car battery woes

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Established Member
20 Feb 2004
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In the eternally wet North
Woke up this morning to find the car battery dead as a dodo. Dead as in 3v across the terminals. Wondered as to why and so disconnected the battery and stuck it on charge (although with that low terminal voltage I think its days are numbered).

Next, measured the resistance looking back into the car and got 3 ohms....mmmm..that would explain why it had drained. Bit of Googling and the pessimist in me starts to think the alternator is shot. So disconnect the heavy duty cable on the alternator (not the other smaller cables) and measure again...still 3 ohms...thinks 'alternator OK'.

Remove the plastic gubbins around the battery +ve and see a fat wire (to the starter I am guessing) and also a 60 amp fuse. Remove 60A fuse and measure resistance again. High resistance. Replace fuse and measure again. Still high resistance. Que? I have done nothing. I have not rolled the car forward. Nothing and yet now the resistance has changed (unless the wee multimeter is junk and gave me a false reading).

Don't really understand what is going on here. Reluctant to buy a new battery especially if there is this 3 ohm (intermittent ? how?) load which will crucify any battery.

One hour later, disconnected the charger and measured across the battery which gave 12v....surely the voltage isn't back up 12v so soon (as in an hour) or is it that it might be at 12v but the internal resistance is still very high and so not be able to deliver a full discharge when needed for starting etc.
Have you measured the resistance across the fuse? have you put the fuse back right?
Mmm something has happened if the resistance had gone up, did any thing feel hot? starter, alternator?

Last battery that died on me went open circuit, one click then nothing, had to get all my bike gear off and go on the bus.


P.S. you should get 14.4V at least from a lead acid battery
my 2pence worth :)
Its more than likely that the battery has come to the end of its life but just in case I'd check that no interior lights (including boot, glove box etc) have been left on. sat navs, phone kits etc are powering off as and when they should.
any auto electrical/battery/tyre? place will be able to test your battery in a matter of minutes and tell you whether its duff or not, (btw it can still show 12v even if its duff, but it wont have the strength to turn the engine over) and then you can start any investigations into the car.
I take my car to a local dealer who sticks a computerised meter across the terminals which instantly gives a printed readout of exactly what's right or wrong with the battery. He doesn't charge (bad pun!) as he sells the new ones, and he's the cheapest in the area, anyway.
When you measured resistance ... did you pay attention to the polarity of the meter ? You might expect a low resistance one way, and a high resistance the other. You aren't measuring a simple resistive load.
Thanks for all the advice, guys. The battery checked out OK. I couldn't repeat the 3 ohm measurement and so I'd put that down to cold fingers or operator error :oops: ....currently sits at around 160 ohms mark. I do now remember that when I started messing about and checking things that the boot was open. That really should have given me a clue as to the culprit....but [sigh] ever one to over-complicate things...[/sigh].

Anyway battery charged up went vroom vroom...all sorted. Time now for the Mazda to be SORNd and the Honda to come back into service for the [hollow laugh] summer [/hollow laugh]

If you have wifi as it pulses if the car is within range it can actually keep waking up the BECM which will drain battery if left for a day or so - had exactly this happen with a 1999 P38 Range Rover outside our house a few years ago - There is a retro fit filter that can be fitted to prevent this happening.

Just a thought and may be the reason for your flat battery that has recharged fine - Have a read here ... 06648.html

That's a fascinating link, Rog..

There was an interesting article in the FT the other week about how vulnerable to hackers cars are going to become as they become more 'intelligent' with connections to the Internet etc. Couple that with the prospect of 'driverless' cars......scary.