26, 2011)

EVfun said:


> I've got this crazy thought... I was considering a "stink pack" (as in, go like...) for the Buggy when I just wanna have more fun. The buggy packs 32, 60 amp hour TS cells that can dish out 450 amps at about 84 volts. I was thinking about adding a single string of 32 Headway 38120P cells for an extra 200 amps on demand. That is 200 amps more fun in an 1100 lb. car. The whole second pack could sit where the original 12 volt battery used to be located. It would only weigh about 25 pounds and add 40% more power.
> 
> I know many have paralleled cells to make a higher capacity pack but has anyone paralleled 2 separate series strings? Since I only charge to 3.5 vpc I think charging would be safe, even if 1 string lost 2 cells the remaining cells would average less than 3.75 vpc at the end of a charge. It would seem that they would automatically share the load. Would be a good idea to only have the strings in parallel when charging or driving, or could they could stay together all the time?
> 
> ...


You are not Crazy. I am not sure how much gain you will see from it. It could be like a poor mans version of adding a bank of supercaps since the Headways are supposed to be stiffer than the 60AH TS cells. For sure you will see extended range.

As for paralleling different batteries. You should have the same basic chemistry in both packs. LiFePo4 in both packs so that the fully charged and discharge voltages are compatible. You should have the same number of cells in series in both packs. I would leave them connected all the time. They should probably be within 15% SOC of each other when you hook them up. This is easy though since you just charge both packs before you hook them together. If the SOC is too far off then you will see a rather healthy spark and possibly a few hundred amps flow for a few seconds so you do want to be careful. I would double check the voltage of each pack before I cabled them together to make sure they are within a few volts of each other. With 32 cells difference if you hook them up where one is full charged and one is empty you would see a voltage difference of 14 volts. Guessing the series resistance of the batteries might be as low as 0.064 ohms (0.032 ohms per pack) the inrush current would be 219 amps. But that would only last a fraction of a second. However it is enough to possibly weld the contacts and surprise the heck out of you. That is probably worst case. As long as the voltages are close you should have no problems.

Let us know how it works out.


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## DIYguy (Sep 18, 2008)

I have interest in this thread also. I've been pondering the idea of paralleling my CALB LiFePo pack with a very high discharge Lipoly pack. I know, I know, save your breath on warnings of different chemistry etc. I've been "thinking myself smart" on this for a while. lol. 

For me, this is for "events" like a day at the strip or a chance meeting at a stop light with a muscle car. Most EV's are made for daily drivers. A few are made for racing only. I'm after a bit of both here. Yes it will be heavy... but no worse than many lead racers. I'm not after world records either. 

I think the important points are matching voltages. . as has been pointed out. I'm pouring over the implications of charge, resting and discharge level potentials with the two chemistries. Of course I can vary the number of lipoly cells to match lifepo pack voltage. 

My set up will include a 13" motor with a gear vendors overdrive, and a Big Sol (Shiva) also.

I recall a thread where someone was starting to look into this, with RC cells. Can't seem to find it though. They gave up on it before testing I think.


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## dougingraham (Jul 26, 2011)

DIYguy said:


> I have interest in this thread also. I've been pondering the idea of paralleling my CALB LiFePo pack with a very high discharge Lipoly pack. I know, I know, save your breath on warnings of different chemistry etc. I've been "thinking myself smart" on this for a while. lol.
> 
> For me, this is for "events" like a day at the strip or a chance meeting at a stop light with a muscle car. Most EV's are made for daily drivers. A few are made for racing only. I'm after a bit of both here. Yes it will be heavy... but no worse than many lead racers. I'm not after world records either.
> 
> ...


The discharge curves are enough different that I dont think you are going to gain much. I don't know what you have right now so Ill do a hypothetical. Lets say you have a "HV" type motor and you are doing a 340V LiFePo4 setup. This would be 94S (3.6Vpc) pack of some capacity so as to not exceed the motor controller voltage at the fresh off the charger pack voltage. Equivalent LiPo would be 81S (4.2Vpc) pack. If you assume they are both empty at 2.8Vpc then your main pack voltage will be 263V and your Lipo pack would be 226V. The empty higher voltage pack is the one we need to go with. Based on this your Lipo pack would have an average cell voltage of 3.25 when the main pack is flat. So as a booster it seems like it would work out ok. To tell what the ratio of current provided by each pack would be under a several thousand amp load you would need more information than we have available. The idea of course would be that at low currents they would share in relationship to their relative capacities. And this would happen naturally. At high currents the one with the lowest internal resistance would provide the juice. For the booster pack to be useful it would need to be significantly lower in internal resistance. I am thinking the LiPo pack would in practice only ever use about half its energy.

Headways or A123's would be better since you don't have the different discharge curve problems and you get to use all the capacity.

I might not be looking at this the right way either. I am thinking that you might already have a hot setup and want a little more. If you are running a non HV setup say 144V and you need lots of amps for torque this might help more. Ill have to think on it.


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## DIYguy (Sep 18, 2008)

Thanks for the reply Doug. 

I have 66 Calb 180 Ah for a 211 volt nominal pack. I'll be looking for a high current, mid voltage set up. I am currently running a Soliton1 and it sags to about 170 volts (when it is warm out) at 1000 amps. That about 2.6vpc. This is right around where I want to be with motor volt limit. (Planned  ).

I'm thinking that I need to look a little closer at voltage sag characteristics at full load, and match these fairly closely and let the lower resistance cells do most of the work. I'm looking into sag voltage at high load for different cells and then thinking about the other states. (idle, charge - separate/together, low power... etc.) Also thinking about continuous connection vs temporary. 

It's getting to be a bit of a cloud... lol


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## EVfun (Mar 14, 2010)

Wow Major, thanks for that link. Those are some serious resistors and available in values between 0.069 and 0.185 ohms. If mounted open for convection cooling how many watts do you think they are good for? That would make it easy to change configurations and provide a nice toasty radiant warmth in the garage. 

I guess a first test is in order. I can take 4 TS cells and charge 2 up to 3.32 volts resting and discharge 2 to 3.27 volts resting and see how much current flows and how long it keeps up (how many amp hours quickly move to the low side.) I know when 2 cells are real close it is very little, but if just a little further apart exponentially speeds it up this may work.


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## major (Apr 4, 2008)

EVfun said:


> If mounted open for convection cooling how many watts do you think they are good for?


They are rated at 700 to 1000 watts, IIRC, depending on the exact model. Depending on how long and how much air over, I see no problem taking them to 3 or 4 kW, maybe higher. In that cabinet I have 24 of them. I built it to discharge large packs. I think the most I have done is 180V at 300A. So about 54kW. Over 2 kW apiece. Had the fans blowing and still discolored the paint up top on the cabinet. Those tests lasted about 15 minutes.

I guess I'd go 1 kW each in dead air. 2 kW with good fan. That is for long tests. Short tests, pour it on until they glow


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## rwaudio (May 22, 2008)

DIYguy said:


> Thanks for the reply Doug.
> 
> I have 66 Calb 180 Ah for a 211 volt nominal pack. I'll be looking for a high current, mid voltage set up. I am currently running a Soliton1 and it sags to about 170 volts (when it is warm out) at 1000 amps. That about 2.6vpc. This is right around where I want to be with motor volt limit. (Planned  ).
> 
> ...


Just to give you some data to play with, A123 20ah pouches sag to about 2.95v after 10 seconds at around 12C. At 5C they are still up over 3.1v, at 1.2C they maintain 3.3v till about 65% SOC and over 3.2v till around 35% SOC.

That's on the bench, I should have some data from cells in the car soon.


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## Sunking (Aug 10, 2009)

Well experience tells me with working with Telco battery plants when you add a parallel string of batteries you had better well make sure they are equalized. We use disconnects, very very expensive disconnects to take batteries on and off line. If the batteries are not within 1/4 volt of less on a 48 volt string and close in the disconnect, you will weld the contacts on your $5000 disconnect from the EQ currents. 

The way we EQ the string going back into service is with a portable charger. We charge up the battery to reach the plant operating voltage of 54 volts. Once it reaches 54 volts volts and the charge current taper off, we close it in.


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## DIYguy (Sep 18, 2008)

Sunking said:


> Well experience tells me with working with Telco battery plants when you add a parallel string of batteries you had better well make sure they are equalized. We use disconnects, very very expensive disconnects to take batteries on and off line. If the batteries are not within 1/4 volt of less on a 48 volt string and close in the disconnect, you will weld the contacts on your $5000 disconnect from the EQ currents.
> 
> The way we EQ the string going back into service is with a portable charger. We charge up the battery to reach the plant operating voltage of 54 volts. Once it reaches 54 volts volts and the charge current taper off, we close it in.


Thanks Dereck. yes, I could see where a votage imbalance at connection time could be spectacular. The first thing that comes to mind is that you may be referring to a lead acid application?? . . . If so, I can see that because of a steep charge/discharge curve, that SOC is more significantly represented by a voltage difference therefore making the problem that much more of one. With lithiums' fairly flat curve, this should be a lot less of a problem. . certainly within one chemistry, and perhaps a little more of a challenge with different ones, . . albeit still a rather flat curve that shouldn't be too hard to match. Obviously, the best way would be a more permanent connection.. .if possible. I dunno yet...


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## tomofreno (Mar 3, 2009)

There is this post on a string of LCP in parallel with a string of LFP, one 100Ah, the other 200Ah:
http://electric-vehicle-discussion-list.413529.n4.nabble.com/Thanks-Daddy-td3697491.html#a3702493

and this follow up:
http://electric-vehicle-discussion-...P-LFP-Reconfiguration-td3755643.html#a3757918


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## Sunking (Aug 10, 2009)

DIYguy said:


> Thanks Dereck. yes, I could see where a votage imbalance at connection time could be spectacular. The first thing that comes to mind is that you may be referring to a lead acid application?? . . .


Yes lead acid chemistry. With that said lead acid discharge curve is also pretty darn flat. In fact I believe if look across 0 to 100% charge levels lead acid is quite a bit flatter. 

Using static SOC levels a fully charged 48 volt lead acid battery is 50.8 volts, and 10% = 47.7 volts. LFP (16 series cells) at 100% is 67.2 down to 44.

The only thing that limits or regulates the EQ currents is the cable and connection resistances. No big deal with respect to cables, but is significant problem with the connection device, especially mechanical relay or switch types like the disconnect I mentioned earlier. I think it can be worked around like a solid state relay or some means to limit the EQ currents. 

But I can see where that might cause some problems. If you take current from one battery to EQ the added parallel battery will take current from the motor. If you are paying Peter, Paul has to be robbed.


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## GizmoEV (Nov 28, 2009)

Sunking said:


> Yes lead acid chemistry. With that said lead acid discharge curve is also pretty darn flat. In fact I believe if look across 0 to 100% charge levels lead acid is quite a bit flatter.
> 
> Using static SOC levels a fully charged 48 volt lead acid battery is 50.8 volts, and 10% = 47.7 volts. LFP (16 series cells) at 100% is 67.2 down to 44.


If that is true why is it that I see LESS voltage difference from full to empty with my LFP pack than I did with my lead acid pack? A fully charged 16 cell pack with out the surface charge is actually at 53.44V and at empty it is at 48V if you don't run them into the ground. The difference is that the slope of the discharge curve of a lead acid pack is steeper than the majority of the discharge curve of a LFP pack.

What you are confusing is the charge cutoff voltage and the fact that charging is supposed to stop when the cutoff voltage is reached AND current has tapered to 0.05C. Do that and you will see what I'm talking about. Besides, if you continually charge your LFP pack to 4.2V with a low ending current you will shorten their life. Less than 1% of the capacity of a LFP cell is above 3.5V.

When I used to charge to 4.00V with a minimal ending current I was actually overcharging my cells. The energy between 4.00V and 3.4V would only take my rig ~100 feet!


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## EVfun (Mar 14, 2010)

My pack of 32 cells spends almost all its time between 102 volts and 106 volts when unloaded. Yes, it is a little more right off a charge and some cycles I can get it a little lower, but in any normal situation I'm looking at up to 4 volts to move the charge between the packs. Some rough calculations indicate a max Headway charge rate from the TS cells would be 26 amps at 4 volts. 

That is based on zero connection resistance, the Headway cell straps being about 4 gauge (20mm wide by 1mm thick) and 1.6 inches long each, the TS braided cell straps being 1/0 and 2.4 inches long each, and 18 feet of 1/0 cable between the packs (a slight exaggeration.) I added to that the cell resistance of both strings in the best case (0.145 times 32 for the TS cells, and 0.0032 times 32 for the Headway cells.) 

When both packs are equally charged the Headway pack should supply 31% of the current and the TS pack 69% of the total current (about 200 amps from the Headways when the TS cells are dishing out 200 amps.) But if they are only recharging at 26 amps peak, and if they are close to the same SOC perhaps half of that, I don't see how they could share the load very well.


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## GizmoEV (Nov 28, 2009)

Get a CycleAnalyst and data logger for each string and hook things up and take it for a spin.


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## EVfun (Mar 14, 2010)

I don't own any Headway cells to make the hybrid pack with. I was looking at the Headway cells we are experimenting with at work. I stacked 4 of the 24 volt (8 cell) packs on top of each other to move them and suddenly realized just how small a 102 volt pack of Headway cells was. 

I do plan to pull off a test between 2, 4 cell packs of TS cells to see what kind of cross charge rate there is when one pack is about 20% SOC and the other at 80% SOC.


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## DIYguy (Sep 18, 2008)

Sunking said:


> Yes lead acid chemistry. With that said lead acid discharge curve is also pretty darn flat. In fact I believe if look across 0 to 100% charge levels lead acid is quite a bit flatter.


In the area of typical use... there is no comparison between the discharge curves of lead vs lithium.


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## Ziggythewiz (May 16, 2010)

From currentevtech's site: 
Lithium *Recommended Charged Voltage*: 3.8 *Recommended Discharge Cutoff*: 2.8
Thats a 26% voltage difference from full to empty.

With my lead 13.2 is full, 12 is empty...if you go only 50% which is recommended, that's to 12.7v. That's 4%. Pretty dang flat.

We're not trying to say that lead acid is better, just that it will have less % voltage drop over use than lithium.


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## DIYguy (Sep 18, 2008)

Ziggythewiz said:


> From currentevtech's site:
> Lithium *Recommended Charged Voltage*: 3.8 *Recommended Discharge Cutoff*: 2.8
> Thats a 26% voltage difference from full to empty.
> 
> ...


Yes, I understand and I would say this is still not really correct for practical usage. I think the to clear up the point of the discharge curve discussion as it relates to this dual pack issue one must focus on the voltage level during the bulk (90%) of practical usage area. After lifepo is removed from charge, it sits right around 3.35v or so IIRC. As soon as you start to use it, it hangs around 3.2 volts for most of its "life". Lead acid starts a steady and fairly linear decline though out it's discharge "life". Herein lies the real difference. If you take the pics above and lop off the beginning and the ends then compare the two you see really the part that is always used.


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## dougingraham (Jul 26, 2011)

Ziggythewiz said:


> From currentevtech's site:
> Lithium *Recommended Charged Voltage*: 3.8 *Recommended Discharge Cutoff*: 2.8
> Thats a 26% voltage difference from full to empty.
> 
> ...


I think your numbers for the LA are valid only for C/20 discharge. Under a 1C useful load they are empty at maybe 6v? This would be more like 50%. And it gets lots worse if you go to 2C or 3C. For LiFePo4 the recommended charge voltage has nothing to do with the discharge. The LiFEPo4 cells full charge resting voltage is 3.45 and they are pretty much depleted at 3.0 volts. That is only 12%. At higher discharge rates the differences between the two chemistries only increases.


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## GizmoEV (Nov 28, 2009)

dougingraham said:


> The LiFEPo4 cells full charge resting voltage is 3.45 and they are pretty much depleted at 3.0 volts.


If you have a LiFePO4 cell resting at 3.45V it has been overcharged. Resting voltage is below 3.4V. It should be around 3.34-3.38V.

As for pack voltage differences between lead acid and LiFePO4, if lead acid batteries are not used beyond 50%SOC for the voltage difference then LiFePO4 batteries shouldn't be used beyond 80%SOC. By direct measurement after a long hard run and with a 6A load still on a 200Ah pack with 179.6Ah removed the cell voltages were between 3.177V and 3.192V with an average of 3.186V. Full voltage of 3.35V or even 3.38V to 3.18V is a difference of only 0.20V between 100%SOC and 80%SOC. This isn't made up. This is direct measurement. Lead acid never did this low of a voltage drop for me. Look at the graphs that were presented. If you don't believe them or my measurements then go measure them yourself. Play with some LiFePO4 cells so you can get a direct knowledge of what these cells are really like.


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## CFreeman54 (Jan 14, 2009)

I have a AGM pack (13 Exide Orbitals) paralleled to a Thundersky pack (52 100 Ah cells). They are connected when the main contactors are engaged. The fully charged resting voltage of the AGMs is about 170V and the LiFePO4s about 175 V. The TS keep the Orbitals recharged initially. When the Thunderskys get below about 3.25 per cell then the Orbitals start to drain. The main problem I have encountered is that the EV Display monitor measuring the Li pack SOC seems to be confused by the exchange of current between the packs.


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## GizmoEV (Nov 28, 2009)

CFreeman54 said:


> The main problem I have encountered is that the EV Display monitor measuring the Li pack SOC seems to be confused by the exchange of current between the packs.


How does it get confused? It should just act like it should during a discharge and regen or charging. My CA doesn't have any problem with either situation but then I don't have two different packs. It just does fine with regen and charging.


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## EVfun (Mar 14, 2010)

tomofreno said:


> There is this post on a string of LCP in parallel with a string of LFP, one 100Ah, the other 200Ah:
> http://electric-vehicle-discussion-list.413529.n4.nabble.com/Thanks-Daddy-td3697491.html#a3702493
> 
> and this follow up:
> http://electric-vehicle-discussion-...P-LFP-Reconfiguration-td3755643.html#a3757918


Thanks for the links. There is an interesting experiment there. It doesn't seem to reach a conclusion. 

One thought is a pack of 30 Headway cells that are only connected to the 32 TS cells when the car is on. Under 100 amps the TS cells should actually be charging the HW cells (up to about 3.54 vpc for the HW cells at no load) At about 150 amps the HW cells should be doing nothing. At 450 amps from the TS cells the HW cells should provide an extra 150 amps. Well... that's the theory. 

This is starting to seem like a lot of trouble for 1/3rd more power.


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## DIYguy (Sep 18, 2008)

GizmoEV said:


> If you have a LiFePO4 cell resting at 3.45V it has been overcharged. Resting voltage is below 3.4V. It should be around 3.34-3.38V.
> 
> As for pack voltage differences between lead acid and LiFePO4, if lead acid batteries are not used beyond 50%SOC for the voltage difference then LiFePO4 batteries shouldn't be used beyond 80%SOC. By direct measurement after a long hard run and with a 6A load still on a 200Ah pack with 179.6Ah removed the cell voltages were between 3.177V and 3.192V with an average of 3.186V. Full voltage of 3.35V or even 3.38V to 3.18V is a difference of only 0.20V between 100%SOC and 80%SOC. This isn't made up. This is direct measurement. Lead acid never did this low of a voltage drop for me. Look at the graphs that were presented. If you don't believe them or my measurements then go measure them yourself. Play with some LiFePO4 cells so you can get a direct knowledge of what these cells are really like.


+1 Exactly my experience also. I had two lead packs b4 the lithium.


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## DIYguy (Sep 18, 2008)

CFreeman54 said:


> I have a AGM pack (13 Exide Orbitals) paralleled to a Thundersky pack (52 100 Ah cells). They are connected when the main contactors are engaged. The fully charged resting voltage of the AGMs is about 170V and the LiFePO4s about 175 V. The TS keep the Orbitals recharged initially. When the Thunderskys get below about 3.25 per cell then the Orbitals start to drain. The main problem I have encountered is that the EV Display monitor measuring the Li pack SOC seems to be confused by the exchange of current between the packs.


Interesting. Thanks for posting. I like real world data/experience.  How long/how many miles have you been running this dual pack? I assume you are charging them separately?


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## CFreeman54 (Jan 14, 2009)

The EV Display quickly goes to zero SOC with about two miles of driving. A multimeter check of the Lithium cells still shows them at about 3.31 V or so. Sometimes overnight the SOC increases.....very mysterious since the circuit is broken at the main contactors. I am thinking it may be a computer board problem in the EV display rather than confusion with the parallel packs. I would also think it should handle it just like discharge and regen or charging. For whatever reason, it is not though.

Yes, I am charging the packs separately, but on short trips (12 miles or so) there is no need to recharge the AGM pack. The Li pack keeps it topped off very well until its voltage sinks down to around 170 V, close to the same as the AGM pack. The first test drive was a year ago, but due to some brake/suspension issues that were just resolved I only have about 130 miles on it so far. The longest trip to date without recharging was about 40 miles. At that point the low Li cell was at 3.17 V and the AGMs according to the pack tracker were at about 70% SOC according to my memory....I can check the log and tell you exactly.


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## dtbaker (Jan 5, 2008)

CFreeman54 said:


> The EV Display quickly goes to zero SOC with about two miles of driving. A multimeter check of the Lithium cells still shows them at about 3.31 V or so. Sometimes overnight the SOC increases.....very mysterious since the circuit is broken at the main contactors.


I am remembering the evDisplay should be powered on the pack ALL the time as it counts on the end-of-charge voltage to 'reset' itself and accurately count ah in and out and maintain temp compensation. the charge amps IN have to go thru the evDisplay meter, and it needs to hit the peak voltage to reset and re-zero from what I have read.

A cell showing 3.31v after resting really is not much indication of the actual SOC, although its certainly not 'empty'.


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## dtbaker (Jan 5, 2008)

CFreeman54 said:


> I have a AGM pack (13 Exide Orbitals) paralleled to a Thundersky pack (52 100 Ah cells). They are connected when the main contactors are engaged. The fully charged resting voltage of the AGMs is about 170V and the LiFePO4s about 175 V. The TS keep the Orbitals recharged initially. When the Thunderskys get below about 3.25 per cell then the Orbitals start to drain. The main problem I have encountered is that the EV Display monitor measuring the Li pack SOC seems to be confused by the exchange of current between the packs.



no doubt the evDisplay is confused by the current flow between packs... I am too!

Seems like the combination of different rest voltage of the packs, and different sag under load and voltage drop of the AGMs during discharge would make it pretty hard to tell who was charging or draining whom at any given moment when driving under load....


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> I am remembering the evDisplay should be powered on the pack ALL the time as it counts on the end-of-charge voltage to 'reset' itself and accurately count ah in and out and maintain temp compensation. the charge amps IN have to go thru the evDisplay meter, and it needs to hit the peak voltage to reset and re-zero from what I have read.


I bet that right there is the problem. Raise the voltage at which the display resets to something above what you usually see when not charging and see if it starts to work properly. If you can reset it manually then raise the reset point way above anything you see from the pack to make sure the auto-reset doesn't function and then go for a drive. Maybe that will "fix" the problem.


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## CFreeman54 (Jan 14, 2009)

The EV Display is hooked to both the most + and - of the Li pack and to the 12V auxiliary battery for power. Its reset voltage is at 190V. When charging the Li pack reaches about 188V. It drops to about 180 V after a few hours. As soon as the contactors are engaged it drops to 175V. The hall effect sensor for the EV Display is only seeing current flow from (and to) the Li pack. Since it reads it as + in one direction and - in the other, I do not understand why it can not "keep track" of current going to the other parallel pack, or even coming back from it. It is still basically charging and discharging......


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## CFreeman54 (Jan 14, 2009)

Also, the other day I only engaged only the Li pack (with its main contactor) and took a short drive around the block. The EV Display still went to zero SOC. There is no way 17 KWH from the Li pack can be used up on a 2 - 3 mile drive! Since the circuit was broken there should have been no way for current to shuttle to the AGM pack either, which makes me doubt the EV Display.....?


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## dtbaker (Jan 5, 2008)

CFreeman54 said:


> ....which makes me doubt the EV Display.....?


I still am having trouble visualizing what you have going on...

I would suggest sketching out your schematic, including voltages and config of packs, where the evDisplay is connected, post here and perhaps contact Dimitri directly and invite him to comment in this thread as a matter of public interest, information, and education....


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## Coulomb (Apr 22, 2009)

CFreeman54 said:


> [ EV Display ] Its reset voltage is at 190V. When charging the Li pack reaches about 188V.


Well, that's why it won't zero automatically; you want the reset voltage to be between 180 and 188, say 184 V.



> It drops to about 180 V after a few hours. As soon as the contactors are engaged it drops to 175V.


Is that because it then has the lead acid pack across it? 

So it seems to be saying that the pulse of current charging the lead acid pack is enough to drain the pack.

Surely it's not using the voltage of the pack as an SOC measure? Maybe it's tricked by the sudden dV/dt (rate of change of voltage).

I assume that you have set the capacity of the pack correctly in the EV display? If the capacity is set way too low, that would help explain the symptoms.


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## CFreeman54 (Jan 14, 2009)

The directions for the EV display say to set the voltage setting as close above the max charge voltage seen as possible, therefore I chose the 190V setting. 

Yes, the voltage drop from 180 to 175V is because of the Pb pack, but the EV Display is showing only 6.0-7.0 amps of current being pulled. At least 2.5-3.0 amps of that must be from from the DC-DC converter, because it shows that 2.5-3.0 amps even without the Pb pack contactors engaged. 

Yes, the EV Display capacity is set for 100 Ah cells.


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## CFreeman54 (Jan 14, 2009)

Here is the battery wiring schematic. I haven't drawn in the EV Display hall effect sensor yet.


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## dimitri (May 16, 2008)

CFreeman54 said:


> The directions for the EV display say to set the voltage setting as close above the max charge voltage seen as possible, therefore I chose the 190V setting.
> 
> Yes, the voltage drop from 180 to 175V is because of the Pb pack, but the EV Display is showing only 6.0-7.0 amps of current being pulled. At least 2.5-3.0 amps of that must be from from the DC-DC converter, because it shows that 2.5-3.0 amps even without the Pb pack contactors engaged.
> 
> Yes, the EV Display capacity is set for 100 Ah cells.


First off, pack voltage setting is irrelevant to the issue at hand. Pack voltage is just another way to sync a full charge, but it has nothing to do with quick drain of SOC value. SOC is purely based on current, so you have to set display to show current and observe its values as you drive, to make sure its not grossly out of range, which could indicate a noise on the line, causing current reading to glitch.

The whole EV Display issue is really off topic for this thread, I stumbled on it accidentally. Perhaps you can contact me directly for troubleshooting at [email protected]

I hate to hijack the thread with unrelated issues. You setup EV Display correctly and it should not be draining SOC that fast. So we have to check all other settings and make sure current is reading correctly.


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## dtbaker (Jan 5, 2008)

CFreeman54 said:


> Here is the battery wiring schematic. I haven't drawn in the EV Display hall effect sensor yet.



....well, thats a pretty key piece of info to make sure it is actually measuring the current in/out of the Li pack. Are you zero-ing after full charge?

at end of charge, what is the highest voltage on the AGM pack and on the TS pack?

at rest after charge before driving, what is the no-load voltage on each pack?

after driving a couple minutes, what is the no-load voltage on each pack?



what I am trying to get at is to understand what is most likely going on as current shuttles between the Li and AGM packs as you close contactors and as the system comes under load, and then as the AGM voltage drops during use from its start voltage to significantly lower end voltage, and sags (more than Li) under load.

what I would be afraid of is that the Li are draining in an attempt to parallel with the AGMs which are falling to a lower voltage during the normal discharge, or sag under load, and the current from the Li pack is draining into the AGMs.

Have you disconnected the AGMs to see if the evDisplay and range makes sense on the Li pack alone?


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## EVfun (Mar 14, 2010)

CFreeman54 said:


> Here is the battery wiring schematic. I haven't drawn in the EV Display hall effect sensor yet.


That is a key piece of information. I don't see anything wrong with the schematic but since the SOC tracks amp hours it is hard to say what might be wrong without knowing where it is reading.


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## CFreeman54 (Jan 14, 2009)

Thanks Dimitri. I suspected the pack voltage setting was not the problem, but was responding to someone's question. I will contact you at cleanpowerauto.com to see if you can help me figure this out. 

The hall effect sensor is between the two cells in the group of four in the right lower- most corner of the schematic. Also, before someone notices the discrepancy, two additional Li cells were added to the group in the lower left hand corner of the schematic since this schematic was drawn. In any case, the sensor is reading current through the Li pack going to both the AGMs and the motor. 

At end of charge the highest (temporary) voltage is 188V Li and 172V AGM.
At rest before driving (and before closing contactors) voltage is about 180 (Li) and 170 (AGM).
On closing contactors voltage is 175 V (Li) and 173 V (AGM) [assuming AGM pack was fully charged].
They stay close to that for several minutes of driving. After 15-20 miles the Li pack is around 170 V and the AGM pack is around 169 V. 

The digital volt meter on the Li pack shows transient sags down to around 155V under hard acceleration. The analog voltmeter showing combined Li and AGM voltage shows almost no sag at all. The analog meter is marked in 5V increments, and needle sag is less than that. The Orbitals can apparently give bursts of 1800 A, so pulling 800 A or so when the Li is contributing some current too doesn't seem to effect them much.

Not engaging the AGM pack (at the contactor) does not seem to resolve the Lithium pack monitoring glitch, so the problem may not be pack interaction. Other than for that, the parallel packs seems to be working fine. It complicates charging a bit; I charge the packs separately. On the up side if a cell goes bad in one pack you can still drive home on the other pack.


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## dtbaker (Jan 5, 2008)

CFreeman54 said:


> At rest before driving (and before closing contactors) voltage is about 180 (Li) and 170 (AGM).
> On closing contactors voltage is 175 V (Li) and 173 V (AGM) [assuming AGM pack was fully charged].
> They stay close to that for several minutes of driving. After 15-20 miles the Li pack is around 170 V and the AGM pack is around 169 V.


this sounds bad to me.... the voltage mismatch means as soon as you put them in parallel, the Li is dumping like crazy into (over) charging the AGMs rather than doing anything useful. The evDisplay may even be right since it sounds likely you are dumping most of the Li energy into the AGMs.

I don't see why you want to run in parallel with two different types having different voltage, IR and peukarts anyway.... seems it would be way better to run one pack down, then switch to the other as a reserve... never in parallel.

if you run the packs sequentially, you don't see performance you expect?


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## CFreeman54 (Jan 14, 2009)

Dan: What leads you to believe the AGMs are over charging? If the EV Display is accurate only 2.0 -3.0 Amps of current are flowing to the AGMs (when at rest). That is well below what the charger gives them unless they are full. The shunt resistors on the BMS could burn off that amount if needed, but since the AGMs aren't going overvoltage the resistors never turn on. The AGMs are basically never above float charge voltage the whole time (13.8 V). The AGMs are only 100% full and receiving current from the Li pack for the few seconds if they have been fully charged and before I start driving. Several miles of driving does take them down to 99% or 98% according to the Paktrakr....still basically full, but not overcharged as far as I can tell....

WHen driving the Li sags to 155V or so, below the AGM voltage (160-170V) and the AGMs are supplying current. At rest the Li are recharging the AGMs. The Lithiums are protected from a 12C draw during full acceleration and the AGMs are protected from full discharge down to 20% SOC that would occur if they were not being continually recharged. It seems to me both batteries benefit from the paralleling. 

The idea was to protect my 100 Ah LiFePO4 cells from 1200A draws..... that will surely shorten their lifespan.


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## dimitri (May 16, 2008)

There is nothing wrong with what you did. You have well matched voltages between 2 packs such that current exchange between them is within typical levels for these batteries. Same approach is used in PHEV kits for Prius, it works fine there too. There is nothing wrong in general with paralleling 2 different chemistry packs, as long as you did your homework and ensured proper voltage and current matching to keep both packs happy. I don't know what the fuss is about.

Paralleling 2 identical chemistry packs is a different story, since its better to parallel individual cells rather than strings of cells, but only because its easier to manage them in single string.

About your EV Display, please Email me all your settings as they show in setup menu and make sure test mode is not on, as that would make SOC swing 100% to 0% and back every 100 seconds.


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## dtbaker (Jan 5, 2008)

CFreeman54 said:


> Dan: What leads you to believe the AGMs are over charging?


I don't know that much about AGM charge curves because I never used them.... but it seems to me that if the Li comes online at 180v, and the AGM at 170v and drop steadily during use, that the Li will be constantly dumping amps into the AGM trying to balance the pack's voltages, and the 'middle' at 175v is probably up above the useful charge voltage for the AGMs and into the land where they are 'full' and just getting a surface charge.



CFreeman54 said:


> WHen driving the Li sags to 155V or so, below the AGM voltage (160-170V) and the AGMs are supplying current.


this sounds really weird to me. I think its a bad sign for your Li to sag from 180v to 155v except perhaps under the heaviest loads.



CFreeman54 said:


> The idea was to protect my 100 Ah LiFePO4 cells from 1200A draws..... that will surely shorten their lifespan.


you pull 1200 amps very often? if racing, it seems that carrying all that lead around would be a big liability?


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## dimitri (May 16, 2008)

dtbaker said:


> this sounds really weird to me. I think its a bad sign for your Li to sag from 180v to 155v except perhaps under the heaviest loads.


Do the math and pay attention to what he said. 50 LiFe cells at 155V is 3.1V, so no heavy sag to speak of. 180V is just off the end of charge, at 3.6V per cell, so no issue there either.

His setup is fine for what he wants to do, despite being unusual. If you can't afford large Li pack and happen to have good AGMs on hand, why not? Sure its ugly and heavy, but it does the job for him. I'm sure one day when AGMs bite the dust he will get more LiFe cells.


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## DIYguy (Sep 18, 2008)

I find this very interesting right now. I think your set up is fine also CFreeman. I will be installing a small (in the beginning) li-poly pack with my lifepo prismatics. I am looking at voltage levels at charge, discharge and idle. For my 66 cell lifepo pack, I will probably use a 55 cell poly pack. I will not have this pack connected all the time though, only for fun . 

My charger set to 3.45 vac for the lifepo will charge the poly to 4.1vpc which is 92%... and as much as I would want in them anyways. The sag voltage should match up pretty close also when drawing a few thousand amps. Im a little concerned about flipping the contactor on for the poly pack when it's sitting at 4.1vpc and the lifepo pack has discharged down to say 3.2vpc. That would be a difference between 211 and 225 or 14volts. Like your system, the voltage will stabilize somewhere in the middle and there will be some discharge...but by then, the lights will be green. 

I'm still considering separate charging also. I will start with one 11 series (x5 series) pack and then parallel a few more after I have a warm and fuzzy. Thoughts??


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## dimitri (May 16, 2008)

14V difference at 200V levels is nothing to worry about. It will equalize almost immediately and inrush current won't be too large. Difficult to calculate in theory due to many unknown factors, but experience tells me it will be just fine.


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## DIYguy (Sep 18, 2008)

dimitri said:


> 14V difference at 200V levels is nothing to worry about. It will equalize almost immediately and inrush current won't be too large. Difficult to calculate in theory due to many unknown factors, but experience tells me it will be just fine.


Thanks Dimitri. Fully charged and at rest, 3.35 lifepo vs 4.1 poly the voltage will be only a couple volts apart.


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## Yukon_Shane (Jul 15, 2010)

great thread.

Any updates from EVfun or DIY on this? Have either of you pursued this idea any further?

I won’t know for sure until my build is on the road but I think I might find my available pack power to be a bit of a limitation on my daily commute. 

I currently have a 160V, 100ah LiFePO4 pack that’s probably 2-3 times what I need from a range perspective; however, I have a nearly 1 km long, 8% grade hill that I need to climb at around 80km/h on my way home from work every day. My concern is I’ll regularly have to exceed the 3C rating of my cells to make that climb.

If I paralleled a small pack of headways (or A123 pouch cells) I could maintain the 3C limit on the Winston batteries and still put 600amps to the motor (96 kW or 130 hp which should be plenty).

I’m surprised this approach isn’t taken more often!


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## DIYguy (Sep 18, 2008)

Yukon_Shane said:


> great thread.
> 
> Any updates from EVfun or DIY on this? Have either of you pursued this idea any further?
> 
> ...


YEs, I've been building a performance add on pack. After doing more research though and some help from a more experienced battery guy. . . I decided that the lipo is not a good match and that you need to match the chemistry. Charge/discharge curves need to be close/same. So, I am building an A123 high discharge pack to put with my CALB's. Same voltage. So, it will be seamless. It will charge with the Calbs and discharge hard when needed, recharging directly after. The A123's I'm using are 35C continuous, 70C peak. I confirmed this with testing.


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## ga2500ev (Apr 20, 2008)

dimitri said:


> Do the math and pay attention to what he said. 50 LiFe cells at 155V is 3.1V, so no heavy sag to speak of. 180V is just off the end of charge, at 3.6V per cell, so no issue there either.
> 
> His setup is fine for what he wants to do, despite being unusual. If you can't afford large Li pack and happen to have good AGMs on hand, why not? Sure its ugly and heavy, but it does the job for him. I'm sure one day when AGMs bite the dust he will get more LiFe cells.


After a long hiatus off the list, I was thinking about coming back and starting a new thread about a very similar setup. I'm glad that I found this one. I just wanted to clarify exactly how things work.

13 AGMs @ 13.2V (100% SOC) -> 171.6V
52 100 Ah LiFePo4 @ 3.2V (under load) -> 166.4V

The resting voltage for the LiFePo4 pack is a bit higher.

One question: what the total energy (in wH) of each pack?

My main question is what exactly does each pack contribute under load? From a voltage standpoint the LiFePo4 pack will sag less under load than the AGMs right? Or will the AGM's pack voltage remain above the Lifepo4's steady voltage under load.

I know that the setup is designed to have the AGM's do the heavy lifting of the load, and the energy of the LiFePo's pack to replenish the AGM's as their SoC drops. But exactly how does the setup protect the LiFePo4's pack from excessive current draw?

I'm extremely interested because this looks like a path for integrating a smaller capacity Li pack (20-60 Ah cells, which are cheaper individually) to a cheaper to capitalize Lead/Acid pack. It's possible to get a vehicle out on the road with $1000 worth of lead acid. Not so much with Li.

Thanks for any info that you can offer...

ga2500ev


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## DIYguy (Sep 18, 2008)

ga2500ev said:


> After a long hiatus off the list, I was thinking about coming back and starting a new thread about a very similar setup. I'm glad that I found this one. I just wanted to clarify exactly how things work.
> 
> 13 AGMs @ 13.2V (100% SOC) -> 171.6V
> 52 100 Ah LiFePo4 @ 3.2V (under load) -> 166.4V
> ...


you need to get a lot more specific with each chemistry. There are AGM's that can really belt out the current. There is also Lifepo that can really belt it out, and others that can't.

I know how mine will load share at a specific voltage due to testing each one. . . but these are both lifepo. . . different designs...


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## Yukon_Shane (Jul 15, 2010)

DIYguy said:


> YEs, I've been building a performance add on pack. After doing more research though and some help from a more experienced battery guy. . . I decided that the lipo is not a good match and that you need to match the chemistry. Charge/discharge curves need to be close/same. So, I am building an A123 high discharge pack to put with my CALB's. Same voltage. So, it will be seamless. It will charge with the Calbs and discharge hard when needed, recharging directly after. The A123's I'm using are 35C continuous, 70C peak. I confirmed this with testing.


This sounds very promising. Keep us updated. I'm keen to hear the specifics of your design and how it all works out.


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## ga2500ev (Apr 20, 2008)

DIYguy said:


> you need to get a lot more specific with each chemistry. There are AGM's that can really belt out the current. There is also Lifepo that can really belt it out, and others that can't.
> 
> I know how mine will load share at a specific voltage due to testing each one. . . but these are both lifepo. . . different designs...


I had a feeling that this wasn't going to be easy. Let me shed some more light...

Lithium is better. Period. I understand that. However, my wallet cannot handle the capitalization costs of a Li only pack. I figure once the price comes down enough that a $1000 Li pack that can go 20 miles exists, that lead acid will be toast. A $1000 LA pack is doable, but it'll be $1000 every year or two.

So I'm going to start out with Lead Acid. I'm trying to figure out how to supplement that pack with Li as I can scrape up more funds. This is why I'm interested.

CFreeman pointed out the exact reason why a hybrid LA/Li pack seems attractive: by sharing the current load, the LA pack can protect the Li pack from high current draws while the Li pack can lessen the currrent load of the LA pack, thus reducing the Peukert effect on the LA pack, extending its range. Literally I'm looking for a Li range extender to a LA primary pack.

The missing component is how to current limit the Li pack under load. I want that pack to contribute its energy to the system, but not at an excessive current rate which can shorten its life. a 2C-3C continuous draw on a 40-60 Ah pack isn't sufficient to motivate a standard conversion long term. That's why Li packs are made of cells of 100 Ah and up. At $125 a cell typical cost and a requirement of 30-50 cells for a pack, that really adds up.

A reasonably priced 40 Ah cell is a third of the price with a 2C max continuous current draw. The question is how can a pack be constructed such that the power can be extracted without exceeding that current limit.

Some more research seems to point to putting a DC-DC converter between the auxillary Li pack and the rest of the system. Since the Li pack isn't the prime motivator and must be current limited, there isn't too much of a problem with a converter inbetween. Opens up the possibility of having a lower voltage pack with a flyback converter upping the output voltage to the primary pack voltage. That system can call for the Li pack to provide constant current limited power to supplement the primary pack.

Looks like more reading is in order...

ga2500ev


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## cruisin (Jun 3, 2009)

ga2500ev said:


> I had a feeling that this wasn't going to be easy. Let me shed some more light...
> 
> Lithium is better. Period. I understand that. However, my wallet cannot handle the capitalization costs of a Li only pack. I figure once the price comes down enough that a $1000 Li pack that can go 20 miles exists, that lead acid will be toast. A $1000 LA pack is doable, but it'll be $1000 every year or two.
> 
> ...


Waiting for the price to come down on Li-ion? Thats like waiting for the cows to come home. With ALL car manufacturers ramping up ALL of their models as a EV, just when do you think the demand will be less than the supply which will bring the prices down? Dont hold your breath, buy the lead acid and save up so you can replace them in a couple of years. My 2 cents worth towards whatever you buy.


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## DIYguy (Sep 18, 2008)

ga2500ev said:


> I had a feeling that this wasn't going to be easy. Let me shed some more light...
> 
> Lithium is better. Period. I understand that. However, my wallet cannot handle the capitalization costs of a Li only pack. I figure once the price comes down enough that a $1000 Li pack that can go 20 miles exists, that lead acid will be toast. A $1000 LA pack is doable, but it'll be $1000 every year or two.
> 
> ...



If u want to start with lead and mix chemistries, just buy enough A123 Amp20 pouch cells to match ur pb pack. U can do a 144 volt 20ah li pack for $1000. Just add to it when u can afford it. Not sure how u can do a decent lead pack for $1000 tho. ?? You don't need to limit current and u don't need anything between the packs like a dc/dc. The trickiest part is making sure u charge each chemistry appropriately.


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## ga2500ev (Apr 20, 2008)

DIYguy said:


> If u want to start with lead and mix chemistries, just buy enough A123 Amp20 pouch cells to match ur pb pack. U can do a 144 volt 20ah li pack for $1000. Just add to it when u can afford it.


I've been looking. There's a trailer full of issues there. First off at that price point (45 cells @ $22 each) they are definitely gray market. Reports have been sporadic with some folks receiving duds. While the 20C draw makes them useful, to get the full benefit, the pouches have to be properly encased and clamped into battery boxes. The final issue is that for all that work it only gets you 2.8 kWh of capacity. 



> Not sure how u can do a decent lead pack for $1000 tho. ??


Decent is debatable I admit. To balance cost and weight I'm thinking of starting with 12 8V GC8 batteries for a 96V pack. Nominal 15.8 kWh right at $1000 from Sam's Club. Probably only will last a year but gets my foot in the door. With an expectation of getting half the available energy in usable form I guessing that the range will be a bit north of 25 miles @ 300 Wh/mile.



> You don't need to limit current and u don't need anything between the packs like a dc/dc. The trickiest part is making sure u charge each chemistry appropriately.


Charging is of course a separate (and as you point out, tricky) issue. But I don't understand why current limiting isn't needed. What determines the current load sharing arrangement between the two packs if they are simply paralleled. Every Li cell I see has a continuous and pulse C rating. For low Ah packs, the motor current draw will often exceed those ratings. As Cfreeman pointed out, he wanted to protect his 100 Ah pack from 12C (1200A) draws. My objective is to create a system will 80A max is drawn from the 40 Ah Li pack and any remaining required current is drawn from the LA pack.

30 40Ah cells is in the ballpark of $1600, adds 3 kWh of usable energy, and weighs a bit more than 100 lbs. Added to the LA pack, it would have more total energy and less than half the cost of the cheapest 30x100Ah pack I could find.

ga2500ev


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## Ziggythewiz (May 16, 2010)

ga2500ev said:


> 30 40Ah cells is in the ballpark of $1600, adds 3 kWh of usable energy, and weighs a bit more than 100 lbs. Added to the LA pack, it would have more total energy and less than half the cost of the cheapest 30x100Ah pack I could find.
> 
> ga2500ev


Why not just do 30 x 60 ah (less than the $2600) for starters, and skip the lead?


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## ga2500ev (Apr 20, 2008)

Ziggythewiz said:


> Why not just do 30 x 60 ah (less than the $2600) for starters, and skip the lead?


Because I have access to $1000 now but not $2600. I'm not planning on building this dual pack anytime soon. All of this is trying to manage cash flow.

If I had enough cash, I'd go buy 40 160 Ah cells currently on sale at CurrentEv. It would run $7k, gives a 20 kWh pack, and solves all my problems for $0.34/Wh.

There's no question what to do if you have the capital up front. My question is what do you do when you don't. Wait a year? Two? seven?12 8V GC batteries are a bit over $1000 with nearly 8kWh or usable energy. That's $0.13/Wh.

I understand that over a 5 year period that the lead cost more since it'll require 2 replacements to none over the lithium. And that the pack has 1/3 the usable energy of the $7k pack. But the overriding variable is that I have access to the $1k now, not the $2600 or $7k.


My handle on this board has my budget: $2500 for the initial conversion. So far I have the glider, motor, adapter plate stock, and most of the electronics to build a starter controller and charger using designs from this board. I'm in about $1200 so far. And that's taken nearly 4 years to scrape up.

All of this is modular pack management from a cost/time standpoint. What is the best that I can do with today's dollars, and dollars that I may have 2 or three years from now, as opposed to capital that I don't have access to. BTW I doing this cash and carry, no credit allowed.

I'm open to suggestions...

ga2500ev


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## DIYguy (Sep 18, 2008)

ga2500ev said:


> I've been looking. There's a trailer full of issues there. First off at that price point (45 cells @ $22 each) they are definitely gray market. Reports have been sporadic with some folks receiving duds. While the 20C draw makes them useful, to get the full benefit,


Apparently, there are prices now down below $20. $17.50 or something like that if I recall correctly. Watch JR show. From what I have seen/recall (I'm not pursuing it but...) there is every indication that these cells are absolutely fine. The few slagging them have other interests perhaps? 



ga2500ev said:


> the pouches have to be properly encased and clamped into battery boxes. The final issue is that for all that work it only gets you 2.8 kWh of capacity.


 It's true, there is some work there. There are other options also though. As in cylindrical lifepo.




ga2500ev said:


> Decent is debatable I admit. To balance cost and weight I'm thinking of starting with 12 8V GC8 batteries for a 96V pack. Nominal 15.8 kWh right at $1000 from Sam's Club. Probably only will last a year but gets my foot in the door. With an expectation of getting half the available energy in usable form I guessing that the range will be a bit north of 25 miles @ 300 Wh/mile.


 Aren't the SC 8v batteries floodies? I thought u were talking AGM, which had me confused because a decent set of AGM's are a lot closer to Li prices. Additionally, AGM's require balance BMS and most can dish out high current. . . albeit some don't last long doing it. These benefit by paring with Li mostly on the peukerts front.
Floodies balance by boiling, don't need a bms and can benefit not only from peukerts when paired with Li, but also make up for their limited discharge rates.



ga2500ev said:


> Charging is of course a separate (and as you point out, tricky) issue.


It's only tricky in so much as the packs can't be done together with the same charger. You need separate chargers, or one that is set-able for chemistry. . . and one at a time.



ga2500ev said:


> I don't understand why current limiting isn't needed. What determines the current load sharing arrangement between the two packs if they are simply paralleled.


Simply internal resistance. They load share according to the resistance at a specific voltage drop. After each high discharge, and subsequent "unbalance" due to IR differences, they re-balance due to parallel connection. 



ga2500ev said:


> Every lithium cell I see has a continuous and pulse C rating. For low Ah packs, the motor current draw will often exceed those ratings. As Cfreeman pointed out, he wanted to protect his 100 Ah pack from 12C (1200A) draws. My objective is to create a system will 80A max is drawn from the 40 Ah Li pack and any remaining required current is drawn from the LA pack.


 The upper limitation pulse C rate is usually a function of, again, IR. They just won't give you more than that. As long as you don't run high continuous rates and cause excessive heating, short duration bursts are likely of little if any harm. Some brands exhibit this more than others.



ga2500ev said:


> 30 40Ah cells is in the ballpark of $1600, adds 3 kWh of usable energy, and weighs a bit more than 100 lbs. Added to the LA pack, it would have more total energy and less than half the cost of the cheapest 30x100Ah pack I could find.
> 
> ga2500ev


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## Ziggythewiz (May 16, 2010)

ga2500ev said:


> Because I have access to $1000 now but not $2600. I'm not planning on building this dual pack anytime soon. All of this is trying to manage cash flow.


Roger that. I thought you were planning the lithium addition soon. I just upgraded most of my pack to GC8s, I'm expecting them to last around 2 years.

What range were you looking for? Also, keep in mind a GC8 is more like $105 out the door.


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## ga2500ev (Apr 20, 2008)

Ziggythewiz said:


> Roger that. I thought you were planning the lithium addition soon. I just upgraded most of my pack to GC8s, I'm expecting them to last around 2 years.
> 
> What range were you looking for? Also, keep in mind a GC8 is more like $105 out the door.


I was hoping to convince them to waive the $9 core. Also I have a raft of dead car and UPS batteries that I'm hoping to be able to trade so that I can get out the door for under $100 a battery.

I'm still thinking that a flyback DC/DC converter can have quite a bit of utility to an add-on pack in several different ways. The first is that the total output power of the lithium can be managed. So if you have cells that can only output 3C continuous then you can limit the output power to Vin * 3C. The second is that since a flyback DC/DC can serve as a voltage converter it would be possible to put together a lower voltage pack than the primary and scale up the voltage to match the primary pack voltage. So for example a nominal 36V add-on pack can be created with 12 Li cells and then scaled up to 96V using the DC/DC. The next item is that it would be possible to manage the power outputs of two packs with with different energy capacities so that the total system depletes its energy at approximately the same time. So if the primary pack has 3/4 of the total energy of the system and the add-on pack has 1/4 the total energy, that energy is contributed in that 3-1 ratio from each pack. Finally whatever current reduction an add-on pack contributes to the system reduces the Peukert effect on the primary LA pack, thus extending the usable energy of the LA pack, which further extends the range. All of this can be done in smaller chunks of cash so that a pack can be built out in parts, instead of having to purchase everything all at once.

The additional complexity is terrible, I know. The energy loss in doing the conversion sucks. Having to manage multiple chargers isn't good. But at the end of the day it does facilitate adding the much needed Lithium based energy to the system without having to capitalize the entire system up front.

I think there's definitely some work to do on this front.

ga2500ev


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## DIYguy (Sep 18, 2008)

Hey it's a free country.. . u get to spend your $2,500 on anything u want.


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## octagondd (Jan 27, 2010)

I am interested and confused at the same time. Let's say you have a TS 100AH battery and you want to add an A123 20AH pouch in parallel for a performance boost. Now, in essence, you have a 120 AH battery, correct? When you load this new battery with continuous 600Amps, what happens to the TS which can do the 600, but for cycle life should be kept in the 100-200 range? Do the TS and A123 both see 600 Amps of draw?


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## Ziggythewiz (May 16, 2010)

They will split the 600 proportionally compared to the internal resistance of each.


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## octagondd (Jan 27, 2010)

Ziggythewiz said:


> They will split the 600 proportionally compared to the internal resistance of each.


Got it. Thanks. So it could be something like 400 on the A123 and 200 on the TS depending on what the IR is of each of those cells.


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## Ziggythewiz (May 16, 2010)

Yeah, kinda a double edged sword on the A123, they can put out a ton of power, but that means they're likely to be the one putting out a ton of power.


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## octagondd (Jan 27, 2010)

Ziggythewiz said:


> Yeah, kinda a double edged sword on the A123, they can put out a ton of power, but that means they're likely to be the one putting out a ton of power.


I was more concerned about the TS cells with a situation where 600amps continuous is being drawn (going up a steep grade at freeway speed), the A123 is doing 300 and the TS is doing 300. This would put strain on the TS batts as far as cycle life is concerned.


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## EVfun (Mar 14, 2010)

I think I would mostly be concerned with what happens when the low IR 20 amp hour cell runs low while you are drawing 600 amps. With a little more voltage sag the load will shift sharply over to the TS cells as the smaller cells go flat.


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## Ziggythewiz (May 16, 2010)

octagondd said:


> I was more concerned about the TS cells with a situation where 600amps continuous is being drawn (going up a steep grade at freeway speed), the A123 is doing 300 and the TS is doing 300. This would put strain on the TS batts as far as cycle life is concerned.


Your first guess of 400 200 is probably more likely. How long is this steep grade? The 20ah pack would be drained in about 3 min. Maybe go up the hill slower?


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## Yukon_Shane (Jul 15, 2010)

octagondd said:


> I was more concerned about the TS cells with a situation where 600amps continuous is being drawn (going up a steep grade at freeway speed), the A123 is doing 300 and the TS is doing 300. This would put strain on the TS batts as far as cycle life is concerned.


My understanding is that ts cells have no problem pulling 3c continuously and lots of people seem to be regularly exceeding 5c for short periods. Limiting a 100 ah pack to 200 amps max seems a bit conservative


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## Yukon_Shane (Jul 15, 2010)

Ziggythewiz said:


> Your first guess of 400 200 is probably more likely. How long is this steep grade? The 20ah pack would be drained in about 3 min. Maybe go up the hill slower?


3 minutes is a pretty long time really and once you take your foot off the gas the 100 ah pack would start to recharge the 20ah one. 

3 minutes at 600 amps is probably about all your average dc motor can handle anyway. According to the evnetics owners manual the warp motors are only good for 5 minutes at 500 amps and 20 seconds at 1000 amps


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## octagondd (Jan 27, 2010)

Ziggythewiz said:


> Your first guess of 400 200 is probably more likely. How long is this steep grade? The 20ah pack would be drained in about 3 min. Maybe go up the hill slower?


I was just coming up with a possible situation where there could still be a high continuous draw on the TS batts which could hurt cycle life and capacity in the long run. I actually built my pack to be twice what I need for commute and plenty power for the 3 mile 2% grade hill along the way. I have 44 TS160s in my build.

I can see that if someone had a pack of TS 100's, these A123 cells might be a nice performance booster if you had a parallel string, as long as the motor and controller can handle more amps. Also a little range extension.


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