[Nut-upsdev] Megatec and Batteries

[Zeljko Baralic]

> -----Original Message-----
> From: Arjen de Korte [mailto:nut+devel at de-korte.org]
> Sent: 27. oktobar 2008 23:54
> To: Zeljko Baralic
> Cc: nut-upsdev at lists.alioth.debian.org
> Subject: Re: [Nut-upsdev] Megatec and Batteries
> 
> Citeren Zeljko Baralic <zeljko.baralic at pakom.com>:
> 
> >>      You can't calculate battery charge of a
> >>      UPS by looking at the battery voltage alone
> >>
> >> It physically just isn't possible.
> 
> In order to measure battery charge, knowing how much Watt hours it has
> delivered doesn't help much. We want to know the remaining charge
> (which is load independent), not the amount of power we already
> extracted (which is load dependent). Having said that, the efficiency
> of the energy conversion is unknown anyway (depending on size of the
> UPS) and so is the charge the battery can hold. Note that NUT won't
> remember state/values between restarts, so anything that isn't stored
> in the UPS, will be lost forever.

This is a good pint.

> 
> > What is good about megatec is that you can always get Load% and you
> > know rated power/voltage out of UPS.
> 
> Not at all. Quite a number of UPS'es will happily report a load of 0%
> (or any other fixed number) at all times. The protocol may be able to
> deal with this, but many implementations don't.

You are right. Here is the real life case: Fenton Lestar MD-800E and
Inform Guard Compact 800AP are 90% the same devices from some third
manufacturer of controller. In nut they both identify themselves as
VT02052N and report constantly 0% load no matter what is connected load.
 
> The power factor is dependent on the load and this isn't reported, so
> this value isn't available for most ordinary users. Therefore, I
> restate that it is impossible to calculate the battery current from
> the values provided by the megatec protocol and hence coulomb counting
> is not possible.
> 
> > Anyway Arjen, from my point of view creators of megatec protocol
> > where tried to make something in between contact-clousure and
> > inteligent signaling and I think that they succeded in this. They
> > give some minimal info about a state od device for disaster
> > protection.
> 
> Absolutely. I'm not trying to put down the makers. In most cases,
> knowing the battery charge is irrelevant, since NUT will only use the
> low battery warning signal for shutting down. But in some cases, where
> the remaining runtime on battery after signaling low battery is
> insufficient for an orderly shutdown of the connected systems for
> instance, this is not enough. In that case, a ballpark figure isn't
> good enough.

It looks like that this needs some deeper consideration.

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