[Nut-upsdev] REGRESSION: New "megatec" driver does't work for UPS that was managed by old fentonups driver

[Arjen de Korte]

>> It may appear to be working for you, but only now. As soon as load,
>> temperature and/or battery age (!) change, the calculation will be off
>> (and more often than not by a landslide). The only way to accurately
>> measure the amount of charge in battery [...]
> I get the point, but actually don't care about measuring anything
> "accurately" but I'm interested in measuring something "roughly" (even
> "very roughly").

No, you don't get the point.

Like I explained, what you're doing now (or with any other scheme that is
based on battery voltage alone) may seem to work now, but may (will) fail
horribly if any of the above mentioned conditions change. What we
certainly don't want in NUT anymore, is giving people the false impression
that the value reported is a measurement value and not a guess made by the
driver. Granted, we can't guarantee that a UPS is doing these measurements
in a correct manner, but the least we can do is making sure that the
conversions we do have a sound basis. And as explained now (and many times
before on this list), this is not the case for voltage based battery
charge estimates.

In NUT we have 'upssched' for situations where the grace period after
signalling low battery is too short for properly shutting down a server
(see the FAQ and 'man 8 upssched'). This won't deal with (many) repeated
power failures in short succession, but neither does a 'guesstimate' of
the capacity left. This would let you ride through short power
interruptions, while allowing to reliably shutdown the load for longer
outages.

Best regards, Arjen