Extending home UPS capacity - Page 1
Forum section: electronics / general

author: bitwelder
author_rank: Super Contributor
timestamp: October 10, 2022, 01:24:04 pm
content: 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.

author: tom66
author_rank: Super Contributor
timestamp: October 10, 2022, 01:32:19 pm
content: 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.

author: jonpaul
author_rank: Super Contributor
timestamp: October 10, 2022, 02:48:00 pm
content: UPS designed for short-term outage 30 s...15 minLonger, get nature gas/ propane generator5..15 KVA$3...12kJon

author: SmallCog
author_rank: Frequent Contributor
timestamp: October 11, 2022, 12:10:53 am
content: Keep an eye out for a more commercial sized UPSThe 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 hoursIt 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.

author: Red Squirrel
author_rank: Super Contributor
timestamp: October 11, 2022, 12:52:16 am
content: 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.

author: PaulAm
author_rank: Frequent Contributor
timestamp: October 11, 2022, 02:07:27 pm
content: Not sure where you're at, but used UPSes can be extremely cost effective. Many organizations just surplus them when the batteries die.I bought a small pallet of UPSes for around 100USD a few years back. One of them runs my server rack with a 45 minute run time.

author: Red Squirrel
author_rank: Super Contributor
timestamp: October 11, 2022, 02:22:34 pm
content: If you're lucky enough to come across a good local deal it works but typically the shipping is the issue with used UPSes. I've see $600+ shipping costs through ebay since they're so heavy and bulky.

author: BradC
author_rank: Super Contributor
timestamp: October 11, 2022, 02:30:51 pm
content: It depends. As previously mentioned UPS are often limited by thermal characteristics. If you significantly under-rate your UPS, and its firmware isn't stupid then it's doable.I have 2 UPS at home. Both APC SmartUPS. One 3rd Gen, and one 4th Gen. The 3rd Gen is a 2200VA and the 4th Gen is 3000VA. Both sit between 10-20% load, and both have ~420% of their rated capacity. I get between 9 & 13 hours from both of them and none overheat during a full discharge. Both have 17AH from the factory, and I add 55 AH to that.APC do an XL version with bigger transformers, heatsinks and fans for extended full load use. Right tool for the right job.First UPS I added an extra set of batteries to melted.