Can a battery be tested while flat?

If I ran a garage and was presented with a car battery at 0v, I'd replace it regardless what a tester said, unless the person dropping it off signed a waiver acknowledging that my recommendation was replace but their intention was to charge and use it against advice to the contra

Sometimes garage owners won't want to line themselves up for being blamed/badmouthed because "they charged my daughter a fortune to change the alternator and they still didn't fix the problem" when the complainer quietly omits the fact that the mechanic told them the battery was shagged but they opted out of replacing it..

It's fairly universally accepted that a lead acid cell discharged to 0v will not recover its former capacity no matter when you catch it - and i think you've even managed to determine that this battery is well ruined from your own testing. Might start a car all summer but just when daughter needs it in the first day of a new job, mid winter when it's persisting it down at the motorway services..

Sometimes you buy the peace of mind and willingness for someone else taking liability. If the old battery screwed the new alternator up would you be back insisting it be fixed on their dime? Quite sure you wouldn't but there are an unscrupulous minority who would, they being the crows we must fly with, and be shot with
 
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I have recharged batteries that were zero volts, and they have given good service after, however using them in the caravan is one thing, using them to start the car is another. And there is the problem of time.

For years under 10 volt may as well scrap it was my thoughts, and commercially that is still the case. About a year before my father-in-law died he had turned off the battery charger to his stair lift, and the German unit seemed to have no manual option to wind it back to the base so it could be charged, so only option was go out and buy two new batteries. However the old batteries vanished, and were only found on his death, so when found I thought they were just about right size for my 2 way radio, so decided to put them on charge, however the smart battery charger would not charge a completely discharged battery, under 7.5 volt it would see a battery as being 6 volt, so only method was to connect it in parallel with a good battery so kid the charger that a 12 volt battery was being charged.

I was a little nervous about doing this, so plugged the charger into an energy monitor so I could check what was going on from my PC.

It sat there for 10 days doing nothing, it had charged the battery in parallel, then gone into maintain mode, which is basic switch off at 14.4 volts until battery voltage drops to 12.8 volts then recharge again, and this can be seen as a small spike on the monitor, then about day 11 it started to charge, just as if some one had flicked a switch. The charge time an rate matched a good 7 Ah battery, and the small VRLA seemed as good as new, so tried same with second battery, (lift was 24 volt) and it did exactly the same, I have since done it many times, Colvid resulted in no access to caravan and that battery did same did have one fail, but most recovered.

However to be sure the battery has been fully charged i.e. all sulphur now back to acid, it needs to be left on charge for 2 weeks, with a smart charger so the battery is not damaged by the charger, and carefully monitored as if there is a shorted cell the battery will over charge and a 35 Ah VRLA (AGM) warped the plastic with just a 5 amp charger with the mobility scooter when one of the 12 cells when short circuit, (24 volt) and it then damaged both batteries.

So unless carefully monitored not really an option, so on a commercial basis the batteries once discharged that low are scrap.

Jump starting a car which has a discharged battery due to say lights being left on, may get the car going, and the alternator may in the fullness of time recharge the battery, but the charge rate is down to the battery, not how big the alternator is, so with a stage charger fastest you can recharge is around 8 hours, with a float charge from most alternators looking at more like 12 to 14 hours, and if the charge is not completed the sulphur starts to harden, so only option to save the battery is a smart charger for 24 hours.
 
Renault4 said:
Have just been speaking to the garage which diagnosed a "cooked" battery along with a failed alternator on my daughters car.

After they replaced those two items, I got the old battery back: it was at 8V, but took a good charge and is still well above 13V a few days later.

The garage said they had tested the battery while flat and found it needed to be replaced.

Is there really such a battery tester? - ie one which can test a totally flat battery?
Click to expand...


UPDATE
Today I've tested the battery (fully charged about 3 days ago) with a TOPDON BT100 Battery Tester. It says it is good.

I've also tried the tester on an old discarded battery which was at 1.5V . It wouldn't work until I'd got the battery up to several volts at which point it asked for the battery to be charged and retested (after which it said 1% health, 83mΩ & REPLACE)

I would not have expected the battery to be bad after being flat (for the first time in its short life) for 3 days, but am surprised it tests so well after being left uncharged for another 10 days on top of that.

The battery is a DriveTec from GSF.

My post was not to determine the condition of the battery but as per title. I'm adding the results of this further test due to the interest shown in the condition of the condemned battery.
 

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