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Just about everyone has heard of the problem with mixing new and old batteries, but is it safe to mix various old batteries with each other, regarded that they are the same model and brands, and read with a similar voltage on a multimeter?

I have a number of batteries of the same type (voltage rating, brand and size) in storage. When measuring DC voltage output by the batteries straight from the positive and negative terminals, my multimeter gives readings ranging from ~ 1.30 V to ~ 1.57 for some used batteries, and it quite consistently gives 1.606 V as the reading for fresh batteries of the same type. Would it be fair to assume that in most cases the output voltage gives a general idea of the charge left, and would it be safe to mix all batteries of the same type (as explained above), as long as the voltage output measured was similar for the batteries to mix, and how much difference is recommended before you should be encouraged to not mix the batteries (if that is the case)?

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  • \$\begingroup\$ It's also recommended not to mix new batteries of different types. I believe this has to do with mixing different internal resistances for the batteries, which is something you might run into with different old batteries. \$\endgroup\$ – Samuel Aug 2 '15 at 3:02
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You are right in saying that the voltage across the cell gives an indication on its charge status, but the problem is that that indication is relative to the specific model of the battery. For example, two AA cells both having, say, 1.35V don't need to have the same amount of energy stored. That depends on many variables related to the specific chemistry used by their manufacturer.

Therefore, although you won't damage two batteries with the same voltage reading, they will discharge at different rates, probably. After a while one of the two will have a much lower voltage and a correspondingly higher internal resistance and will act as an additional load for the other.

Probably the only safe approach is to use cells of the same model from the same manufacturer and built almost in the same period: manufacturers improve their chemistries from time to time and I won't trust two batteries to be identical if one has been built two years after the other.

As for the range of the difference in voltage, 1%-2% is probably safe, i.e. from a practical POV if their voltage differ only by a couple of digits in the third digits you are ok: 1.25 and 1.23 is OK, 1.25 and 1.29 is borderline, 1.25 and 1.35 is NOT ok.

Anyway, I recommend to do this mix only with low power gadgets, in order to reduce risks to abuse a cell. That is, if you draw very little current, that's ok. If you try to use that mix-up in a power-hungry device it could overstress the weakest cell in the set and cause leakage or something worse.

EDIT (prompted by a comment)

The source for what I said above is just personal experience. After all, you are asking something that is contrary to every industry best practice, thus I interpreted yours as an hobbyists' question and answered in that context. If you asked me for a professional advice about that I'd replied not to ever mix and match cells like that. No one in a professional context would do that.

Don't get me wrong, I also reuse old cells, both because of environmental concerns and to avoid wasting usable energy, but I rarely mix two old cells "having a different history". When I change batteries from an "high-power" gadget I measure the old ones, if they are still usable, say V>~1.1V, I put them away sticking them together with a rubber band, so that they don't mix with other old cells. Then, if I have a low-power gadget (wall clock, small LED flashlight, digital thermometer, etc.) that needs that number of cells I'm sure they have all the same level of charge.

In rare cases I have done the mix and match, measuring their voltage as I told you before. What's the rationale of my 1%-2% rule of thumb? If the difference in voltage is so low, I can safely assume (being the same model and coming from the same batch) their internal resistance is different roughly by a similar percentage. That means that the load unbalance when used in a gadget in percentage will be roughly of the same order of magnitude (a few percent). Assuming I use them in low-power gadgets, i.e. in devices that draw say 5%-10% of the cell's max current, the cell with higher resistance will dissipate a few percent more than the already low power average dissipation of each cell, which is well below the safe threshold, since I'm working with loads that draw much less than the max current a cell can stand.

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    \$\begingroup\$ The question does ask specifically if it is alright for batteries of the same model later into the body. \$\endgroup\$ – micsthepick Aug 2 '15 at 3:25
  • \$\begingroup\$ can you provide a source for the amount of safe voltage difference you have stated? \$\endgroup\$ – micsthepick Aug 2 '15 at 7:46
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It is possible to mix any kinds of cells if you are prepared to put up with the problems. If you were to monitor the on load voltage of each cell and flag it as it fall below the useable level, and switch it out of circuit, you could use any mix of cell age, chemistry, size, age or manufacturer. Best value is gained by using compatible cells as this way all of them get used up instead of just the weakest one. Such a cell drainer system could be used to charge up your powerpacks and get free power from the near end of life chemistry. Having a string of 20 or 30 cells and then using a buck converter to charge your energy pack would put very little load on the cells and get the last drop of power out of them. Making the circuit to moniitor and switch the cells might never be recovered unless you had a endless source of used cells. (latching DIL relays to not waste power)

Using cells that have had a matched lifecycle means that you don't have to concern yourself with all the potential problems.

There are devices like an IR remote that will never stress a pair of cells but they may leak from old age before there could be any danger from using a new and old mix of different technology cells, they are an ideal place to put in almost dead cells. Using 12 cells in a motorised device with a 1 hour operating life and having one cell different may easily cause it to fail (perhaps spectacularly) if it is weaker than the others.

I rutinely take the 4 best AA cells (penlight) of matching chemistry (zinc or alcaline) from my box by voltage and have not had problems, I sometimes test the available on load current (0.5 to 2.0A depending on cell size) to detect weak cells if I hope to get long life from the set of cells. Generally if you have 4 or less cells in a device and one is completely discharged the device will fail to work and one automatically changes the cells before the weak cell is reverse charged to destruction, also remember that there is very little voltage and curent available to reverse the weak single cell if there are just 3 half emplty cells and the device load in series to try and do the job. I usually find that the top 4 cells in my box will be a type matched set that were probably used together in some device and will serve well again.

Some of the problems are

Internal resistance mismatch, the high ISR cells will heat up more

Reverse current possibility, cells are not designed for this and my get hot or generate gas

Minimum voltage not maintained in Lithium Ion cells can cause fire hazard

Any cells that are recharged togeher should have a similar state of charge before going back into service or the above or other problems may surface. Lithium Ion packs are monitored on a per cell basis to prevent these issues, other types are often over charged to balance the cells if possible.

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