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Extending home UPS capacity
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bitwelder:
I was thinking about extending my small UPS capacity (making it last longer during power outages) by adding extra batteries in parallel to the existing ones.
Would that generally possible, and if so, is there any special attention to take while connecting batteries in parallel, apart obviously using same technology and voltage? E.g. is there a need for balancing resistors?

If I remember correctly the UPS uses two sealed lead acid batteries 12V 6 (or 7?) Ah connected in series.
tom66:
The biggest issue is probably the thermal limit of the transistors and transformer.  Most UPSes use very small heatsinks for the power transistors, the logic being the time they are supporting the load will usually be short.  Example for an APC UPS:
https://surdu.me/assets/images/apc/bad-caps.jpg (not my image, found on the internet)

If you use them for a long time you may discover the FETs go pop shorting out your battery bank and blowing the fuse.

So at the very least I would suggest upgrading heatsink there and/or adding a fan.  The transformer is a harder limit to work with, it is probably built a bit like a microwave transformer build quality wise, in so far as it only needs to run for 10 minutes at a time so cheap insulation and thin gauge wire will be used.  Once the temperature gets too high and the insulation fails, game over.

Also typically the charge rate of the battery is not very good for consumer UPS, my one is for instance 10 watts when charging, which for a 7Ah battery takes roughly 8hrs to recharge.  If you increase that 10x you are looking at multiple days to recover charge for another  blackout.

A better option may be to buy a second hand server UPS and build your own battery array for it, or a true sinewave inverter; you could use your existing UPS to give you time to activate a manual or automatic changeover switch for such a capability.
jonpaul:
UPS designed for short-term outage 30 s...15 min

Longer, get nature gas/ propane generator

5..15 KVA

$3...12k

Jon
SmallCog:
Keep an eye out for a more commercial sized UPS

The model I use is now discontinued, but running a typical desktop PC with an LCD screen plus a modem (cellular/satellite/ADSL depending upon site) it will last for a few hours

It also has a socket on the back to attach additional batteries.

Given that the one I use is discontinued and we interface to it via serial modbus I've been contemplating making our own in-house. If we go ahead with it these will most likely be based on a bank of 105AHr 12V AGM batteries with a pure sinewave inverter continuously supplying the load and battery charger keeping the batteries full. Monitoring will mostly likely be an industrial grade datalogger of some sort that can take care of analogue measurements and talk modbus.
Red Squirrel:
One option is to look at an inverter-charger. It's basically like a UPS except it's designed for longer run time. You can connect a couple larger flooded acid batteries to it.   I have a setup like that for my servers with 3 12V RV batteries.  I had 4 but one battery died and instead of replacing it  I just decided to stick to 3. My goal is to upgrade that to a 48v dual conversion setup but now that I have solar I might look at just upgrading the solar batteries instead and setting up a transfer switch to transfer to solar. I eventually want to set something up like that so when I'm producing excess power it will switch to solar anyway to take advantage of that power.
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