Absorbed Glass Mat battery - how to recharge when completely flat

See the link "Care Instructions, Click Here" on that very page. It suggests a *long* charging time. If that doesn't help, you could ask the manufacturers at

Andrew

Almost everything drains the battery if you don't use it for months & if it stays flat for weeks it's often dead. You can sometimes resurrect them with a normal Electronics PSU, charge at 30V limited to 1A until the voltage drops to 14V , then charge at 10A limited to 14.4V.

Virtually all high amperage lead acid batteries hate prolonged deep discharge. I suspect it's now a dud.

In future see if you can attach a float charger to the battery when not used for more than a couple of weeks. Ideally with an output of 13.8V or so. Cheap trickle chargers often can give too higher a voltage and this also degrades the battery.

Don't forget that the car electrics also discharge the battery over a long period.

Yes - I'd read that but I thought it might have a bit of life after three days!

The charger is a CTek XS800 charger. Should be fine for the Westco battery.

Tony

What, even an AGM battery? Sounds like a recipe for a large explosion to me...

This isn't a plain lead-acid jobbie with dilute sulphuric acid sloshing around you know!

Tony

This is my worry but I understand that AGM batteries may be less susceptible to this sort of death?

Indeed - I did recommend this back in January! Neighbour has now bought a CTek charge which ought to be just fine for this purpose.

Indeed - I'm sure that is what will have happened.

Thanks,

Tony

Well if you check you'll find it is. There's some differences but not many. & that's all the posh chargers do, it's just automatic.

Most modern cars should have the battery disconnected if left for more than a couple of weeks or so, if a permanent float charge can't be arranged.

An SLA charger isn't suitable for trying to re-claim a knackered battery. It is limited to 13.8 volts. You really need to have at least double this voltage to try and reverse the chemical process. An ancient basic charger that has a maximum output of about 5 amps is ideal, as the off load voltage will be high. As (if) the battery recovers, its internal resistance will lower and bring the voltage down. As soon as it starts taking a reasonable current, change to a modern regulated charger.

In general you're right, but optimates & the like have a 24V sulphated battery mode, where they stick 100mA through at upto 24V. Which works fine on little SLAs but may not be enough to salvage a large one.

I'm not sure that genuine SLA (gel) types can be saved. But no cars to my knowledge use these anyway.

FWIW, the last sulphated battery I tried to reclaim took nothing like

100mA at 30 volts to start with. And had to be left on charge for over a week before recovering - after a fashion.

Unless you don't need the vehicle a new battery is probably the simplest option - and take more care in the future. ;-)

I'm guessing a AGM battery will have similar charging characteristics as a gell battery, which means that if they've been left discharged for any length of time, then the only way to get them to accept a charge is using high voltage (typically twice the normal charging voltage).

As others have said, a proper charger designed for gel batteries will charge at high voltage/low current first, before dropping to bulk charge mode with medium voltage/high current, before then dropping to a float charge low voltage/low current. Or cheaper chargers will use a constant current power, and keep forcing the same current through regardless of voltage.

How does that work then?

Even a regulated bench supply has to step the voltage down to limit the current, you can't give it '30V at 100mA'

Or is it that you have a very high internal resistance at first, so you have to limit it to 30V *or* 100mA (say) whichever comes first, then let the current increase as the internal resistance lowers with normal battery function returning?

In my day, SLA or gel battery chargers were constant voltage...

Normally there's a microcontroller which switches in a 100mA Constant Current source, so as the battery terminal voltage drops the current remains constant, till you hit the point where normal charging should resume.

Most modern ones are constant current to a preset voltage & then drop to a lower voltage limited maintenance current, then cycle between the two modes.

Sorry Duncan you are talking rubbish. The electrolyte is not free liquid at all.

Go and read about Absorbed Glass Mat batteries.

Tony

How many times! It's not my car - it is my neighbour's and I advised her to get a trickle charger back in January the last time I replaced the battery for her!

Tony

Keep your hair on. It was just a comment as part of the general discussion.

Despite all the claims for this sort of battery you treat it much as a liquid acid type. It is *not* an SLA type which uses gel.