When is a lead-acid battery charged?

[stan]

=A0 London SW

We have a fast garage style charger that we salvaged and modified. It came equipped with a simple timer.

[Dave Liquorice]

So I start it, let it idle for 30s switch off, every 20 days? It's been "run". B-)

To keep things happy, engine gearbox etc included, I'd say a car needs to be driven so that it is fully hot for 10 mins or so about once every two to three weeks.

[Harry Bloomfield]

on 11/10/2009, stan supposed :

I agree pretty much with the rest, but not with the above.

A good 12v battery should still be above 12.5v after standing for that period. Less than that and it has problems.

[Dave Liquorice]

Reference is made to it in my Discos user manual. Not sure if it gives details on how to put it to sleep or just on how to wake it up.

More than enough to flatten a battery in a month or so.

That would make sense for loose batteries. You don't want loads of H2SO4 sloshing about in case of a leak or accident. Not quite the same when fitted into a car though. And it would be a right PITA to have to keep swapping batteries, or just fitting one and removing it afterwards, each time you wanted to move the car. Much simpler to have a "deep sleep" mode for the electronics.

[Fredxx]

A lead-acid battery terminal voltage soon drops to 12V on discharge. It really depends on the "period" you mention and the self discharge rate.

[David Hansen]

On Fri, 9 Oct 2009 18:08:09 +0100 someone who may be "Fredxx" wrote this:-

An assertion which is not correct as far as I can see from a quick skim, but it is not worth any more than a quick skim. However, even if I have done so there is nothing wrong with that if done properly. Paragraphs are not limited to one concept, at least in writing intended for people who can read more than a newspaper.

Which suggests that more work on comprehension of English is necessary.

By the way, your other assertions indicate a certain level of knowledge, but one which repeats some commonly held "truths" which were overtaken by better knowledge some time ago.

I'll keep charging the lead acid batteries concerned (which are a little different to the car batteries which the OP mentioned after my posting) in the way I have been. They are coming up to their estimated lifespan soon. When they do start to be less effective I will charge their replacements in the same way.

You will probably want the last word.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Dave Plowman (News)" saying something like:

Ah, a self-proclaimed 'expert' whose knowledge is derived from sales brochures, perhaps? Hmmm... now who does that remind me of?

[Andy Dingley]

Agreed, except that the voltage should change, as the battery accumulates charge. A higher voltage is reasonable into a flat battery, reducing gradually to the appropriate float charge voltage. Starting out at the float charge voltage wouldn't get your battery charged in any sensible timescale.

Note #2 is that this ideal voltage is slightly temperature-sensitive

Note #3 (important) is that float voltage is less than trickle voltage. Constant trickle use kills batteries, just more slowly.

[Andy Dingley]

That means that the charge voltage is sufficiently higher than the terminal voltage to push 3A into the battery

Can't tell. Is the battery flat? Is the charger high?

On anything less than a submarine, 3A is a reasonable charge current but a bad float current. So either your battery isn't charged yet, it's dead and never going to charge, or the charger is over- enthusiastic and is killing your battery as we speak. Without knowing either the voltage, the design of the charger, or the specific gravity of the battery electrolyte, it's impossible to tell.

With experience, you might learn that charge current for that charger does indeed drop into a charged battery - but we can't know that yet. If it normally does, then your battery is dead. If it never does, then your battery _will_ be dead soon, once the charger has finished killing it.

On the whole, batteries are expensive, Lidl's nice voltage-controlled auto-charger is =A313. Your call.

[Andy Dingley]

Then you've been float charging, not trickle charging. The difference is fractions of a Volt.

[Andy Dingley]

I've an 8-year old, almost-unused, car battery in the garage (not on a cold floor). It's checked regularly (hydrometer eye in the top) and re- charged about twice a year. It still has sufficient power to jump- start anything that needs it. Funny how long they can last, if not abused. It _is_ worth looking after them.

[Andrew Gabriel]

I saw a few in Aldi at lunch time, 12.99.

[Dave Plowman (News)]

That's actually what Lidl sold them at. Must have been a mistake on their website. Different colour from the old ones, though. Now grey.