Testing Lead Acid Battery capacity

[davidmh]

I have a 80 AH lead acid domestic battery that is now quite old, On my home smart charger it does not take long to reach a fully charged condition from a standing voltage of around 12.2 volts. In use the charge decreases quite quickly I suspect that is is getting sulphated.
I would like to check what capacity I have at present. My plan is to fully charge the battery and then discharge it at a known rate. I can connect 2 x 21W bulbs (3.5 amps) across the battery is that load sufficient or should it be be larger. If I monitor the voltage of the battery under load at regular intervals that should give me a good Idea I think using 3.5 amps per hour. What voltage should I stop the test at.

It seems very simple to me but am I missing something, are there any fudge factors to take account of.

Regards
David MH

 

[lw395]

See the graph:
http://www.nilar.com/factory-technology/skarmklipp-discharge-rates/
If you take 3.5A out of a notionally 80Ah battery, that is a low discharge rate, 3.5/80= 0.044C or a '23 hour rate'.
So, a simple crude test, see how long it takes to get down to 12.5 volts, that will roughly take it to 65% discharged.
That should take about 15 hours if it really is still 80Ah capacity.

I took a battery out of my neighbours car, it only gave about 6Ah capacity, but amazingly, it still started the car unless the car hadn't been run for a fortnight, the alarm etc then flattened it.

 

[Hadenough]

So what does “the panel” think about this method of testing?
https://m.youtube.com/watch?v=NSZlVCBsxs4

 

[lw395]

So what does “the panel” think about this method of testing?
https://m.youtube.com/watch?v=NSZlVCBsxs4

Good way of taking the platng off someone else's meter probes?

 

[RichardS]

So what does “the panel” think about this method of testing?
https://m.youtube.com/watch?v=NSZlVCBsxs4

"A nearly fully charged battery, 12V" ..... when the battery reads 12.89V.

"Whatever you do, don't get any contamination in there" ..... and then sticks a steel probe into the acid.

"The voltage from each cell should be around 2V but might be up or down depending upon where you touch the plate" ... or even 3.6V 'cos it's just "bleed over" from the other cells.

I've never heard such a load of bollocks.

Richard

 

[VicS]

"A nearly fully charged battery, 12V" ..... when the battery reads 12.89V.

"Whatever you do, don't get any contamination in there" ..... and then sticks a steel probe into the acid.

"The voltage from each cell should be around 2V but might be up or down depending upon where you touch the plate" ... or even 3.6V 'cos it's just "bleed over" from the other cells.

I've never heard such a load of bollocks.

Richard

+ about a million

 

[William_H]

As said the Utube is a load of garbage. Essentially all he did was check the open circuit no load voltage. Always useful and easy but not justifying a long Utube.
OP question re battery condition. Yes checking with the bulks will be very useful. The load that you test a battery with should best reflect the kind of load you have in service. 3.5 amps OK perhaps more would be good.
He does need to be assured the battery is fully charged. I am a bit doubtful of the modern chargers which drop back to low charge current/voltage when it detects voltage has risen so battery is charged. I suspect with an old battery that voltage will rise to cut back current point before the battery is really charged. I would try pushing current in at a high voltage until you have put in something like AH rating of the battery. Use an old fashioned charger or even a solar panel with no regulator. (many may disagree with this idea)
Do the discharge check. Discharge IMHO to about 11.5 volts. I think a dud battery will be very obvious. Any AH delivered around 50% of rating would be good. Anything less might call for a replacement decision based on actual need on the boat. So ultimately regardless of testing a battery is no good when it will not give performance you need on the boat. ie good engine start and adequate life to service your needs. The use of a dual battery system means you can squeeze out the last of the battery life. ol'will

 

[GHA]

Somewhere on your battery or datasheet if there is one should be how the 80Ah is measured - good chance it will be 20 hour rate. Which means it should take 20 hours at a constant current in amps of 5% of the Ah number for the voltage to fall to 10.5V. So 4A for 80Ah, your 3.5A isn't far out, though if the batteries are getting on a bit then the capacity might be well under 80Ah. 10.5v is generally taken as a cut off point, if the batts go straight back on charge they shouldn't come to much harm discharging all the way once.

http://all-about-lead-acid-batterie...acid-battery-fundamentals/what-are-amp-hours/

 

[lw395]

Somewhere on your battery or datasheet if there is one should be how the 80Ah is measured - good chance it will be 20 hour rate. Which means it should take 20 hours at a constant current in amps of 5% of the Ah number for the voltage to fall to 10.5V. So 4A for 80Ah, your 3.5A isn't far out, though if the batteries are getting on a bit then the capacity might be well under 80Ah. 10.5v is generally taken as a cut off point, if the batts go straight back on charge they shouldn't come to much harm discharging all the way once.

http://all-about-lead-acid-batterie...acid-battery-fundamentals/what-are-amp-hours/

True, but if you don't need to know exactly, discharging down to some intermediate voltage usually tells people what they need to know, i.e. the battery is either quite useful or not very useful.
The 3,5A won't be exact or constant anyway, the current will drop as the volts drop.
Either way, it's important to recharge the battery straight after dischrging it a long way.