Charging UPS batteries? - Page 1
Forum section: electronics / projects

author: iXod
author_rank: Frequent Contributor
timestamp: April 04, 2020, 04:54:02 pm
content: I’ve got a 1500w APC UPS that uses two series 12v gel-cell batteries.I also have a 1600w emergency generator that outputs 120 vac and 12 vdc.Is there a straightforward way (ie, that doesn’t involve rewinding the generator coils) that will allow me to use the 12 vdc gen output to charge these batteries? I am dedicating the 120v output to keep the refrigerator cold.Thanks.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 04, 2020, 05:19:19 pm
content: Plug in the UPS on the 120V. The UPS has it's own charger on board.Power is power, if you can't run both the fridge and the UPS on 120V you still can't if you move the UPS to the 12V outlet.But, 12V to 28V for charging the batteries can be done with a dc-dc boost converter. If you can't find a current limited one you also might need to add a resistor.There also is one other caveat. The UPS might not be able to run longer than the batteries are intended to last. This is a thermal limitation.So you may need to hack some forced cooling somewhere. In that regard I highly recommend using the AC out of the generator so the UPS goes into bypass again.

author: iXod
author_rank: Frequent Contributor
timestamp: April 04, 2020, 05:48:48 pm
content: This generator is an early consumer model about 20 years old. It's not rated (recommended) for electronics power. I've heard that a "conditioner" could make power better suited for electronics. That's really why I wanted to use the UPS as a conditioner, powering electronics from the generator, indirectly.Thanks.

author: Nusa
author_rank: Super Contributor
timestamp: April 04, 2020, 05:53:27 pm
content: Generally the 12V output on cheap generators is limited to 8 amps or so, and is often unregulated (that means the actual output voltage changes with generator speed, is likely electrically noisy, etc.). The point is, even if you can deal with dc-dc issues, you probably only have about 100 watts total available on that outlet. Not enough for the scenario you describe.It's best to use the UPS as designed. Which is to run your equipment long enough to allow for a clean shutdown if 120V isn't going to be available soon.On the other hand, the refrigerator is a cold bank. If the situation demands, you could get away with not running it at all for a several hours (if nobody opens it), then running it for a few hours to cool down. How long depends on the insulation of the unit in question and the thermal mass of its contents.If you have to choose, well, just live without your gaming machine for a while in such a situation. Putz around on your cell phone or laptop.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 04, 2020, 06:46:37 pm
content: Quote from: iXod on April 04, 2020, 05:48:48 pmA typical consumer UPS won't be a very good conditioner. When AC is present, i.e. when the generator is running, the UPS just switches the input through to the output. There will be a bit of noise filtering, but that's all. A more expensive UPS will always convert the incoming AC to DC and then run an inverter to recreate the AC. Units like that are described as "Dual Conversion" or "On-line".The other thing to consider is the vicious starting load that a fridge compressor puts on the power source. An ordinary fridge that has a one or two amp running load could easily draw 20 or 30 amps on startup. Neither a generator nor a UPS appreciate that sort of thing. Many will just collapse - maybe permanently!Ed

author: iXod
author_rank: Frequent Contributor
timestamp: April 04, 2020, 07:50:10 pm
content: No intent to run fridge from UPS, only direct from generator. It’s a small fridge—will test starting up.Intent was not to use UPS as conditioner for gen’s AC output. Rather to use gen’s 12 vdc to charge batteries, so the term “conditioner” was a euphemism...At least that was my initial intent...

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 04, 2020, 08:54:45 pm
content: If you're trying to make sure the UPS doesn't add to the inrush draw, just add a relay and on delay timer.

author: iXod
author_rank: Frequent Contributor
timestamp: April 04, 2020, 09:26:51 pm
content: No, it’s that this gen isn’t friendly to electronics, apparently, same as all cheap gens. I want to run electronics from UPS and somehow charge batteries safely with the gen.

author: trobbins
author_rank: Frequent Contributor
timestamp: April 04, 2020, 10:50:30 pm
content: Perhaps use a 12Vdc inverter connected to a standard charger designed for 24Vdc charging of VRLA batteries. The 12Vdc input inverter may be able to protect itself from disturbances and variations on the genset 12VDC supply. The charger should provide an acceptable float voltage for the UPS batteries, especially if the charger has an external temp sensor to locate with the batteries (as they may well be in a higher temp ambient). Hopefully the inverter output AC is compatible with the charger mains supply input. Don't expect a long service life (ie. longer than 5 yr) from UPS style new 12V 7Ah batteries, and don't leave them in a discharged state.