Battery chargers

it won't hurt and you will get a little extra going in, not the full 5 plus

4 but maybe 4 plus three. A faster recharge is to now put the battery back on the car, get a push start if it still won't go and drive it around, you then get thirty or so amps going in.

As stated, you may get a little extra with both chargers, but not much. Assuming your battery is at least 40 AH capacity, then at the max 5 Amps, it will take at least 10 hours to charge up properly, probably more as the charge rate drops as the battery volts rise.

I'll remember that trick the next time I try charging a completely dead battery. Last time I tried it it blew the bridge rectifier on my donkeys years old charger as it tried to grab more current into the battery than the charger was capable of supplying. If the battery is too flat (as was in my case) you may find that the cars alternator will go into protection mode and won't attempt to charge unless there's already a few volts in the battery. Although a battery allowed to go so flat has probably got some permanent damage even after fully charged.

Bill

I'd be inclined to charge it slowly if I had the time to do so.

I've not had much luck with car batteries which have gone flat. They don't seem to last very long after getting the car started and using the alternator to put the charge in.

The one exception, where a dead flat car battery lasted for years afterwards, is the one I charged over a couple of days from a 3 amp regulated charger.

John

That'ws probably luck though, unless you let it get frozen & buckle the plates it shouldn't make a big difference.

Maybe, but I'm not so sure. I've seen a few flat batteries (lights left on) in 40 years of maintaining cars. None of these has lasted more than a couple of months after a fast (alternator) charge - even if they were only a year old.

The usual reason for final battery failure is the internal short caused by the build-up of conductive material in the space below the plates. This manifests as a cell which won't hold charge for very long, followed quickly by other cells as their plates shed material. I'm sure their being massively overcharged in trying to bring the failing cell(s) up to voltage doesn't help.

The failures of flat batteries after being fast-charged seem to show this failure mode (excessive shedding of plate material).

I'd assumed that going completely flat somehow softened the plates. So I was surprised when an already 2-year-old battery survived a complete discharge followed by a very slow charge. And this battery was completely dead - the headlights had been left on for days.

This is conjecture, but maybe fast charging from flat doesn't allow time for the plate material to physically harden properly.

Of course, others might have had better experiences with flat batteries which have been fast-charged. And I'm quite happy to be proven wrong - it's understanding what's happening that's important.

John

Most of the ones I've seen have had obviously sulfated plates, I suppose it depends on how you let them go flat :-)

You are quite right john- well expressed and you really know your stuff. In had a similar experience- had an excellent battery-then flat battery- lights left on all night- battery picked up charge but lasted no more than two days- cell/ cells dead- needed a new battery