possibilty of overcharging batteries??

[Dan2]

i have built up a portable sound system using a car radio\cd player and 4 speakers. it all runs off a computer power supply and works well. the only problem was that when you unplug the system from the wall, the radio lost it's memory and i would have to set everything up every time i turned the system on.


i have put in an old battery pack from a cordless drill. although the radio can run off the batteries, the main function is just to keep the memory on the radio. so this is how it's hooked up:

batteries are connected to the radio via a relay when power is off.
when the PSU turns on, it activates the relay, and the PSU is connected to the radio, leaving the batteries completely disconnected.
the charger is also hooked up so when the system is plugged in the batteries are charging. (the charger is just a transformer with rectifier diodes, it came with the drill. it is a slow charger, but it doesn't say how long the charge time is)

so if the batteries are fully charged (+- 13.6 v) and i run the system off mains (ie the charger is on), can i overcharge the batteries???

[Frank Berry]

Probably not.
The charging circuit was designed for the battery pack.
Many folks leave their battery pack in the charger for days or weeks at a time with no bad results.

[walkura]

I am not so sure .
Battery packs for drilling machines aint typicly used to be charged and used like that .
For a drilling machine you use the battery and charge it to disconnect it from the charger and use it again .
Battery's you intend to keep on the charger for a longer time (or permanent) usualy get connected to a slightly lower voltage .
I don't know what chemistry your battery is but you will be able to find some more information @ www.batteryuniversity.com
Maybe its a idea to build some charge controller .
Either dedicated chip or something with a microcontroller.
A good charger definetly will extend the lifecycle of your battery.
Those chargers for the standard accu driller is just transformer + diodes .

[Dan2]

thanks walkura for that link, i will look at it soon.

i am not going to have the radio plugged in all the time, just when i use it and if the battery needs a charge. but if the battery is almost fully charged and i use it (charge the battery) for 6 or so hours will that cause damage?? i suppose the answers will be on that battery university site.

there is no reference to what type the battery is, but if i would guess it should be Ni-cd (i think most cordless drills are)

[dangus]

Feel the battery... if it's warm, it's being overcharged. If that's the case, the easiest solution is to just add a series resistor, maybe across a schottky diode so that when the power supply is off you can still run the radio.

If you had a sealed lead-acid battery instead, they can be floated for long periods on standby, at about 13.8 to 14.4 volts.

[Dan2]

i have opened the battery pack (to get to the wires) so you can see the cells. they never get warm while charging.

i was thinking of putting a diode in series with the charger, but then i figured the rectifier is doing that job already, so i left it.

well i left it at a friend's place and he left it on charge for 2 days (by mistake) and nothings blown up yet

[amc184]

It is definitely a very bad idea to leave batteries on this type of charger for a long time. Leaving it on charge for two days will likely have robbed it of a significant amount of its life and capacity.

With fast chargers, they can be left on, but with the older, simpler slow charger, you've got to remove the battery.

[Nordic]

Funny, my drill says clearly not to charge for longer than 5 and a half hours... look in TI.com, I often see battery chargeing controler chips in their newsletter...

[Dan2]

well i have the charger connected to the battery pack permanently, but its not always on (connected to mains). i am not expecting the batteries to perform well, after all they came from a cordless drill that was pretty much dead. plus it never gets completely dicharged, it is used just to hold the memory of the radio.

i was just worried of the batteries exploding or leaking onto the radio, amps etc, but seeing that nothing happened after 2 days of being on charge i guess its allright.

thanks for all the help guys