Car battery left standing flat

Joe_Horner

Strider590 wrote: »

The gassing is due to the fact the battery is no longer capable of being charged to 14.4v. Any decent charger will work by holding it's output voltage at 14.4v (or 14.8 for AGM) this will lead to the charger applying maximum current until it reaches that voltage..... Simple ohms law applies here.

Not quite. That's how most (if not all) commercial chargers work but, for lead acid, the ideal charging regime is split CC/ CV charging with a two stage topping / float CV phase.

The initial CC is done at a constant current of around 0.2C (so 10 amps for a 50 Ah battery) over long enough to reach about 2.3 - 2.4V per cell (13.8 - 14.4V on a 12V battery).

The charger then switches to CV mode and maintains that constant voltage until the current drops to about 3% of the Ah rating (so 1.5A for a 50 Ah battery) which typically takes a further 5 - 10 hours.

The correct voltage for the switch and CV phase depends on the individual battery, and charging temperature (by about 0.2V between a cold shed overnight and a warm one in sunshine - warmer needs LOWER voltage !) The commonly set 14.4V (top end of the voltage range) will over charge a lot of batteries, especially if there are variations in cell strength, and cause plate corrosion and gassing.

Once the current drops to the second switching level the charger should finally switch to a float voltage of about 13.5 - 13.6V, again temperature dependent. That float charge can be held indefinitely and will maintain against self-discharge. Obviously, if it's going straight into service, that stage isn't needed.

Strider590

Different chargers operate differenty, but all that means nothing when the initial bulk charge takes the battery up to 14.4 before it even thinks about changing over to a maintenance charge.

The charger that was gassing the old 015 battery, it was a Ctek mxs 5.0. (The 5 obviously indicating it's maximum current of 5amp). This also means that once the max current limit is reached, the output voltage has to drop anyway, as the only way to maintain CV is to have no current limit.

Charging a lead acid battery isn't as complicated as the charger manufacturers would have us all believe.
All it really needs is a bulk charge to 14.4, then switch to a float charge of around 13.5, if it wants to get really clever it could cut the float charge every 24hrs and monitor to see if the voltage drops below 12.8v after a given time.

Joe_Horner

Strider590 wrote: »

Charging a lead acid battery isn't as complicated as the charger manufacturers would have us all believe.
All it really needs is a bulk charge to 14.4, then switch to a float charge of around 13.5, if it wants to get really clever it could cut the float charge every 24hrs and monitor to see if the voltage drops below 12.8v after a given time.

No it doesn't need that.

First, 14.4V (2.4V per cell) is too high for a lot of lead acid batteries, especially allowing for any imbalance that's appeared over time. The exact voltage varies depending on exact plate make-up, electrolyte chemistry and additives and physical construction but 14.4V is right at the top end of the range. If you have one cell lagging behind on charge then reaching 14.4V across the battery will mean over-volting the other cells.

Second, whatever the peak voltage is for the bulk (CC) charge, it will reach that voltage at around 70% charge. It then needs to be held at that voltage for about the same time again or you're running it at a permanent partial discharge and it'll suffer sulphation. That's the reason for the 3-stage charge: Bulk charge to 70% (approx), top-up at max safe voltage for the remaining 30% capacity (measured by the drop in charging current), then switch to float charge of around 13.5V.

The alternative (old school) way is to trickle charge it right the way through at a very low current (less than .05C or so / 2.5A on a 50Ah battery) but that takes considerably longer - typically 24 hours + and nearly 2 days in this case. It's not quite as good as the theoretical model but it comes close.

It can be done with a simple transformer + rectifier which, if selected properly, comes fairly near to a CC / CV output because the transformer winding resistance limits the current at low battery voltage, then the transformer ratio limits the output voltage as the current drops.

Richard53

Since the battery experts are out to play tonight, can I interject a quick question?


I've just replaced the battery on the Merc after the starter motor started spinning slower over a period of a few days. Then, after a week parked up, it wouldn't turn the engine fast enough to start it and soon died. A new battery has restored the lively spin and quick starting. However, when I check the charging voltage it is only 13.8V at a fast idle, going as low as 13.5V and as high as 14V at times while running. (Checked with a voltmeter in the cigar socket and confirmed with a multimeter across the battery.) I always thought that 14.4V was the ideal. Does this sound as though something is wrong, and the new battery is going to be undercharged, or is it within normal limits?

Joe_Horner

No, 13.8V is fine, it's just at the lower (and safer) end of the range. Anything over about 13V will put some charge into a car battery, just not fast enough to keep up with repeated starter loads in most cases.

almillar

So, in the interests of money saving ..., yes it is worth charging before condemning a flat batter

100%. If the battery sits connected to a car for a long time, it'll go flat - there will be SOMETHING in the car to drain it, from the alarm, immobiliser, to a clock. If it goes flat, charge it! It's not the best way to treat a car battery, but it's not suicide!
If the battery dies whilst the car is in daily use, that's different. If you notice the car is slower to turn over in the winter, and is getting good long journeys, plan for a new one before next winter so you're not left stranded.

Richard53

Joe_Horner wrote: »

No, 13.8V is fine, it's just at the lower (and safer) end of the range. Anything over about 13V will put some charge into a car battery, just not fast enough to keep up with repeated starter loads in most cases.

Thanks Joe.