Can some chargers prevent balancing?

rg12

100 kW
Joined
Jul 26, 2014
Messages
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Hey guys,

When a pack finishes charging it usually (even when the charger is disconnected) balancing high cell rows down by bleeding them slowly.
I've had this way out of balance pack, it didn't charge all the way up so I connected it to the charger and left it connected for about 3 days and it was perfectly balanced again.

Now I have this pack which is out of balance (16S charges to 65.9V) and it's connected to the charger for 24 hours now and it hasn't moved even 0.01V.

It's pretty logical to think that some chargers when the light turns green just go on idle and never pass any current after that so the only balancing that is possible in this scenario is bleeding down but never raising up low cell rows.
From experience, the bleeding down happens at the end of the charge whether the charger is connected or not and it never fixes large differences, it stops after a few minutes, sometimes an hour or so, so if there is one row at 4.2V that stopped the charge and the rest of the pack is at 3.9V then that 4.2V will never be bleed down to 3.9V.
 
Your assessment sounds correct.

Most chargers will terminate charge when the charge current drops below a given set point, usually around 1/10 of the CC level. If the cells are badly out of balance, the BMS might cut off the charge current when the higher cells reach the limit. This may also trigger the charger to shut down. As soon as the charger stops, any cells that are over the balance voltage will slowly bleed down. Any low cells will just sit there and do nothing.

To force balance, ideally you would want a charger that stays on regardless of current (like a Meanwell). Even better if the current limit is near or below the balancing shunt current, which is usually pretty low, like 40 - 100mA.

On one of my packs, I let the Meanwell go until cells started hitting HVC and the BMS would cycle on/off about once a second as they would bleed down and reset the HVC.

If a pack has a serious imbalance, restoring it at 40mA is going to take a really long time. Maybe days. If your charger cuts off and doesn't come back on, you only get a tiny amount of balancing on each cycle.

Another approach is to let the charger finish, then discharge the pack just enough to make the charge cycle start again. It may take a LOT of cycles to restore a large imbalance.

It may be possible to spoof a regular charger by adding a resistive load to the output (light bulb or large resistor) to keep the current above the level where it cuts off. This way the charger will stay on as long as you want.

Of course we all know that holding a pack at the maximum voltage for any length of time is bad for the cells, but if this is only done occasionally and for only as long as needed to balance the cells, I don't think it would make a significant difference.
 
fechter said:
Your assessment sounds correct.

Most chargers will terminate charge when the charge current drops below a given set point, usually around 1/10 of the CC level. If the cells are badly out of balance, the BMS might cut off the charge current when the higher cells reach the limit. This may also trigger the charger to shut down. As soon as the charger stops, any cells that are over the balance voltage will slowly bleed down. Any low cells will just sit there and do nothing.

To force balance, ideally you would want a charger that stays on regardless of current (like a Meanwell). Even better if the current limit is near or below the balancing shunt current, which is usually pretty low, like 40 - 100mA.

On one of my packs, I let the Meanwell go until cells started hitting HVC and the BMS would cycle on/off about once a second as they would bleed down and reset the HVC.

If a pack has a serious imbalance, restoring it at 40mA is going to take a really long time. Maybe days. If your charger cuts off and doesn't come back on, you only get a tiny amount of balancing on each cycle.

Another approach is to let the charger finish, then discharge the pack just enough to make the charge cycle start again. It may take a LOT of cycles to restore a large imbalance.

It may be possible to spoof a regular charger by adding a resistive load to the output (light bulb or large resistor) to keep the current above the level where it cuts off. This way the charger will stay on as long as you want.

Of course we all know that holding a pack at the maximum voltage for any length of time is bad for the cells, but if this is only done occasionally and for only as long as needed to balance the cells, I don't think it would make a significant difference.

Where exactly do I add the load? in parallel between the pack and the charger?
I have 4pcs 20ohm 100w resistors bolted to a heatsink and all connected in parallel (5ohm total), would that work?

Also, I didn't quite get the balance procedure.
If I charge it and a cell hits 4.2V then charging stops while lets say the rest of the pack is at 4.0V so then it bleeds the 4.2V cell for a bit and stops at lets say 4.15V, then I discharge the pack a bit to have another charge cycle, put it back in the charger and now that cell hits 4.2V again and the rest stay at 4.0V.
The question is, what would make the 4.0V ever advance towards the 4.2V cell if the charger never enable balance after the BMS sends a signal to stop charging?
 
rg12 said:
Where exactly do I add the load? in parallel between the pack and the charger?
I have 4pcs 20ohm 100w resistors bolted to a heatsink and all connected in parallel (5ohm total), would that work?

Also, I didn't quite get the balance procedure.
If I charge it and a cell hits 4.2V then charging stops while lets say the rest of the pack is at 4.0V so then it bleeds the 4.2V cell for a bit and stops at lets say 4.15V, then I discharge the pack a bit to have another charge cycle, put it back in the charger and now that cell hits 4.2V again and the rest stay at 4.0V.
The question is, what would make the 4.0V ever advance towards the 4.2V cell if the charger never enable balance after the BMS sends a signal to stop charging?
the load would be in parallel. so directly connected to the battery terminals. it's just to "fight" the charger.
a 5mOhm load would generate a 12A current (U = I * R, I = U / R = 60 / 5 = 12A), which equals to 720W heat to dissipate. that quite a lot, and you need a big heatsink, or maybe just put it in a bucket of water.

the algorithm of your BMS is unknown, but most of the cheap "cell bleeding types" do it that way:
you set a HVC (high voltage cutoff = max voltage ever to be reached) per cell. let's say this is 4.2V. and you may be able to set "balance start voltage" and "overcharge release voltage". let's assume balance start = 4.0V and overcharge release is 4.15V.
so your battery is empty, all cells get charged, once the first cell reaches 4.0V the balancing starts, and this cell, as it's the highest one, will be pulled down by activating the balance circuit, and it will be discharged by this super small resistor (some mA bleeding current). but it's still charged at the same time (some hundred to thousand mA)!!
you weakest cell will reach 4.2V quite fast and the bms will stop charging AT ALL. it will discharge ALL cells that are above 4V down until this cell reaches 4.15V. this can take AGES if the battery is big, and the bleeding resistor is small). once the cell is there it will start charging again.
if all works well more and more cells will catch up with the first one, and raise their voltage until all are where you want them to be.
got it? i hope it was understandable.
and to be honest: your bms' balance logic MAY work that way, and it's quite likely, but there are way more clever ways to implement this - they are more expensive though.

i personally woud check every cell's voltage by hand with a good digital multimeter. the note the suspect cell and charge it DIRECTLY with a rc charger. connect the small pins and charge/discharge this cell with 1-2A and it will be done pretty soon.
 
Got it, thanks alot :)
I usually fix big gaps like you said with an rc charger but am concerned about the packs out there that use shitty chargers that don't allow balancing after the green light.

btw, can I add the load on the battery load side or does it have to be on the charger side?
 
rg12 said:
Got it, thanks alot :)
I usually fix big gaps like you said with an rc charger but am concerned about the packs out there that use shitty chargers that don't allow balancing after the green light.

btw, can I add the load on the battery load side or does it have to be on the charger side?

It should probably be on the charger side as the BMS may disconnect the pack when cells hit HVC.

The resistor just needs to draw enough to keep the charger from shutting off. This is typically around 100mA but varies with charger model. You could place all of your 20 ohm resistors in series and be in the ballpark depending on your pack voltage. An old 100W incandescent light bulb might work too.

The handiest way would be to build a little in-line adapter that has plugs that match your charger and place it between the charger and pack when needed.
 
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