Balancing a 20s battery back

Hi lithium enthusiasts...

After the new 20s3p 84v lg mh1 pack is finished (and hooked up to the original bms) I will have balancing leads going out from the pack.

I've heard that this original bms dosn't really balance the cells. Even if it does it can't handle large imbalances. Anyway I need to keep it as it's communicating with the controller of the vehicle.

The aim is to get the pack very balanced cell-wise and then balance it with the four 5s balance leads on an external hobby charger every few weeks/months.

Because there arent any 20s balance chargers, and because it won't be balanced often I was thinking of putting together a simple cheap solution of several smaller chargers.

To avoid ground shorting the pack through the chargers I need them run directly on the mains so they are isolated from eachother (behind a transformer).

I know this sounds crazy but there are 3-4usd 3s chargers (using only the balance leads) that run directly from the mains. Lets say I get 2/4/7 of these and balance 5/10/20s at a time.

To minimize the risk of one of the 20 charging circuits to fail I was thinking I can minimize risk by getting two 3s chargers and test the circuits and pick the 5 best ones of the 6 available (ending voltage as equal as possible).

Would this be a feasible solution? I know buying cheap chargers sounds crazy but only the cheap balance lead chargers have the individual charge circuits. I don't want to serial charge 3s @ 11v at a time then wait forever for balancing to bleed power around.

I don't really want to arrange an array of twenty 1s charger modules and twenty power supplies either. Seems these 3s ones are the largest ground isolated chargers available. Do you know of a better solution to balance the pack every now and then?

Thank you very much for reading and commenting. I really appreciate it..

/a

BC168 (or clones thereof) is almost mandatory for DIY battery builders. I highly recommend one.

I think there are an I-charge that can do 2x10s. I hope it is just a matter of having one extra lead in the mittle of the pack to use it on 20s. Electric god has made a revew of the charger somewere on the forum.

Una9plus + they also have a battery nanny with power supply for 100.00 . I have one and just use it as a 6s balancer for my 24s pack with four 6s sense wires. This charger charges at 8amp thru the sense wires. I never used the battery nanny or totally understand or need them as my battery never goes out of balance 1,390 cycles 5yr. Its only out of balance if I screw with it. No bms no bms problems. Know your battery use it within its limit. Get a 1,000 cycles. Yes the charger does only 9s at one time. The battery nanny does up to 8 - 6s but one at a time which I don't know if this is good way to balance charge a 20s pack.
So a 18s pack would be easiest for this inexpensive charger

@999zip999
What is a battery nanny?

A battery nanny is sold with the una9plus+ kit and only with the kit. It connects to the 9s charger and has in 8 times of columns going around this box that are three 3s', two 4s, one5s, then one 6s plug. Now this stack goes around the Box 8 times. So you can plug in eight 6s or eight 5s. It charges one column at a time one through eight. I don't know if you can plug. I don't think you can put Halo battery in one brick and plug it it's for 5S sense wires but I thought you could use it to balance at top of charge.
Look up una9plus+ with battery nanny.
I have never use my nanny yet. As my pack of 5yrs 1,380 cycles only get out of balance if I forget to unplug a cellog.

999zip999 said:

Una9plus + they also have a battery nanny with power supply for 100.00 . I have one and just use it as a 6s balancer for my 24s pack with four 6s sense wires. This charger charges at 8amp thru the sense wires. I never used the battery nanny or totally understand or need them as my battery never goes out of balance 1,390 cycles 5yr. Its only out of balance if I screw with it. No bms no bms problems. Know your battery use it within its limit. Get a 1,000 cycles. Yes the charger does only 9s at one time. The battery nanny does up to 8 - 6s but one at a time which I don't know if this is good way to balance charge a 20s pack.
So a 18s pack would be easiest for this inexpensive charger

Click to expand...

Hey thanks for the info...

Do you disconnect the 24s pack's internal serial connection before hooking it up to the battery nanny?

Because I have my balance leads in parallel with the stock bms I'm not sure I can disconnect the serial connection or that disconnecting it won't still create shorts through the stock bms.

I need a charger/batterynanny that can switch between the balance leads sequentially (and reliably) and not have the ground pins all interconnected. I think the Radiolink cb86 plus can do this but I've seen a picture of one burned up into a pile of ash that I'm afraid to get one.

I've seen the un-a9 and I'm kicking myself for it not being only once cell more at 10s. It'd be a nice solution at 10s x2 for a 20s battery. Still if a battery nanny can switch sequentially through a couple of 6s balance leads it'd be great too. Just hook it once and let it finish.

Thanks for helping me find a solution.

j bjork said:

I think there are an I-charge that can do 2x10s. I hope it is just a matter of having one extra lead in the mittle of the pack to use it on 20s. Electric god has made a revew of the charger somewere on the forum.

Click to expand...

I think the ichargers and imax chargers all rely on serial charging to do the bulk of the charging and do only a fraction of the power through the balance leads at a very slow rate, constantly switching between serial charging and bleeding power from the other overcharged cells. This is useless for my application.

The main problem is that a large imbalance will take a crazy long time and I'd have the trouble of having to connect the battery pack serially aswell as with balance leads.

I need individual cell charging, no serial charging. That means pure balance lead charging.

Un-a6 has this, un-a9 has this, Radiolink cb86 has this, B3AC 3usd chargers have this. The question is if I can sequentially have a charger/battery nanny charge four 5s blocks one after another without connecting all the balance lead ground pins together (shorting the pack).

Ideally i'd like the charger to run dirctly on the mains as I don't like to have exposed 12v terminals and power supplies. I'm not really running a hobby lab here. I just want to balance my 20s pack every few months.

I don't like the B3AC chargers for two reasons: 1. who knows when they break 2. end voltage might differ a lot = useless balancing.

I also considered buying only an end voltage discharger. Stop discharging at 4.0v for example. Sure the pack wouldn't be top-balanced anymore but I'd have less risk of burning down my house without active equipment that can overcharge or short the pack.

As I said I have NEVER used my nanny part of the charger. I have two 12s packs I bulk charge as two 12s I just unplug the main pos. wire . Then I have neg. and pos 10ga charge wire and charge with a 24v meanwell and 12v hp power supply for 42v @ 16amps ea 12s module. Then check cells each time with a cellog as four 6s sense wire. But only use the thru the sense wire una9plus + as a 6s thru the balance wires as its thru the sense wire charger only no mains on the charger. Did you look up the charger on line ? I having same problem as I will step down to 20s same voltsge as my 24s lifepo4 . So 18s is a step down is making me sad.
By the way what you using a 3p battery for ? I only don't use a bms because I haul around 20ah of heavy lifepo4. But very reliable and heavy.

I did have the hyperion 1420 it was 14s and it broke. I bought a Thunder Power 1220 12s charger it broke. Yeah so bought another one and it broke. They don't sell either one of these Chargers anymore probably because they break. If you're using the battery pack properly it should not go out of balance. It might be one set of your 5S that is our little bonkers or else you're over just discharging the pack at too high a rate for what it's meant for. I mean you are using new cells and what are you using a 3p pack for it that's awful tiny.

999zip999 said:

I did have the hyperion 1420 it was 14s and it broke. I bought a Thunder Power 1220 12s charger it broke. Yeah so bought another one and it broke. They don't sell either one of these Chargers anymore probably because they break. If you're using the battery pack properly I might be one set of your 5S that is our little bonkers or else you're over just charging the pack at too high a rate for what it's meant for. I mean you are using new cells and what are you using a 3p pack for it that's awful tiny.

Click to expand...

It's an electric unicycle. 800w nominal/2000w peak. Original it comes with a 20s2p battery. I have two unicycles so I can work on the new battery pack as I'm riding the other. I'm going from 40 to 60 cells to increase the range and also because thats the only amount that will fit inside of it.

The range will increase to maybe 45km which is good.

The reason I don't want to split the pack is because it's a safety sensitive issue. I can't change the bms because if power cuts out I crash. When braking the motor regenerates power that goes into the pack. I imagine the power spikes are very high when it does.

Sounds brutal but it's perfectly reliable as long as the connections are all stable and solid. I've already done 6000km on it, from minus 15C to +30C, riding up and down mountains with 20% inclines, and it's been solid. Never once has it cut out on me.

Anyway. The chargers you mentioned aren't really much better than serial chargers with a weak balance circuit. To only charge the cells that are below 4.2v you need individual cell chargers, like the un-a6 un-a9 cb86.

The way a professional EV works is with a very balanced battery. Imbalances happen only after many cycles and so it's ok to run them with weak bms's and on a small serial charger. When you start to connect longer chains of cells in series the risk increases. I think thats why we don't see any 20s chargers out there.

So you only use your un-a9 to balance the cells occasionally. Got it. That's what I want too but I'd rather have a smaller/cheaper/simpler solution. Maybe b3ac is it. I'd have to monitor the voltages carefully and only use the channels that I approve of with a small end voltage deviation.

I decided to go with the 3s charger. Seems the new 20w version has a better end voltage. 4.19-4.20v

To keep things simple I'll hook up seven 3s leads hanging out of the pack. That way I can't connect the chargers wrong.

I'll test the chargers end voltage before trying it on the pack.

If I ever feel boored one day I can just get seven of these chargers and charge the whole pack at once. They cost literally nothing.

3p shouldn't take long to balance. and if you built it well and if the cells are all from the same batch you don't even have to balance for like a year

It's about how low you go or how hard you hit them. This is when unbalance starts. Can you put an alarm for lbefore lvc on your uni. Pic of uni.
You have a unicycle like Justin ( are leader ) 30mph ? You bulk charge with what ?

999zip999 said:

It's about how low you go or how hard you hit them. This is when unbalance starts. Can you put an alarm for lbefore lvc on your uni. Pic of uni.
You have a unicycle like Justin ( are leader ) 30mph ? You bulk charge with what ?

Click to expand...

Bulk charging also unbalances a pack if the IR/capacities are off.

Bulk charging, using a high voltage dc power brick and charge to 84V. (4.2x20)

No it's a 30km/h unicycle. Light one that can be carried up an occasional stair without breaking your back. (Inmotion V8)

Yes since I will have the seven 3s leads I can easily connect my voltage alarm to the weakest group and become notified when they start to deviate enough for me to balance the pack. Because the unicycle becomes unrideable long before reaching 20% battery capacity I'm not afraid to go under the cutoff voltage. My fear is for the weak groups to bulk charge over 4.2v. That's a problem.

The unicycle is 800w nom 2000w max which means the original battery 20s2p sees 800/84 ~ 10amps nominal (5a per cell, cells rated at 10a) with bursts up to 2000/84 ~ 25amps (12.5a per cell).

After I rebuild the pack with 20s3p I can get less voltage sag, less power draw per cell = more cycles = cooler pack, higher charge rate: 3.3amps nominal 8amps burst.

I will charge with a twice as powerful charger. From 1.5A to 2.7A. Here is the cherry on top: the new charger has a pot where I can dial in the end voltage. It means if I set it to 82V instead of 84V I get cells to stay around 4.1v max. This will increase the cycle life considerably.

I can also use the adjustment to keep an unbalanced weak cell group from charging above 4.2v by setting the end voltage to a point where that group doesn't go over 4.2v. This way I can keep riding an unbalanced pack and postpone the rebuild until I see fit.

I have the old charger so if I know I need to take a long ride I can use it to give me max range.

With the new battery I can ride until the motor turns to dust.

Yes and the closer you get to end voltage the more you see diversity in the voltage number. Meaning at 3.9v they are all at 3.9v and may only show voltage more different at top of voltage and at 4.3v only one will race to 4.5v and above. Meaning you will drive out the weak cell at lvc and hvc andabove or below. You will find that one weak one.
you have 60 cells and one might show it weakness.

By top balancing the pack at 4.2v this should be helped. Also that there are three cells in parallel can help minimize a small imbalance.

The original charger assumes the pack to always be balanced at 4.2v which is unwise. By lowering the voltage to 4.10v you have a small safety buffer. You make a good point that going from 4.1 to 4.2v takes almost nothing so it creates a false sense of security.

I've also heard that some bms don't start the balance process until the voltage goes above a certain level so by charging to 4.1v I might actually perpetuate an imbalance grow.

In any case by monitoring the cells throgh the balance wires I can see what works best.

I tested my 4 slot 18650 charger (liitokala 500) by capacity testing the same four cells in each slot. I noticed some slots measure high and some low. I took all the measurements and worked out a coefficient for each slot to minimize it's error. Each time I capacity test a cell I multiply the result with the slots coefficient so I know I can use that measurement with the other slots measurements. I recommend anyone that builds a pack to do the same. If not I would on my charger have differences of 3%.
slot1 measures on average 1.8% too low,
slot2 0.69% too high
slot3 0.10% too low (best slot)
slot4 1.2% too high

Thanks for your patience and interest in the thread 999zip999. I hope your pack works as planned without surprises.