Pismo Battery Mystery

despite being fully charged, the battery only shows one light.
Jonny Stephens wrote on :

I've just bought a Pismo in beautiful, hardly used, condition, but there's one minor anomaly: despite being fully charged, the battery only shows one light.

When running from battery the control strip displays more time left than it should, 12 hours at full charge. It seems to be holding a fair amount of juice though and can comfortably run a full-length DVD movie in fullscreen.

When connected to the power supply and fully charged, the lightning bolt occasionally appears in the battery menu item for a few seconds.

The manual suggests doing a full battery discharge to cure unusual behaviour but it's had no effect in this case.

Have also tried PMU reset.

Anyone seen this before? (OS 9.2.1)

Jonny