[mythtv-users] Uninterruptible Power Supply

Simon Hobson linux at thehobsons.co.uk
Wed Dec 23 15:18:15 UTC 2009

Brian Wood wrote:

>  > I second this suggestion. Some APC UPSes will appear to be completely
>>  dead, just because the batteries are no longer accepting a charge.
>Some UPSes use the same batteries that are used in emergency Exit signs, these
>are readily available and relatively cheap. Others use hard-to-find and
>expensive batteries. Sorry I don't have model numbers handy.

Most will use some form of Sealed Lead Acid Battery - but often the 
'off the shelf' batteries are combined into proprietary packs. For 
example, many of the common small APC units use two 12V batteries 
assembled into a 24V pack with a connector attached.

Beware of just fitting a huge battery to a small UPS. Whilst large 
UPSs are often designed to have additional batteries added for longer 
runtimes, smaller units may not have the thermal management to cope 
with this - they are designed on the basis that the battery will run 
out in <some short time> and so may not be able to dissipate the heat 
for longer. They may be OK for longer runtimes if you derate them (eg 
only put 500W load on a 1000W unit) but you'd have to determine that 
for yourself.

Also, if you fit batteries that are very much larger than standard, 
the UPS might not detect end of charge properly due to the increased 
current taken by the bigger battery. A typical charge cycle goes :

* bulk charge at constant/limited current until the battery reaches a 
certain voltage. This will typically take the batteries to 80-90%.

* top off at constant voltage while monitoring the decreasing 
current. This will take the batteries to full charge and level the 
charge across cells by promoting a limited amount of gassing (must be 
very limited to avoid drying out a cell).

* when the current drops to a threshold, switch to a float mode where 
the battery is held at full charge but would not reach 100% (hence 
the previous step). The voltage is carefully controlled to avoid any 

I have seen situations where using too large a battery (for the size 
of charger) has prevented the charger reaching the full charge state 
and switching to float mode.

I've had some success at work in retrieving batteries that have gone 
bad in our own or customers UPSs. Often only one battery (or probably 
only one cell in a battery) in each pack is bad - so I've been able 
to recycle a few that are OK for other uses (and weigh in the bad 
ones at the scrapyard). Sadly it's likely to be a few years before we 
have to swap out the nice large (68AH AGM batteries IIRC) batteries 
in the big UPS in the server room :-(

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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