Battery Testers?

At a guess, perhaps it only pulls high current for a few milliseconds. I assume that this is the way the microprocessor-based "mains" testers test for maximum fault current.

Good idea. The display indicates 'Testing' for a short period (perhaps

It does have (fairly) decent leads, suggesting it does drain a largish current.

You are going to need a nice firm clamp to make sure you get a low resistance contact. If the test is quick enough you don't need thick leads. Easy enough to work out the adiabatic formula from first principles!

?conductance? its all on the interweby thing if you look

I've got an ACT here which gives you a direct readout of the battery capacity. Cost an arm and a leg a few years ago, but wouldn't be surprised if they came down in price. Superb bit of kit.

If you don't have decent leads, unless you calibrate the beast for naff ones, then you answers wouldn't be much good. Likewise you need to allow for the contact resistance between the clamps and terminals. Some kind of 'Kelvin sensing' may be an improvement- two wires to 'carry' the test current and two to do the measuring. Even that wouldn't compensate for contact resistance- just lead resistance.

I agree, to do it properly you need Kelvin contacts. Some of the expensive commercial stuff claims to do this although I have not seen the hardware. Kelvin leads *do* reduce the contact resistance effect because you don't take any significant current down the voltage leads.

My only point was that you can do dV correction for thin or thick leads as long as you know their length and area. You don't need thick leads for a quick enough test. I agree, contact resistance particularly for a typically oxidised battery post is likely to be the main problem.

But they don't compensate for the contact resistance at the point were the high current leads connect- just minimise the effect the where (extra) sensing leads connect and compensate for the high current lead resistance.

The crock clips on my ACT have a sort of nest of needles to punch through any corrosion, etc.

True, crock clips do have teeth which tend to cut through corrosion but sometimes you need to give them a wiggle to ensure they do so- I've noticed this once or twice when jump starting cars with battery terminals that haven't been well maintained. As I'm sure you know, it is surprising how little 'gunge' you need to cause a problem. One more than one occasion, a car which seemed to have a 'flat' battery needed no more than the terminals cleaning. (Clean battery terminals are one of my 'soap boxes'.)

(well, I have two of them). ;-)

Cheers, T i m