Battery drain

[Galaxie390]

Hi folks,

It's good to see the board up and running again. It's been terrible without it.......I've had to think for myself!!

Just looking a bit of info on a charging problem I've been having.

The battery just doesn't hold it's charge or the alternator isn't charging. The car runs fine once it's going, inc all the accessories so I don't think it's the alternator.

The alternator is charging at about 12.8 volts, a bit low but fairly ok I reckon. The battery shows 12.2 volts when the engine isn't running, but drops to 2.5 volts when the starter is going.

Hmmm, the more I type the more I think the battery is duff!

I also noticed that about 0.8amps is being drawn from the battery with everything off? Is this normal?

The alarm is disconnected, there's no lights on and the only draw I can think of is the radio. Is there anything else on the car that would use current when not in use?

Thanks,

Richard

[CTCNetwork]

Hi,

Have a read through this topic:
Linky....

There will be a drain on the system, even with "everything" off. Alarm, clock and radio are still drawing current.

Des. . .

[BillB]

12.8v is low, should be higher i.e. 13 and a bit. Check connections to alternator including ground. You'll get some current draw with everything "off" e.g. clock, bonnet light. Regards.

[Galaxie390]

Hi,

Thank you both for the replies.

Des, your link was very good. I'll follow it up.

BillB, I went through all the alternator connections and earths and cleaned them up well. I also took out the diode (At least that's what I think it is. Small black disc bolted to back of alternator) and cleaned up the brushes and connections some were very grotty - haven't had chance to put the meter on it yet. The commutator was slightly grooved, I've seen worse. I cleaned it up as best as I could in-situ.

I've swapped in another battery off my Chevrolet Kingswood station wagon and put the other on charge to get it tested.

Thanks again guys, all the best.

Rich

[ozznut]

has it got a tow bar ,mine was shorting instead of negative ,it was live hence draining battery ,it was connected to alternator wire by scotch lock connector .make sure alt connections are,nt touching

[StanC]

Hi,

Firstly, 12.8v with the engine running is low, but it will be if the battery is shot - even a healthy charging system will give a low voltage across the terminals if the battery is flat and drawing a high enough current from the alternator. With a healthy, fully-charged battery the alternator should give around 14.5v across the terminals with the engine running. A fully charged battery will give about 12.7v across the terminals when 'at rest'.

Secondly, a residual drain of 0.8 volts is high. I did a bit of research on this last year regarding my 960 and found that residual drains across a range of different car makes and models were typically between 10 and 100 mA. On my 960 it is 40 mA, or 27 mA if I isolate the power seat adjustment circuit (the seat memory function draws about 13 mA). 0.8 v is 800 mA. A 40mA drain equates to about 1Ah drain for every 24hours the car is parked up. If, as in your case, there is an abnormal drain somewhere in the car, that could explain why the battery is shot - 800 mA would equate to 5Ah for every eight hours the car is parked. That might not sound like much, but it adds up rapidly over the weeks and months and soon drains a battery.

As the others have said, you need to find out what the residual drain is - and then you might need a new battery if your present one is beyond redemption.

Stan.

[StanC]

Originally Posted by StanC

Hi,

Firstly, 12.8v with the engine running is low, but it will be if the battery is shot - even a healthy charging system will give a low voltage across the terminals if the battery is flat and drawing a high enough current from the alternator. With a healthy, fully-charged battery the alternator should give around 14.5v across the terminals with the engine running. A fully charged battery will give about 12.7v across the terminals when 'at rest'.

Secondly, a residual drain of 0.8 volts is high. I did a bit of research on this last year regarding my 960 and found that residual drains across a range of different car makes and models were typically between 10 and 100 mA. On my 960 it is 40 mA, or 27 mA if I isolate the power seat adjustment circuit (the seat memory function draws about 13 mA). 0.8 v is 800 mA. A 40mA drain equates to about 1Ah drain for every 24hours the car is parked up. If, as in your case, there is an abnormal drain somewhere in the car, that could explain why the battery is shot - 800 mA would equate to 5Ah for every eight hours the car is parked. That might not sound like much, but it adds up rapidly over the weeks and months and soon drains a battery.

As the others have said, you need to find out what the residual drain is - and then you might need a new battery if your present one is beyond redemption.

Stan.

Sorry - I'm mixing my units - but it was very late at night when I was writing it!! What I meant to write is that a residual drain of 0.8Amps is high - not volts. Also, I should have written 0.8A is 800mA.

Stan.

[minstrel]

Hello Stan,

I've just read your last post and it just shows you alway see what you want to read. I read your post and knew exactly what you meant and never spotted you put Volts and not amps.

Cheers

Minstrel

[rippedoffagain]

With the engine running you should be measuring about 13.8 volts off the battery terminals if all is well.

12.8v suggests maybe one cell in the battery is bust.

When you turn the starter, the voltage should drop very low, so 2.5V with the starter cranking wouldn't worry me. It all comes down to the magic of ohms law and internal resistance of the battery. When the resistance of the load (the starter motor in this case) becomes lower than the internal resistance of the battery, the voltage across the battery will drop drastically. The voltage in theory stays the same, but what you're measuring with your meter is the equivalent of measuring one part of a potential divider, so you get a much lower reading.

[Galaxie390]

Hi Folks,

Thanks for the input on this, I'd forgotten all about it as the car was running ok. Or at least it was until the dash wiring meltdown and the engine swap! It's about to go back onto the road, so I'll keep you all posted.

Thanks again,

Rich