Compaq LTE Elite 4/75CX not charging battery (anymore)

[3lectr1c]

I was surprised to see that my LTE Elite came with an aftermarket Energy+ battery. More surprised to see it charge, right up to 12V, and then it ran the laptop for a short time before dying. I figured it just needed longer to charge, or it was old and tired.
It hasn't run the laptop since then though. Normally, I'd just think that the battery failed suddenly due to age/developing a shorted cell, but that doesn't appear to be the case. The battery still measures 11.5v on my multimeter. I did a test, and left it to charge for a while, and then checked and saw it went up a tiny bit. Don't remember the exact numbers but it was something like 11.55 to 11.62 or something similarly small. So, it is charging, just extremely slowly.

It wasn't doing this before - it charged up quick the first time. It also isn't getting warm at all anymore. It got quite hot before, which I do believe is normal for NiMH batteries while charging.

My main guess at this point is that for whatever reason, it got too hot (it did get very toasty), and then blew a thermal fuse or something. But if this was the case, why would it still take a very small amount of additional charge?
I don't have another LTE Elite to test in unfortunately, so I can't rule out whether it's the battery or the charging circuit in the LTE at fault. I really don't feel like the cells are bad though, as they seemed healthy right before this happened (I monitored the voltage while it was charging several times for that first time), and in the past, when batteries have had sudden death, they don't generally keep that much voltage. It's nearly enough to run the computer. The drop over time is also slow and seems normal, I don't see it suddenly going down while measuring.

The charge light on the LTE does come on orange.

Any thoughts?

 

[modem7]

More surprised to see it charge, right up to 12V, and then it ran the laptop for a short time before dying.

'12V' is a nominal figure. From various sources, I see that the battery is ten NiMH cells in series. A fully charged NiMH cell outputs about 1.4V. So ten of those in series equates to about 14V, and that is confirmed by information in the computer's Maintenance and Service Guide.

... and then blew a thermal fuse or something ...

If that blew, then I would expect that there would be no voltage on the battery pack's output terminals.

 

[Svenska]

You can try measuring the charging current (may or may not be easy) to see whether the system actually tries to charge the battery.

If the battery hasn't been used for a long time, you may have to deal with some faulty cells in the whole arrangement. NiMH cells somewhat tolerate discharge down to zero, but they get damaged from reversed voltage. In other words, weaker cells may have been destroyed by their stronger neighbors while in storage.

The only way to find out is to measure the voltage across each cell separately and balance them manually. You could tricke-charge the whole stack at very low current for a very long time (should not damage NiMH cells), and that would eventually balance all cells at maximum voltage - however, that only works if all cells are healthy and not too different.

Check your cells.

 

[3lectr1c]

Charging is working again seemingly, after leaving it plugged in for a while it’s up to 12.78 volts. We’ll see what condition the cells are in. I’d sooner recell the battery than try balance charging potentially 20 year old cells. I’ve got no clue how old this pack is, but it can be that recent.

 

[GiGaBiTe]

It wasn't doing this before - it charged up quick the first time. It also isn't getting warm at all anymore. It got quite hot before, which I do believe is normal for NiMH batteries while charging.

It is NOT normal for NiCD or NiMH batteries to get hot while charging. If they are, they're either grossly overcharged, or are being charged too fast. If you just slapped full charge voltage across an old battery pack, I'd wager that one or more of the cells had shorted out and the rest of the cells were being pushed far over their max voltage rating and trying to burn the excess energy as heat.

I would recommend just re-celling the pack, assuming it doesn't have smart circuitry in it to prevent it from being re-celled. Just because a battery outputs a voltage, doesn't mean there's anything behind it. I see batteries all the time at work that put out proper looking voltages, but fall flat on their face when you put a load tester across them.