Battery buffer charging/maintenance

On a sunny day (Sat, 7 Aug 2010 17:49:18 +0200) it happened "Henrik Nowak" wrote in :

It is very simple, look up the battery specs. Manufacturers give a charge voltage and max current, sometimes different for different applications, like for example alarm system standby or full cycling. Sometimes there is a limit to the allowed charge time too. For example for cycle applications 14.4 to 14.7 V, current limted to a few amperes, until current drops below some (milliamps) value. And for standby (as in your case perhaps) 13.5 to 13.8 V. Again, look up the battery specs.

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It sounds like you can use an off-the-shelf DC/DC converter, with some isolation (simplest is two diodes) for the multiple inlets. UPS software can handle the shutdown, you'll want to detect battery drain (voltage drop on the battery's diode will do that) and generate some kind of interrupt or squirt an error code into a serial port.

There are a LOT of computers wired this way, in telco offices and in Google farms, I hear.

Look at this pico-ups from mini-box, might be just what you need (and a small SLA battery)

Cheers

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.

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.

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.

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.

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