Lead-acid charger

[ivak245]

I have a lead acid (car type) battery charger given to me to repair. I found that it had been subjected to a high load on the output, which cooked the ammeter and permanently open circuited the bi-metal overload protector. I measured the output before this mess, and on the 12volt setting, I am getting about 20 volts, open circuit. I reckon that was the cause of the overload which took the protection out, but I am at a loss as to why the output is so high. The circuit has only a transformer & bridge rectifier and a couple of switches (HI/LO) and a 24 volt output as well. Would a l 12 volt lead acid battery charged at 20v or so become an acid boiler? And would that lead to a low internal resistance in the battery?

 

[Martaine2005]

I would first place the battery on the charger and then measure the voltage across it. It might stabilise around 14-15 volts.

Martin

 

[dave9]

The voltage on an unregulated design like that will drop to the battery voltage plus a small amount over it. The battery electrolyte shouldn't appreciably boil away until the battery is fully charged and there's nowhere else for the extra power to go besides electrolysis of the water. That is one of the main drawbacks of the unregulated chargers if capable of much current (so they retain a fair voltage margin over battery peak charge voltage at the end of charge cycle).

I agree with Martaine, you have to test under load.

 

[bertus]

Hello,

There are many schematics for lead acid chargers on the internet.
From very simple to very advanced:
lead acid battery charger schematic

Bertus

 

[Bluejets]

Breaker will be available on Ebay etc as below as I doubt original type is still available.
Ammeter the same.
You will need to select appropriate size though.
Forget your voltage readings as they are not the cause of your problems.

 

[ivak245]

I just tried the open -circuit voltage of my good chargers, and they are all around 14 volts, that's why I thought the 20v was a bit high. The open circuit voltage of the 24volt section is about 40V.! This charger is (going by what was left of the ammeter) good for 15 amps, and has no regulation or filtering. I will give it a shot under load of around 5-10A ( not a battery, just in case) and measure the volts.
Thanks for the replies.

 

[Bluejets]

Testing on a battery would be ok.
Your ammeter size would not indicate the output capability of the charger.

 

[ivak245]

Just tried it on a car battery, voltage dropped to 15v under load, which should be OK. I measured another couple of chargers under load, and they both came up to around 14volts.

 

[Bluejets]

Depends on the charger and the state of charge/condition of the battery.