Battery Testers?

I recently purchased a (car) battery tester. While it was to test leisure batteries, they are mainly aimed a car batteries and are widely advertised on Ebay/Amazon etc for about £25/30. A typical one is the BU101.

They can test the battery in or out of the vehicle. The out of the vehicle test involves inputting the CCA rating, whether the battery has just been charged, and off it goes. You get a state of health reading and a Replace / Good battery reading and, what seems to be, an actual CCA reading, and the internal resistance on an LCD display.

(In vehicle testing involves using the lights as a load etc but I've not explored this mode.)

Unlike other testers I've seen before, these don't seem to have a load resistor- at least not one that could load the battery seriously (they are too small and would 'cook' if they had).

On a car battery I'm sure is good, it shows good.

On a couple of leisure batteries I thought were on their way out back in Nov but Halfords tested as good, it says Replace. (There were in our old motorhome, I replaced them just in case before I sold it, I didn't want to sell it with a questionable battery).

I'm curious, has anyone else found these beasts to be reliable?

Has anyone any idea how they test the battery, given they don't seem larger enough to have a load resistor inside- at least not one which could stress the battery.

Reply to
Brian Reay
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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.

Reply to
newshound

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

10 sec, although that is a guess-estimate from memory). I should see how easy it is to have a look inside.

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

Reply to
Brian Reay

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!

Reply to
newshound

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

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Reply to
Mark

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.

Reply to
Dave Plowman (News)

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.

Reply to
Brian Reay

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.

Reply to
newshound

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.

Reply to
Brian Reay

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

Reply to
Dave Plowman (News)

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'.)

Reply to
Brian Reay

+1

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

Cheers, T i m

Reply to
T i m

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