I have a huge stack (400+ or so) of used AA Alkaline batteries (Duracell Industrial).
Remaining capacity left varies anywhere between 0% and about 30%.
I have no way of knowing which ones are really empty and which ones are at 30%.

I've got a couple of small gizmo's (containing a few LED's and a tiny DC-motor) that can be driven by any DC power-supply in the 4.5V-10V range and which draw only 10-20 mA current each.
I currently run these gizmo's from a USB charger wall-wart with a DIY Frankencable.

I would like to find a way to use those AA batteries in stead. Seems a waste to just send them in for recycling.

Putting 4 AA's in series would do the trick, but I'm well aware that putting mismatched batteries in series is potentially dangerous and certainly not efficient due to the weaker batteries having higher internal resistance.

Any ideas how to do it? Or is the 4 mismatched AA's in series not problematic enough to cause issues (besides draining the batteries quicker than normal)?
A way to identify the really empty ones would be nice too. (I'm using a 1.4V LED to test if a AA cell can light it up. But that only tells me which batteries are really dead.)

  • 1
    \$\begingroup\$ Buy a multimeter to measure the voltage. Once you have one, and have used to to measure other things, you'll wonder how you did without. Even something that costs 5euro/dollar/pounds is useful for the basics. Match cells by voltage to get the best out of them. \$\endgroup\$ – Neil_UK May 12 '17 at 14:26
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    \$\begingroup\$ This is your chance to build a sorting robot. Like those for Skittles, but now for AA's. \$\endgroup\$ – Jeroen3 May 12 '17 at 14:27
  • \$\begingroup\$ @Jeroen3 That did play though my mind... :-) \$\endgroup\$ – Tonny May 12 '17 at 14:31
  • \$\begingroup\$ @Neil_UK I've got a decent multimeter, but those voltages hover closely around the 1.5V mark unless the battery is really bad. In that case the LED test is just as valid and quicker to apply. \$\endgroup\$ – Tonny May 12 '17 at 14:33
  • \$\begingroup\$ I agree with @Neil_UK. Get a Voltmeter and sort them into bins by voltage. \$\endgroup\$ – mkeith May 12 '17 at 14:33

You can test V with a 1A analog current meter for a blinking fraction of a second and bin them, as this will allow you to measure (almost) short circuit current and thus capacity. Combine say 10 bins into fewer bins as needed to optimize matching.

Mismatched cells are useless as the series capacity and ESR is controlled by the weakest cell. Car batteries have 6 cells and if not matched within 0.1% when new, it will become 100% mismatched sooner than expected lifetime. This is measured by specific gravity measured with 4 significant figures which is an indication of both SoC and ESR from sulphation.

The same idea is true for LiPo's in Tesla cars and many battery recycler's on Youtube videos yank out the bad 18650 cells from modules and measure the fully charged , settled Voc and bin them into similar tight ranges to maximize the useful total capacity.

Remember : Series cells follow the weakest link and dead cells can become reverse polarized with high ESR as another Op reported in a 3x AAA situation.

  • Fresh AAA cells are equivalent to xxx Farads with xxx. mOhm ESR in small batteries and scale this for large capacity cells like lead acid and LiPo.

  • Dead cells are equivalent xxx microFarads and xxx Ohms ESR with no capacity except as a very lossy capacitor.

All others are in between so matching series cells is very important.

I suggest <10% Voc for AAA but pref <1% and for LiPo I suggest < 1%Voc and pref. less. variation for longer life.

Also understand

  • the cell with the highest ESR gets warmest and ages faster when used high high motor current applications.
  • otherwise they age or deplete linearly and if you can't afford new cells just replace the dead one with a similar one or a new one and expect life to be limited by the weakest cell.
  • \$\begingroup\$ Better use the 20A range. Fresh AA alkaline cells can easily supply 5A. \$\endgroup\$ – Spehro Pefhany May 12 '17 at 15:52
  • \$\begingroup\$ I was assuming 0 to 30% \$\endgroup\$ – Tony Stewart EE75 May 12 '17 at 17:08
  • \$\begingroup\$ Bulk Alkaline batteries are only what 15 -20 cents each or so by the hundred?? so how much is your time worth? \$\endgroup\$ – Tony Stewart EE75 May 12 '17 at 17:19

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