Battery charger keeps blowing fuses ?

[Vince70]

Last night I was asked if I could help a friend of a friend as her car battery was as flat as a pancake and needed to come out of her very autoshite 2005 Rover 45 auto.

 

So I got the battery out of the old girl and connected it up to my trusty halfords battery charger and all it kept doing was blowing the 20amp blade fuses in the charger (and before you ask I did check the polarity lol).

 

The thing is the battery wasn't that old and is a decent make one when bought but I only used the charger a couple of weeks ago and have no reason to expect it to be knackered but wondered if a knackered battery could do this..

 

But I will say the car does live up to expectations as it's a low mileage motor and had a new head gasket a couple of years ago and looking under the bonnet revealed an oil leak coming from the head which would of given the Exxon Valdez a run for its money and a not very clean looking expansion tank but I thought I keep quiet about that as ignorance is bliss..

[Pillock]

There'll probably be some sort of breakdown within the battery and it's shorting. The fuse is there to protect against clanging the leads together and melting the cables but if there is a short circuit inside the battery, same effect.

[castros_bro]

Get a 5w 12v bulb and connect it between your charger and the flat battery to regulate the current, leave for hours and IF the bulb gets noticeably dimmer the battery is charging THEN you can connect the charger directly to the battery.

[spike60]

Try the charger on a known good battery to see which is at fault. If the battery has a short inside blowing the fuse it's shagged.

[spike60]

Try disconnecting the battery and charging it, perhaps the car has a dead short which is why the battery is flat anyway.

[mat_the_cat]

The internal resistance of a battery drop with how charged it is, so it is possible for a very discharged battery to have a sufficiently low resistance that it would blow a 20 amp fuse. As castros_bro suggests, limiting the current initially will get some charge into the battery (assuming it isn't knackered) and then hopefully let you charge normally. I'd expect the lifespan to be reduced by running it that flat though, if that is the case.