Battery float charger

Anyone here ever used a battery float charger before? I picked one up from HF. I have a car that I let sit with a new battery I just put in, I’m not sure if I should use it or not for this car. Can it cause any damage to the battery? Also if they battery dies and no one is around to help jump it, can I use the float charger to jump it?

Depends on the design, but you probably can’t jump start a car with just a float charger. But you might could charge the battery up enough with a floater if you gave it enough time. Usually you’d use a plain old battery charger for that though. There are battery chargers that also include a jump start function I think.

It’s fairly simple math. A battery hold about 60 amp hours of charge. So it takes 10 hours to charge it at 6 amps. That’s a typical charging rate for a shop battery charger. How many hours it takes using a float charger depends on how many amps the float charger is capable of.

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This is the one I got

“Maximum amperage output (amps) 400ma”

if you had 40 Amp-Hours missing in your battery:

40 / 0.4 => 100 hours to charge

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The purpose of these chargers is to make up for the constant drain all modern vehicles have even when shut off (ECM, radio memory, etc.) as well as self-discharge of the battery, and little, if any more. In other words, it won’t recharge your battery.


It won’t recharge it quickly… but given enough time it will bring a battery all the way back. I used just one of these devices to bring back a dead battery on a friends stored car I was looking after. The engine wouldn’t even turn over. Hook this up, come back a week later, starts right up!

This thing is designed to keep a batteries charged on things that aren’t started much - Like a motorcycle in winter, a generator or a classic car - and that’s really all they do well.

I keep an 85cca lawn tractor battery connected to a float charger similar to that and the voltage remains between 13 and 14 volts. An emergency light and radio are connected to the battery and occasionally run. If the power goes out the light automatically comes on. The current battery is 2 years old and it was installed when the 8 year old battery died. I have used the battery to start lawn mowers and a generator on several occasions and then reinstall it. When reinstalled the voltage will be down noticeably but seems to recharge eventually. The contraption is in the laundry room and I don’t often see the voltmeter. I would imagine it would require several days to recharge a 500cca automobile battery. And it would be useless trying to jumpstart a car using it.

Should you disconnect the float charger before starting the car?

in the ideal world, it would have an overload protection, but given it is from HF, I would disconnect it

I have an old battery maintainer, It stays at the cabins, not sure the amps but it usually takes about 8 hours to bring the deep cycle boat battery up to full charge, the battery is probably 12 years old, without charge 90hp cranks fine after sitting over winter, but I do it anyway. It is an outboard so it does not need that much juice I guess, now my wifes van had a sticky brake switch, the brake lights on would kill the battery as I eventually figured out, but 1/2 hour on the old diehard, maybe 75 amp judging from ebay offers was enough to start the car.

There is no electricity at the cabins october through June so it would do no use to leave it connected

And Yes I prefer to disconnect it before starting the boat.

Here’s the smart charger I use.

Set it to two amp charge rate, and sit back and watch the charger float the battery.