Testing the battery

[WhatNext?]

looking on you tube, the instructions says leave headlights on for 2 minutes before using a Voltmeter set to 20V to test the battery

Wound the same apply to an SPA car? surely when I open the car door again, it will trigger a load as the accessories become live?

What would you recommend

[gaby]

To test a (fully loaded) battery, a significant load has to be applied.
While the load is connected, voltage should not drop below a given voltage.
Afer the load is removed, voltage should return to or above a given minimum.
The specific voltages depend on the technology of the battery and the temperature
at which the test is performed.
I don't think that turning the headlights on for a few minutes is a significant load.

[Clan]

Originally Posted by Bloke69

looking on you tube, the instructions says leave headlights on for 2 minutes before using a Voltmeter set to 20V to test the battery

Wound the same apply to an SPA car? surely when I open the car door again, it will trigger a load as the accessories become live?

What would you recommend

There is a built in battery test in the car which the Volvo VIDA diagnostic tools can activate , It works with the considerable data which the battery monitor collects over a period of usage and uses a 20A load when the lights and ignition are on .

[WhatNext?]

Thanks

but for simple folks, can I just use a Voltmeter as above to measure the Voltage?

Would an OBD II reader be another way?

[cheshired5]

Originally Posted by Bloke69

can I just use a Voltmeter as above to measure the Voltage?

Yes. My video here shows you what to look for.

Originally Posted by Bloke69

Would an OBD II reader be another way?

It's possible but it isn't strictly accurate as it shows the voltage at the OBD port as opposed to the actual battery voltage.
If you want to test a battery, test at the battery + and - posts.
If you want to test alternator output, test at the alternator b+ post and alternator casing.

[Lexman8]

Here's a table for State of Charge readings when open circuit (ie no load on the battery). I've found it reasonably accurate when measuring across the battery terminals.

Note that the measurements should be taken some time after the car was last used (and therefore the battery was charged) because of surface charge effect.
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