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So, I have a number of high-capacity, Amazon AA NiMH batteries with unknown health and charge cycles. They’ve been in use for quite sometime and continue to work well for my purposes.

I have been charging them in groups of 4 at a constant current of 200 mA with a max voltage of 6.4 V.

I recently discovered that some cells will have very high voltage in the group, like 1.7 V or more. This causes the entire group to reach the 6.4 V limit and charge rate will drop down. I typically will just trickle charge the batteries in this condition for a couple of days until the charge current bottoms out around 20 mA. The batteries seem to work well after this.

However, I went one step farther and tested applying the full 6.4 V (200 mA limited) charge to individual cells. I found about half my cells stay at expected voltage ranges 1.2-1.5 V. Other cells jump very high. Even over 3 V. And if I leave the charge for several seconds the voltage will slowly drop. But I never leave them charging like that. This occurs even if the battery is already trickle charged as described above.

I separated out every battery with this behavior from the batteries that have “normal” behavior. When I group 4 “normal” batteries I can charge them easily below 6.4 V at 200 mA for most of the charge cycle.

On a side note, prior to discovering this latest behavior, I had noticed that the devices these 4 battery groups run in would start misbehaving when the batteries were low and if I checked the voltage on each cell, most would be normal 1-1.2 V but usually one will be floating, or even reverse voltage, until I apply some charge to it then it reads more normal again.

Why do some batteries show this high voltage when charging while many others stay within acceptable ranges?

Is there something wrong with the battery?

I will continue to do more testing to check output capacity and internal resistance, but I thought it was very strange behavior to see these cells jump so high in voltage and couldn’t find anything on the internet to back this up.

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    \$\begingroup\$ What you see is common behaviour with charging batteries in series without balancing. Small differences in capacity will cause some batteries to deplete earlier than others, causing (at first) slight differences in voltage. When these weaker batteries are part of a fixed set that is permanently placed in series, they will structurally discharge more than other batteries in the set, causing them to age faster and the differences in capacity to grow. The solution is to individually charge the batteries in the set, or use a proper balance charger. \$\endgroup\$
    – StarCat
    Commented Jun 20, 2022 at 6:38
  • \$\begingroup\$ Welcome! TL;DR Sounds like a bad cell. Get yourself a battery analyzer and test capacity in ESR and replace if bad. Alternately just replace the suspected bad cell. \$\endgroup\$
    – winny
    Commented Jun 20, 2022 at 6:38
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    \$\begingroup\$ Once a NiMH cell has been reversed, it's suffered permanent damage. It's not worth keeping in a series set, because it will dominate their capacity, and continue to wear more rapidly itself. High voltage on charging is a symptom of this damage. \$\endgroup\$
    – Neil_UK
    Commented Jun 20, 2022 at 7:55
  • \$\begingroup\$ As already stated either some cell faulted or some cell has vastly different specification and unbalances the charging process; a four pack of NiMH usually charges without special balancy circuitry \$\endgroup\$ Commented Jun 20, 2022 at 8:06
  • \$\begingroup\$ I'm not sure what sort of charger you're using, but the good ones charge cells individually, not in series, for reasons described in EJE's answer. You should get one of those. \$\endgroup\$
    – cjs
    Commented Sep 25, 2022 at 7:14

2 Answers 2

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Your questions:

Why do some batteries show this high voltage when charging while many others stay within acceptable ranges?

Is there something wrong with the battery?

The answer is related to another investigation you did:

I had noticed that the devices these 4 battery groups run in would start misbehaving when the batteries were low and if I checked the voltage on each cell, most would be normal 1-1.2 V but usually one will be floating, or even reverse voltage, until I apply some charge to it then it reads more normal again.

Short answer: Rechargeable batteries HATE being reverse charged and accumulate damage, increasing the symptoms of internal damage for the next use.

Then how this damage happens?
A common battery-killer is to drain a battery pack below a given minimum voltage, where one battery cell becomes fully discharged, reaches Zero Volt, then the other cells continue delivering current - and that poor battery is forced to pass current in the fully discharged state = charges in reverse polarity.
In this 4 pack NiMH battery, discharging starts at 4x1.2V = 4.8V but should be interrupted when voltage reaches 3x1.2V = 3.6V, or in the worst case, 3x1.0V = 3.0V. In case this is a conventional incandescent flashlight, or an electric motor-powered toy, they may not cut the use at 3.0V~3.6V and there is a risk to reverse charge a weaker cell.
I have a cordless vacuum when any cell depletion happens, it can be heard as the spinning motor lowers the sound pitch due to drastic voltage drop from the weaker cell. Draining current in that portable vacuum is quite high, about 8A. So, the continued use under this condition would mean an ultra-fast reverse polarity charge at -8A: something that will leave some internal damage.
in summary - A weaker battery gets damaged when reverse-charged, the greater the damage the longer it takes to be properly recharged, or how frequently this happens. My past experiences have shown it has a non-linear cumulative behavior.

Battery damage symptoms are:

  • Increased internal resistance (charge & discharge) - sometimes when discharged it is higher, then seems to “improve” when charged, but is just temporary;
  • Faster self-discharge;
  • Lower Ah capacity;
  • Increased self-heating (charge & discharge);
  • A combination of all of the above.

Most of these symptoms appear gradually for normally aged batteries, after long service use.
But when few cells gets worse quickly or faster than its “sister” cells in the same pack, suspect of being damaged by reverse charging or overcurrent discharge - another possibility, but you did not mention in your “case”, so it seems less likely for you.

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3 V, or even 1.7 V, appears to indicate a damaged cell. (EJE's answer gives lots of detail on what may be damaging them.) You shouldn't see more than about 1.55 V across a cell while charging it.

In the following chart shows typical cell voltage vs. charge input as % of typical capacity for charging at a moderate constant-current charge rate. The solid line is NiMH the dotted line is NiCD.

Cell voltage vs. sharge input, % of typical capacity

Source: David Linden, Thomas B. Reddy, Handbook of Batteries, 3rd ed.. Figure 29.16a (§29.5, p. 882).

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