Charging SLA batteries

31 Mar 2006
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United Kingdom
hello all.

I've got a 3.2Ah SLA battery. The sort you'd commonly find in a burglar alarm panel. It's powering an LED strip and some ancillary devices on a theatre set. I need to charge this battery after each show.

I've got a car battery charger, but it says on the front for 32Ah to 60Ah.

Will it be safe to charge a small battery with this or do I need a specific charger for it?

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TBH, I'm not sure, Rob. Its charging capacity is 10 times bigger. But I have successfully charged my 8Ah Yuasa with my CTEK MXS 5 charger.

The bumpf goes as follows:

The MXS 5 is an advanced microprocessor controlled battery charger with automatic temperature compensation built in providing unrivalled performance on lead-acid batteries from 1.2Ah up to 110Ah.

The MXS 5 solves a broad range of battery problems and is the ideal charger for the user with high demands. Features of the MXS 5 include diagnosis of battery condition to establish if it can receive and retain charge, patented automatic desulphation step and a special reconditioning step that will revive and restore deeply discharged and stratified batteries.

AGM option is perfect for maximising performance and life of most Stop - Start batteries.

The built in temperature compensation ensures ideal charging performance even in the most extreme conditions. Patented Float/Pulse maintenance makes the MXS 5 ideal for long - term maintenance.

The entire process of battery testing, charging and maintenance is easily followed on the clear LED display.

Supplied with fully insulated clamps, Comfort Connect permanent connection (M6) and nylon storage bag.
automatic temperature compensation

How does it measure the temperature of the battery ? Some of that description sounds more like marketing hype than actual functionality.

Almost all types of Sealed Lead Acid batteries can be charged by applying the correct constant voltage to them

Assuming the charging period between performances is twice as long as the performance time then cyclic charging of the battery from a fixed voltage source should be adequate.

The battery data sheet should give the voltage required. Many batteries have the charging voltages printed on them

The RS 537-5488 (12volt7.0Ah) requires :-

13.5 to 13.8 volts in standby use ( continuously charging )
14.4 to 15.0 volts in cyclic use
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A 'bog-standard" (cheap) car battery charger (which invariably consists of simply a transformer and rectifier) will be unsuitable for charging an SLA type battery and will in all probability destroy the battery, even though it appears to have been charged. SLA types require a specific charge process and you should get a charger that's specified as suitable for SLA batteries.
A straight 13.8V power supply will do the job with no ill effects.
Lead acid batteries (12 volt nominal) are considered charged at 13.2 volt, to charge them over 13.2 volt is required, by time you reach 14.8 volt it can damage the battery, but the higher the volts the faster it charges, as they reach 80% charged then they slow down in their ability to accept charge, so there are three systems used to regulate battery charging.
1) Constant voltage normally around 13.8 volt.
2) Stage charging starts at max output of charger (stage 1) then has a voltage limit 14.4 with VRLA and 14.8 with batteries you can top up, but at the same time it monitors current, at current drops to between 1/10 and 1/20 of amp-hour rating it drops the volts to between 13.4 and 13.8 volts, so the last 10 to 20% of the charging is done at a slower rate. Because it measures current the charger has a range of AH batteries it can charge, too small of a battery and too large of a battery can be over charged.
3) Pulse charger these send a pulse of charge and measure voltage decay between pulses, these are more suitable for when the battery is being charged and used at the same time. Solar panel and wind charger units are normally pulse chargers, also the units used with boats to combine the outputs of two generators and charge multi-battery banks. The CTEK charger I think uses this system, I had a cheap Lidi charger that did this, but wrecked it by putting another charger on the battery at the same time.

So if you have a 13.8 volt power supply that will do the job, even if over 13.8 volt you can use a 7812 voltage regulator and a red LED between Com and real negative. CB power supplies were made using that idea, I use a ham radio power supply and it works A1 when I have the time. What you need to work out is how fast do you want to charge the battery?

The small VRLA (valve regulated lead acid) the valve is simply a rubber cap, when they have been over charged I have been able to add a few drops of distilled water, but really you should not open them. My stair lift has two 12 volt VRLA and there is 29 volts on the charge rail so 14.5 volt per battery, they last around 2 years, I am sure if the volts were dropped the batteries would last longer. It seems therefore that they will stand some abuse, I consider 14.5 volt abuse, I am sure the idea is the lift is used every day so it needs that little extra, but my lift is used once a week to transport the washing.
The danger with using a "too powerful" charger is that the initial current will be too high for the small battery.

If you have a multimeter, connect it in series and measure the current for a minute or two. For a 3.2 Ah SLA battery it should not exceed 0.8 A.

As aptsys says above, this is unlikely to be a problem if the charger simply supplies a constant voltage of 13.8 V. But many chargers will supply more, either deliberately because they are "fast chargers", or unintentionally because they are badly regulated. Again if you have a multimeter you can measure the voltage to check.

Warning: if you wake up in the middle of the night and decide to go and check on your battery, and find that overcharging has caused it to swell like a football, do not be tempted to try to release the pressure by prising it open - or if you do, put some clothes on first. Someone I know did exactly that with a caver' lamp and ended up in A&E. Funniest part was that he was a junior doctor and had to explain his idiotic behaviour to nurses he knew....
Yeah, I know someone who had a car battery explode in their face.

Two colleagues picked him up and dunked him head-first in a bin full of water.

He escaped serious injury from the explosion but nearly drowned!

It says Temperature Compensation Built in charge voltage compensation according to ambient temperature

Which could be fine but only if the the battery was at ambient temperature. If it had just come "off service" it could be considerable hotter ( or in some uses considerably colder ) than the ambient temperature around the charger.

Sensible temperature compensation would require at minimum a temperature probe to be attached to the battery casing and to be be really effective then a third connection on the battery to an internal temperature sensor that the charger could then read.
How does it measure the temperature of the battery ?
Ford tractors with Lucas ACR regulators had a battery temperature sender which sat under the battery, but it was really there because the engine heated the battery not becase of the heat generated due to charging, some fast chargers do have probes you stick in the electrolyte but clearly this will not work with a valve regulated lead acid battery as you have no access to the electrolyte, the case of the battery will lag behind the actual electrolyte temperature so is not really any good to control charging, with NiCad batteries where the case was metal so heat moved quicker then yes they used temperature switches to control the battery, but not seen this done with lead acid.

The idea is when electrical energy is converted to chemical energy the battery is cool, but once charged the electrical energy is instead converted into heat so the temperature raises as soon as charging is complete, this was used a lot with drill batteries with NiCad but today more normal to use Delta V chargers which look for the voltage peak just as it hits full charge, however I have not seen many constant current chargers used with lead acid batteries, the fork lift charger was in a way current controlled, but not constant current.

The CAV AC208 bus alternator had current control, but other than that odd one out I have not seen any current control on vehicle alternators, and I have worked on many vehicle alternators, the latest does have connections to engine management to switch off alternator when engine is under load and switch it back on when vehicle is on over run, but this is still not common, in the main lead acid charging is simply voltage regulated. Specials using both stage and pulse charging are found on narrow boats, and if time to charge is important then narrow boat, model car racing, and wind charging modules may be worth looking at.
Yeah, I know someone who had a car battery explode in their face.

Two colleagues picked him up and dunked him head-first in a bin full of water.

He escaped serious injury from the explosion but nearly drowned!
Working in Algeria with the heat I saw far too many batteries explode, most dramatic was a 966 loading bucket where it exploded under the seat and ejected seat and driver, the alternator was over charging and there was a large space for the hydrogen to build up in. Driver landed in sand heap and was then subjected by me of having a carboy of water poured over him. He was lucky as not much water around there.

Even a small Landrover battery had enough force to re-open the bonnet when it went, I saw the flame moving cell to cell and dropped the bonnet quick, it re-opened quicker. And that was Falklands which was quite cool, it was my fault, I was jump starting with 24 volt.

Even AAA batteries can explode, I had a mouse explode, not injured, two non rechargeable batteries with a case which looked like rechargeable has been charged in a standard AAA charger, then put in the mouse, they went bang nearly an hour latter.

However the VRLA has very little acid in it, just enough, and I would think the chance of this type of battery exploding is low, not saying it can't happen, but must be rare.
Yeah, I know someone who had a car battery explode in their face.

Two colleagues picked him up and dunked him head-first in a bin full of water.

He escaped serious injury from the explosion but nearly drowned!
I knew someone who lost an eye.

Wasn't a charging problem explosion - it was a non-sealed car battery, and he decided to see if it was still charging by looking for bubbles. It was too dark to see properly, so the fool struck a match...

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