Chinese magic charging lithium cells in series...

dl324

Joined Mar 30, 2015
14,897
I don't know of any way to charge batteries in series without being able to monitor the voltage of each cell independently. The same goes for charging batteries in parallel.

I suspect it's just another product that actually doesn't work and they don't care because their business model doesn't depend on repeat customers.
 

MisterBill2

Joined Jan 23, 2018
12,685
I have several power tools that appear to charge the litium cells in series. And I have built and used series strings of lithium cells with no problems. Of course in my applications the cells are charged at much less than the highest rates, and in the battery powered tools the cells were probably matched. so it is possible and it can work but of course it is not as god, nor does it aid in having maximum life. So there is a trade-off.
In addition, keep in mind that not all products are designed well. Especially from some parts of the world.
 

bassbindevil

Joined Jan 23, 2014
497
Don't take the illustration literally. It's your responsibility to use a protection board or protected cells. Unless otherwise specified, chargers have one job: charging to the appropriate voltage for the number of lithium cells, along with current limiting and charge termination detection stuff. No different than a zillion wall-wart/power-brick lithium-ion chargers. If you want a charger that includes balancing, you need a "hobby charger" which is very versatile (and belongs on every electronic hobbyist's workbench), but not as foolproof as "plug it in and wait for LED to turn change colour".

But I have opened up name-brand tool batteries (not "serious" tool brands, but home/garden stuff sold in big box stores) that have NO cell balancing, and sometimes no protection beyond a fuse. They get away with it by using reasonably well matched cells, and, a warranty that expires before the battery pack does.

I've bought a couple of those boards; haven't tried them yet, but they seem like a good option for charging 2S/3S/4S packs from USB when you'd rather not lug around a dedicated 120V charger.
 
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BobTPH

Joined Jun 5, 2013
5,458
I don't know of any way to charge batteries in series without being able to monitor the voltage of each cell independently. The same goes for charging batteries in parallel.

I suspect it's just another product that actually doesn't work and they don't care because their business model doesn't depend on repeat customers.
Cells in parallel always have exactly the same voltage, they behave as a single battery and can be charged as such. Even if they are of different capacity, the current will split to keep them charged to the same voltage.
 

dl324

Joined Mar 30, 2015
14,897
Cells in parallel always have exactly the same voltage, they behave as a single battery and can be charged as such. Even if they are of different capacity, the current will split to keep them charged to the same voltage.
This is a common misconception.

If you're trying to charge each cell optimally, cells in parallel will never allow you to do that. They won't split current equally because they'll never be matched closely enough. Even if they started out that way, they won't stay that way. The weak cell could cause the stronger cell to fail prematurely, or vice versa.
 

BobTPH

Joined Jun 5, 2013
5,458
This is a common misconception.

If you're trying to charge each cell optimally, cells in parallel will never allow you to do that. They won't split current equally because they'll never be matched closely enough. Even if they started out that way, they won't stay that way. The weak cell could cause the stronger cell to fail prematurely, or vice versa.
please explain how one cell can be at a different state of charge than another one when their terminal voltages are exactly the same.
 

dl324

Joined Mar 30, 2015
14,897
please explain how one cell can be at a different state of charge than another one when their terminal voltages are exactly the same.
Here's an example. Say I have 2 12V SLA batteries. One is dead because it won't charge or hold a charge and the other is still good, but I think I can charge them in parallel.

In parallel, both will have the same voltage, but the dead battery will not have the same state of charge as the battery that's still good.

The same will apply in single cell configurations. The cell voltage is the same, but that doesn't tell you anything about the state of charge of each cell.
 
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bassbindevil

Joined Jan 23, 2014
497
Well, they both kind of have the same state of charge: 100%. It's just that for the bad battery, 100% of nothing is zero.

Now, if you have two healthy batteries of wildly different capacity in parallel, it'll be difficult to predict how much charging current each battery will take. So, best to err on the side of caution and charge at a rate that's safe for the smallest battery. But that's an unlikely situation, unless you're off-grid and want to charge a small 12V battery for some portable device. Then, the right answer is a DC-DC charger (or solar charge controller) that can charge the small battery separately.
 

MisterBill2

Joined Jan 23, 2018
12,685
Batteries in parallel need to be matched, and probably will seldom be at the same state of charge. I have serviced battery packs mage for lighting up crash-test impacts. The lights were arrays of 24 lamps, 1000watts each, running all in parallel. The battery pack had a series string of 24 of the BIG 12 volt batteries, and the output was fused at 700 amps, far below what it could deliver. The arrangement was 12 pairs of paralleled batteries, with one charger for the whole string. That was about 156 volts, charging at a max of ten amps. The batteries would last at least two years, often as many as five years. Quite a bit of caution is needed working on a system that can evaporate the wrench out of your hand.
Individual battery failures were very seldom, but always spectacular. 24 KW at 120 volts DC is a LOT OF CURRENT.
 

BobTPH

Joined Jun 5, 2013
5,458
If course you cannot charge dead cells.
You cannot charhge them in series, you cannot charge them in parallel, and you cannot charge them individually. if you have two good cells of the same type, you can charge them in parallel. I would not charge them at the sum of the nax currents, because yes, they would not split the current exact correctly, but if one cell is getting too much charge, its voltage will rise faster, which will shift current to the other. Thus negative feedback will accommodate small differences
 

dl324

Joined Mar 30, 2015
14,897
If course you cannot charge dead cells.
I used an extreme case to simplify the thought process.

The same problem occurs in unmatched cells. The stronger cell will charge faster and the weaker cell will never get fully charged. That will continue the weaker cell's downward spiral until it really is dead. If you're lucky, that will just result in reduced capacity. If you're unlucky, the dead cell will short the good one.

I don't know of any battery chemistry where you don't need to monitor the cell voltage to determine what the state of charge is. When you put two batteries in parallel, the voltage across them will be the same, but you can't determine the actual voltage of either of them. Some battery chemistries require detecting mV changes in cell voltage and you can't do that with parallel batteries without some scheme to isolate/disconnect them for charging.
 
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MisterBill2

Joined Jan 23, 2018
12,685
I have recharged a 24 volt pack with one failed cell. But I have no idea as to what the failure mode of that cell is. Clearly it has not failed as an open circuit. So the battery pack will deliver about 21 volts. Not sure how this relates.
 

BobTPH

Joined Jun 5, 2013
5,458
The same problem occurs in unmatched cells. The stronger cell will charge faster and the weaker cell will never get fully charged
That is where the negative feedback comes in. If a cell falls behind, its internal voltage lags the voltage at its terminals, thus there is a greater charging current to catch it up. In reality, it never falls behind because this process is contnuous.

Now, a severely damaged cell will cause problems. It could hog the available current and overheat, but that is different than what you are saying. The same thing would happen if you charge a the damaged cell by itself.
 
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