Computers & Hardware > Networking

UPS recommendations

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Weaver:
Any recommendations for a cheap really fat (long runtime) UPS? Maximum runtime or is it J, or C, per £. Not going for max current delivery. Basic, no shutdown notification i/f required as it isn't for a server.

I'm sure I've asked about this before but just I can't find the thread. I was asked to try and evaluate the current draw, on which I failed. It will be three dsl modems, one low-power Firebrick router, one HP 24-port switch, one or two WAPs. Iirc we might have tried to do some guesswork.

Second issue: I'm wondering if I should partition the whole lot into two groups supplied by two UPSs. Instinct says 'no' because of unequal usage of the UPS. But I'm the other hand I could keep the modems alive longer than everything else so as to avoid upsetting DLM by letting the modems go offline. (If you believe in last-gasp awareness, then this is a non-issue.) Having multiple UPSs might be a desperate way of getting more runtime, but again it represents uneven usage. Also I don't know how the runtime vs current works, nor how the J / C / runtime vs £ economics work, whether getting twice as much runtime costs you more or less than twice as much money when purchasing a fatter UPS.

A really stupid idea (rule out, sanity-check). Would it make any sense at all to have one UPS driving two or more UPSs in turn? This daft cascade idea gets rid of the unequal usage thing but presumably the wastage due to double power conversion inefficiencies is stupid and just wastes the root UPS, or even worse, completely overloads it.

jelv:
I can remember the previous discussion but can't find it. I'm sure for devices which run on 12V DC we suggested running them direct from 12V leisure batteries (which can have over 100Amp/hour capacity) to avoid the conversion loss.

Increasing run time would be just adding more batteries in parallel (making sure the state of charge was equal before coupling them for the first time).

For non-12V devices you'd need an inverter to be able to plug their bricks in.

Weaver:
I'd be a bit nervous about putting batteries in parallel in case for some unknown reason they discharge at different rates - does that even make sense / is that even possible though in this configuration? After all you did say to equalise them to begin with.

And indeed, apart from the modems, the other kit needs mains, so there's nothing I can do about that. And I’m not up to diy as a healthy enthusiast might be. By power conversion I meant the generation of mains twice if we feed one ups from the output of another.

Can anyone give me any tips for models? And how to choose commercial UPS kit for maximum runtime per £.

nallar:
Parallel batteries have to stay at the same voltage (since they're in parallel...), so it's safe to put in parallel any batteries of the same chemistry even if they're of different capacities.

Series batteries have balancing problems, however for lead acid batteries in a UPS this isn't typically a problem as they spend most of their time at a "float charge" which will tend to even out the cell voltages. Lead acid cells can tolerate a slight overvoltage MUCH better than lithium ion/polymer, which will tend to vent and/or explode.

By default APC 3000VA UPSs come with four 12V lead acid batteries in series, each of which is composed of multiple cells already in series.

Weaver:
Thank god someone knows what they're talking about.

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