Why is it bad for one cell to charge another?

[mvyrmnd]

I know, and accept, the common knowledge that is it bad for one cell to charge another, but I don't understand why.

Say for example, you have 2 Li-Ion cells in parallel. One is charged to 4.2 V and one is charged to 4.0V.

Wouldn't the charge just balance out over time? Can't that be harmless?

Could someone explain the process of it all going wrong?

 

[wyager]

I'm interested as well.

Also, why is it bad to stack different voltage batteries in series?

 

[Mr. Tone]

This is something I have been wondering for a while as well. Hopefully Silverfox or someone with this understanding will enlighten :candle: all of us.

 

[Mr Happy]

It really depends on which way the current is flowing. See the diagrams below.

In the first case, the current is flowing into the positive terminal of the lower voltage battery. This is mostly OK, as it is simply charging the battery. The only thing to worry about is whether the voltage difference is too large, in which case the instantaneous current flow might also be too large. But over time the charge states or EMFs of the two batteries will equalize and all will be OK. (In this situation the batteries must be rechargeable. If they are not, the charged battery may leak, or if it is a lithium battery it may explode.)

In the second case, the current is flowing out of the positive terminal of the lower voltage battery and thus flowing in the wrong direction, driving the battery voltage negative. This can happen when two batteries are in series and one battery discharges first. If this situation is allowed to continue, the reverse driven battery will suffer irreversible damage, and in the case of lithium batteries it might even explode.

 

[SilverFox]

Hello Mvyrmnd,

As Mr Happy pointed out, charging is not the issue. Reverse charging is what ruins cells, and with lithium chemistry it can become explosive.

Tom

 

[k1rod]

Indeed. It is not the problem of a like polarized source equalizing with another like polarized source (true parallel). It is because batteries will discharge at somewhat different rates and eventually one will become reverse polarized. At that point the battery that still has correct charge is causing problems with the one that has started reversing polarity by forcing current backwards into it through the bulb. This will not happen if the switch of the flashlight is off, nor will it happen with a led flashlight (a diode only conducts in one direction).

 

[Mr Happy]

It is because batteries will discharge at somewhat different rates and eventually one will become reverse polarized. At that point the battery that still has correct charge is causing problems with the one that has started reversing polarity by forcing current backwards into it through the bulb. This will not happen if the switch of the flashlight is off, nor will it happen with a led flashlight (a diode only conducts in one direction).

Sure it will happen with an LED flashlight. See below:

The current is not being forced backwards into the discharged cell (this would be charging, and is fine); rather the current is being forced forwards into the cell, the same direction as normal, and this is what is bad. Current should never be forced through a cell, the cell should always be doing the forcing using its own voltage.

 

[k1rod]

You are correct. I stand corrected... Thanks.

 

[mvyrmnd]

OK. So running cells in series increases the risk of once cell discharging faster then the rest, creating a scenario where it is being reverse charged by the cell that still has a higher voltage.

Parallel is OK, as long at the charge states and capacities are similar (or the same)

Correct?

 

[k1rod]

Voltage across a parallel circuit by definition has to be the same at all nodes across the parallel circuit (excluding large I2R drops which are probably not a problem here). In this configuration it would not be possible for two batteries to be at different potentials (voltages). I don't see any way to harm the batteries in a parallel circuit unless... You start off with something like a fully charged battery and a completely dead reversed polarity battery paired together in parallel. Then I am not sure what would happen.

 

[Lynx_Arc]

If the cells are close enough together in voltage like the diagrams it is ok, but get a cell that is nearly discharged and connect it to another freshly charged cell to charge it and the current flow can heat things up rather fast in some cases, possibly to dangerous levels.

 

[Mr Happy]

OK. So running cells in series increases the risk of once cell discharging faster then the rest, creating a scenario where it is being reverse charged by the cell that still has a higher voltage.

Parallel is OK, as long at the charge states and capacities are similar (or the same)

Correct?

Yes, spot on! :thumbsup:

 

[Mr Happy]

Voltage across a parallel circuit by definition has to be the same at all nodes across the parallel circuit (excluding large I2R drops which are probably not a problem here).

Voltage yes, but internal potential (EMF or charge state), no. You have to take internal resistance into account.

In this configuration it would not be possible for two batteries to be at different potentials (voltages).

If you connect two similar batteries in parallel and one is at a lower state of charge than the other, then current will flow from the more charged battery to the less charged battery. This current will continue until the batteries are equalized. If the batteries have a low internal resistance and you connect them with a heavy gauge wire the equalization current may exceed the rating of the batteries. It may be rare for this to be a problem, but in the case of lithium ion cells it is as well to be cautious.

 

[mvyrmnd]

With lithium ion cells in series, how far apart is too far apart?

I have a Mag625 running on 3 26650's in series. The cell at the switch starts to get pretty hot from being closer to the bulb. As it gets hotter, it discharges faster.

I've only ever used the light in short bursts, but if I were to run it for a full burn, I imagine the first cell will start to become dangerously different to the 2nd and 3rd cells.

As a rule of thumb, should I allow .05V? .5V?

What's your best educated guess?

If the batteries have a low internal resistance and you connect them with a heavy gauge wire the equalization current may exceed the rating of the batteries. It may be rare for this to be a problem, but in the case of lithium ion cells it is as well to be cautious.

I see. So an IMR cell could discharge at say 10C into another cell that is only designed to charge at 1C, causing

 

[bobrez]

Heyas, question on this subject.
I have 2 batteries series for 24v for my electric motor on my boat, 1 batt is slightly older,(1 to 2 years older) fully charged is 12.54v.
Newer batt is 12.86v full.
Will this slight difference cause any problems, is there a best way to hook them up?
I also connected an led spotlight to 1 of the batts, choosing the newer one.
Will the led drain cause problems to this setup?
I charge the batts individually, with a auto plugin charger.
Thanks All..

 

[SilverFox]

Hello Bobrez,

Welcome to CPF.

You are hooking up a battery that is at 100% with one that is around 80%. As long as you don't over discharge, you should be OK.

However, in a perfect world it would be better to have both batteries at the same voltage.

Tom

 

[TakeTheActive]

...I have 2 batteries series for 24v for my electric motor on my boat:

• 1 batt is slightly older,(1 to 2 years older) fully charged is 12.54v.
  .
• Newer batt is 12.86v full.

...You are hooking up a battery that is at 100% with one that is around 80%...

SilverFox,

Please explain how you arrived at 100% and ~80% from 12.86v and 12.54v.

Thanks!

 

[bobrez]

Please explain how you arrived at 100% and ~80% from 12.86v and 12.54v.
Thats about what i would call it too going by this chart- says bout 85%http://www.mmbalmainauto.com.au/PDF/State_of_charge_12_volt_batteries.pdf

Also connecting led in that way would cause any problems?
Thanks.
ps these are deep cycle walmart batteries

 

[SilverFox]

Hello Bobrez,

It would depend on how much you use the light. If it is on all the time it will drain the one battery.

Once again, if you shut things down before they are totally discharged, there will be no problems.

In a perfect world you would have a seperate battery for the light.

Tom

 

[SilverFox]

Hello Bobrez,

It would depend on how much you use the light. If it is on all the time it will drain the one battery.

Once again, if you shut things down before they are totally discharged, there will be no problems.

In a perfect world you would have a seperate battery for the light.

Tom