My friends Peugeot van wouldn't start yesterday. This morning i tested the battery with my multimeter and it showed 12.9 V, but just a single click on turning they key.

Popped by starter pack on and it fired up, while running was showing 15v.

So why would it show good volts but just click? Would a knackered cell do that?

Thanks!

Check the earth lead, how old is the car ? Can the lead to the engine be corroded ? I-pace First Edition Santorini & 2nd car Mazda MX-30 Advantage red/black
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Car is a 2020 van, couldn’t see any corrosion but I will have a proper look tomorrow.

Thanks 👍

It could be the battery, corroded connectors (remember the battery pack goes on the outside, the vehicle battery goes on the inside) or the starter drawing more current than the battery can provide by itself.
Test 1 measure battery voltage at the battery clamps when attempting to crank.
Test 2 measure battery voltage at the battery posts when attempting to crank
Test 3 measure voltage between the -ve battery clamp & a good ground point on the engine when attempting to crank.

You may well see the issue if the weather is cold enough for the glow plugs to draw enough current to show a low voltage (<10.5VDC) on any of the above tests.

Great, thanks for the advice.