couple of questions re car battery

[rhinosaw]

Hi
I have a 3.0 lt, Toyota supra which I do not use very often sometimes only 30 miles in a week.
The battery is about 2.5 years old and I charge it up every 6-10 weeks depending how often I use the car.

Questions are:

1/ Recently when charging the battery will get to 12.4v and then the indicator on the battery charger comes on to say battery full. Is this a sign that the battery is on the way out?

2/ When starting the engine from cold is it more likely that the car wouldn't start if the battery voltage was only showing 12.28 v compared with 12.65 ? ( bearing in mind that it is trying to turn over a 3.0lt engine) I think that I read somewhere that a battery under 12.30v is only at a qtr capacity?

Thanks

 

[Burnerman]

Its a bit difficult to assess battery condition using voltages - a battery that is absolutely crocked will happily show 12v or more when charging or standing, but when the starter is used the voltage can drop dramatically, even to zero if its really wrecked.
Average battery chargers have a current meter which measures the amperage that the battery is accepting at the time of charge. If the battery accepts a good few amps for a while it may be in good condition, but if it accepts charge for only a short time then it could well be goosed.
The best device to have is a 'smart charger' which is able to assess what condition the battery is in, and charge it accordingly....prices around £60.
Unlike a car alternator which charges at around 14.5v, the smart charger won't be as high as this and will clip off at around 12.5v, I've found.
For your second question, the voltage you have is enough to crank the engine, providing that the voltage doesn't drop much below 11 when cranking.....that's the important bit.
Your battery isn't old, but if it's ever been allowed to go flat then it will be below standard - but a smart charger may well recover it but allow this to happen regularly then its had it.
John

 

[rhinosaw]

Thanks John
The battery has never gone flat and I have had the battery checked ie cell check, seems ok.
Just that if I don't drive the car for, say, 10 days and I check the voltage before starting from cold voltage may only read 12.20v.
Just thought that could be a bit low to start a 3.0 Lt enginebut see what u are saying about not necessarily down to voltage.

 

[Harry Bloomfield]

Unlike a car alternator which charges at around 14.5v, the smart charger won't be as high as this and will clip off at around 12.5v, I've found.

I'm sure you meant a value much higher than that? More like 14.2 to 14.5v to stop charging. My own smart charger can optionally charge to 14.6v in cold weather. Even a trickle charger will charge to 13.8v.

Most any reasonable battery will present more than 12.5v of charge, so a smart charger would not charge it were that true..

For the OP - Modern car's electronics and alarms systems can easily draw 30 to 200mA of current when parked up, which can flatten a battery in a matter of weeks, unlike older cars which would draw nothing when parked. You are nearing the limits expecting a battery to start a car after being stood for 30 days unused and each time you leave it for that long without it being charged, you are seriously reducing the batteries life. If you are going to leave it stood for that long, consider disconnecting the battery, then giving it a full recharge, then leaving it disconnected until needed.

 

[rhinosaw]

Thanks Harry, I get what u are saying, not sure if a Supra would be classed as a modern car or older car in terms of the electrics

 

[Harry Bloomfield]

Thanks Harry, I get what u are saying, not sure if a Supra would be classed as a modern car or older car in terms of the electrics

Well, does it use points for ignition or an engine management system?

 

[rhinosaw]

No points Harry, and petrol injection, does that mean it's using battery even when off?

 

[Harry Bloomfield]

No points Harry, and petrol injection, does that mean it's using battery even when off?

Yes. Some would fine tune the operating parameters of the engine and fine tune the timings to the engine automatically, with the settings stored while ever the battery remained connected. Disconnect the battery and the settings go back to factory settings, but are relearned in the first few miles of operation.

 

[Burnerman]

Measure the voltage when the engine is cranking, Rhino......it'll give the best indication of the battery condition. Motor factors will do the same for you, using a heavy discharge meter that connects to the terminals. On occasion a bad cell will start to fizz but its not a good idea to peer in!
John

 

[rhinosaw]

Can I use a hand held multimeter?

 

[Harry Bloomfield]

Yes, but you may need to prevent it firing, so it cranks for long enough to get a sensible reading on the meter.

 

[rhinosaw]

Ok and what voltage should it be, also what voltage to indicate a battery on the way out
Thanks

 

[Harry Bloomfield]

Ok and what voltage should it be, also what voltage to indicate a battery on the way out
Thanks

Difficult one - I would suggest 10.5v, if cranked over from cold, but the higher the better.

 

[Burnerman]

+1, not below 10v.
John

 

[sircerebus666]

Aldi/Lidl do a smart charger a couple of times a year for the bargain price of £13!

Cracking bit of kit and has recovered a couple of batteries for me that I thought were done for....