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Everyone knows about this but I'm curious about why one should never mix batteries of different types. By type I mean the same shape of the cell like AAA but with different chemical reactions and voltages inside.

Would it just mess up the voltage and the instrument that is operated with the battery? Or would it lead to unexpected reactions in the battery itself that could lead to leakage/explosion or other catastrophies?

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  • $\begingroup$ Theoretically there must not be any issue because only electrons are transferred through wires. You can connect different voltages of different batteries but here comes an only issue- internal resistance, for dry cell it is too high for li-po it is very low. So if you connect a li-po battery that supply huge current with a dry cell then dry cell may reach very high temperatures and may explode. I am not sure of it so added it as a comment but you can surely use different battery types with almost same values of internal resistances and voltages. $\endgroup$ – Saurabh Singh Apr 29 at 16:45
  • $\begingroup$ If run in parallel, one might try to charge the other one if the voltages don't quite match. I'm not aware of applications where batteries are run parallel. Usually they are in series, to get a large voltage. $\endgroup$ – Karsten Theis Apr 29 at 17:20
  • $\begingroup$ They are used in parallel when you want a bigger capacity of USB power banks. They use then convertor of 1 LiIon cell voltage to 5.1V. generally, it the cell voltage difference is less then twice of their voltage raise/drop during safe cell charging)/discharging, such connection is safe too. $\endgroup$ – Poutnik Apr 30 at 5:31
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Serial usage

For the serial usage, one must avoid combination of cells of different kind and health. They should be as identical as possible.

The best is using

  • the same chemistry
  • the same format
  • the same vendor
  • the same brand
  • the same lot
  • the same aging

Otherwise the cells will have the different capacity and different discharge profiles under the load.

If the serial cell chain is deeply discharged, the "weakest" cell gets discharged completely.

And what is worse, other cells start to electrolyze the unlucky cell in the opposite direction that would be its charging.

This would lead almost inevitably to heavy damage or more likely destruction of the cell, what is unwanted for rechargebles.

Physical destruction with electrolyte leakage is also possible.

For primaries, it would be rather annoyance of not fully used better cells and no easy possibility to find a matching cell.

Parallel usage

It is generally adviced not to use it, unless you know exactly, what you are doing. See also notes for non rechargeable cells below.

Dangers

If cells of different chemistry and therefore voltage range are used in parallel, they overcharge or overdischarge each other when their common voltage does not fit the overlapping of their save voltage ranges.

But even within the overlapping voltage range, cells can be overheated and thermally functionally damaged/destroyed or can even explode, depending on the cell voltage difference and cell internal resistence.

The last paragraph is also valid for cells of the same chemistry. They cannot overcharge each other, but the thermal damage from above applies to them as well.

How to do it well

Only cells of the same chemistry and performance class with about the same actual voltage (check it ) should be connected in parallel.

The voltage difference must respect expected internal resistance and acceptable resulting balancing current. Note that typical internal resistance of Modern AA NiMH cells is $\pu{30-100 m\Omega}$, so $\pu{60 mV}$ difference can cause the current $\pu{1 A}$.

Once connected and balanced, they are safe, as they autobalance themselves during charging and discharging, but consider notes below.

Cell autobalancing of already connected parallel cells

This analysis is aimed for rechargeable cells., supported by Excel simulation.

Cells in an open circurt

Balanced parallel batteries have always the same external (behind their internal resistance ) and internal voltage.

Cells under load

The initial current is in reciprocal ratio of their resistances, so high performance cells would get drained initially faster despite their lower capacity.

The cells with lower $resistance * capacity$ parameter gradually get the internal voltage lover than the others and provide therefore gradually lower current than the other cells.

This leads to the cell balancing, with the current ratio possibly even getting reversed to the initial one. The current ratio slowly converges to the capacity ratio.

Important parameters are the ratio of cell capacities and the ratio of cell internal resistances.

The best results are achieved if these ratios are reciprocal. In such an ideal case, cells are discharging in ratio of their capacities and no internal autobalancing ever occurs.

The good thing is, that the cell resistance increases with the degrading the cell capacity, as cells are aging.

Best autobalancing is achieved on low currents. High currents lead to higher internal voltage disbalance and are more demanding for good cell matching.

If the cell resistance ratio is not optimal for the capacity ratio, autobalancing kicks on, but does not fully compensate. The internal voltages get therefore little disbalanced.

After disconnection, there is ongoing small temporary compensating current.

It is not good to combine lower capacity high performance cells with low resistance with high capacity low performance cells, especially for high loads. As it significantly raises the internal voltage disbalance and after load compensation

Cells under charging

It is generally adviced against parallel charging, as it affects charge stop detection and can cause overcharging of some cells.

That affects serial charging as well.

Non rechargable cells

Parallel connection is not generally good for different . As there can occur small balancing load after disconnection of the powered device.

OTOH, if the identical cells ( like in the serial section) are used in parallel connection all their lifetime, it is not a big deal, as these currents are minimal and short time.

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  • $\begingroup$ And that's also why you must remove empty batteries from an appliance. If a small current can still flow, the batteries will destroy each other one by one. $\endgroup$ – Karl Apr 29 at 18:01
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    $\begingroup$ Also, the longer the serie is, the bigger danger, as it is a fight N-1 versus 1. With 2 cells only it is not a big deal. With 4 or 6, the fun may begin. $\endgroup$ – Poutnik Apr 29 at 18:04
  • $\begingroup$ If you put different cell types in parallel, the one with highest voltage will almost definitively destroy the others. unless each of them has a dedicated overcharge protection (very unlikely). $\endgroup$ – Karl Apr 29 at 18:04
  • $\begingroup$ Not necessarily, if they are of the same type and about the same charge and voltage. The big power banks uses e.g 5 3400mAH 16550 type heavy duty LiIon cells in parallel. With the open voltage getting closer, the risk of damage dramatically decreases. $\endgroup$ – Poutnik Apr 29 at 18:07
  • $\begingroup$ You wrote above that mixed cell types in parallel may overcharge each other. I think that's nearly a cert. ;-) Same cell type and age not so problematic, but those powerbanks have a controller that monitors each cell separately. You can buy these circuits on ebay in packs of ten from China. $\endgroup$ – Karl Apr 29 at 18:26

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