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There are battery adapters that take e.g. three smaller cells (e.g. AA) and form a battery of a larger format.

Like that one:

enter image description here

The batteries can be parallel or in series.

For the parallel kind, what happens if the batteries inserted have very different voltages, e.g. two are full and one is empty?

Or what happens when using rechargeable batteries of different sizes, e.g. 1.2 V 1300 mAh and 2000 mAh, and the smaller one is depleted earlier?

Or if you combine a 1.5 V non-rechargable with a 1.2 V rechargable one?

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  • \$\begingroup\$ The only way I could see this product working safely and efficiently, is if the individual cells are all "protected" Li-ion cells of the same exact model, from the same manufacturer, and fresh, from the same lot. Even then, after several charge/discharge cycles, the cells will likely get dissimilar, and lead to what mkeith described below. \$\endgroup\$
    – Rich S
    Commented Oct 1, 2022 at 21:54
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    \$\begingroup\$ I wonder if the positive terminal has reverse polarity protection. If not, that is probably the number one hazard. You can put in one battery correctly and the second one in reverse polarity. \$\endgroup\$
    – user57037
    Commented Oct 2, 2022 at 6:06
  • \$\begingroup\$ Do you have a reference for the parallel kind of battery adapter? I've never met one. It seems a bad idea, for the reason in the question, and also because the power density of the combination is bound to be poorer than that for a single larger cell. That's also true for the series kind of battery adapter, but at least this kind achieves something over a larger single-cell battery: boosting the voltage. \$\endgroup\$
    – fgrieu
    Commented Oct 2, 2022 at 8:24
  • \$\begingroup\$ You can find many of them on Amazon or Ebay, "battery adapter aa mono". There are the ones that convert only size and take a single cell and parallel or serial ones. \$\endgroup\$
    – Joe
    Commented Oct 2, 2022 at 11:35
  • \$\begingroup\$ Indeed, found one of these adapters. I understand your concern. I indeed fear there would be at best a waste of energy, at worse some level of heating, if different technologies/freshness/charge levels of batteries/accumulators are mixed into these. \$\endgroup\$
    – fgrieu
    Commented Oct 2, 2022 at 17:35

3 Answers 3

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Current will flow from the higher voltage cell or cells into the lower voltage cell or cells until the voltages are equal, or until one of the cells is disconnected. In general you are not supposed to allow current to flow into primary (non-rechargeable) cells. I don't know for sure what could happen if you do this, but the manufacturers don't recommend it.

From a practical perspective, there is no reason to put cells in parallel unless they are close to the same voltage. A lot of useful energy will be lost as the cells equalize voltage. It would probably be better to discharge the highest voltage cell alone until the voltage drops, then add in the parallel cell or cells (if possible).

If you do it accidentally, I think the chances of fires, explosions, etc are probably remote, but I have never done any testing of this configuration so I cannot be sure.

If you put a fully charged NiMH cell in parallel with a fully discharged NiMH cell, there is a possibility of pretty large current flowing while the voltages equalize. This could cause one or both cells to heat up, and it could also cause the battery contacts to heat up, possibly even hot enough to be a hazard. NiMH cells are capable of delivering very high currents. The worst type of battery contact is the helical wire type (the ones that look like coil springs). I have seen these glow red hot during AA battery short-circuit tests.

Usually the user manuals for battery powered devices specifically say not to mix cell types and to always change out all the cells at the same time.

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    \$\begingroup\$ Alkaline cells, the most common type of primary cell, can handle current flowing backward through them reasonably well. They actually can be recharged, but very inefficiently (meaning you have to do it very slowly to avoid heating the cell) and only about five or six times; there are (or at least were) devices on the market to do this. I don't think it would cause any major problems to parallel them as long as they're at a reasonably similar SoC. Some consumer devices actually put batteries in parallel, though that's gotten less common. \$\endgroup\$
    – Hearth
    Commented Oct 2, 2022 at 15:48
  • \$\begingroup\$ @Hearth: From my understanding, what prevents akaline batteries from being recharged isn't that the chemical processes can't run both ways, but rather that the shapes of the electrodes will change as cells are charged and discharged. \$\endgroup\$
    – supercat
    Commented Oct 2, 2022 at 21:07
  • \$\begingroup\$ @supercat Yes, this is why they can only be recharged a half-dozen or so times before they become completely unusable. They can be recharged, but it's not recommended. \$\endgroup\$
    – Hearth
    Commented Oct 2, 2022 at 21:08
  • \$\begingroup\$ Yes. I worked for a toy company for years. We went to great lengths to make sure that alkaline cells NEVER had charge current applied to them. It was strictly forbidden by QA. Likewise, we had to have very low current once cell voltage was around 0.9 or 0.8 V. If you have ever had a device where the alkaline battery leaked, that was probably due to cell inversion (discharge continued until cell voltage is < 0). This happens when two cells are in series and one has fully discharged and one has not. The fully discharged cell inverts and spills its guts. We didn't want that to happen. \$\endgroup\$
    – user57037
    Commented Oct 10, 2022 at 6:39
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For the parallel kind, what happens if the batteries inserted have very different voltages, e.g. two are full and one is empty?

When you connect things in parallel, you force the terminals to be at the same voltage. As mkeith says, the higher voltage one will try to charge up the lower voltage one. That could lead to overheating or leaking batteries.

Or what happens when using rechargable batteries with different sizes, e.g. 1.2 V 1300 mAh and 2000 mAh and the smaller one is depleted earlier?

Provided all the cells are fully charged, that should work. All the cells are forced to discharge together, and all remain at the same voltage as each other.

Or if you combine a 1.5 V non-rechargable with a 1.2 V rechargable one?

That definitely wouldn't be good. An alkaline cell stores more charge than an equivalent NiMH one. So it will try to charge the NiMH one up to 1.5V. Expect overheating, leakage, or at least a destroyed cell.

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For the parallel kind, what happens if the batteries inserted have very different voltages, e.g. two are full and one is empty?

The full ones will try to charge the empty ones.

  • if alkaline, the full one will discharge, the empty one will warm up a bit, just like a resistor (P=V*I)
  • if NiMH, the full will discharge, the empty will charge somewhat, with some energy wasted by the heat produced.
  • if Li-Ion, the full will charge the empty with a large current, the empty will be damaged, and very hot (say 4V * 5A = 20W), possibly explode.

Or what happens when using rechargable batteries with different sizes, e.g. 1.2 V 1300 mAh and 2000 mAh and the smaller one is depleted earlier?

Nothing will happen. If they all start at 1.2V, each battery will supply current about proportional to its capacity, until all are depleted more or less simultaneously.

Or if you combine a 1.5 V non-rechargable with a 1.2 V rechargable one?

Waste of energy, 1.5V will try to charge the 1.2V one, possibly too much heat, but fire unlikely.

BTW, it is not a question of "more charge" in one battery or other. The variables are:

  • the "full" battery ability to supply current
  • the "empty" battery resistance to be charged

The fire happens when the "full" battery can supply large current, and the "empty" battery resistance is low, so the large current can flow.

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