Recently, it have been discovered, that N900 kernel (or system? no idea) doesn't handle this capacitor battery properly, i.e. it never recharges it. That explains, why it fails after a year or so, on every N900.

I may not have most actual info, but when I last checked, freemangordon and Pali were to do something about it.

downside: it won't resurrect already dead batteries - to benefit from not-dying standby battery, one would need to replace faulty one (after fix in kernel gets introduced, or was it already?). They're cheap, and quite easy to buy, but still.

/Estel