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Voltmeter


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This may be an extension of my thread on choosing batteries - not sure.

Anyway when running previously with, let's call it battery 1, as main battery and battery 2 as radio battery my voltmeter read about 13 amps but dipped when headlights, wipers and fans were on. This led me to think that the voltmeter was giving me a reading on the surplus output from the alternator.

Today with battery number 2 as main, and no secondary (awaiting new one), the reading sat hard on 12v.

This made me wonder whether the voltmeter is giving me the voltage across the two poles of the battery rather than looking at the charging voltage???

If so, then doesn't this suggest battery 2 is in poorer condition than battery one???

Sorry my understanding is so shaky but never really had any need to think about voltmeter so long as it sat over 12v.

Can anyone enlighten me please?

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A charged battery should sit at about 12.5 volts without any load.

A dead battery will not be able to get over 12 volts

11.5v is considered run down.

With the engine running the battery should sit at 13.5 volts. This is beacause you are reading the voltage of the alternator not the battery.

As soon as you turn the engine off the battery will hover around 13v for a while but if you turn the lights on it wil dip to about 12.3 volts and turn the lights off again and it will rise back to its charged state of around 12.5 volts.

It is always the same for a good battery.

A bad battery may charge to 12.5v but put it under load and it will drop below 12 volts and not recover when the load it taken off, ie not go back up to 12.5v

If over night your battery drops to under 12volts with no load then bin it.

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When you did each test did you have the engine running.

If the engine is running you are in no way measuring the battery.

Only with the engine off can you assess it.

Batteries can read good voltages with no load but the moment you add a load they collapse.

The only way to really assess the battery codition is to use it for a while and see how it performs and to which voltage it settles at after doing some work some time (24 hours) after a good charge.

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