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From: Grant on 7 Aug 2010 18:05
On Sat, 7 Aug 2010 17:49:18 +0200, "Henrik Nowak" <hno(a)impc.dk> wrote:

>Hello,
>
>I seek advice on the following:
>
>We are designing a computer system based on a ETX computer and Windows
>Embedded.
>
>The power we get (15-30VDC) can disappear suddenly, so we thought about
>fitting our computer with a small 12V lead-battery and when input power is
>OK, we charge the battery, and when mainpower disappear, we use the battery
>to gracefully close files, and shut the system down. The ETX runs off a 5VDC
>so we can easily design a SMPS for 5V with a 8-40V input range, allowing for
>a seamless transition between mainpower and battery.

How much power, amps at 5V? Suggest you time the outage and only
shutdown system if it seems power is lost the greater than some short
time, assuming you're mains powered. Ignore this if you know each
power failure will likely be longer term, for example, user pulls
power to finish a session.
>
>So the battery is actually only used maybe once a day or something like that
>and only for about 2-3 minutes or so. So it never really gets fully
>discharged.
>
>We don't want to do all sorts of fancy charger circuit if it is not
>neccesary, but it seems that there is a lot of different opinoins about what
>and how to accomplish what we want. Ranging from full-featured BQ231-based
>chargers to a simple LM317 constant voltage solution.

Constant voltage is the easiest for standby float applications, if the
battery not taken too far into discharge. Add a boost option to bring
the battery back from deeper (longer) discharge.
>
>I have a feeling that the appropriate solutions lies somewhere in between,
>but I would like to have a few expert opinions on this as well. I dont want
>to damage the battery and I dont want to find out that it didn't work
>someday and the system crashed. So I want to be as kind and gentle to the
>battery as possible, but I don't want to more than what is neccesary. We
>expect to use a 12V 2AH small sealed lead-acid battery.

Trouble is how much power you want the battery to supply, most standby
UPS type applications woefully undersize the battery, and wreck the
battery after a few power outages.
>
>Preferably this should be a "no-maintenance" product, we dont want to do
>batterychanges every year or so. Expected lifetime for a unit is approx 5
>years.

For max battery life, don't exceed the battery's max current draw, and
size battery for expected outage time power draw at say 4C max (check
battery spec), charge at 1C max (check battery spec). For longer outages
you need to boost charge the battery (to higher voltage) when power comes
back on, implies some form of battery supervisory circuit or software
to time the outage thus depth of discharge.

Even boost charging SLA batteries takes some hours, allow for this when
designing worst case outage repeat capacity. Proper boost charging
relies on some memory of depth of charge.

Measure battery temperature and adjust charging and boost voltage.
If you want 5 years battery life it implies you properly design
battery charger, power circuit from the varying battery voltage,
that's two power converters. Using a separate battery charger
is an extra converter in either case.

Don't over-discharge the battery, so shutdown before the battery gets
too flat for reliable shutdown and repeat power failures.

Tradeoff then for the real design factors ;) How much for battery
replacement versus proper battery care?

One system I worked with used a 12V 1.2AH battery for a four hour
standby capacity, and the charger was a LM317 type circuit with a
small 3W lamp as battery ballast. Was a common circuit in the
'80s ;) Worked okay as a long power outage was rare, so the battery
was never deeply discharged.

If you undersize the battery, expect to take more care of it to
get a decent life out of the battery. Battery end of life is at
some defined reduced capacity level, not so much a complete
failure.

Grant.
>
>
>Thanking you all in advance for any comments and pointers.
>
>Best regards
>Henrik
>
From: Rich Grise on 11 Aug 2010 19:21
On Sat, 07 Aug 2010 17:49:18 +0200, Henrik Nowak wrote:

> We are designing a computer system based on a ETX computer and Windows
> Embedded.
>
> The power we get (15-30VDC) can disappear suddenly, so we thought about
> fitting our computer with a small 12V lead-battery and when input power is
> OK, we charge the battery, and when mainpower disappear, we use the battery
> to gracefully close files, and shut the system down. The ETX runs off a 5VDC
> so we can easily design a SMPS for 5V with a 8-40V input range, allowing for
> a seamless transition between mainpower and battery.
>
> So the battery is actually only used maybe once a day or something like that
> and only for about 2-3 minutes or so. So it never really gets fully
> discharged.
>
13.8V, constant voltage, with a current limit at .1C. Lead-acid batteries
can sit at a full float charge almost indefinitely. (think of your car.)
But still use deep-cycle batteries - evan SLA (sealed lead-acid) are happy
with a float charge.

Good Luck!
Rich

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