I have a toy that works with 3 AA Batteries in series. I switched to NiMH batteries but my voltage is less now.
Do you think that the middle Battery in the series can be swapped to a 14500 Li-ion?
That way it runs in between 2 NiMH batteries and the total voltage would be 5.1 in a perfect world, with more mAh.
What would happen? The toy is a GPi Case running a Raspberry Pi Zero, or upgraded to a CM3 module if I wanted.
Best Answer
As a general rule, it is not adviseable to mix batteries' or cells' chemistries.
(edited after a comment)
And any kind of mixing should be avoided: don't mix cells of same chemistry, but different capacities; don't mix cells of different brands; don't mix cells of different age; don't mix cells of different charge levels...
In short: if there is any difference between cells or batteries (capacity, age, chemistry, brand, charge level), don't mix them.
The different internal resistance of the different kind of batteries may increase the discharging of one type in relation to another, and you can end with some of the batteries completely depleted before the others.
When using Li-Ion, is adviseable to run them with battery managers IC's, which assures that the battery is not depleted below the voltage levels that could damage the battery in an irreversibly way. Of course, if you mix different chemistries, you won't be able to use a battery manager IC.
By the way, besides these reasons for not doing this, I should add that the position of the Li-Ion relatively to the NiMH ones is irrelevant. You could put Li-Ion at the start of the series, in the middle of two NiMH, and at the end of the series, and the final result would be the same: bad things.
As a final warning: although I wouldn't recommend this solution that you describe, I still have to say that, if you want to try this solution, NEVER EVER recharge this trio of different batteries together: when charging, Li-Ion would go to a Li-Ion charger, and NiMH should go to a NiMH charger. But the best, rational and safe option is really DO NOT DO this.