Hi. Does anyone here know how to calculate approximately how much power an online/double-converting/sine wave UPS consumes from the mains? I guess that, if it has to keep N 12V batteries charged, the most power it can consume is (a bit higher than, depending on efficiency) N*12V*c, where c is the maximum charging current. Am I missing anything?
(For my 1KVA online UPS, which has 3 65AH/12V batteries and charges them at up to 6A, I speculate that the maximum power used is 6*3*12=216W, and when the charger is "floating" to keep the batteries charged, the power use is pretty much negligible.) Second, can anyone comment on the advisability of charging SLA (sealed lead acid) batteries at currents of >=0.1C (i.e. 10% of their rated Ah capacity or more)? My UPS vendor says they've had problems with current that high, and the battery FAQ says <0.1C for SLAs, but battery vendors seem to recommend 10-20% even for SLAs. Does this differ significantly for VRLA batteries? Third, can anyone explain in detail the internals of a constant-voltage battery charger? I understand the basic idea: provide as much current as needed to maintain the voltage across the terminals at the recommended charging level, but I don't understand enough about how batteries work to figure out what happens when you apply a higher current, or how the voltage is measured or maintained. Also, does anyone know how much I should expect to spend on a basic DC ammeter (the nifty clamp-on hall effect ones, not an in-circuit one), preferably in India? -- ams
