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rp181

Fri Dec 07 2007, 01:24AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

i got a 12v 3A sealed lead acid battery, the charging circuitis broken, how do you charge them? I read somewhere you can just feed 7.5v 300ma into it and itle charge. any sugestions?

i have a universal adapter 1.5v to 12v 300ma regulated, so ile go with that. thanks.

[Mod Edit to fix thread format - Dave]

Mates

Fri Dec 07 2007, 08:17AM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

12V is not enough... You really need at least 1V over. I use 14V and have 10W 1ohm resistor in series and it works good.

Dr. Slack

Fri Dec 07 2007, 08:44AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

There's a lot of technology surrounding charging SLA's, it's worth visiting a few manufacturer's web sites to see what they say in detail, especially the manufacturer for the particular battery you have, however here's a summary

1) If you're charging a sealed battery, it must not gas.
2) To prevent it gassing, the final charge rate must be low enough so that the oxygen recombination cycle can absorb the overcharge
3) Depending on the battery manufacturer, the recombination limit may be anywhere from C/100 to C/1000
4) Constant current charging at either of those rates would be tedious (unless it's a float application), so we need to constant voltage charge
5) Most batteries (again depending on maufacturer) will drop to a suitable recombination rate if the terminal voltage is held at 13.8v for a 12v battery.
6) Most batteries will still be fairly slow to reach their full capacity if only charged at 13.8v
7) So many recommend a timed blast of 15v, current limited to somewhere between C/3 (the excellent but expensive Hawker Cyclons) and C/10 (most other rubbish), dropping back to a 13.8v float once the current has fallen below a threshhold value.

BTW, SLAs don't have memory effect and do get damaged by deep discharge, so whereas NiCds would be fully discharged then fully charged for best life, SLAs like to be topped up as often as possible for best life.

Steve Conner

Fri Dec 07 2007, 10:50AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I've always charged those little SLA batteries by plugging them into a regulated lab power supply set to a voltage of 14.4V, and a current limit of one-eighth of the battery's Ah rating, and leaving them overnight. The 7.5V you quoted would be about right for a 6V battery, but useless for a 12V one.

This probably won't be much help if you don't have a lab power supply - maybe your school science lab might have one?

The only other advice I can give is don't use a car battery charger, unless the battery is old and you don't care about it. It'll probably just wreck the battery after a few charges, and might even make it gas and blow up.

Dr. Slack

Fri Dec 07 2007, 05:20PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

My car regulates its battery charge at 14.1v.

The advice from Hawker is for float applications use 13.8v, for cyclic use 15v followed by 13.8v, and for general purpose applications, err on the side of over-charging rather than under-charging, as it will give you more cycles of decent battery capacity. The loss of capacity through under-charging is worse than the steady loss of electrolyte through slight over-charge, for a lifetime of several years.

So 13.8v is bomb-proof, around 14v -14.5v will cause tolerable loss over a reasonable lifetime, much more will need to be actively terminated.

ConKbot of Doom

Fri Dec 07 2007, 05:33PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

For a quick and dirty charger, a 12vdc (on the label) unregulated wall wart, and disconnect it when it reaches 13.8-14v

Dr. Drone

Fri Dec 07 2007, 06:01PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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Posts: 1673

rp181

Fri Dec 07 2007, 11:03PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

hmm, i dont have any supply higher than 12v ;(
as for school, i cant as im in junior high (8th grade) =p.

if i use 12v, will is still charge up to 3A? im using the battery more for the current (glow wire ad electrolysis) than the voltage.

the gel cell charger from bikes is where i got the 7.5v, i saw a website mention those.

Firnagzen

Sat Dec 08 2007, 01:42AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Hm well, I can reccomend a site with good reading about all kinds of batteries, here:

Shaun

Sat Dec 08 2007, 05:10AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

For hot-wire cutter and electrolysis use, a 6 volt battery would probably be better. Barring that, I would just do what Mates said and use a 14V or so supply with a power resistor in series.

But thats only because I'm not patient enough to order a premade charger off ebay, even though it would work better.

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