18v Ni Cad batteries and charger

[joinerjohn]

Any of you sparks explain how this works? I have an old battery charger and 18v batteries, 2 of the batteries won't charge up at all. The charger switches to the fully charged light as soon as I plug them in. The other two work ok and do charge up.
I took one of the batteries apart, and there are just the two connections on the batteries + and -.
When the battery is plugged into the charger, how does it determine if the battery is charged or not? Does it work off the resistance of the cells or some other means of determining the state of the battery?.
I know on some of my other cordless gear, the batteries have three connections and the charger uses one of these to determine the state of the battery, but how does this one with only two connections work?
Any help appreciated chaps.

 

[ricicle]

What voltage is the battery charger ? Once the battery voltage equals the charger output voltage it cannot charge anymore. If it is a 12V charger then the batteries will only be charging to 12V (or just above).
If the 18V batteries are well below 18V but at or just above 12V they will show charged on the battery charger.

 

[joinerjohn]

It's an 18v charger and 18v batteries both bought years ago for a cordless reciprocating saw/cordless drill. I have not used these in years, but now find that two batteries show fully charged when plugged in, but actually have none or very little charge in them. Probably the batteries are FUBB,,,,,

But, here's the thing,,, I've managed to charge them up by jury rigging them via some wires to another charger (18v too but different fittings altogether. I need the recipro saw tomorrow to cut some pipes away,. Just didn't want to find the batteries die on me with no way to charge them without messing about jury rigging a different charger for the two batteries that don't charge. (not very professional I know)
I now have all 4 batteries charged. think I'll leave one on the trickle charge overnight though (just to err on the side of caution)

 

[ericmark]

There are a number of ways that a charger can work out if the battery is charged.
1) Temperature sensor in the battery. This will often include a button either pressed by user or the battery being placed in charger. Idea is when sending a constant charge into the battery while charging the energy is turned into chemical energy. Once charged it turns into heat so as battery warms up the sensor disconnects the battery. The charger senses the disconnection and latches so even when it cools down it will not start charging again. If the battery will not accept the charge then the charger will think the temperature has been reached and turn off.
2) Voltage sensing. The voltage of the battery will rise with constant current charging until just before being fully charged. Then it drops slightly. The charger monitors the voltage and once it peaks it starts a timer then switches off shortly after. Again faulty cells can fox charger to thinking fully charged.

There are other methods including very low rate of charge often designed to take 16 hours. There are a number of problems with batteries. Two common ones are will not take the required charge which will give over voltage and second crystal growth which shorts out the cell. Sometimes we can charge batteries on another charger and they will come back to live. But normally it's a bin job.

 

[big-all]

nicads should be used charged every couple off months to prevent crystals building up
if you leave it any longer i would go through 2 cycles as in use charge use charge but definitely make the effort at least twice a year

and don't completely flatten as this can cause reverse polarity in a cell so aim for about 97% flat this will be when an unloaded drill is still turning fairly fast in top gear but can be stopped quite easily by grabbing the chuck

you can also try several cycles off charge discharge over a number off days which may bring a bit more life into the batteries but any improvements are likley to be short term

 

[ban-all-sheds]

I don't want to read about battery packs. One of my Bosch 18V NiMHs has died.

First foray into googling for a replacement showed that the price is £100-120.

Cheaper on eBay, but even so, I paid less than £165 (in today's money, £140 when I bought it) for the drill, 2 batteries and a charger....

 

[joinerjohn]

Have to agree BAS. Sometimes it's cheaper to buy, let's say, a new drill c/w 2 batteries , than to just buy the 2 batteries on their own.

Thanks for everyone's help.