NiMH Battery Question

That depends on its level of charge.

Possibly, if it's newly manufactured, and had been built and shipped in a discharged state.

NiMH cells have a nominal voltage of around 1.2 volts per cell, so a "3.6-volt" nominal battery consists of three cells in series.

They're often rather higher when freshly/fully charged... I see around

A NiMH or NiCd battery should not be discharged to below around 1 volt per cell. Doing so risks fully discharging one cell before the others; as the others continue to discharge the weaker cell will be "reverse charged" and this can permanently damage it.

So, 1.76 volts in a three-cell battery would probably indicate one of two things:

(1) Newly made, and the cells were never charged (or were test-charged and then individually discharged, or were charged and then self- discharged over time), or

(2) A battery which has been discharged to below the safety limit, and might have been damaged.

Manufacturers often ship NiMH and NiCd cells and batteries in a discharged condition. Safer that way - less risk of fire if something accidentally shorts one or more cells. Older-style NiMH cells have a high self-discharge rate - they'll lose 10% or more of their charge every month even if not being used at all.

You'd need to actually test it to determine. Charge it fully (according to the manufacturer's recommendations). It should rise to at least 3.6 volts very quickly, then probably to 4 or 4.2 volts as it continues to charge. If it doesn't behave like this, it may well be damaged or defective.

When fully charged, discharge it at a safe rate (5-10 mA in this case) until it drops to 3.0 volts, then disconnect. Multiply the discharge time by the discharge current, to get the capacity. If it's not in the 50-to-70 mAh range, it may be bad.

Expect that you may not get the full rated capacity until the battery has been charged and discharged several times - NiMH cells have a distinct "break-in" effect.

Bad battery. It should read 1.43 volts per cell immediately after it is fully charged. Yours is a 3 cell battery, nominal voltage 3.6 V, voltage immediately after a full charge should be 4.29 V.

Ed

Dave & ehsjr,

Thanks for the quick response. What I suspected was a bad battery. The detail you provided was an extra bonus. I purchased from a local "Battery" store for a replacement backup battery that needs to be soldered onto the motherboard.

The guy at the store said it just needs to be charged and shrugged his shoulders. The others that were checked in the bin all had the same voltage. I ended up taking one of the others and only checked the voltage when I returned to by office. This one was for what ever reason at 3.47vdc. Which makes more sense.

What frustrates me is that the "Battery Store Expert" doesn't know batteries.

BTW as a side question. Are the individual cell welded together in the shrink tubing or are they just held together by the shrink tubing??

Thanks,

Les

This one was for what ever reason

Well, that has a much better chance to work OK.

No great surprise, unfortunately.

They would most likely be tabbed, with a single tab folded between cells. If mass-produced, it is possible they are welded together at the factory. You can easily tell by gently flexing the stick. If it is totally rigid, they are welded. If there is a little flex to it, they are tabbed.

Jon

Jon,

I replaced the battery on Friday and all is good. I did a autopsy on the removed battery and they are tabbed together.

I was going to post then but got distracted. (which happens often) Phone calls, and other stuff.

Another interesting side note. The battery was replaced primarily due to age and the suspected system trouble due to a power outage. The system was new when I installed in 2000. That is 18 years.

I just checked the battery and the voltage is 3.826vdc I realize that it has been sitting on my desk with no actual load attached.

But still, I guess it was not a weak battery after all.

Les

Those figures sound about right - but don't forget the temperature coeficient that makes the terminal voltage to drop slightly at full charge.

They're slightly endothermic while charging - at full charge the energy put in starts producing heat.

But he hadn't charged it then!

An enthusiastically charged cell could read as high as 1.47V - settling to the nominal 1.2V should take QUITE a while.