[Nut-upsdev] Megatec and Batteries

[Zeljko Baralic]

> From: Arjen de Korte [mailto:nut+devel at de-korte.org]
> Subject: Re: [Nut-upsdev] Megatec and Batteries
> 
> Citeren Zeljko Baralic <zeljko.baralic at pakom.com>:
> 
> > This gets me to megatec driver and how it see battery voltage. I
> > think that this is job for someone to improve megatec driver in
> > order to look at some table to see battery charged value or to
> > create it with parameters to driver.
> 
> Thanks for this information. We know that the way the battery charge
> is calculated in the megatec driver is crude. For your information,
> there has been a lot of debate about this at the time this was added
> to the sources. The consensus was, that a crude way of determining the
> battery charge was better than nothing at all.
> 
> > Since I think I am becoming boring if you are interested how to
> > determine charge level I can continue writing... but this involves
> > real time clock inside of driver, some counters... lots of work for
> > someone.
> 
> Not at all. I have written countless times before already (maybe it is
> time to turn this into a FAQ entry)
> 
>      You can't calculate battery charge of a
>      UPS by looking at the battery voltage alone
> 
> It physically just isn't possible.

But we may get some very close values with accuracy of maybe 90%.

> 
> Although in principle it would be possible to get a reasonably
> accurate figure if there would be no current going into or out of the
> battery (also known as the open cell voltage), this is impractical in
> a real life UPS application. You can't just switch off the load, wait
> for a while to redistribute the chemicals inside the batteries and then
> measure the open cell voltage.

You can never get actual health state of battery by measuring just open cell voltage because some bad batteries can suffer from voltage memorizing effect. Known battery load must be attached to it when measuring its voltage.

> 
> There are quite a couple of UPS'es out in the field that do indicate
> battery charge in a fairly accurate manner, but these will invariably
> calculate charge remaining in a totally different way. By periodically
> discharging the batteries, it is possible to determine the total
> charge the battery can hold. From that moment on, all you have to do
> is to do some book keeping on how much charge (Ampère x seconds) is
> going in or out and you have a very accurate figure.
> 
Periodical discharge is recommended by some battery manufacturers because it influences on redistribution of chemicals in positive means.

> The problem with the megatec protocol is, that there is no way to
> determine the battery current by the driver. Not through a direct
> method (the megatec protocol simply doesn't report this) and also not
> via an indirect method. Although the battery charge calculation in the
> megatec driver might be improved somewhat, it will never reach the
> level where it will be nearly as accurate as coulomb counting. If
> accurate battery charge figures are important to you (in some
> applications, it is), you just shouldn't by a megatec protocol UPS.

When UPS operates on battery, this energy is mostly used for Inverter (part that creates OUT when there is no IN power to UPS) than for supplying internal logic of UPS and also some goes to thermal loses of conversion...
What is good about megatec is that you can always get Load% and you know rated power/voltage out of UPS.

This means (simple example): UPS load 50% on 220V and max out power is 1000VA you have 1000VA * 0.5 = 500VA. You can say that this is energy from batteries. If bat bank is 24V nominated and you have read with megatec 24.4V than divide 500VA with 24.4 you will get 20.5A current from batteries. You will have more accurate value if 1000VA * PF of 0,6 = 600W than * Load% 0.50 = 300W than add 10% on internal looses you will get cca 330W divided by 24.4V gives some current 13.55A from batteries. Since there is a big difference I will check in my lab which is more accurate and give a post.

> 
> Best regards, Arjen
> --
> Please keep list traffic on the list

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.

Best regards,
Zeljko Baralic