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car battery very flat. Will it be terminal?
smart51 - 31/12/10 at 05:28 PM

I cam to start my tin top today after being stood for a week. It didn't even try to start. I jump started it from another car but it wouldn't turn over until I got someone else to rev the other car quite hard while I started mine. It's been fine since but if the battery was that flat, will it be damaged beyond repair?

matt_gsxr - 31/12/10 at 05:29 PM

I had it with mine when the alternator shorted itself. The battery was and still is fine in my case.

big_wasa - 31/12/10 at 05:33 PM

Could be either way. The wifes cooked the alternator two weeks before xmas after a flat battery.

Surrey Dave - 31/12/10 at 05:43 PM

You could try an intelligent battery charger

CTEK XS3600
or
Oxford Oximiser
or
Optimate
or
Accumate

ReMan - 31/12/10 at 05:50 PM

quote:
Originally posted by smart51
I cam to start my tin top today after being stood for a week. It didn't even try to start. I jump started it from another car but it wouldn't turn over until I got someone else to rev the other car quite hard while I started mine. It's been fine since but if the battery was that flat, will it be damaged beyond repair?


All sounds normal behavioor to me, so unless the battery was exibiting signs of old age prior to theis then it should be fine once run and charged properly.
Might be worth cheching for anything draining slowly though

That's my locost answer

Bluemoon - 31/12/10 at 05:51 PM

I would not be worrying about the battery so much as if the car has a fault that has caused the battery to go flat. If so this will eventually kill the battery even if you have got away with it once. I would be tempted to disconnect it and measure the residual current drain with a DVM.. For a 35ah battery over a week to be completely discharged only needs 200mA load (i.e. 35/(7*24))..

Dan

britishtrident - 31/12/10 at 06:12 PM

Slow charge the battery for 24hrs, let it rest for a few hours then test the volts drop under 200amps load.

stevegough - 31/12/10 at 06:14 PM

quote:
Originally posted by big_wasa
Could be either way. The wifes cooked the alternator two weeks before xmas after a flat battery.


Really? the wife did pork for us!

hillbillyracer - 31/12/10 at 06:18 PM

I bought a car that had been standing & the batterey was totally flat, it was fine & I used it for 7 years & once in that time it went flat again due to lack of use but again it was fine & was still as good as ever when I sold the car!

britishtrident - 31/12/10 at 06:19 PM

quote:
Originally posted by big_wasa
Could be either way. The wifes cooked the alternator two weeks before xmas after a flat battery.


Chicken and the egg.
More likely the alternator goosed the battery, a completely flat battery will only accept a tiny current for the first hour or so after that it slowly draws in more current.

Peteff - 31/12/10 at 06:32 PM

When you jump a car battery it's better to connect the two together and let the running car charge the flat one for ten minutes before trying to start the flat one by revving the nuts off the jump car. I had a battery which died and got stuck in the shed after it was replaced. I put the Optimate on it and it went into desulphate cycle and after 3 days it charged up. It ran on my daughter's car for two years before she sold it.

britishtrident - 31/12/10 at 06:59 PM

Surprisingly those jump start cables the connected between cigar lighter sockets work pretty good if you give them a few minutes to work.

Of course nothing beats real pro HD jump leads and a big battery.

big_wasa - 31/12/10 at 07:44 PM

Batery is fine after an over night charge still had to do the alternator in the snow ....

My guess was short trips to work and cold = flat batery and like you say the alternator didnt like the very flat batery.

prawnabie - 31/12/10 at 08:02 PM

Probably too late now but i wouldn't drive it until fully charged as Im sure you can knacker the regulator on the alternator from it working its knackers off trying to recharge a flat battery.

Ninehigh - 31/12/10 at 09:04 PM

I remember something about making sure the electrolyte is topped up...

britishtrident - 1/1/11 at 01:13 AM

quote:
Originally posted by prawnabie
Probably too late now but i wouldn't drive it until fully charged as Im sure you can knacker the regulator on the alternator from it working its knackers off trying to recharge a flat battery.



Batteries are not like capacitors, they are also are different from a resistor a flat battery only initially accept charge slowly .
Put a full 14.8 volts across a flat battery and it will accept a very low current, only when the charge builds up will the battery start to "suck" in current from the alternator, the current falls back again as the battery voltage rises to meet the alternator charging voltage.

If you try and force in charge by increasing the charging voltage (as in a garage boost charger) the battery voltage increases but the battery only gets a comparatively small amount of charge that isn't spread evenly across the plates. This is why if a battery is given a high rate boost charge after charging the voltage at the battery terminals will increase to a health 12.5 to 12.8v but the voltage will fall back after a couple hours as the charge spreads more evenly across and through the battery plates.

[Edited on 1/1/11 by britishtrident]

Paul_C - 1/1/11 at 09:57 AM

In my experience a completely flat battery (so called deep discharge) is a bad idea for lead acid batteries.
As previous posts say they sometimes recover taking only a small charge current to start with.
If it does recover you are lucky.

BenB - 1/1/11 at 01:23 PM

In my experience car batteries are able to survive a deep discharge better than a bike battery. I know the lead plates can buckle when deep discharge and short- perhaps because the bike battery is smaller the plates are closer together so buckling is more of a problem? But fundamentally deep-discharging non deep-discharge batteries is a bad idea.