LiPo BMS, balancing and bulk charging safety questions

[keru]

Hi,
I currently have 2x5s and 3x2s lipos (because I wanted to try voltages from 24v to 48v+).
I've been charging up to 4.10v using a fake (I think) iMax charger, and not discharging under 3.75v, controlling with alarms on the balance plug.

I had a few frights while unplugging my batteries from series, to charge the 5s in parallel, then the 2s. Nothing happened yet, but I still get worried.

So I'm wondering if I could safely bulk charge the batteries, 12s for example, assuming I always keep a balancer plugged on each battery pack, or maybe a BMS. I don't know what would be more likely to fail between a fake iMax charger, a balancer or a bms.

Also, considering I would use the battery almost every day, can I keep the balancer connected all the time, over nights and weekends, or would it be safer to unplug it when the battery isn't used ? Are there any ways I could have an On/Off button for those, as I'm not sure if they alternatively use the power from every balance pin, or just one (in that case it's easy)

Also, with the alarms, I'm not sure I have them setup correctly, as they start beeping at 3.75v under load. But my battery is not even half empty when that happens. Should I set the alarm lower ? What is a safe low voltage alarm setting on load ?

Thanks for the help,
Keru.

 

[dnmun]

you can use a 16S lipo BMS if you have a bulk charger that can make the 4.2x16S voltaage, 67.2V in order to balance the pack. you don't have to always charge to full voltage but to balance it it is best to charge to full voltage and let the BMS balance the pack from time to time. eventually the cells start drifting out of balance and you have to rebalance. but in between you can charge to less voltage and then turn off the charger.

it is best to not charge the battery until you need to use it.

the cycle life is consumed by charging the battery up and leaving it charged.

if you want cycle life then wait to charge just before use.

if you leave the balancers plugged into the sense wires they will soak up power from the cells and is not needed if you use a BMS in any case.

 

[Ykick]

BMS bulk charging/discharging is a great idea. Although, I'm not impressed with 84mA balance current for 5-15Ah cells but if you already have an RC charger you can do much faster balance work if needed. Start with everything balanced using similar cells it shouldn't drift out much, if any. If it does, some cell is probably weak and should be culled out anyway.

Bulk charging and discharging, the main thing is to have HVC/LVC - BMS balancing is something I simply don't care much about. (see above). I would not leave RC balancers connected to my pack.

In fact, I would like to get rid of balancing altogether on my BesTech BMS boards. Good boards, 'just don't like/trust the minimal balancing function. Got one channel that sucks a cell down, thinking of removing the bleed resistor?

Under load I'm using 2.8-2.9V/cell LVC. Installing BMS is a very good way to go.

 

[dnmun]

are you certain that there is a drain through one channel on the BMS? can you measure the voltage across the balancing shunt resistor? it may be a self discharging pouch that is drawing down the voltage. if you can detect a voltage across the shunt balancing resistor when it is below the 4.2V level then the transistor on that channel may be shorted. it can be checked too.

 

[Ykick]

are you certain that there is a drain through one channel on the BMS? can you measure the voltage across the balancing shunt resistor? it may be a self discharging pouch that is drawing down the voltage. if you can detect a voltage across the shunt balancing resistor when it is below the 4.2V level then the transistor on that channel may be shorted. it can be checked too.

Thanks Dennis, I don't wanna de-rail this thread but 100% certain not self-discharge. Ch #10 loses about 800mAh every 10hrs when left connected. I'll start a new thread and please come help me figure it out? It's a D167 BMS, btw.

 

[motomech]

Hi,
I currently have 2x5s and 3x2s lipos (because I wanted to try voltages from 24v to 48v+).
I've been charging up to 4.10v using a fake (I think) iMax charger, and not discharging under 3.75v, controlling with alarms on the balance plug.

I had a few frights while unplugging my batteries from series, to charge the 5s in parallel, then the 2s. Nothing happened yet, but I still get worried.

So I'm wondering if I could safely bulk charge the batteries, 12s for example, assuming I always keep a balancer plugged on each battery pack, or maybe a BMS. I don't know what would be more likely to fail between a fake iMax charger, a balancer or a bms.

Also, considering I would use the battery almost every day, can I keep the balancer connected all the time, over nights and weekends, or would it be safer to unplug it when the battery isn't used ? Are there any ways I could have an On/Off button for those, as I'm not sure if they alternatively use the power from every balance pin, or just one (in that case it's easy)

Also, with the alarms, I'm not sure I have them setup correctly, as they start beeping at 3.75v under load. But my battery is not even half empty when that happens. Should I set the alarm lower ? What is a safe low voltage alarm setting on load ?

Thanks for the help,
Keru.

This is the only 12S Lipo balance charger available, but it is unfortunately out of stock at the moment;

http://www.hobbypartz.com/75p-1220-charger.html
It is inexpensive, simple to use and sturdy(mine is 2 years old and going strong).

As for a battery pack, 4) Turnigy 6S\5Ah configured into 12S\10Ah(2S\2P) would be the simplest and most cost effective way to go.
Given what you have, your best bet is to buy 2) 6S bricks to match you 5S bricks and configure to 11S\10Ah.
As for your 3) 2S mini-bricks, they could be combined into a 6S brick, but it is tricky to splice together the balance wires into a 6S JXT. Read up on this if you try it. It might be better to just use them to power lights or something.
3.75 per\cell does NOT leave half battery capacity. That is about the end of discharge. My Turnighy 20C cells start to stray badly at 3.65V.

another option would be to buy 2) 6S bricks and configure 5S+2S+6S=13S. You would have to splice the 2S and 5S balance wires together.
There is a Thunder 14S balaNce charger available, it's about $150 plus the cost of a power supply.
One nice thing about the Thunder 12S charger is, it can be powered but a single $20 server PS.
Here is my charger set-up;

Simple "plug and play", no need to dis-connect anything.
I use Battery Medics instead of connecting balance leads.

 

[dogman dan]

The ideal charger for 12s is that thunderpower one. Balance charges quickly every cycle. No waiting for one cell to discharge, so others can fill later.

I would like to just say that Lico is not safe just because you have a good bms, the volatile and cheaply made RC lithium cobalt cells are never truly safe.

So you still need to practice a lot of vigilance, then store and charge them where you'd build a fire. And the other thing that makes running lithium cobalt at least a little bit safer still needs to be done. Sort through your packs, and remove any cells that have any tendency to drift way out of balance, or have puffed significantly. Protect the cells, don't let corners get dented, or holes poked in them. They need more than the shrink to keep them whole.

At that point, with careful attention to not over discharging, you can bulk charge with very little need for balancing every cycle.

What I do.

I don't use a bms, but don't mean to discourage you from doing it. Mostly I want that flexibility to swap packs around, run various voltages, etc. I have 36v bikes, 48v bikes, and 24v lawn tools. But by all means use a bms if you will run 12s permanently. Or use the TP 12s charger, for a half bms for the charging.

Since I don't use a bms, I practice a slight undercharge. I charge either to 4.1v per cell, or to 4.15v. This gives me quite a bit of wiggle room at the top, just in case my pack is not as balanced as I thought. In two years, I have seen cells at 4.23 v but that's nothing to get scared about. When I do balance, I charge them to 4.2v to balance them best.

I try to never discharge below 3.5v per cell, resting. When I see below 3.5v per cell under load, I start slowing down to reduce the load. This happens only on the very longest trips. 90% of cycles, I carry plenty of capacity and end up stopping with voltages in the 3.7-3.8v range. This easy use tends to let the packs remain in balance for months at a time. Right now, last time I balanced was in May.

That DOES NOT mean last time I checked them was May. I check all cells top of charge voltage very often. I am the bms. So it's not no bms really, it's human bms. I check cells with a cellog 8 every few cycles. Ends up about weekly.

I use alarms only when I know I will push a packs discharge to 100%, and have them set to 3.3v. I don't want to hear beeping half the ride. Mostly I just watch the full pack voltage on my cycleanalyst, which has a note stuck to it reminding me of the ideal stop now voltage. I trust my packs to be balanced enough, since I tossed out any bogus cells years ago. So for most rides, I know I have the range, and just watch the CA for any unexpectedly early low voltage.

My favorite bulk charger is a 5 amps charger from EM3ev. With a flick of a switch, I can charge my 14s batteries to 3.8v, 4.1v or 4.15v. I have a 36v ping charger set to 4.15v for 10s, and my 7s 24v stuff still charges with an 8s RC charger.

 

[keru]

3.75 per\cell does NOT leave half battery capacity. That is about the end of discharge. My Turnighy 20C cells start to stray badly at 3.65V.

I meant 3.75v under load. But after reading other replies I see the low voltage alarm is set too high, thanks all for the information

I use Battery Medics instead of connecting balance leads.

I like this idea. Would there be any advantage balancing with the charger instead of the battery medics ? Does the charger also control something on the balance port, or its balancing function is not related to its charging function ?
Also if you use a battery medics instead of balancing via the charger, your 12s charger acts the same way as a bulk charger ? Do you happen to also balance with the charger from time to time ?

Sort through your packs, and remove any cells that have any tendency to drift way out of balance, or have puffed significantly.

For now I don't see any puff, but I didn't check cells individually. There is one cell that gets 0.03v out of balance, is that 'way out' ? else, how much is 'way out' ?
Just in case, to remove the cells, is it as simple as opening the shrink and cutting a few wires ? how are each cells connected together ? soldered directly on the tabs or wires ?

Protect the cells, don't let corners get dented, or holes poked in them. They need more than the shrink to keep them whole.

Yeah, I already put corrugated plastic around them, attached them so they don't move around in the bag. I wouldn't mind having some kind of fire retardant around too for peace of mind, but I didn't find anything I liked.

Thanks all for the help,
Keru.

 

[dnmun]

voltage has nothing to do with balance. balance is a term to describe when all of the cells in a battery pack have stored an equal amount of charge. it is unbalanced when one cell does not hold as much charge as the others. voltage has nothing to do with it, you have to charge the cell up to fully charged at 4.20V and then discharge it down to 2.7V to determine the total amount of charge the cell holds and then if all of the cells hit the low voltage of 2.7V together then the pack is considered balanced.

 

[keru]

ah I see, thanks for the details!
2.7V ? wouldn't that be an issue for the packs ? would 3.7V be good enough ?

I meant, when I balance charge the pack, ride, then the cells are all at 3.90V, except one that would be at 3.87V. That's why I thought it is unbalanced, except that I didn't go to 2.7V (or 3.7V if that's what you meant). Was I mistaken ?
How much difference in the ending voltage is considered too unbalanced ?

 

[dnmun]

if you charge to full charge, 4.20V then that is considered to be full. discharge to 2.7V and that is fully discharged. the amount of discharge in Ah is the capacity of the cell. a battery is balanced when it has all the cells storing the same amount of charge.

 

[keru]

Ok, thanks for the information. I thought that below some voltage (not sure how much) it was dangerous for the battery, risking it to blow or reduce it's life.

 

[dnmun]

do not discharge a battery at high rates when it gets down to the 3.5V level but for discharge tests you would always use about .2C for the discharge in any case.

what these guys are talking about is a balancing charger. these types of chargers charge all the cells up to the same voltage specified in the menu. in most cases not all cells have the same amount of charge stored so it has to remain on the balancing charger long enuff for the cell with the lowest voltage to reach the same setpoint. so the cell that reads .03V less than the others requires another 2-3% of charge to match them in voltage. so it has to remain on the charger longer.

if that cell never climbs to the set voltage then there is defect in the charger that prevents it from charging that cell up, or the charger display voltage measurements are defective, or the cell has a drain down rate higher than the balancing current of the charger.