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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.)

Tonny
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    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. – Neil_UK May 12 '17 at 14:26
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    This is your chance to build a sorting robot. Like those for Skittles, but now for AA's. – Jeroen3 May 12 '17 at 14:27
  • @Jeroen3 That did play though my mind... :-) – Tonny May 12 '17 at 14:31
  • @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. – Tonny May 12 '17 at 14:33
  • I agree with @Neil_UK. Get a Voltmeter and sort them into bins by voltage. – user57037 May 12 '17 at 14:33
  • I have never used a Duracell industrial, but I have used many types of alkaline batteries and I have never seen one that hovered around the 1.5V mark. I think you may want to double-check that assumption. – user57037 May 12 '17 at 14:36
  • @Tonny: around 1.5V sounds pretty full for an AA... The multimeter should be much more accurate than your LED test though. Also for best results do it while applying a load, like 1kΩ or so. – PlasmaHH May 12 '17 at 14:37
  • @mkeith I checked about 20 using a multimeter. Might have been a too small sample size, I admit. Will do some more substantial testing this weekend. With load as PlasmaHH suggested. That makes sense. – Tonny May 12 '17 at 14:38
  • @mkeith: My new ones are ~1.6V and the "used ones but useful" is around 1.3V and I have everything in betwee, I don't think 1.5V mark is in any way unusual, unless you mean for an empty battery. – PlasmaHH May 12 '17 at 14:38
  • @PlasmaHH, I don't think we are disagreeing. The OP was suggesting that OCV reveals no useful information about SOC because the OCV hovers around 1.5V until the battery is essentially fully discharged. I have used many types of alkaline batteries (and designed products using them, and set the voltage levels used for reporting battery status to the consumer) and my experience has been the voltage declines steadily as the battery is discharged. – user57037 May 12 '17 at 14:44
  • And, yes, I know, the voltage rebounds substantially when the load is removed, etc., etc. – user57037 May 12 '17 at 14:44
  • Alkaline batteries are chargeable. But only special alkaline batteries are **safely** chargeable. If you don't mind leaking and exploding batteries you can recharge the ones that didn't go poof a few times. You should definitively monitor the batteries, do the potentially dangerous thing in a safe outdoors area, have a way to prevent potential fires from spreading from the charger, have a way to shutdown the charger without electrocuting or burning yourself, and wear safety googles. – Oskar Skog May 12 '17 at 14:46
  • You're not trying to estimate SOC by terminal voltage, you're a) trying to sort the sheep from the goats (in the same circumstance I tend to throw anything <1.4v after 24h off load) and b) trying to match cells with a similar SOC after a similar history at a similar temperature (for which voltage will give a better than nothing estimate) – Neil_UK May 12 '17 at 14:48
  • I do the same, @Neil_UK – user57037 May 12 '17 at 14:51
  • The ESR and thus the SoC and current is a better indicator for Alkaline, but not best indicator for large cells. as Ah and ESR age in different mechanisms.So I prefer I to V, just as Eveready did with the thermal colour resistive load cell on the package strip, which is another valid test for matching but slower. – Tony Stewart EE75 May 12 '17 at 17:16

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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.
Tony Stewart EE75
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