NiMH Battery Pack Charger

[CPW]

Do you think you could give me a link to one of those datasheets? Thanks.

Before charging begins, a voltage measurement should be made that the battery isn't already charged.
During charging, a voltage measurement should be made to determine if the battery has a shorted cell.

So if at the beginning of the charge the voltage is already 1.2 volts, it should cancel the charging? Also, how would I detect a shorted cell by measuring the voltage?

One more question. Let's say you have a battery that is partially worn down, to about 80% or so. Would the charger still work right or would it detect a sharp voltage increase to late?

Again, thanks everybody for the help.  ;D I will probably be able to make the first charger soon. It won't be perfect, but it will be a start.

[audioguru]

Do you think you could give me a link to one of those datasheets?

Go to www.maxim-ic.com and look at their 48 battery charger ICs.
http://para.maxim-ic.com/cache/en/results/5099.html
You could also see the rechargable battery tutorials at www energizer.com and at other battery manufacturer's websites.

So if at the beginning of the charge the voltage is already 1.2 volts, it should cancel the charging? Also, how would I detect a shorted cell by measuring the voltage?

Without a load, a discharged battery will probably measure 1.2V per cell. So some battery chargers load the battery then measure its voltage.
During charging, if the voltage doesn't increase to what is expected then the battery must have a shorted cell.

Let's say you have a battery that is partially worn down, to about 80% or so. Would the charger still work right or would it detect a sharp voltage increase too late?

It should detect the voltage rise when the battery is fully charged.

Again, thanks everybody for the help.

[CPW]

Go to www.maxim-ic.com and look at their 48 battery charger ICs.
http://para.maxim-ic.com/cache/en/results/5099.html
You could also see the rechargable battery tutorials at www energizer.com and at other battery manufacturer's websites.

Thanks. I'll take a look at some of those datasheets.

Without a load, a discharged battery will probably measure 1.2V per cell. So some battery chargers load the battery then measure its voltage.
During charging, if the voltage doesn't increase to what is expected then the battery must have a shorted cell.

It may seem like a stupid question, but how would I put a load on a battery? Would I use a resistor? Also, how long would it take before I could be sure there is a short? If the voltage doesn't increase at all, would I be able to tell right away?

It should detect the voltage rise when the battery is fully charged.

So if the battery is 95% charged, I will still be able to detect a sharper than normal increase, indicating a full charge?

Thanks.  ;D

[audioguru]

how would I put a load on a battery? Would I use a resistor?

Yes.

Also, how long would it take before I could be sure there is a short? If the voltage doesn't increase at all, would I be able to tell right away?

If the vol;tage doesn't rise to the volts that are expected then the battery must have a shorted cell. It shouldn't take much time but you need to experiment to find out exactly how long. Maybe a charger IC has the amount of time.

So if the battery is 95% charged, I will still be able to detect a sharper than normal increase, indicating a full charge?

I think the charger circuit will miss detecting the full charge. That's why the battery charger ICs have many redundant circuits (temperature and pressure etc.) to detect just before a battery cell explodes. hopefully their designers have thought of everything and you are just copying their ideas.

[CPW]

Thanks again for your help.

Yes.

Sorry to keep asking questions about something so simple, but what value do think I should use? 1K maybe? Or is that too low?

I think the charger circuit will miss detecting the full charge. That's why the battery charger ICs have many redundant circuits (temperature and pressure etc.) to detect just before a battery cell explodes. hopefully their designers have thought of everything and you are just copying their ideas.

So maybe I could check and see if the battery is almost fully charged, and if it is more than, say, 90% it will just use a trickle charge, or maybe a timed charge, then a trickle charge. That way I don't have to worry about missing the increase in voltage.

Thanks.  ;D

[CPW]

If anyone can help me out with the charger design (circuit), I would appreciate it.  ;)

Sorry about the double post, but I would really like to complete this project and needed to get it back on the first page where people will see it.