# Cost of a LiFePO4 pack for my range/speed requirements?



## dimitri (May 16, 2008)

Lets round up to 40 miles and assume 300 Wh/mile for Honda, or 450Wh/mile for pickup. You need a pack holding 40 * 300 = 12,000 Wh for Honda or 18,000 Wh for pickup. Since you don't want to exceed 80% DoD, multiply by 1.2 , so 14,400 Wh pack size for Honda or 21,600 Wh for pickup.

Now, for your speed I would recommend minimum 96V pack, preferrably more, like 120V if you want a peppy EV which can go faster sometimes to get out of traffic.

Since LFP cells are 3.2V nominal, lets say a pack of 35 cells ( they come strapped in groups of 5, so its easy to mount ). 

35 * 3.2V = 112V , perfectly between 96V and 120V 

Now, for cell size 14,400 Wh / 112V = 128Ah for Honda, or 21,600 Wh / 112V = 192Ah for pickup.

Considering you don't want to exceed 3C currents during hard acceleration, I would say that 100AH cell is too small, even for Honda, although some people have done it.

I would recommend 160AH cells for Honda, which will give you more range than you need ( never a bad thing ) and enough current 3C = 480Amp to accelerate.

For pickup you can use 200Ah cells.

Check EV Components for cell price and specs ( weight and dimensions ).

Simple BMS will add up to $20 per cell, or less if you can DIY ( see VoltBlocher kits ).

Few chargers are available, depending on features and price you are willing to pay.

Hope this helps, good luck.


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## samborambo (Aug 27, 2008)

Since your range requirements aren't very ambitious, your limiting factor will be power, since LiFePO4's maximum discharge is usually kept at 3C, or 3 times the capacity. 8.7kWh of capacity will cover your commute in the CRX. Configured for 144V, that's 44x TS LFP60Ah. If your commute is fairly straight, even road, you may get home with 70% DOD. That's a good utilization of your pack and should last 15 years if treated well.

The power available with that pack is 40hp. This may be a little low. 0-45mph would be around 14 seconds. You could pull 4C out of your pack (other's have) and do the same in 11 seconds.

Have a look at evcomponents.com for good prices on LiFePO4 cells.


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## dimitri (May 16, 2008)

60AH is too small for DC conversion less than 200V, he will be constantly exceeding 3C rates , which will murder the pack too soon.

If you go for higher voltage ( more cells, more BMS cost, more expensive controller ) , then you can probably get away with 100AH cells.

Its a tradeoff between voltage ( number of cells ) and current ( cell size ) and range ( both voltage and cell size ).

Sam, your numbers assume 100% DoD and 250Wh/mile, both of which aren't very practical.

My numbers were overshot, but with so many variables at hand its better to overshoot than undershoot


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## maggler (Oct 26, 2009)

OK, If I understand correctly, samboramba is suggesting 44x 60Ah, at $66 each, for a battery pack price of $2904. That is an incredibly low price! Its cheaper than the lead acid route -- that seems almost too good to be true. And it would weigh only 242 lbs? wow.

samborambo's rig is configured at 144v, for a power output of 40hp (not very much, I agree).

dimitri is suggesting (I think) 35x 160Ah, at $176 each, for a battery pack price of $6160. That's about twice as much as samboramba (unless I've muddled things up), so we have a major divergence of opinion here. 

Why the big difference (or am I confused)? I guess dimitri rounded up the mileage (a good idea), so that would account for some of the difference. Also, dimitri suggested 80% DOD versus samboramba's 50% DOD. Why the big difference of opinion on what an acceptable DOD is? Also, does 80% DOD mean you used up 80% of the charge, or only 20% of the charge? If its 20%, then that would explain the rest of the price differential. If not, then I'm even more confused now 

dimitri's pack weighs 431 lbs, which is also fantastic, given that the donor has had its ICE removed. Its darn near close to weight neutral versus ICE -- very suprising. dimitri's rig is 112v -- how much power does that yield (I am not yet clueful in figuring such things out)?

Edit: my post crossed with dimitri's. I think I understand the difference now. I'm still pretty suprised that lithium is only, 50% to 100% more than a roughly equivalent lead acid pack. I had no idea.


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## dimitri (May 16, 2008)

maggler said:


> I'm still pretty suprised that lithium is only, 50% to 100% more than a roughly equivalent lead acid pack. I had no idea.


Considering LFP pack will last 3-4 times over LA pack, its actually cheaper in the long run.

As you found the weight is about half of LA, but volume is about the same, due to prismatic LFP relatively low energy density, but hey, can't have your cake and eat it too


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## Crash (Oct 20, 2009)

Maggler, a very basic way of figuring out how much HP a pack can handle and what kind of range it *may* give you is by doing this math:

All batteries have a rated and maximum current. The LiFePO4 batteries from thunder sky have a maximum continuous current of 3C (Which is 3 times the rated AH). It's not good to run these batteries at 3C for long periods of time because it kills the life of the battery in the long run. 1C (1 times the rated AH) is OK for longer battery life. 

So, that in mind let me give an example.
Dimitri says you should use 160AH batteries (which I agree with). 160AH at 3C will give you up to 480 amps constant current when needed. 35 cells will give you 119 volts if you're charging to 3.4V per cell (which is what I recommend to keep the battery life long). 

To figure out how much power your battery pack can handle, you multiply the amps by the volts. That will give you the wattage power. Then you divide by 746 to figure out how much horse power.

SO! When you're running your pack at 3C, it would be 480 * 119 / 746 = ~76.6HP. More than enough to get up to speed. Then while you're cruising you will not need nearly as much power as you're not accelerating, so you can drop down to 1C which would be 160 * 119 / 746 = ~25.5HP. 

This doesn't mean that's the power you're going to make to the ground, it's just the power in watts that your pack can handle.

As for range, you'll need to figure out how efficient your entire car is, from the electronics to the tires. Weight obviously makes a big difference, but low resistance tires, aerodynamics, manual/auto/direct drive also are a big deal when it comes to efficiency.

You can figure out how many watt hours your traction pack holds by multiplying the traction-pack voltage by the AHs. So, if you have 119V system with 160AH batteries, you're looking at (119 * 160 = 19040) 19040 (wh) watt hours. Then you need to figure out how many wh your going to use per mile. VERY efficient systems are in the low-to-mid 200's. Lower voltage systems will be above 300. As a rough estimate (since I don't know your EV setup) I'd do the math at about 330 wh/mi. So, you divide the total wh by 330 and get (19040 / 330) ~57.7 Miles range.

The math is easy to do when you have all the numbers. 

I think Dimitri's suggestion sounds best, IMO.


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## dimitri (May 16, 2008)

Crash said:


> 35 cells will give you 119 volts if you're charging to 3.4V per cell (which is what I recommend to keep the battery life long).
> 
> SO! When you're running your pack at 3C, it would be 480 * 119 / 746 = ~76.6HP.
> 
> ...


Actually, using 3.4V in power calculations is wrong. LFP sags to 2.9V-3.0V at 3C, so use 3.0V when figuring max power. At 1C use nominal 3.2V. Also use 3.2V when figuring pack size and expected range.


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## samborambo (Aug 27, 2008)

A few more details on my estimates:

Cell voltage of 3.3V for range calcs
Range based on the aerodynamics of a CRX, ie: Cd=0.3, CSA=1.77m2
Crr=0.007, low rolling resistance tyres.
Weight = 1450kg
Knocked 30km off the range for stop-and-go.
Battery to wheel efficiency of 85%.
Efficiency estimate may be a bit too high - DC series wound motors aren't very efficient at light loading.


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## tomofreno (Mar 3, 2009)

Dimitri, I think you may be overestimating the energy he needs a bit. He said:


> I need to travel 36 miles per day minimum, at around 45mph


 In a small car I think 45 mph should only require around 225 to 250Wh/mile, not 300. What do you get in your 3000 lb car, around 275 at 45 mph? Your numbers seem more in line with 55 mph or so to me. Pretty much agree with the rest of your reasoning.

Tom


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## dexion (Aug 22, 2009)

If it helps, I am converting my lead powered saturn to Lifepo4. It is about 2200 lbs with the Lifepo4 and 2650lbs with the lead installed. I have lrr tires and basically downhill 40% and 60% flat to work (27 miles) and 60% flat and 40% uphill (27miles) from work to home. I arrive at 40% soc with 12x12volt 105AH Lead batteries going an average of 35mph. I can charge at work for 9 hours. I arrive with about 20% soc going from work to home (hills are icky) so doing the numbers:

(105Ahx144V).5 (p effect)= a pack of about 10K

I used about 6K going to work = 222wh a mile
I used about 8K going home = 296wh a mile
Average is 259 wh a mile round trip.

To figure my round trip needs:

wh*miles*1.2 for lithium.

Now I am going to say 10% less power needed because there is
1. Less weight
2. Less Sag at what I am pulling from the pack

So lets figure average 240wh a mile
240*54= 12960
12960 * 1.2 = 15552
So I would need a 15.5 K pack to do round trip to 80%dod but, luckily I can charge at work so I am going to use 43 100Ah cells. That gives me about 47 miles range so as long as I can get an hour or so charge at the office I am fine. Your numbers would be MUCH cheaper if you could charge at work. No chance of that happening?

Your cars are heavier and less aerodynamic and I do less stop and go driving. So I think you want to figure 350wh a mile to be save for the honda and 450wh a mile for the truck unless you can find someone with a close set up and quiz them. 

A 50 mile round trip is 21K (I left you a little for heat in that on about 30 minutes or so) for the honda and 27K for the truck.

So 32 (106V) 200Ah cells for the honda and 41 (136V) for the truck 


At the going rate of about $1.45 per ah (You have to include delivery, taxes, customer and shipping over the pond)

Thats: $9280 for the crv and $11890 for the truck 

Once you learn to drive efficiently with an electric motor you may be using less power. So, perhaps you could get away with 160AH cells.

Thats about 7400 for the honda and 9500 for the truck

But thats also a reduction of 20% of your stored power. So if my numbers are 20% high you could do it cheaper.


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## dreamer (Feb 28, 2009)

dimitri said:


> Considering LFP pack will last 3-4 times over LA pack, its actually cheaper in the long run.
> 
> As you found the weight is about half of LA, but volume is about the same, due to prismatic LFP relatively low energy density, but hey, can't have your cake and eat it too


LeadAcid batteries only have a short life if they are significantly discharged. They'll last many thousands of cycles if they are only discharged a few percent before recharging -- as shown by regular car starting batteries. So ... how about a combined battery pack of LeadAcid batteries that can withstand high amp discharge rates and LiFePo batteries for the driving range ?

I'm thinking a 120V system consisting of ten Odyssey PC925 batteries, each one paralleled to four LiFePo 90AH batteries. The PC925 can maintain over 500amp discharge for more than 30 seconds -- longer than needed for acceleration. It would be recharged by the four LiFePo cells each time its voltage sagged from a heavy amp discharge. The LiFePo cells should be protected against high-amp discharges and not have their lives shortened like they would if you tried to pull 500A from them regularly.

PC925 batts cost $140 + four LPF90AHA cells at $99 ea, for a total cost of $540 x 10 sets = $5,400 and usable energy of 12KWH and 40 mile range at 300wh/mile. This pack would weigh 520lbs.

Seems OK on paper. What experience is out there on combined battery packs ?


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## maggler (Oct 26, 2009)

Crash said:


> You can figure out how many watt hours your traction pack holds by multiplying the traction-pack voltage by the AHs. So, if you have 119V system with 160AH batteries, you're looking at (119 * 160 = 19040) 19040 (wh) watt hours. Then you need to figure out how many wh your going to use per mile. VERY efficient systems are in the low-to-mid 200's. Lower voltage systems will be above 300. As a rough estimate (since I don't know your EV setup) I'd do the math at about 330 wh/mi. So, you divide the total wh by 330 and get (19040 / 330) ~57.7 Miles range.
> 
> The math is easy to do when you have all the numbers.


Thanks. My last major concern is safety. It seems like a guy like me, who's never done much electrical work, could figure out a variety of ways to fry himself to a crisp with a 480 amp rig that no doubt has lots of huge capacitors. How safe is it really to work on this stuff? I would be very cautious, but I'm quite worried that sheer ignorance could end up costing me quite dearly.


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## dexion (Aug 22, 2009)

I am an IT guy. I would classify myself between novice and experienced when working on cars. My first line of defense is research. Learn everything you can (youtube, here, other ev forums) about what you are planning on doing. Give youself PLENTY of time then double it to get a part of the project done. Start at one end of the car and work back one thing at a time until you are done. Use the one hand rule. If possible just work with one hand on anything metal at a time. Treat the other one like it doesnt exist. DISCONNECT the traction pack (remove a terminal connector, activate an E-switch, disengage a contactor etc.) Before working at the top end of the pack voltage (ie anywhere the thick black cables go to.) Wear eye protection, rubber gloves etc. But so far learning all I can learn about and mapping out a project with a computer (you dont need autocad I am mapping my batteries with paint  ) really help in keeping things safe. Be safe treat everything as live and that it will zap you dont get distracted or blase' about what you are doing. Oh, Ive forgotten. Take pictures/notes of everything. I have a detailed picture (or several) plus notes on everything I do. So, if I have to do it again or If I have to remove a piece and reinstall it (like my controller I sent back to have upgraded) I can follow the notes, pictures to make sure everything is back together. Its not too complicated (the controller wiring) but better safe than sorry.


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## dimitri (May 16, 2008)

maggler said:


> Thanks. My last major concern is safety. It seems like a guy like me, who's never done much electrical work, could figure out a variety of ways to fry himself to a crisp with a 480 amp rig that no doubt has lots of huge capacitors. How safe is it really to work on this stuff? I would be very cautious, but I'm quite worried that sheer ignorance could end up costing me quite dearly.


Most of people on this forum never worked with high DC voltage before, but I bet many of us replaced light switch or receptacle at home. That kind of work can kill you too, but you understand basic safety rules and respect electrical power. Same goes for EV, basic safety and respect to power.

It doesn't matter if its 500 amps or 50 amps, if you grab + and - of EV pack with your hands, you are dead, end of story, so just don't do it 

Always disconnect the pack from the rest of EV when working with batteries, use isolated handle tools, etc etc.


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## dimitri (May 16, 2008)

tomofreno said:


> Dimitri, I think you may be overestimating the energy he needs a bit. He said:
> In a small car I think 45 mph should only require around 225 to 250Wh/mile, not 300. What do you get in your 3000 lb car, around 275 at 45 mph? Your numbers seem more in line with 55 mph or so to me. Pretty much agree with the rest of your reasoning.
> 
> Tom


Sure I overestimated, I even admitted to it . But I always try to go over to see the top cost and size, then pull back if needed. I know he said 36 miles at 45 mph, but lets face it, once you get that EV grin on your face you tend to want more and it would suck if you got a bit overzealous with your throttle and got stranded half mile from home, how many times you read those stories here? And that's only from people who aren't shy to admit it. My point is, get the largest pack you can fit and afford, you will not regret it.


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## maggler (Oct 26, 2009)

dexion said:


> Your numbers would be MUCH cheaper if you could charge at work. No chance of that happening?


No, unfortunately. My company rents part of the 4th floor of an office building, there is simply nowhere to plug in.



dexion said:


> Your cars are heavier and less aerodynamic


The truck is definitely heavy and non-aerodynamic, but I'm ditching that idea since I don't need lead batteries. A CRX SI has a curb weight of around 2100 lbs (with ICE), and a cD of 0.32. Those are some really good numbers -- a Saturn SL2 has the same cD, with a curb weight of 2400.


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## dexion (Aug 22, 2009)

Ah every time I read crx I read cr-v (the suv.) I have seen/driven in crx's they are light and hatchbacks are always more aerodynamic. I think you could safely use my numbers of 240 wh a mile then. Sorry, I was thinking the suv so figure 1kw per 4 miles or so and then add 20% to the pack. So right where I am pretty much 40 100 AH is about a 42 mile range. $5800 delivered or so. The thing about 100AH cells is the maxing of about 300 (3c) draw from the pack. For me its not an issue (and I plan to add another 43 100ah cells later to give me 100miles range and then 600amps max draw.) So you cant dump the pedal or you will be abusing the batteries. I just program the controller to max at 300amps and thats all Ive got. Thats about 57Hp thats plenty for flat roads anyway with rush hour traffic I rarely get over 40mph and most of the time travel at about 30. My ice has a computer it tells me all sorts of stuff about my trip. My average mph on this trip in today was 28mph max speed 43mph.

So since its a crx it just got cheaper.


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## Crash (Oct 20, 2009)

dreamer said:


> LeadAcid batteries only have a short life if they are significantly discharged. They'll last many thousands of cycles if they are only discharged a few percent before recharging -- as shown by regular car starting batteries. So ... how about a combined battery pack of LeadAcid batteries that can withstand high amp discharge rates and LiFePo batteries for the driving range ?
> 
> I'm thinking a 120V system consisting of ten Odyssey PC925 batteries, each one paralleled to four LiFePo 90AH batteries. The PC925 can maintain over 500amp discharge for more than 30 seconds -- longer than needed for acceleration. It would be recharged by the four LiFePo cells each time its voltage sagged from a heavy amp discharge. The LiFePo cells should be protected against high-amp discharges and not have their lives shortened like they would if you tried to pull 500A from them regularly.
> 
> ...


IMO, an ultra-cap pack would serve as a better battery hardener.


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## MN Driver (Sep 29, 2009)

I'd have to agree with the ultracaps being a better battery hardener but those are very expensive and then you add wasteful capacitor aging to the mix as well.

I was thinking about this for the past two days and was wondering if A123 M1 cells could be used in parallel with 3C max cells. The A123's are the same chemistry and voltage max/min are compatible. A123 cells have a max voltage rating of 3.65volts which should be enough for a full charge with Thunder Sky cells, if using Sky Energy then the A123s would probably be plenty charged. The A123 cells have a max burst rating of 120 amps for 10 seconds. So if you needed the acceleration performance but didn't need to pull a constant high C draw to drive the car then this seems like a good idea on paper. Say you need to pull 600 amps, you could put 5 A123 cells and 1 prismatic cell together and you would be more than set, but you could likely use less of the A123s but with the deeper sagging prismatic cells I think the A123 cells would be loaded heavier as they have a higher discharge curve profile, but maybe not at 120 amps. It would take some math and likely a decent amount of over engineering to make it work and it would add a decent amount of cost too. Then again I don't know if it would be best to have the A123 pack as a seperate booster in parallel or to parallel individual LiFePO4 cells.


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## dreamer (Feb 28, 2009)

MN Driver said:


> I'd have to agree with the ultracaps being a better battery hardener but those are very expensive and then you add wasteful capacitor aging to the mix as well.
> 
> I was thinking about this for the past two days and was wondering if A123 M1 cells could be used in parallel with 3C max cells. The A123's are the same chemistry and voltage max/min are compatible. A123 cells have a max voltage rating of 3.65volts which should be enough for a full charge with Thunder Sky cells, if using Sky Energy then the A123s would probably be plenty charged. The A123 cells have a max burst rating of 120 amps for 10 seconds. So if you needed the acceleration performance but didn't need to pull a constant high C draw to drive the car then this seems like a good idea on paper. Say you need to pull 600 amps, you could put 5 A123 cells and 1 prismatic cell together and you would be more than set, but you could likely use less of the A123s but with the deeper sagging prismatic cells I think the A123 cells would be loaded heavier as they have a higher discharge curve profile, but maybe not at 120 amps. It would take some math and likely a decent amount of over engineering to make it work and it would add a decent amount of cost too. Then again I don't know if it would be best to have the A123 pack as a seperate booster in parallel or to parallel individual LiFePO4 cells.


Using A123 cells rather than the Odyssey cells in an interesting idea. But ... for the same price as an Odyssey PC925, you could only buy 12 A123 cells. Placing 3 of the A123 cells in parallel with each Thundersky unfortunately would not be enough to absorb a 500A discharge. That would require the A123 cells to operate at 75C discharge rate. They would have short lifespans at 75C.


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## MN Driver (Sep 29, 2009)

I don't think they would absorb the full discharge though, both chemistries will have voltage sag and they should support eachother to a point, it would be hard to know how much of the load would go toward the A123's. The lifespan issues with high discharge are typically tied in with exceeding internal cell temperatures of 60 degrees C(140 degrees F). The idea was to have less voltage sag under load to allow for an undersized pack, right? If that is the case the heavy load toward the A123s should be limited to about 10-15 seconds.

Granted I don't think it is the best solution, I think that the best solution is to have a pack that can support the discharge rate, but I see issues with mixing lead acid in with LiFePO4, the charging voltages for both chemistries aren't the same, they may be close when comparing 4 in series but what is ideal for one isn't ideal for the other so you would likely compromise and charge for what is best but also compatible with the other. If you need to charge the lead acids to desulphate them, it requires about 15 volts which is 3.75 volts for the lithium but when charging the whole series string, it will be hard to hold every cell at 3.75 since they don't accept much amperage without going over that voltage but the lead acids will be thirsty for more amps at that voltage. It might become difficult without dumping amperage through shunts and you do't want to charge at the desulphating voltage every time or you get grid corrosion so you don't have the same charging voltage every time either. I also wouldn't want to do it for the added weight of the pack adding the lead acid batteries in there, sure they add to capacity but it adds to the cost and requires more performance. The lithium cells are going to tolerate 80% depth of discharge from time to time, the lead acid will not.

I also see a bigger issue here, when I put a heavy load on a lead acid battery, the voltage goes lower than it would with a lithium pack. If drawing current and 4 lithium cells are at 3.2 volts, the lead will be at 12.8 volts which won't put them under load, in fact that is the voltage they would likely be sitting at not too long after they are off a charge, my car battery outside is likely between 12.6 and 12.8 volts and it's going to be real close to fully charged. When under a heavy load and lithium at 3 volts per cell, the lead will be at 12 volts. I'm not seeing the lead acid cells taking the load that you are expecting them to. I think that in order for this to work and for you to effecively monitor and balance the lead pack they will need to be in a seperate string with the voltage of the lead acid pack being higher so that it can actually receive some of the load otherwise I think you might just be lead sledding a lithium system without them helping much.


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## dreamer (Feb 28, 2009)

Keep in mind that Odyssey batteries are AGM and not flooded. They are designed for both high-discharge and deep discharge. An 80% discharge doesn't bother them. They also do not need a desulfating cycle. They require 14.1V to 14.7V for charging to full capacity, which is defined as anything over 12.8V. At 75% SOC, they are 12.5V and at 50% SOC 12.2V, and at25% SOC they are down to 11.9V. In contrast, a series set of four LiFePo batts will be 15V at full charge, and still above 12V at 20% SOC. Batteries connected in parallel will attempt to balance out to the same voltage, right ? So as the Odyssey batteries are tapped by the controller, and their voltage sags, charge should flow from the LiFePo batt and into the Odyssey because the LiFePo batt will be at a higher voltage than the Odyssey.

The Odyssey will sag all the way down to 7.2V after 30 seconds if you try to pull 925 amps from it. This makes it sound like after each acceleration, it will have sagged enough for the LiFePo batt it is paralleled to to "balance" voltage, and bring it back up above 12V.

As far as charging goes, I think it would be easier to charge individual battery packs. By charging each Odyssey to 14.7 volts, each LiFePo cell in the batt paralleled will be charging to 3.675V which should be safe for them.


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## dimitri (May 16, 2008)

You guys keep talking about voltage, but never mention internal resistance. LA has significantly higher IR than LFP, so despite your desire to draw most acceleration current from LA, the opposite will happen, you will make LFP do twice the work, run the motor and charge LA at the same time, I'm not even talking about lugging extra weight around.

Take 2 resistors of different values and put them in parallel, take low value resistor of low wattage and high value resistor of high wattage, put large current thru them. Does it help that your high wattage resistor is there if most current goes thru low value low wattage resistor and burns the hell out of it?

Granted I am not a huge expert and never tried it in real life, so this is just my semi-educated guess 

Please feel free to try your idea and prove me wrong 

Also, if you combine high cost of A123 cells and medium cost of good AGM, then you might as well get prismatic LFP cells and get the best and the worst of both worlds in one package.


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## MN Driver (Sep 29, 2009)

Your resistor idea is different than batteries since batteries have energy capacity and resistors do not, electricity takes the path of least resistance so yes you are correct about resistors. ...but the saying goes that electricity takes the path of least resistance to 'ground', in this case when you parallel two batteries they seek 'common' since ground isn't actually the ground, it's isolated to the common connections between the pack which would be anywhere you have positives and negatives that are producing a parallel circuit.

High voltage and low voltage essentially flow together, I can take a small consumer grade 9 volt battery and connect it to a 1.5 volt AA battery with the positive leads connected to each other and the negative leads connected to eachother. The terminal voltage of both will be in the middle, meanwhile the 9 volt battery will be drained will the 1.5 volt battery will generate heat and probably start leaking all over the place but due to the high internal resistance the energy exchanged probably won't cause them to vent or get hot enough to burn someone. Granted it's not the best example but if you parallel a series pack of Lead with a series pack of Lithium of similar voltages, both packs will match voltages if their terminals are touching. So what I'm saying is if you take one battery chemistry that at a certain amperage discharge would hang around 12.8 volts and another pack that would hang around 11.8 volts for the same amperage discharge then the one with the higher amperage draw will be the one that has less voltage sag and will take on more of the load because the voltage of the other pack wouldn't drop to the level to where it would be discharging its amps. It's like trying to charge a lead acid battery at 12.6 volts or a LiFePO4 battery at 3.4 volts, neither battery will ever get completely charged but if you raise the voltage a bit then they will. A similar effect happens on discharging, if you don't reach an effective voltage that pulls energy from the pack the battery won't discharge.

The internal resistance makes a much bigger difference if you decided to take one cell type and put them in series with another cell type for example taking a 12 volt lead acid battery and putting it in series with 4 LiFePO4 cells. The one with less internal resistance will draw more power and unless the one with less internal resistance has more relative capacity it will be drained sooner. Yet the one with high internal resistance will have more voltage sag. In short mixing internal resistances can cause trouble if it imbalances the pack too much.

Putting two chemistries together in a single series string is obviously a recipe for failure but in parallel they will support the voltage sag but I don't think that putting a lead pack in series with a lithium pack is going to work the way you'd think it would unless that lead pack has similar voltage sag to it at the same amperage but I think one won't find this because the LiFePO4 has a flat discharge curve, lead certainly does not.

I tried to explain how electricity finds a common voltage in a parallel circuit, I think I covered the topic pretty well, does that make sense?


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## icec0o1 (Sep 3, 2009)

Well I was curious here so I did a simple experiment with a liIon 18650 battery and 1.5v alkaline batteries tested with a small motor. Here are my results:


3.7V LiIon drew 0.25 amps and voltage dropped to 3.5V = 0.875 watts
3.0V alkaline battries drew 0.26 amps at vdrop of 2.7V = 0.702 watts
4.5V alkaline batteries drew 0.25 amps at vdrop of 4.1V = 1.025 watts

Hooked in parralel: 

LiIon - 0.12 amps at 3.2V = 0.384 watts
Alk (3V) - 0.16 amps at 3.2V = 0.512 watts
total = 0.896 watts

LiIon - 0.13amps at 3.9V = 0.507 watts
Alk (4.5V) - 0.09 amps at 3.9V = 0.351 watts
total = 0.858 watts

I don't know what the internal resistance is of the batteries as my multimeter can't go very low. Therefore I'm not sure I can conclude whether the difference in amp draw is dependent on voltage differences or resistance changes. I would venture a guess that it's the voltage difference as i'm increasing the internal resistance from 2 to 3 alkaline batteries by 50% while the amp draw changed from a ratio of 0.75 to 1.4 LiIon:alkaline. 

So if you had lead acid batteries paralleled to LiFePO4, you wouldn't be able to choose where you pull the amps from. You can't pull 500 amps from the lead acids while only pulling 100 amps from the LiFePO4's unless you have a huge voltage difference like having only 2 lithium batts to one lead acid but I'm pretty sure that would go very well.


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## dreamer (Feb 28, 2009)

icec0o1 said:


> Well I was curious here so I did a simple experiment with a liIon 18650 battery and 1.5v alkaline batteries tested with a small motor. Here are my results:
> 
> 
> 3.7V LiIon drew 0.25 amps and voltage dropped to 3.5V = 0.875 watts
> ...


What were the AH ratings of the LiIon batteries vs. the Alkaline batteries ? Assuming Amp draw is a function of overall capacity, you should test a parallel string of 18650 cells that total an AH rating of 4x the Alkaline batteries ratings and keep voltages close. Even that might not tell us much, because the Alkaline batteries are not rechargeable and have a very different discharge curve than an AGM lead acid battery would.


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