AA NiMh Battery Question

[George7941]

These four AAs were used in a electric deadbolt lock for a couple of months. 2nd one is an Amazon Basics NiMh dated Dec 2017, the other three are Eneloop NiMhs which I purchased around 2012. There is no date on the Eneloops.

The Weiser deadbolt warning of low battery went off (goes off prematurely because it is set up for the 1.5V of an alkaline AA). The Eneloops took 260mahr charge but the Amazon Basics took 370 mahr charge.

Considering that all four batteries were in series in the deadbolt and subject to the exact same discharge current, does the discrepancy in the recharge currents provide any information on the batteries?

 

[DemoFly]

NiMH cells have an internal self discharge rate in addition to draw.

Eneloop whites are 2000mah and Amazon whites are 2450mah.

So the eneloops took ~13% to charge and the amazons took ~15%, which is expected as their voltage drop off will be similar per % capacity lost.

 

[George7941]

NiMH cells have an internal self discharge rate in addition to draw.

Eneloop whites are 2000mah and Amazon whites are 2450mah.

So the eneloops took ~13% to charge and the amazons took ~15%, which is expected as their voltage drop off will be similar per % capacity lost.

I can see voltage drop offs being relevant if the cells were in parallel. Here they are in series and I fail to see the role of voltage drop off during discharge.

 

[DemoFly]

Series or parallel makes no difference. Voltage decreases as you deplete the charge in a cell.

(one AA = cell, multiple in series or parallel = battery)

Even though the amazon whites have a higher internal self-discharge but higher capacity, the depletion rate of the cells in the battery will be similar because the battery will draw more from cells with the highest state of charge. (ignoring internal resistance, etc as these are tiny low voltage batteries)

To simplify, the amazon cells were at 75% capacity when you put them in the charger, and the eneloops were 77% capacity. When removed from the battery and left to rest they likely would read similar voltages because they are at similar state of charge.

If you're thinking each battery should have charged the same amount of mah because of a misconception of them being drawn from equally in series, that isn't the case. The battery is drawing more from the cells with higher state of charge and there is also a measurable amount of cell equalizing (though small).

 

[JMJNet]

No, it is called tolerance.
There are a lot of variable to see if that indicates the health of the batteries.
The best way is load test but I am not sure how to do it in AA batteries.

 

[George7941]

Series or parallel makes no difference. Voltage decreases as you deplete the charge in a cell.
If you're thinking each battery should have charged the same amount of mah because of a misconception of them being drawn from equally in series, that isn't the case. The battery is drawing more from the cells with higher state of charge and there is also a measurable amount of cell equalizing (though small).

The current draw from each one is exactly the same in a series configuration.
Equalization will only happen when they are in parallel.

 

[DemoFly]

No, the theoretical demand from each is the same. But that is not what is delivered.

You can literally charge a dead cell by putting it in series with two charged nimh cells.

 

[y_p_w]

There should be a date (something like "07-12UR" for Dec 2007) on the Eneloops if they're actually made by Sanyo. At least until 2014 (even under FDK ownership and Panasonic branding) they should have a consistently looking date that's impressed into the plastic wrap. I have some Sony CycleEnergy branded batteries that have the same date imprint style. The way it's impressed changed (smaller), although I'm not sure when.

It can be a little bit hard to see though.

As for the amount of charge that they took, the information you have might not have enough to go on. There could have been discrepancies in how the original charger and the current charger determined the "full" point. There could be differences in self-discharge characteristics. There could be differences based on manufacturing lot or just randomness.

That does seem to be odd with a device that relies on a higher voltage. I do remember having a Sony portable TV using 3 AA batteries. The useful time with most alkaline was about the same as with 1600 mAh NiMH batteries - around 3.5 hours. The alkaline would get really hot by the time the backlight stopped working and the open voltage was around 1.25 V. But the NiMH cells would still be working at a much lower open voltage.

 

[George7941]

Just checked - there is 12-02AD branded on the Eneloop. That makes it from 2012.