Parallel batts, diff voltages

[ltiu]

I know if in-series, batts with different voltages will reverse charge each other.

But how about batts in-parallel? What happens if the batts in parallel banks do not have matching voltages?

 

[2xTrinity]

I know if in-series, batts with different voltages will reverse charge each other.

But how about batts in-parallel? What happens if the batts in parallel banks do not have matching voltages?

In series, you can get away with batteries with different voltages in certain circumstances (note: still not a good idea). The voltages will simply add up. The problem is if the two have dramatically different capacities, then once the weaker one is spent, the other will still be able to conduct current through it. The one with the lower capacity will then become reverse charged. Note - this is true even of cells with the SAME nominal voltage -- throw a NiMH D-Cell in series with a AA, and once the AA runs out, the D-Cell will begin to reverse charge it. This is also the most common failure mode of laptop battery packs -- one battery craps out faster than the others, and becomes reverse charged by the others. Most of my 18650s are left over from old thrown-away laptop packs, where only 1 out of the 6 or 8 cells is truly bad.

Throwing batteries of different voltages in parallel is a problem no matter what. When two components are in parallel with each other, there MUST (by definition) be the SAME voltage across them. Which means the lower voltage cell will be overcharged (forward-charged, not reverse) by the higher-voltage cell.

Doing this with a cell that has a low internal resistance, like a NiMH and LiIon in parallel with each other, would cause a short circuit and (very problably) a venting of chemicals in short order as you're basically applying 3.7V to a cell only meant to be charged with 1.2V.

 

[ltiu]

Throwing batteries of different voltages in parallel is a problem no matter what. When two components are in parallel with each other, there MUST (by definition) be the SAME voltage across them. Which means the lower voltage cell will be overcharged (forward-charged, not reverse) by the higher-voltage cell.

Doing this with a cell that has a low internal resistance, like a NiMH and LiIon in parallel with each other, would cause a short circuit and (very problably) a venting of chemicals in short order as you're basically applying 3.7V to a cell only meant to be charged with 1.2V.

Thanks for the answers.

How about just plain regular NiMH. If one bank happens to have 1.35v and the other bank has 1.30v.

Is that OK?

How much difference in V does it take to be a problem?

 

[Mr Happy]

How about just plain regular NiMH. If one bank happens to have 1.35v and the other bank has 1.30v.

Is that OK?

How much difference in V does it take to be a problem?

It's easier to say when there won't be a problem.

If you take two cells of the same age, type and condition, and both freshly charged on the same charger, then connecting them in parallel will not be a problem.

If you fail to meet those requirements, there might be a problem or there might not. The best answer if you are not sure is, "don't do that".

 

[ltiu]

It's easier to say when there won't be a problem.

If you take two cells of the same age, type and condition, and both freshly charged on the same charger, then connecting them in parallel will not be a problem.

Just that I notice it is impossible for me to maintain the exact same voltage for my NiMH. I do not have a super smart charger like that Maha charger. I just have off the shelf dumb chargers and I use my batteries all over so the same set/pairs are rarely used together/used the same way/used on the same equipment.

My batteries are typically off by a max of about 0.05 volts from each other. I hope the difference is not too much to be a problem.

 

[Mr Happy]

Just that I notice it is impossible for me to maintain the exact same voltage for my NiMH. I do not have a super smart charger like that Maha charger. I just have off the shelf dumb chargers and I use my batteries all over so the same set/pairs are rarely used together/used the same way/used on the same equipment.

My batteries are typically off by a max of about 0.05 volts from each other. I hope the difference is not too much to be a problem.

Notice that I didn't say anything about voltages. Voltage is not the important thing. It is more important to match capacity and state of charge.

If you take two cells of the same kind that you bought at the same time and that you have used in similar ways, and if you charge both of these at the same time on the same charger, then it will be OK to use them in parallel.

If you take any two random cells that might be different brands and/or charged at different times and/or of different capacities and put them in parallel, then you are not likely to get good results.

With NiMH cells nothing terrible will happen if you don't follow this advice, but you might not get good life from the cells and you might not get the run time you expect from the equipment.

(If you are wondering about what 2xTrinity said, that two cells must have the same voltage when you put them in parallel, he didn't mean they must have the same voltage before you put them in parallel, he meant they will have the same voltage after you put them in parallel. This is because any two points in a circuit that are connected together will have the same voltage by definition. If you join two unmatched batteries then current will flow from the stronger battery into the weaker battery until the voltages equalize, which will waste energy and may cause an overheating problem in extreme cases.)

 

[Timothybil]

When I had my Apple Newton, I used rechargables since it ate batteries for breakfast, lunch, and dinner. When I put a set into service for the first time I labeled them as Set 1, Set 2, etc. That way I could always keep them matched up and used evenly. I used a fine Sharpie, but a Labelmaker label would have been neater.

 

[ltiu]

If you take two cells of the same kind that you bought at the same time and that you have used in similar ways, and if you charge both of these at the same time on the same charger, then it will be OK to use them in parallel.

I find this impossible for me to do. Keeping track of close to a hundred NiMH batts being used for various assorted hobby purposes is hard.

If you take any two random cells that might be different brands and/or charged at different times and/or of different capacities and put them in parallel, then you are not likely to get good results.

With NiMH cells nothing terrible will happen if you don't follow this advice, but you might not get good life from the cells and you might not get the run time you expect from the equipment.

OK, this makes me feel better.

I will be using same brand, type and capacity NiMH together, just at slightly different charge states and usage history.

As long as things don't explode and vent. I'm ok with less efficient performance or slight chance of slighty damaging the cells.

(If you are wondering about what 2xTrinity said, that two cells must have the same voltage when you put them in parallel, he didn't mean they must have the same voltage before you put them in parallel, he meant they will have the same voltage after you put them in parallel. This is because any two points in a circuit that are connected together will have the same voltage by definition. If you join two unmatched batteries then current will flow from the stronger battery into the weaker battery until the voltages equalize, which will waste energy and may cause an overheating problem in extreme cases.)

OK, this makes sense. Thanks for explaining.

 

[barnefko]

Hi
Interesting thread - thx for all the answers.
one question though:
I have two AW`s 18650 i used in my single cell Ultrafire WF400 for 12 months now. My charger is the ultrafire WF-139.
I now want to use the two cells in parallel in a self build battery holder for a DIY headlamp. Knowing that i absolutely have to have the same capacity and state of charge i assumed i can take the voltage as an indicator (two batteries of same manufacturer,age and capacity).
To my problem:
After being fully charged (light at the charger turned to red) i noticed a difference in Voltage. Cell 1 measures 4.15V, Cell two 4.05V.

Is this difference dangerous? Do they have to have exactly same voltage to use them in my 2P battery holder? Which voltage difference is acceptable?

It just would be annoying to stop the cells in my single cell flashligt for 2,3,4,5,6... minutes before use to match the voltage in both cells or take them out of the charger every 5 minutes to check on them...


Thanks in advance for bringing a little brightness in my unenlightened brain...

 

[SilverFox]

Hello Barnefko,

First of all, a Li-Ion cell that only charges to 4.0 volts is used up and ready to be recycled. Your cell that is charging to 4.05 volts is very close to this point.

There should be no problems hooking those cells up in parallel after charging them individually. There will be a "sudden" current inrush from the higher voltage cell to the lower voltage cell when they are initially connected in parallel, but the difference you have is not that extreme. The larger the difference in voltage, the larger the initial current inrush will be. After a few seconds, the voltages will tend to equalize and the current will greatly reduce.

It would be dangerous to hook those cells up in series and charge them as a battery pack, but that is not what you are doing.

You can expect the lower voltage cell to empty first, so pay attention to the low voltage cut off. It will also run a bit warmer during discharge because of the increased internal resistance.

Tom

 

[SilverFox]

Hello Barnefko,

First of all, a Li-Ion cell that only charges to 4.0 volts is used up and ready to be recycled. Your cell that is charging to 4.05 volts is very close to this point.

There should be no problems hooking those cells up in parallel after charging them individually. There will be a "sudden" current inrush from the higher voltage cell to the lower voltage cell when they are initially connected in parallel, but the difference you have is not that extreme. The larger the difference in voltage, the larger the initial current inrush will be. After a few seconds, the voltages will tend to equalize and the current will greatly reduce.

It would be dangerous to hook those cells up in series and charge them as a battery pack, but that is not what you are doing.

You can expect the lower voltage cell to empty first, so pay attention to the low voltage cut off. It will also run a bit warmer during discharge because of the increased internal resistance.

Tom