Why is my 'flat' battery still starting my engine?

Bav34

Bav34

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Odd one this.

Most of my battery knowledge comes from this forum.

Simplistically I have read that:

An engine battery will give a lot of power instantly, charge back up quickly, but doesn't like slow (service) discharge.

A service battery is the opposite ... it likes slow release ... lights, instruments etc, but also only charges back slowly.

Regardless of just how accurate the above is ... and that's NOT the question, I have always understood that a battery should not drop below 12.2 volts as it's now half charged/discharged.

Any battery in the 11s is dead.

So.

This is my car battery but the same rules must apply whether it's starting a car engine or boat engine.

When we got back to our car this year after 5 months away the battery was flat. Wouldn't even operate the central locking let alone start the engine. A volt meter showed it to be 11.5 volts !

Nearest new battery was 4 miles away (boat/car in Brittany)

So, thought its worth a try ... took it back to the boat, boat voltmeter showed it to be 11.4/5 ... topped up a couple of cells ... charged it overnight ... at rest in the morning ... 12.7 !

Waited 24 hours ... 12.6.

Took it to car ... started first turn of the key. Impressed but confused. It should be dead.

Took car for run. Didn't use it for the next week but checked it every day ... it drops literally 0.1 v every day.

Bought new battery but held off fitting it ... wanted to see what voltage was insufficient.

It started first turn of the key with a reading of just 11.9 volts. Using 2 different volt meters!

So, how can this be? Battery obviously dying but recovering from 11.5 and starting at 11.9?

For info it's a 096 battery ... Audi diesel.
 
It has some self discharge, but still has enough capacity and low enough internal resistance.
Might go on like that for years, if you keep it on a charger when not in use, I've done that with a bike battery and got another year out of it until I left it for 3 weeks and it went completely flat, never to recover.

But batteries are generally not that expensive compared to the grief of breaking down, and the possible trauma you might inflict on the starter motor.
 
I had a similar situation on my Audi 2.5 diesel, they need a good fast spin from the battery to start. Mine failed after I sat in the car for an hour listening to the radio. It had been working fine before and having been saved by the AA continued to work for some time after but... The capacity of the battery had reduced significantly therefore it could deliver a very short high power burst to spin and start the engine but if there had been any drain, even a few Amps it would not have enough residual to spin the engine fast enough. I suggest yours is knackered with a very much reduced capacity due to sulphation, it can still show 12.7-8 volts but it may only have 5% or less of it's available 'umph'
 
Batteries can have odd characteristics.
My car battery started every time , no probs until one day having started, still driving after less than a mile the radio died then the car and the clock. RAC jumped the car with a battery they put in the footwell and followed me to halfords, where I replaced the battery and all was well. No fix to the generating was needed.

I would have expected the car to run on the generator output alone, and was surprised the battery died so suddenly.
 
I would be careful nursing a knackered battery. I had one on a camper van as the leisure battery. I knew it couldn't hold a charge, but was waiting until the spring to buy a new one. Was on the A 34 heading for Oxford when the top blew off it.
 
Bav34 said:
So, how can this be? Battery obviously dying but recovering from 11.5 and starting at 11.9?

For info it's a 096 battery ... Audi diesel.
Click to expand...

Probably a geared starter motor, small electric motor running at high speed, low amps, geared down to turn pinion at low rpm.

11.9 volt is about 10% capacity so will still supply power, the on load voltage drop ( engine start ) is getting rapidly larger, by 11.8 it is very big so cannot turn the engine.

It is all about starter motor type, old type slow speed motor used high amps so hit the volt drop step earlier, hence boat problems due to old type starters.

Brian
 
I was once driving a bread round delivery van, (about 55 years ago) when it suddenly stopped. I rang the depot and the fleet mechanic turned up with a replacement battery, fitted it and the van immediately restarted. Yet the original battery would turn over the engine, but not start it. When I asked him how he knew it was the battery he replied, "experience, my man"! Strange things batteries!
 
Bav34 said:
When we got back to our car this year after 5 months away the battery was flat. Wouldn't even operate the central locking let alone start the engine. A volt meter showed it to be 11.5 volts !
Click to expand...

I always disconnect our car battery when leaving for months and leave the handbrake off. Starts instantly when reconnected and no brake pads stuck on discs.
 
Thanks all.

Some interesting stuff. I am only keeping the battery on the car to see at what point it won't start.

I was hoping that this would give me some insight into whether or not I am being over-protective with my boat batteries.

We regular anchor for 5-6 days with the fridge running and I monitor the 220 ah service battery religiously. As soon as it hits 12.2 volts resting I swap over to the second 140ah one, and think about moving or turning the fridge off.

Just amazed that at 11.9v it can turn a diesel engine over first turn of the key.

I'll change it today.
 
GrahamM376 said:
I always disconnect our car battery when leaving for months and leave the handbrake off. Starts instantly when reconnected and no brake pads stuck on discs.
Click to expand...

I will in future but worry about the lack of alarm. However, if I was stupid enough to leave something valuable on display, I don't think that a thief would worry too much about smashing a window and legging it ... alarm or no alarm.
 
As sugested a lead acid battery can have various modes of failure. It can have reasonable capacity for low discharge rate but not be able to provide high current on the other hand a battery can be completely discharged then after a short charge with just a fraction of AH replaced can start an engine. So charge percentage does not relate to ability to crank an engine. A near knackered battery can still start an engine when there is very little actual capacity in it.
All you can do is replkace it if it won't do what you want it to do. IE large capacity or crank an engine. But any shortcoming is probably a sign of near death.
if OP has bought a new battery I would suggest fit it. good luck olewill
 
William_H said:
As sugested a lead acid battery can have various modes of failure. It can have reasonable capacity for low discharge rate but not be able to provide high current on the other hand a battery can be completely discharged then after a short charge with just a fraction of AH replaced can start an engine. So charge percentage does not relate to ability to crank an engine. A near knackered battery can still start an engine when there is very little actual capacity in it.
All you can do is replkace it if it won't do what you want it to do. IE large capacity or crank an engine. But any shortcoming is probably a sign of near death.
if OP has bought a new battery I would suggest fit it. good luck olewill
Click to expand...

Another old trick with a petrol engine and low battery is to remove a couple of spark plugs to reduce compression, a couple of minutes running at tickover will usually give enough charge to restart once the plugs are back in.
 
Kurrawong_Kid said:
I was once driving a bread round delivery van, (about 55 years ago) when it suddenly stopped. I rang the depot and the fleet mechanic turned up with a replacement battery, fitted it and the van immediately restarted. Yet the original battery would turn over the engine, but not start it. When I asked him how he knew it was the battery he replied, "experience, my man"! Strange things batteries!
Click to expand...

Cranking amps reduces the voltage so far it won't support the ignition.
 
I had a battery (very old Bosh) that would charge to about 13V and stay there for a few days. Wouldn't even attempt to turn an engine though.
 
I used to run batteries for large car stereos.

They will take a beating! If the cells get low on water they can heat up and warp the plates affecting charge.
Even after running a cell dry it will still take a charge once topped up again, just not a good charge.

I had run 2 high discharge batteries down on a build I did to 9.7 volts before the fuse blew (250a)

The issue with a low voltage is the amps go up creating heat. Ideally unless its a specific grade of deep cycle battery its best kept above 10v and 12.5 is a healthy floating voltage.
 
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