Problems with Micra Battery or otherwise

Pretty much any car made in the last 15 years or so will have some drain when stopped; ECU memory, alarm, radio code and settings, ect.

Reading resistance across the battery cables won't give you any meaningful information. You need to measure the current drain.

This article:

may help.

Any more than 50mA or so might indicate a fault, although if it's intermittent you might not find it right away.

WRT testing the battery, old-fashioned drop testers were just a volt meter and a shunt; they basically test the voltage with a heavy load on. New electronic ones are more sophisticated, but both types will not always show an intermittent fault. (Although that sort of fault is rare with a battery.)

Most commonly, these faults are caused buy a boot, under-bonnet, or glove box light that stays on. Perhaps you could remove all the bulbs from any such devices temporarily, and see if that cures the problem?

Chris

One common cause for this on most cars is a faulty switch on a boot light. Sometimes, the light doesn't turn off, flattening the battery after a while. Of course, if it's daylight when you close the hatch, or the cover's in place, you probably won't notice that the light is on.

Charging a reservoir capacitor on some electronic device.

1K will draw about 12mA, which would effectively flatten the battery over a period of a few hundred hours. There will always be some leakage if there are electronics on the car such as an alarm, immobiliser or clock.

What they should do is put a battery test meter across it, and the tester will give a go/ marginal/ no-go reading. If it's not sealed, then the specific gravity of the electrolyte in each cell can be checked, but that's not likely to be done at Halfords.

Did you also do a voltage test with the engine running? This gives an indication as to the state of the charging system. You should see about

14.4 volts with a low battery dropping to about 13.8 when it's fully charged.

No surprise at all. Lots of things are powered even when all driver controlled things are off. Remote central locking for example requires a always on receiver. The car radio may have a memory which takes some power. The engine ECU may too. Etc.

Resistance measurements are fairly useless. You need to measure the quiescent current flow with all driver controlled stuff off. You may need to wait for half an hour or so for everything to go to sleep as it were. You should then measure a quiescent current of around 0.050 amps (50 milliamps). A fault will likely result in it being very much more.

I'd hope they'd have a decent electronic tester like the ACT I have. But it seems many are using ones of inferior design.

It's far more likely your car has a problem than the battery is faulty, though.

Many thanks one and all. The problem at the moment is that the car won't start because - you guessed it!

I shall put the battery on charge and see what happens. If it is the battery, I'm just a bit concerned that Halfords will say it's fine and then we shall start the cycle again.

WIll post again when the problem is resolved.

Xav

Many thanks one and all. The problem at the moment is that the car won't start because - you guessed it!

I shall put the battery on charge and see what happens. If it is the battery, I'm just a bit concerned that Halfords will say it's fine and then we shall start the cycle again.

Specifically a 'drop test' puts a high load on the battery. After an initial drop, the voltage should remain contant for about 10 seconds.. If the voltage continues to fall, it would indicate the battery is knackered. Obviously a battery needs to be topped up and reasonably well charged before a drop test is carried out. Mike.

There's always something, and it's not going to be resistive.

Well unless it's charged none of the testers are very good, if it's charged then you measure the voltage drop under load.

The last two posts were particularly interesting. I telephoned Halfords to make sure they had a replacement battery in case the new one was duff and that's when they said they'd 'drop test' it. It's flat I replied. That doesn't matter quoth the Halfords man!!!!

Which made me wonder exactly what a 'drop test' was. Given there weren't that many volts to drop, I thought perhaps he was going to use a long ladder and a suitable high place. Or perhaps it would be put on a fast charge first :))

Xav>

It's not posible to do a drop test on a flat battery, but maybe they intend to charge it first

. See my earlier post. Mike.

Chris Whelan used his keyboard to write :

As explained above, you need to check the discharge current with everything turned off. Very modern cars with lots of electronics, can draw 500mA when first turned off - this should gradually fall in stages, as various things go into sleep mode in the first hour or so if left left undisturbed. Eventually it should fall to around 30mA or less.

Aside from the electronics, a faulty diode on the alternator has sometimes been known to flatten a battery, but this would show up via the above test.

Harry Bloomfield formulated the question :

Forgot to point out...

Any disturbance of the leads might wake the electronics back up. It needs a good hour with meter in place and an eye kept on it.

Or just get one of these:-

Makes measuring quiescent current easy - and less chance of blowing the DVM fuse - which on a Fluke can cost almost as much as this device...

Dave Plowman (News) laid this down on his screen :

Mine has an easy way...

The positive battery post is connected to the main under bonnet fuse board by a thick cable and easily removed bolt on the main bus bar. Connect meter at battery post and bus bar, then simply remove the bolt.

[...]

Indeed, although for this to happen intermittently, in the way the OP mentions, would be pretty rare.

Chris