charging SLA batteries

[Wim Hertog]

Hi,

Can I charge an SLA accu (12V 3,2A) with a normal DC adaptor @ 1000mA? I'll just plug it out after his charging time (calculated).

Is this possible or will it damage the SLA?

Thanks!

 

[Frank Schwab]

Probably a really bad idea, unless the DC adaptor just happens to put out precisely the correct voltage when the SLA has reached full charge. A very unlikely occurrence.

You won't instantly damage the SLA, unless the charging current is way too high. What will happen is that you will start boiling off the liquid (actually, breaking the water down into gaseous oxygen and hydrogen) inside the case. The SLA is allegedly sealed (hence the 'S'), but in reality isn't. The gases will escape, the liquid level will drop, and eventually the SLA will fail.

Look up the spec sheet on your SLA, and find out what the terminal charge voltage is (it's probably 13.6 volts, or something like that). Your best approach to charging it by hand is to hook a voltmeter across the battery, and using a low-current DC adaptor (like 300 ma or so), let the battery charge until the voltage reaches 13.6 volts. It will then be fully charged.

/frank

 

[Streak]

There are usually two charge voltages for SLA's. One for a one time recharge after a deep discharge or (cyclic use) voltage between 14.5 and 15v and standbye usage (permanent connection to the charger) 13.6 to 13.8 volts. So it's not the rated capacity of the DC adaptor that can damage the battery but rather the output voltage. If the battery is really flat it will try and draw more current to charge itself up and then the capacity of the adaptor could be an issue.

If the voltage of the adaptor is a little high then add a diode which will drop the voltage by about .9v or so. Alternatively find an old alarm control panel/battey charger, these are designed to maintain these SLA's on standbye at the correct voltage. I have a radio next to my bed running of an SLA on permanent standbye so that when the power fails I still have an operating radio.
As Frank says, use the an DC adaptor but monitor the battery voltage as well. Many of these adjustable adaptors measure quote a lot higher than their rated output even under load so be aware.

Good luck.