Comments on: Custom UPS External Battery Bank

By: Dan

Dan — Wed, 16 May 2012 18:19:24 +0000

Ah, I didn’t know the rack mount external batteries were twice as large. However, as you note, it seems to only affect runtime calculations and tests since I did this install show that I can run longer than it expects anyway. I imagine this is due to the relative load (in C) on the batteries causing longer runtimes and effective AHr ratings.

I don’t know what the charge current is, and I haven’t actually measured how long it takes to bring the batteries back up after a deep discharge. I would expect that APC only shoots for a 12 or 24 hour cycle time with extended batteries, although I haven’t looked. In practice, my commercial power is extremely stable and I almost never need to run on batteries. I live right near a major Intel site and the local power company does not like to credit them for downtime!

I’d be very interested to hear how your usage shifting goes long-term. I’ve seen other folks do it with dedicated 100% duty cycle inverters, but I tend to wonder how well the APC will stand up to continuous and repeated use like that.

By: Martin

Martin — Wed, 16 May 2012 18:09:59 +0000

In case you weren't aware, the external rackmount battery packs normally contain twice as many batteries as the UPS itself (whereas the non-rackmount pack contains the same as the UPS). In most cases for each external rackmount pack you'd tell the UPS that it has two external packs. However, according to the APC literature that's not the case with these 3U units with the twin connection at the rear, and you just tell it how many actual packs it has. The has 1728 volt-amp-hours of capacity (so that's 36Ah at 48 volts). With your 100Ah batteries I'd recommend telling it that it has three packs, although as far as I know that only adjusts the runtime calculations. Do you know how much current your 3000RMXL's charger puts out? I've just acquired a SU2200RMXLI3U with dead batteries and am looking at the feasibility of using it to shift most of my electricity usage to the cheaper night rate (charge it overnight, use it during the day). I've also got a (12kVA transformerless) but that needs 46 12V batteries in series, I don't fancy rigging a string of deep-cycle batteries up to that!