Testing battery capacity.

[aquaplane]

My domestic battery, a normal lead acid 110Ah leisure battery, seems poorly.
It got run down when the alternator packed up so I brought it home to charge up. My cheap Lidl charger had to think about it before it started charging but it did it on it's own.
After between 24 and 36 hours the battery was charged, it was O/N when the green light came on so I don't know exactly how long it took.
After a day resting the battery held very nearly 13V.
I made up a holder for a bulb and put it on last night. This morning, 10 hours later, the battery had no charge (0.6V).
I thought that the 55W bulb would draw 4.6A and the battery should be ok for 12 hours and be about ½ charged.
Should I have used a lower power draw so it took longer? I can find a 5W bulb, it's the holder I didn't have.
Were my sums wrong when I thought 4.6A/110Ah = 24 hrs? I know I should only consider using half the nominal capacity of the battery.
The charger wouldn't charge this morning so I have resorted to banging in a few amps with a dumb charger to get it started.

[Booby Trapper]

The sums sound right I=P/V so 55/12=4.6amps. Sounds like you are going to have to reach into those very deep pockets.

[cpedw]

Those Lidl chargers only supply a few (about 3?) amps so a seriously flat battery could take ~ 30 or 40 hours to charge. That it held 13V for several hours is encouraging but not a guarantee of seviceability.

I made a device that uses a 40W bulb to load a battery until its voltage falls to about 12.2V (40% charged for a lead-acid battery off load) and measures the current consumed. It calculates Ah consumed and estimates battery capacity. There's a full description here http://www.picaxeforum.co.uk/showthread ... oad-tester It's not so accurate as I would like but it gives a clue if a battery is worth keeping.

You're welcome to borrow the prototype - I don't use it often. Only problem is transporting it. Let me know if you want me to put it in the post.

Derek

[DaveS]

My domestic battery, a normal lead acid 110Ah leisure battery, seems poorly.
It got run down when the alternator packed up so I brought it home to charge up. My cheap Lidl charger had to think about it before it started charging but it did it on it's own.
After between 24 and 36 hours the battery was charged, it was O/N when the green light came on so I don't know exactly how long it took.
After a day resting the battery held very nearly 13V.
I made up a holder for a bulb and put it on last night. This morning, 10 hours later, the battery had no charge (0.6V).
I thought that the 55W bulb would draw 4.6A and the battery should be ok for 12 hours and be about ½ charged.
Should I have used a lower power draw so it took longer? I can find a 5W bulb, it's the holder I didn't have.
Were my sums wrong when I thought 4.6A/110Ah = 24 hrs? I know I should only consider using half the nominal capacity of the battery.
The charger wouldn't charge this morning so I have resorted to banging in a few amps with a dumb charger to get it started.

Normally the battery capacity is quoted at the C/20 rate so for 110Ah that would be 5.5A. Allowing for the likelihood that the battery has lost a bit of capacity, then your test load of around 4.5A sounds about right. After you've got the battery re-charged and rested I'd be tempted to re-connect the 55W bulb for a fixed period, say 4 hours, let the battery rest for a few hours then see what voltage it's at. 18Ah from 110Ah would leave it 84% charged and you could expect to see 12.6V or slightly more. If, OTOH, you see, say, 12.4V then you can assume that it is now 60% charged so the original capacity was 45Ah.

There is fairly good agreement between different sources as to high and low rest voltages, i.e. 12.7V = 100%, 11.7 = 10%, but different views as to intermediate values. The curve I use is: 12.7V = 100%, 12.65V = 90%, 12.6V = 80%, 12.5V = 70%, 12.4V = 60%, 12.3V = 50%, 12.25V = 40%, 12.2V = 30%, 11.9V = 20%, 11.7V = 10%. This is the grey line shown on the first diagram of this old, but still relevant, article: http://www.scubaengineer.com/documents/ ... graphs.pdf

I would then re-charge the battery, let it rest again, and work out how long the test load should take to bring the battery down to 50% of its new calculated capacity. Apply the test load for that time, let it rest and see how close the voltage is to 12.3V. You can then re-calculate the capacity to get a more accurate figure.

[aquaplane]

Thanks for the prompt posts gents.

So I'm on the right lines then, it's just that the 4 season old battery is probably just about knackered. I'll do the charging and discharging routine again and take more readings.

Thanks for the generous offer Derek, it looks like I can get the same info with a bit more effort with the kit I have so it's probably not worth troubling you.

I was already thinking about doubling up on the batterys for next season, it's where to put another domestic one, the locker where the existing one is may not fit another in it.

I have found that the 110Ahr size I have is now called a 100Ahr and the local caravan shop has them for £82.

[aquaplane]

The sums sound right I=P/V so 55/12=4.6amps. Sounds like you are going to have to reach into those very deep pockets.

And lets have a less of the Very Deep, thankyou very much!

[Pete Cooper]

You're doing it wrong. To measure capacity you multiply length by height by width.

[Clyde_Wanderer]

I dont see the point in all this testing and calculating etc.
I you apply a heavy load to the batt and one or more of the cells bubble up then the batt is knucking fackered.
Any garage will test it with a loader for you.
C_W

[Booby Trapper]

The sums sound right I=P/V so 55/12=4.6amps. Sounds like you are going to have to reach into those very deep pockets.

And lets have a less of the Very Deep, thankyou very much!

And here was me being generous, I didn't mention short arms

[sahona]

I dont see the point in all this testing and calculating etc.
I you apply a heavy load to the batt and one or more of the cells bubble up then the batt is knucking fackered.
Any garage will test it with a loader for you.
C_W

or,- 1: charge it up. Unpick a wire coathanger and connect a big bulldog clip to one end.
fix a wooden handle on the other end.
Connect a testmeter to the battery and note the voltage.
Now clip the coat hanger on one terminal, hold the other end to the other terminal by the wooden handle (it will get very hot!)
Watch the meter:- if it drops from 12.xVto 10.xV right away a cell is phuqued (multiples of 2Volts drop mean multiple cells dud)
If the battery is reasonably healthy, the coathanger will start to glow dull red after a few seconds and the meter reading will stay above about 10.7v
Disconnect the coathanger using the wooden handle carefully and lay it clear of the terminals. Remove the meter leads .
Remove the big bulldog clip once the coathanger, hereinafter known as a 'test load', is cool.
Or, - 2: if the bottoms of those pockets are in fact within range of your fingers, £25 will buy a drop tester that is safer to use...
but does the same thing.

[Booby Trapper]

I dont see the point in all this testing and calculating etc.
I you apply a heavy load to the batt and one or more of the cells bubble up then the batt is knucking fackered.
Any garage will test it with a loader for you.
C_W

or,- 1: charge it up. Unpick a wire coathanger and connect a big bulldog clip to one end.
fix a wooden handle on the other end.
Connect a testmeter to the battery and note the voltage.
Now clip the coat hanger on one terminal, hold the other end to the other terminal by the wooden handle (it will get very hot!)
Watch the meter:- if it drops from 12.xVto 10.xV right away a cell is phuqued (multiples of 2Volts drop mean multiple cells dud)
If the battery is reasonably healthy, the coathanger will start to glow dull red after a few seconds and the meter reading will stay above about 10.7v
Disconnect the coathanger using the wooden handle carefully and lay it clear of the terminals. Remove the meter leads .
Remove the big bulldog clip once the coathanger, hereinafter known as a 'test load', is cool.
Or, - 2: if the bottoms of those pockets are in fact within range of your fingers, £25 will buy a drop tester that is safer to use...
but does the same thing.

Maybe allow some time between charging and using this method. Remember the one that went bang in the shed at Troon. I think there will be a few sparks when connecting the coat hanger.

[sahona]

Point taken Ian - Though lots of these modern ones don't appear to have vents for the gas.
The Troon incident was due to a grinder being used in the same room as the charging equipment - that's been sorted now, the batteries now live in a 'kennel' outside , and are fed through a hole in the wall.
Anyway, batteries should always be "rested" after charging to stabilise, otherwise the measurements are rubbish - my omission, sorry.