I've got a few very big 12v lead-acid batteries that I want to check the capacity of. I'm thinking of making a tester, comprising of a fat load resistor and my multi-meter.
Can anyone suggest a suitable current to test them at?
Don't they have a "cranking load" of 300A+ for 30 seconds.
Rick
Cheaper to use headlight bulbs etc in parallel to bring the load to the 20 hour rate.
If it meets the 20 hour spec it will likely meet the high current test.
Use a few car bulbs. Then you can vary the current as you wish.
Regards Capitol
Whatever you do, do not emulate my brother in his younger days. He borrowed my multimeter and tested the car battery on the 10 amp range. :-(
John Schmitt
Managed this recently myself. Don't Fluke fuses cost. ;-(
Never mind the fuse, the leads and shunt were totals, as was the meter.
John Schmitt
Ah. No fuse or circuit breaker, then?
Yes they do!!. But I ordered mine through the company.
Dave
Trouble is I own mine so it doesn't help. ;-)
What are you actually testing, cranking capacity or the 20 hour rate?
Also without having any idea of the capacity of the battery how will you know if any test is an indication the battery is ok?
For testing cranking capacity then unless you have the parts to hand it's not really worth building from scratch when you can get a self contained tester for about 25 quid from Machine Mart.
To test the 20 hour capacity, float charge for an extended period, leave off charge for a day or so to remove surface charge and then discharge at 1/20th capacity until you either exceed the stated capacity in time or you hit the end voltage of around 1.96v per cell (11.8v)
The shunt was paralleled with the workings of the meter, which was fused. Being a cheap meter, there was nothing to protect the shunt or leads. The meter needle achieved a near right angle when it hit the stop, illustrating beautifully how a fuse can transiently carry rather more than its rating.
John Schmitt
"Fluke fuses" are 2.99 from Maplin on most bank holidays. They're big, bright yellow and have a bigger display on them than my Fluke. They also protect against dropping, loss or theft - what a bargain!
And so back to the cordless tool argument. ;-)
But I've got several DVMs. Love the Fluke, though.
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