NiMH Rechargeable AA Battery Pack Mystery
August 7, 2017 2:55 AM   Subscribe

What would cause not one, but two different 6xAA rechargeable battery packs used in a portable bluetooth speaker to charge on one occasion for 30 minutes before the speaker indicates the battery pack is fully charged (followed by only app. 45 minutes of playback before "low battery" indicator comes on, without being able to use on max. volume at all), while the next time, it reverts to normal behavior and charges for the expected 4 hours, followed by the usual 2 to 2.5 hours of playback, often on max. volume?

I have a couple of 6 x AA rechargeable battery packs (shrink-wrapped six cell AA battery package with a soldered connector) that I use in my portable bluetooth speaker. Normal charge/use battery pack behavior is 3.4 to 4 hours of charging followed by 2 to 2.5 hours of use (depending on playback volume levels). Recharging is done via the speaker itself, and an indicator light tells both when the battery is low and when recharging is complete.

When I first got the speaker about 1.5 years ago, the batteries would always charge for around 4 hours, followed by 2 to 2.5 hours of playback time. I barely used the speaker this last winter/spring (so the batteries were not really being used or recharged during that period), though, and it's since then that I've noticed this odd recharging behavior. More often than not, the packs are only charging for 30 minutes or so now, and the speaker is never able to perform at full intensity following this. But then I'll plug one in to recharge and it recharges and then performs at full power just like it used to. (I should note that both packs get very hot when charging for the full 4 hours, which I've read may indicate that one or more of the batteries inside a pack are bad.)

Is this typical NiMH battery behavior? Are they known to do a slowfade where they regularly give indications of being on the way out, but then seem to make a full recovery (with this behavior repeating many times)? Is there anything I can do to help them make a permanent somewhat-full recovery?
posted by tenderly to Technology (2 answers total)
I believe that batteries which have been allowed to drain more or less completely can take a number of charge cycles before their capacity is restored. I have an intelligent charger (available under lots of brand names) which has a refresh cycle which depletes and recharges the batteries for a few cycles. I have some NiMh batteries that are over 10 years old, I doubt they last as long as they did when new but are good enough for my purposes,
posted by epo at 5:36 AM on August 7, 2017

Sounds like the speaker has a charging circuit that isn't so good and it is getting confused about when the pack is full. NiMH batteries don't really need intelligent chargers, but they are often used to save energy and prolong the life of the batteries. Problem is, the end of charge voltage increase that NiMH cells exhibit is small at best, so some chargers will cut off early due to being oversensitive. (Some cells never do that and have to be cut off using a timer)

If you can break down the pack and use a quality charger on the individual cells you'll have much better luck going forward, I suspect.

FWIW, if it takes four hours to charge normally, the packs shouldn't be getting very hot during that process. That's a fairly slow rate that should make them warm at most. I've only ever had NiMH cells get hot from charging when using 1 hour quick chargers.
posted by wierdo at 6:40 AM on August 7, 2017

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