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.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?
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.
It's easier to say when there won't be a problem.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.
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.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.
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.
OK, this makes sense. Thanks for explaining.(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.)