# Does BMS upset the "cost advantage"?



## fishguts (Dec 19, 2008)

I know the conventional wisdom seems to be that lithium has gotten cheap enough that it doesn't make sense to buy LA batteries.

For my 72V application, I can get a 100AH lithium pack for about $2500 or so (great price!). That's a little more than twice the cost of 130AH (20 hr. rating for what that's worth) Trojans. I know about the Peukert effect deal and that the Trojans may not quite be up to the 100AH lithiums, but as far as I can tell, they might be in the neighborhood. None of this is easy to figure out as many of you know.

Over the long-haul the lithiums are supposed to be better, right? Bigger up-front cost but longer life and better overall performance.

But wait, the lithiums require BMS and from what I've seen that can cost as much as the batteries (in my my case anyway). Suddenly the lithiums are 4 X the cost of the Trojans instead of 2 X. I'd be changing out the Trojans more frequently, but four sets of them will basically equal the longevity of the lithiums with lower initial cost and simpler electronics. So I'm wondering if the difference in performance with lithium is really worth the big cash outlay for batteries and BMS.

It certainly will be a lot easier $$ - wise up front to go with the Trojans.

So am I missing something?


----------



## speculawyer (Feb 10, 2009)

BMS systems definitely still are overpriced right now. But with newer chipsets coming out, the cost of BMS systems should begin to drop. 

And you can get away with simple systems . . . check out Brian's balancer circuits:
http://www.voltblocher.com/


----------



## MrRoboto (Feb 1, 2009)

Here is another low cost option with a bit of work from the end user.
http://www.tppacks.com/products.asp?cat=26
I'm still researching it, and as long as you don't want to charge too fast, it should work fine. I'm pretty sure I'll be using it.


----------



## hardym (Apr 2, 2008)

Dont lose the faith. Lithium IS the holy grail of EVs. The key is BMS. I am one of those who almost has a BMS ready for production but am having some problems.. etc...

After developing several versions of BMS and riding on Li Batteries for a short while, I'm venting as I think the actual need for BMS may be more simple than I had anticipated. 

Folks like me who have actually been using Li batteries may also believe that the need for balancing may be less than has been socialized. The actual balancing part of the charge cycle happens at the end where only a few watt hours makes a pack equalize. It doesn't add much to the overall pack power. I'm not convinced that lack of balancing leads to even further imbalancing.

So what if the pack stays a little out of balance? Life's not perfect. ;-)

So if we take balancing out of the BMS problem, all we need is cell overvoltage (charge cutoff via relay) and cell undervoltage (operator alert). This simplifies the BMS problem considerably.

While i'm impressed with the simplicity of the volt blocher, im proposing the opposite solution: no balancing , just individual cell voltage detection is all you need. SOC is a bonus.

If balancing is eliminated from the BMS, a BMS can be built more cheaply. 
Mark.


----------



## dimitri (May 16, 2008)

hardym said:


> Dont lose the faith. Lithium IS the holy grail of EVs. The key is BMS. I am one of those who almost has a BMS ready for production but am having some problems.. etc...
> 
> After developing several versions of BMS and riding on Li Batteries for a short while, I'm venting as I think the actual need for BMS may be more simple than I had anticipated.
> 
> ...


I agree that balancing is not the holy grail, but it still has long term value if done correctly. And it only adds $2-$3 per board, so not that much of a difference.

BMS should not cost more than $20 per cell (assuming large prismatic cells) and perhaps $100 for a head unit which connects to charger and the controller, IMHO. I know this because I built one for myself and could market it with little trouble. Of course its cheaper when you DIY it, but I am talking retail prices here.

For large prismatic cells it's impractical to use single board for many cells like the TPPacks product, because you end up with spaghetti wiring and EMI will screw up your voltage readings. TPPack approach was designed for bikes, where everything is close together and cells are smaller.

The key to succesful balancing BMS is perfect matching of the HVC voltage with the charger and careful calculation of shunting wattage. I was able to do it with Zivan NG3 and its working perfectly.


----------



## ClintK (Apr 27, 2008)

(Not that I'm advocating doing this...) but I remember reading somewhere (maybe on this site?) that many early LiFePO4 EVers had no BMS. Despite this their batteries have lasted for years and still going strong. I almost wonder if the "LiFePO4s require BMS" is a legacy from the very explosive / thermal runaway laptop batteries.

I honestly don't know though - just throwing it out there...


----------



## dimitri (May 16, 2008)

ClintK said:


> (Not that I'm advocating doing this...) but I remember reading somewhere (maybe on this site?) that many early LiFePO4 EVers had no BMS. Despite this their batteries have lasted for years and still going strong. I almost wonder if the "LiFePO4s require BMS" is a legacy from the very explosive / thermal runaway laptop batteries.
> 
> I honestly don't know though - just throwing it out there...


Its hard to separate fact from fiction until you actually have these batteries in your garage. I myself have been sceptical of many BMS approaches until I got my cells and started charging them without BMS 

I can tell you one thing from first hand experience, you do not want any cell to go over 4.x Volt , they start to swell very very quickly 

So, how can you accomplish this without any BMS? Typical charger doesn't know the voltage of each cell, it just keeps pumping energy until total voltage is reached, but each cell gets to their upper voltage at different times unless they are balanced.

IMHO, if you can balance cells initially and set your charger cutoff at safe level of 3.6-3.8 times number of cells, then you can go without BMS for a while, but I believe in the long term this is not safe, since they can drift out of balance. It maybe 2-3 years down the road, but its still a risk.

I personally implemented BMS that does balancing, LVC and HVC, and I have comfort level to leave the charger overnight, which I would not have without BMS.


----------



## ClintK (Apr 27, 2008)

dimitri said:


> So, how can you accomplish this without any BMS? Typical charger doesn't know the voltage of each cell, it just keeps pumping energy until total voltage is reached, but each cell gets to their upper voltage at different times unless they are balanced.
> 
> IMHO, if you can balance cells initially and set your charger cutoff at safe level of 3.6-3.8 times number of cells, then you can go without BMS for a while, but I believe in the long term this is not safe, since they can drift out of balance. It maybe 2-3 years down the road, but its still a risk.


I agree with the safe cutoff voltage of 3.6-3.8V on the charger, and I do think balancing is necessary. I like the idea/simplicity of the Volt Blochers - otherwise one underperforming (or overperforming) cell will eventually stray from the pack. But spending hundreds (or thousands?) of dollars to monitor the voltage of each individual cell to cut off the whole pack (which doesn't even balance) seems excessive to me though.

Of course my Lithium cells haven't arrived yet so I haven't learned my hard/expensive lesson yet.


----------



## dimitri (May 16, 2008)

ClintK said:


> I agree with the safe cutoff voltage of 3.6-3.8V on the charger, and I do think balancing is necessary. I like the idea/simplicity of the Volt Blochers - otherwise one underperforming (or overperforming) cell will eventually stray from the pack. But spending hundreds (or thousands?) of dollars to monitor the voltage of each individual cell to cut off the whole pack (which doesn't even balance) seems excessive to me though.
> 
> Of course my Lithium cells haven't arrived yet so I haven't learned my hard/expensive lesson yet.


Even simple Volt Blochers add up to hundreds of dollars when you have larger packs, so there is definitely a cost factor. Also, Volt Blochers were designed assuming that charger monitors each cell, since that what Brian has in his EV. You have to be very careful with Volt Blochers and the charger without HVC feedback, because you risk overrunning shunting wattage and cause overcharge or even fire. The charger must be capable of slowing down before any cell starts shunting.

I think Brian is working on next revision to include LVC and HVC feedback. I couldn't wait, so I built my own revision and its working well, provided that I have a charger that's compatible with HVC feedback.


----------



## bblocher (Jul 30, 2008)

dimitri said:


> I think Brian is working on next revision to include LVC and HVC feedback. I couldn't wait, so I built my own revision and its working well, provided that I have a charger that's compatible with HVC feedback.


Yep, I am working on the next version to include the HVC and LVC feedback, three shunt voltages and direct to cell mounting. I don't have an exact ETA yet, but it's close.

I think this new system paired with a charger like the Zivan which can use this feedback is all that is needed to safely use LiFePO4.

In my opinion the order of importance is HVC, balancing, LVC. The reason I don't put LVC second, or tied for first is this is something you as the EV builder / driver should have already calculated how to avoid and never be close to pushing your batteries to see this on a drive. If the cells are balanced there is less chance to have one cell that was out of balance enough to hit LVC far before the others.

You can get away with no BMS at all, but there isn't enough experience with these to know how much abuse they can really take compared to the older lithiums that couldn't take any. I prefer spending a few hunder bucks for piece of mind myself.


----------



## TheAtomicAss (Feb 19, 2009)

Although one can certainly operate without a BMS, I personally would want to have it for the "set'n'forget" ease of use. I don't know enough about electronics to go designing one, but I do know a good management profile.

(The BMS absolutely has to be integrated to the charger to make this work, though.)

20ga. wires are strung to each of the cell bridges, and the + and - of the pack make up the ends. The charger charges the whole pack until any one cell reaches the set HVC. When this happens the charger shuts down. The cells are then tested for resting voltage, and any that are significantly low are topped up individually, through the 20ga. wires, at a small rate of say 1A. On the discharge side, the system simply monitors for LVC, ONLY during stops or coasting and perhaps if set conservatively, (3.0v for example), could drop the motor controller into a "limp" mode.

That seems to me like the best setup for a BMS. 

As per cost, it would seem to me good economy to simply design the BMS to handle the fine charging control duties and simply actuate the main charger which could be as simple as a plain transformer and recitifier.


----------



## Technologic (Jul 20, 2008)

dimitri said:


> For large prismatic cells it's impractical to use single board for many cells like the TPPacks product, because you end up with spaghetti wiring and EMI will screw up your voltage readings. TPPack approach was designed for bikes, where everything is close together and cells are smaller.


Do you have any reference for believing EMI comes into play significantly?

From my experience with it (which in all honesty probably rivals any EE around here from speaker design/building days) it's much more of an issue in AC current than DC.
I have a lot of reasons for thinking it won't be an issue if leads are kept short (and since you can break the board into sections it probably wouldn't be more than 1 ft long).

However if you were running large spans and worried (not that you should be)... at MOST you'd be seeing 200 watts down each cell (for something like a 6kw -8kw charger) ... almost nothing in terms of EMI. You could do a few simple things to help/eliminate it if your voltage readings were out of sorts.

One of those would be simply to twist the wire sections together.. increasing inductance in the wire (ie making the voltage quite a bit more stable)... You could run a piece of copper/aluminum (foil is fine... you're talking about very little EMI here) around the wire to a ground...

Or you could simply use 16+ awg RF wiring... which I have several thousand feet of laying around at home... but it's incredibly cheap (it has the EMI shorting ground section in it, with aluminum foil around the wires). Many of these are available with two wires twisted together in a common outer shell... used for RCA cabling and HD cabling

www.partsexpress.com you can get rolls of hundreds of feet of the RCA cabling for a few dollars. Try to get 14+ awg though

I would only worry about EMI though if you are measuring the signal and it seems higher at the end of the wires than at the posts of the batteries (unlikely even at these lower voltages). 

My guess is there simply won't be enough EMI for it to make a difference though even in a moving car. If there is, the RCA cabling or sleeving it in aluminum/copper to ground solution (after twisting) will work just fine.

*The TPpacks should still be the first stop for people currently * ... at least if they don't mind a bit of soldering.

Chances of a person carrying more than 100 watts to each cell during charging is very very low...


----------



## EV59RAG (Oct 6, 2008)

bblocher said:


> Yep, I am working on the next version to include the HVC and LVC feedback, three shunt voltages and direct to cell mounting. I don't have an exact ETA yet, but it's close.
> 
> I think this new system paired with a charger like the Zivan which can use this feedback is all that is needed to safely use LiFePO4.
> 
> ...


BBlocher,

I recently came accross of your Volt Blocher and I am very impressed on how you did the ovelall package. If i may point you to a different solution, this might (or May not) help you with your next version.... I came from a solar/wind energy community and we have been using a similar voltage controller like the one you have for some years now. The only difference is yours is cheaper... $8 bucks for a 12 volt controller against his for $12 bucks. But if you have time to check his out, you can find it here... http://ghurd.info/dc.html Very siimilar in function, but the dump controller we are using can handle lots of amps thus no issues of overvolting. And his can be easily be an LVD also. He also have the whole schematic on his website and parts are all listed for all people to diy it.

I look forward in your next version, and can your kit be easily upgraded to 72 volts or 85.8 volts cut-off with hysteresis of 2 volts. And add a relay parallel to th resistor to control on/off of a dumb charger?

Thanks


----------



## bblocher (Jul 30, 2008)

The highest voltage request I've had is for a 12v LA battery. I'm not sure I understand the need for a 72 volt shunt module. My modules are designed to be installed on each battery or cell. Each battery/cell needs to be seperately drained for balancing / HV protection. So, technically, they can handle any voltage in the sense of the total pack/series voltage.

The hysteresis is not adjustable on my modules and have fairly sharp turn on/off. They are designed to work with chargers that ramp down current in HV scenarios instead of just turning off. I can see an adjustable hysteresis being a much needed feature if charging is cut off though.


----------



## EV59RAG (Oct 6, 2008)

bblocher said:


> The highest voltage request I've had is for a 12v LA battery. I'm not sure I understand the need for a 72 volt shunt module. My modules are designed to be installed on each battery or cell. Each battery/cell needs to be seperately drained for balancing / HV protection. So, technically, they can handle any voltage in the sense of the total pack/series voltage.
> 
> The hysteresis is not adjustable on my modules and have fairly sharp turn on/off. They are designed to work with chargers that ramp down current in HV scenarios instead of just turning off. I can see an adjustable hysteresis being a much needed feature if charging is cut off though.


Well, the reason im asking is because i have a 72 volt dumb (not smart) charger. And in my case its a 10 am 72 volt charger.

So if i understand it right.... Even if i have your balancer on each of the 6 batteries i have, when one is fully charge or reached the desired voltage, the load will be diverted to the resistor, so it wont go to the battery thus maintaining that voltage, while waiting for the other batteries to reach theirs.

Now here is my question, since i have a dumb charger, when all the batteries are fully charge, the charger will just keep it charging unil i turn it off.

But if I can put something similar to yours, but instead designed for 72 volts monitoring, then i can make the dumb charger smarter, and make it go off when it hits a certain voltage...

does it make sense? or i just cant simply use your device for my kind of setup?

Also, if i have a 72 volt 10 amp charger, thats equivalent to 12 volt 10 amps on each battery balancer right? will that resistor handle that load/heat? or will i break something? thats 120 watts? am i correct? so i need at least 120 watts of resistor? so i dont over voltage my batteries when they hit their full charge voltage?

sorry for so many questions, im just trying to see if it will work on my current setup...


----------



## bblocher (Jul 30, 2008)

EV59RAG said:


> Well, the reason im asking is because i have a 72 volt dumb (not smart) charger. And in my case its a 10 am 72 volt charger.
> 
> So if i understand it right.... Even if i have your balancer on each of the 6 batteries i have, when one is fully charge or reached the desired voltage, the load will be diverted to the resistor, so it wont go to the battery thus maintaining that voltage, while waiting for the other batteries to reach theirs.
> 
> ...


The charger, as long as it really is designed for the type of battery you're using, WILL turn off when the entire pack reaches it required voltage. Additionally your charger will not be at 10 amps always. The last stage of the charging will continue to drop amperage as the cells become full. So the shunting doesn't require the full charger output. You're, however, correct in that if the charger is outputting more than the modules can shunt, you will still over volt the cell. My current version dosen't prevent this. The next version allows for output, but again to a charger that is capable of input. I'll eventually try to provide a lost cost solution for those with dumb chargers to regulate current in-line, but I haven't started that yet.


----------



## tomofreno (Mar 3, 2009)

I have been looking at bms'. From what I've seen, including this thread, it seems all that is necessary is a shunt regulator with HVC and LVC. The Stybrook (Agnimotors) seems to fit this bill, and I think also the one made by evpower in Australia. Sounds like Brian is working on something similar. I plan to use a Manzanita PFC30 charger. Anyone have experience with feeding a HVC signal to one of these? Evpower also sells the TBS gauge for pack V, I, and SOC. This, with the above seems all you would need.

Thanks,
Tom


----------



## tomofreno (Mar 3, 2009)

One other thing. The Elcon charger sold by KTA is 3kW and can also use 240VAC or 120VAC and is less than half the cost of the Manzanita. However, it has no input to apply a HVC signal. Could I just run AC power to it through a 240VAC solid state relay and have the HVC cut AC power to it, or will transients cause problems?

Thanks,
Tom


----------



## JRP3 (Mar 7, 2008)

I'm leaning more and more towards the "divide and conquer" method a la Jack Rickard. Split the pack, monitor both halves, and use some level of difference between them to shut down the charger if there is a HV problem and pulse the controller or signal a warning on a LV issue. This does not balance but it should protect. (Of course I haven't figured out the best way to implement it.) This works only if the cells are pretty well balanced to begin with, and if you never take the pack to extremes, which should not be a problem for me. Everything I've read suggests that if you don't draw high C rates and stay away from high and low SOC the cells stay fairly well balanced.


----------



## Technologic (Jul 20, 2008)

JRP3 said:


> I'm leaning more and more towards the "divide and conquer" method a la Jack Rickard. Split the pack, monitor both halves, and use some level of difference between them to shut down the charger if there is a HV problem and pulse the controller or signal a warning on a LV issue. This does not balance but it should protect. (Of course I haven't figured out the best way to implement it.) This works only if the cells are pretty well balanced to begin with, and if you never take the pack to extremes, which should not be a problem for me. Everything I've read suggests that if you don't draw high C rates and stay away from high and low SOC the cells stay fairly well balanced.


Just save yourself the time JP and use the Tpacks


----------



## JRP3 (Mar 7, 2008)

My plan should actually be pretty simple, 2 wires from each half of the pack going to 2 volt meters in the most basic setup, then add a switching device in case one half of the pack is out of spec. So with my 34 cell pack at 3.6 volts I get 122.4 volts fully charged, each half should be 61.2. If one half goes higher than that I have an imbalance and it stops charging. On the low end at 2.5 volts total pack is 85, half is 42.5, if either half goes below that I get a warning. From what I've read cells sagging momentarily below that under hard acceleration is not a huge problem, and my planned usage shouldn't put me anywhere near there.


----------



## bblocher (Jul 30, 2008)

tomofreno said:


> One other thing. The Elcon charger sold by KTA is 3kW and can also use 240VAC or 120VAC and is less than half the cost of the Manzanita. However, it has no input to apply a HVC signal. Could I just run AC power to it through a 240VAC solid state relay and have the HVC cut AC power to it, or will transients cause problems?
> 
> Thanks,
> Tom


I've thought of this as well as a easy solution for chargers without input. You'd want a way to delay the toggling on/off. Doesn't seem like it would be good on the charger if you end up toggling on/off too quickly over and over. This would be an easy addition to the relay (or even find a delay relay).


----------



## EV59RAG (Oct 6, 2008)

bblocher said:


> I've thought of this as well as a easy solution for chargers without input. You'd want a way to delay the toggling on/off. Doesn't seem like it would be good on the charger if you end up toggling on/off too quickly over and over. This would be an easy addition to the relay (or even find a delay relay).


This is what im trying to do on my earlier post. Thats why I was asking about the 72 volt (or total voltage pack)... If not then where are you going to get the HVC from? from one of those cell? So if that cell is higher than the rest then it will shutt off the charger even if the rest are not fully charged yet.

am i right? or this HVC you are talking about is the accumulaive voltage of the rest of the pack? How? Please explain.


----------



## tomofreno (Mar 3, 2009)

EV59RAG said:


> This is what im trying to do on my earlier post. Thats why I was asking about the 72 volt (or total voltage pack)... If not then where are you going to get the HVC from? from one of those cell? So if that cell is higher than the rest then it will shutt off the charger even if the rest are not fully charged yet.
> 
> am i right? or this HVC you are talking about is the accumulaive voltage of the rest of the pack? How? Please explain.


The idea is to turn off the charger if any one cell exceeds its "full charge" voltage, so that the cell is not overcharged and destroyed. Typically the BMS modules on each cell have a circuit that bypasses some current past that cell when it reaches a voltage close to it's fully charged voltage, permitting the rest of the cells in series with it to continue charging. This is sometimes called a "shunt regulator". It doesn't work to monitor the entire pack voltage, as some cells may be lower in voltage, some higher, or cells may have different resistance, with the result that you overcharge some cells before you reach the overall fully charged pack voltage.


----------



## tomofreno (Mar 3, 2009)

Yes, you want it to turn off quickly, but stay off for a while to let the "high" cell drop down in voltage. So when do you think you will have this completed with the HVC and LVC feedback Brian? 

Alternatively, if the charging current is less than or equal to the cell shunt current, it seems a cell voltage would not reach the shutoff voltage significantly before any others unless it has significantly different resistance. The Manzanita charges to a user selected voltage, then cuts back to one amp or less. Other chargers, like the Zivan, have programmable charging profiles that might do similar. In this case, it seems a cycling charger would indicate a "bad" cell - one that charges quite differently than the rest. You can see how a bms can get complex if you add diagnostics to monitor things like this, cell temperature...but I think the simple version should work fine. 

Tom


----------

