[mythtv-users] MythTV and power cuts

[Simon Hobson]

Rajil Saraswat wrote:

>Until now I have been using an
>APC ups along with excellent apcupsd software to switch off the
>machine on power failure and bring it back on when power restoration.
>Unfortunately, I have had to change the batteries every couple of
>months since they stop holding their power after a few cycles.

First off, what size UPS ? How big are it's 
batteries ? What is your load ? What's your 
environment like ?


As others have said, the batteries should last a 
lot longer than "a few cycles", but my experience 
with 'dry' batteries is also not good - if you 
think you have problems, at work we have a UPS 
with 1/4 ton of batteries that cost over £1000 to 
replace.

Larger batteries will last longer - both in 
runtime and cycles. Running a large load from 
small batteries is "very hard" on them, and as 
pointed out they will get hot doing it. Heat is a 
killer for these 'dry' batteries ... more on that 
below.
In addition to the heating, when you pull large 
discharge current, it's easier to over-discharge 
one cell which then fails. Charge and discharge 
rates are commonly expressed in units of "C" to 
get a size independent quantity. C is the nominal 
capacity of the battery, and for (say) a 10AHr 
battery 1A would be 0.1C (or C/10) and 20A would 
be 2C.
This allows manufacturers to provide data sheets 
that cover a range of sizes of the same battery 
just by expressing everything as multiples or 
fractions of C - the chemistry is the same, so a 
2AHr battery will behave the same as a 20AHr 
battery in the same range, so they only need one 
set of graphs.
If you study the datasheets, you'll find that 
effective capacity decreases with increasing 
discharge rate - if you discharge at 1/2C instead 
of 1C, you may find the discharge time might be 3 
times as long ! Another reason for using larger 
batteries.


Which leads on to ...

Do you have any control over turn-off point ? If 
the UPS runs the battery down to 10.5V (for a 12V 
block) then that's quite a deep discharge, and 
tough on the batteries (particularly if done at a 
high rate as mentioned above). If you can turn 
that cut-off point up to a higher voltage then it 
will help - at the expense of runtime.

More on temperature. A 'wet' battery can be 
topped up as it loses electrolyte (water) though 
electrolysis. Water is disassociated into 
hydrogen and oxygen and happens in all lead-acid 
batteries during both charge and discharge. For 
SLA ('dry') batteries, there is chemistry in the 
plates designed to promote recombination back 
into water - though there is a limit to this. Gas 
too much in a 'sealed' battery and it's relief 
valve will vent excess gas and you've permanently 
lost water. They start off with very little 
electrolyte (hence the incorrect but common use 
of the description "dry") which is help in thin 
sheets of glass fibre mat between the plates - in 
effect, it's like the water that's help in a damp 
dishcloth !

As an aside, 'sealed lead acid' (SLA) batteries 
are now called Valve Regulated (VRLA) which is a 
more accurate description as they have a valve 
which prevents excess pressure. In small 
batteries, it's nothing more than a rubber cap 
which stretches to let pressure out.

If you are in a hot climate (or just have the 
batteries in a hot location), then even standby 
life is reduced. By 40šC the standby life (held 
at float voltage, not cycled) can be as little as 
12 to 18 months. Deep cycle them and discharge 
them at high rate will make them fail very 
quickly.


So my suggestions would be :
Keep the batteries/UPS in a cool place.
Use larger batteries so the discharge rate is 
lower. Twice the size of battery will halve the 
discharge rate (when expressed in Cs).
Stop the discharge sooner. Sacrifice runtime and 
leave more in the batteries, you're less likely 
to kill individual cells by over-discharging them.

-- 
Simon Hobson

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