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Old 11th October 2021, 08:43   #1
macafee2
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Default Have I a duff battery

I took the battery off the car and put it on charge for a few days in the garage.
As soon as I took it off charge I measured the voltage and it was 12.8v, another battery that had been off charge for about a week measured 13.2v.

The above was about 3 or 4 days ago, I have just measured the suspected duff battery again and it is 12.57v. Not a lot of drop but as it was 12.8 when it came off charge I am suspecting it is duff.

Your thoughts please. If you too think it may be duff I will take it back and the shop can try a drop test.

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Old 11th October 2021, 10:20   #2
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I wouldn't say the voltage you have measured necessarily indicates a duff battery.

I would put a mild load on the battery for a while and recheck the voltage. An old 55W 12V headlamp bulb consumes nearly 5Amps and makes a good test load.
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Old 11th October 2021, 10:39   #3
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I wouldn't say the voltage you have measured necessarily indicates a duff battery.

I would put a mild load on the battery for a while and recheck the voltage. An old 55W 12V headlamp bulb consumes nearly 5Amps and makes a good test load.
At say 2 minutes, what would I expect to see voltage wise?

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Old 11th October 2021, 11:19   #4
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I would put the bulb on for 30 minutes. This will remove approximately 2.5Ah of charge from the battery which will have a capacity of around 70Ah for a 096 type when new.

You should get at least 12.5V which would indicate that the battery is at >90% fully charged.

If it drops to less than 12.5V, the battery is not as healthy as it should be.
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Old 9th November 2021, 23:34   #5
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What was the measurement or conclusion in the end?
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Old 10th November 2021, 08:08   #6
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What was the measurement or conclusion in the end?
Result, undecided about the battery. The voltage seemed not to drop any further, always staying above 12v. I am wondering if the fault is on the car. I kept a headlight bulb connected for well over an hour as suggested but the voltage stayed above 12.
I have just taken the battery off charge and it is showing 13.04v.

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Old 10th November 2021, 15:35   #7
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The reading off charge won't be accurate.--It needs to be left for a few hours to settle.

12.57 is normal for most batteries.---

The very best way to check a sealed batteries capacity is to have a drop test done at a local garage.

Oh for those filler caps where you could test the specific gravity of the acid.
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Old 10th November 2021, 17:08   #8
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The reading off charge won't be accurate.--It needs to be left for a few hours to settle.

12.57 is normal for most batteries.---

The very best way to check a sealed batteries capacity is to have a drop test done at a local garage.

Oh for those filler caps where you could test the specific gravity of the acid.
ok thanks, I'll test it again in the morning

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Old 10th November 2021, 19:48   #9
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An old way that the AA used to test batteries was to charge by running the engine for 5 mins , then disable the engine by pulling a fuse so that the car would crank but not start. Crank the engine for 15 seconds and listen to the cranking speed , and if it starts to drop , the battery is unserviceable . That is a live drop test and is pretty conclusive.
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Old 11th November 2021, 09:16   #10
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An old way that the AA used to test batteries was to charge by running the engine for 5 mins , then disable the engine by pulling a fuse so that the car would crank but not start. Crank the engine for 15 seconds and listen to the cranking speed , and if it starts to drop , the battery is unserviceable . That is a live drop test and is pretty conclusive.
I can try that. What fuse to remove, fuel pump?
15 seconds continuous cranking seems a long time, I just timed it my goodness that is a lot of cranking

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