Charging Sealed Lead Acid Batteries

[Radio Tech]

This may seem a silly question from someone like me, however, I picked up a portable compressor with spotlight that has two 6V5Ah batteries in series but sadly the correct charger did not come with the device. The only charger I have is a 12V 1.5Ah and as the batteries have not been charged for quite a while the charger I have is taking a long time to get the batteries up to what they should be. I would like to know if the charger I have is going to do the trick or do I have to either use a car battery charger to initially get the batteries up.

Ken

[G6Tanuki]

SLA batteries really need a voltage controlled charger, or you can run into issues..

Your car battery charger? Is it a proper regulated one or a vintage brute force transformer and rectifier whose output voltage rises to something crazy at low load-current?

[mickm3for]

Hi if they have not been charged for a time esp, if they are low on voltage they may not take a charge (sulphated) but you can with luck recover them charge slowly over a long time monitor voltage and batt temp. Mick

[G6Tanuki]

The traditional thing with batteries is to apply 1.5C amp-hours of charge, where C is the AH capacity of the battery.

In other words if your battery is 6AH and you have a 1.5A charger it should do the job in 6 hours.


If it doesn't then it's probably new battery time.

[duncanlowe]

Batteries that have been discharged for a time, can be damaged and fail to charge properly. The batteries can also get damaged by poor charging circuits that don't have proper charge voltage regulation. It's surprising how many devices such as yours don't have charge regulation! One I had, without the compressor, had more electronics to display SoC via LEDs, than it did to ensure charge regulation.

[Roger Ramjet]

Hi, These type of batteries were also fitted to burglar alarms, and were charged via a constant voltage supply of 13.5 to 13.8 volts. There was no adjustment on the current which in any event would be limited so the only variable was the time taken to charge fully if discharged due to a power outage.

I would often give my van spares a boost by connecting them to a 12V ciggerette lighter outlet whist driving .... but not for too long.

If your two batteries (in series) give an emf below 10 volts it is unlikely that they could be recovered using the method detailed above, however some current generation car / motorbike battery chargers have a recovery mode to "rescucitate", a dead battery.

Rog

[The Philpott]

SLA's can appear to successfully take a full charge, then start to fail quite shortly afterwards. I had this happen last week. Many are only serviceable for 3 to 5 years, regardless of how you cosset them.

Dave

[duncanlowe]

Originally Posted by Roger Ramjet

If your two batteries (in series) give an emf below 10 volts it is unlikely that they could be recovered using the method detailed above, however some current generation car / motorbike battery chargers have a recovery mode to "rescucitate", a dead battery.

Rog

They do, but have so far failed to resuscitate any battery I have tried with them.

[kalee20]

I've recovered SLA batteries to a certain extent by putting a high voltage across them - 30V for a 12V battery - current-limited to C/100.

What happens is the current starts nearly zero, and then slowly increases - till it hits current limit.

Generally speaking, SLA's need to be charged to 2.33V/cell, no higher - constant-current charging is a no-no when fully charged (unlike nicads, which just start getting warm when fully charged). Ideally the 2.33V should be temperature compensated, at 2.7mV per degree C negatively - the cell voltage falls as temperature rises.

[KeithsTV]

There is an IC designed to control the charging of Sealed Lead Acid batteries the U3906. I've attached the application note which details how it charges the batteries to avoid overcharging.

The batteries can also be charged using a simple linear regulator using a 2.7V zener diode as the reference. This has approximately the same temperature coefficient as the battery (-4mV per degree C per cell). This approach was used in power supplies for fire alarm systems which used SLA batteries as a backup supply for the alarm system.

Keith

[emeritus]

A chemist colleague who had worked on battery research at GEC, said it was sometimes possible to restore some capacity to a SLA that was flat due to disuse, as opposed to having been discharged under load, by charging at a very low rate. I tried it on a 12V 7.5Ah SLA salvaged from some scrap equipment that read 0V, and got about half back, but it took over a month. I used a simple 1960's trickle charger (5A max) with a 12V 2.2W bulb in series. Nothing obvious happened for a week, after which the bulb started glowing dimly and progressively glowed brighter as the charging current increased. This technique would probably not work with a modern smart charger which only generates an output voltage if it detects it is connected to a battery by detecting the battery's voltage, and so cannot charge a completely flat battery reading 0V.

[beamcurrent]

One of the many things to sort out for our MCR21 restoration project is what to do about the battery charging. As built it had 4 sets of batteries; 24V for standby lighting, 24V for standby communications, 18V for standby?emergency Sound mixer supply and the 12V vehicle battery. They were all 100AH ones. These were all charged by transformer rectifier (selenium) unit that makes raw DC.

We plan to use surplus SLA batteries (22AH) for the 24Vs and smaller ones for the 18V. But we have concerns about correct charging and have been contemplating a device to go between the raw DC and the battery to fix the potential overcharge and maintenance problems.

The application note that Keith posted looks to be a good read and a possible build. Job Number xxxxx

We are very keen to retain the original charger for historical accuracy but need not to boil our batteries!

www.mcr21.org.uk

[The Philpott]

Frying tonight!

It's apparent that fashionable calcium or silver doped plates in lead-acids (as opposed to old school antimony doped plates) make a battery that behaves differently in quite subtle ways. They are still a horse, but a horse of a different colour. It's well worth reading up.

I suppose you could insert extra systems into the charger that you have, but a discrete setup 'after' the charger sounds preferable to me!

Dave

[kalee20]

What a super looking charger!

But it would have been designed for open batteries - continuous trickle current when fully charged wouldn't be a problem. The cells would gently start gassing, and periodic adding of distilled water would be the work of a few moments. This isn't an option with SLA's, so some protection is essential!

Personally, I'd consider a shunt regulator, in parallel with the battery, to divert the trickle current and clamp the voltage. It would be a trivially easy bolt-on module. You could even use Keith's UC3906 of post #10!

[KeithsTV]

I built a charger using the U3906 many years ago and found a way of indicating which part of the charging cycle it's in. It uses a SNHC138 decoder and 3 LEDs. but I can't find the details of where to connect it. I'll need to dig out the charger from the garage and reverse engineer the circuit.

I'll post the circuit once I've done that as it's a useful addition to the charger.

Keith

[Radio Tech]

Thanks everyone, I think the info in post 4 sums these batteries up, they would not come up after 6 hours so yes its time to get some new ones.

Best wishes

Ken