Charge a 12v battery

[IgorM]

I've an old 12v 7ah lead acid battery from a security system and I want to reuse it for something else. I tried to charge it using a 12v 2A ac adapter but it didn't held up the charge. The old system has a transformer that outputs more than 12v (about 20v I guess), some relays, ICs and a lm7812 regulator. Can I reuse those parts and what are the ratings to charge that battery?

[GrandNebSmada]

sorry this isnt an answer but this is really funny because i literally have almost the same problem. even got the battery from an old security system haha

[SquintyG33Rs]

to charge a battery you need to give it a higher forward voltage than what it supplies itself. putting 12V to a 12V battery can't charge it. so yes the simplest solution is to reuse the old circuit

[Enderman]

Buy a proper Pb charger or you will either not charge the battery enough, or make it bloat up and possibly explode.

[knightslugger]

1 hour ago, SquintyG33Rs said:

to charge a battery you need to give it a higher forward voltage than what it supplies itself. putting 12V to a 12V battery can't charge it. so yes the simplest solution is to reuse the old circuit

100% accurate. Could be that the battery is totally shot too.

[mariushm]

Read this: http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

 

tldr : a super basic charger would be done by using 13v or even higher voltage.

 

if the battery is dead (like less than ~ 6-8v) it would be best to slowly bring it back to "can be charged" state by pumping a limited current (let's say 100-250 mA at a voltage slightly higher, like 10v for example

 

 

[knightslugger]

nominal voltage on LA batteries is somewhere in the neighborhood of 13V. You need an input voltage higher than this to actually charge one. leaving individual cells at a lower voltage than 2.1V is prime territory for sulfation to occur, which basically renders a battery into a paperweight. you can bring them back by applying a VERY large input voltage, but it is no guarantee.