Can you mix rechargeable batteries with different mAh ratings?
It will definitely reduce the effective capacity of the set as a whole, perhaps not quite to the lower of the two ratings, but very close. You will also be over-dischargeing the lower capacity battery, which isn’t good. A similar argument applies to charging. Overcharging a cell will overheat it, reducing its useful life. On rare occasions it might ‘explode’, which really means gases escaping from safety vents. I don’t see why you would want to mix them. Best case you are voluntarily reducing the capacity of your higher capacity cells… I’m sure there are arguments against mixing that I haven’t discovered… On the other hand, the grave warnings make it seem like a life or death issue, I’m sure it isn’t that serious.
What everyone else said – it’s a good habit to keep batteries in ‘sets’ through their lifetime (elastic bands are handy here). Charge and recharge the same batteries of the same type and spec together at the same time and use them in the same devices whenever possible. Batteries used together should be of the same rating, and the same brand. Manufacturers have been known to be somewhat ‘flexible’ in their self-rating (for marketing purposes) – cells of different brand with the same numbers on the side might not be as alike as we might want.
Do not do this, assuming the cells are in series, which they almost certainly are. The lower capacity ones will discharge first and then you will be impressing the voltage of the remaining cells across the load and the discharged cells. That means you’re reversing the voltage on the empty cells, which is invariably bad. NiCd cells will outgas (burst), NiMH probably the same, Li-Ion will catch fire. It’s exactly the same as charging them backwards and probably at a much higher current than a charger would use, even in the correct direction. So yes, it will work for a short time. Then you will destroy your cells, even though the device may appear to continue functioning since there is sufficient voltage from the remaining cells. Gassing/leaking cells may leak nasty fluids into your precious electronics, making it a terminal event for more than just the cells. This is the same reason you should always charge and use your cells as a set – if you mix fresh & partially-discharged, the full o
