Appliance Battery Backup
June 13, 2022 11:30 AM Subscribe
The local utility has been sending out scare messages about power availability this summer. I have the electrical guts of a camper van that was sold earlier this year. If I wanted to make sure my deep freezer always stayed online through rolling blackouts, could I connect the huge 12v battery to the inverter, leave the smart charger always connected to the battery, then just run the freezer off of the inverter all the time?
The battery is a deep-cycle 255Ah AGM, the inverter is only rated at 300W (360W max, 700W momentary peak) but the freezer is rated at 1.8A on 115V, so 207W. The charger is a fancy charger/long-term storage maintainer/12V power supply, so it will know what to do.
Other than the inefficiency of the inverter, this should work, right? The reason I want to do it is so that if there was an outage while I was in the office, the freezer wouldn't start warming up and then have to work harder off the battery to stay cool.
The battery is a deep-cycle 255Ah AGM, the inverter is only rated at 300W (360W max, 700W momentary peak) but the freezer is rated at 1.8A on 115V, so 207W. The charger is a fancy charger/long-term storage maintainer/12V power supply, so it will know what to do.
Other than the inefficiency of the inverter, this should work, right? The reason I want to do it is so that if there was an outage while I was in the office, the freezer wouldn't start warming up and then have to work harder off the battery to stay cool.
Response by poster: Sorry, forgot to mention the charger is rated at 15A at 120V. We sized it to charge the van's house system up after a weekend of boondocking.
I'll have to see if I can dig up my old Kil-A-Watt to check the startup draw...
On Edit, just found out that 15A is the 12V current into the battery, and NOT the 1800W I assumed off of the AC wall circuit.
posted by hwyengr at 11:46 AM on June 13, 2022
I'll have to see if I can dig up my old Kil-A-Watt to check the startup draw...
On Edit, just found out that 15A is the 12V current into the battery, and NOT the 1800W I assumed off of the AC wall circuit.
posted by hwyengr at 11:46 AM on June 13, 2022
Well, if your battery is 8D sized (i.e. weighs 100+ pounds) it MAY have enough energy to run a deep/chest freezer.
The way I calculate it is I looked up standard chest freezer (10-16 cubic feet) and figure I got is 1.5 - 3.5 amps running 4-8.5 amps starting, consumes about 350 kw a year.
According to DIYSolarshack, 100 Ah deep cycle battery can run a 630KWh/year fridge for 13.3 hours. Scaling the load, rough guesstimate is about 48-60 hours, 80% discharge. but to NOT discharge battery beyond 50% no more than 36 hours. I'm just guesstimating.
I am more worried about your inverter being able to take the load.
posted by kschang at 1:00 PM on June 13, 2022 [3 favorites]
The way I calculate it is I looked up standard chest freezer (10-16 cubic feet) and figure I got is 1.5 - 3.5 amps running 4-8.5 amps starting, consumes about 350 kw a year.
According to DIYSolarshack, 100 Ah deep cycle battery can run a 630KWh/year fridge for 13.3 hours. Scaling the load, rough guesstimate is about 48-60 hours, 80% discharge. but to NOT discharge battery beyond 50% no more than 36 hours. I'm just guesstimating.
I am more worried about your inverter being able to take the load.
posted by kschang at 1:00 PM on June 13, 2022 [3 favorites]
I'm reminded of the old Dan's Data DIY UPS article, which might be worth a read.
posted by offog at 1:52 PM on June 13, 2022
posted by offog at 1:52 PM on June 13, 2022
I'dd add solar panels to this kind of setup, because using a gas engine is so inefficient. Panels have come down in price. Make sure the inverter can handle the maximum draw of the fridge, the electric compressor motor probably has a high starting load. In an outage, open the fridge maybe 2 - 3 times a day, as opposed to several times an hour. Cover it with quilts. I think you'd keep your food safe, and get some ice, and be less miserable in an outage. In Maine, in mid-80s weather, my fridge was fine for 24 hours. While I was out getting dry ice, the power came back.
If an outage is expected, turn the freezer to its coldest setting and fill fridge and freezer with bottles of water. I keep my fridge and freezer full of (recycled) plastic containers of water because it's more efficient anyway. also, I always have a cold pack for a cooler, or nice cool water, etc.
Also, you may want to use some of that power to run a fan to make life bearable.
posted by theora55 at 1:56 PM on June 13, 2022
If an outage is expected, turn the freezer to its coldest setting and fill fridge and freezer with bottles of water. I keep my fridge and freezer full of (recycled) plastic containers of water because it's more efficient anyway. also, I always have a cold pack for a cooler, or nice cool water, etc.
Also, you may want to use some of that power to run a fan to make life bearable.
posted by theora55 at 1:56 PM on June 13, 2022
FYI, Prius owners can get a kit to use the Prius as an emergency generator.
posted by theora55 at 1:57 PM on June 13, 2022
posted by theora55 at 1:57 PM on June 13, 2022
FWIW, a lot of cold packs are going out in those meal kits, or in some food deliveries. IIRC, those are sink-safe corn- slurry based, but if you got some lying around... you can use them to fill the gaps in the fridge or freezer.
As long as you don't open the freezer / fridge the temp will keep for a LONG time. I think I've seen references that say you only need to power up a fridge for 1 hour every FOUR hours if you don't open the door to keep the food safe, but obviously that's a rule of thumb to be broken all the time. :D
posted by kschang at 2:42 PM on June 13, 2022 [1 favorite]
As long as you don't open the freezer / fridge the temp will keep for a LONG time. I think I've seen references that say you only need to power up a fridge for 1 hour every FOUR hours if you don't open the door to keep the food safe, but obviously that's a rule of thumb to be broken all the time. :D
posted by kschang at 2:42 PM on June 13, 2022 [1 favorite]
Interestingly, I recently profiled the power usage of my smallish 4-drawer freezer. This particular freezer is only drawing power around ¼ of the time, using roughly 60W, in a 5min on 15min off pattern. The problem, as AzraelBrown said, is the initial spike each time is turns on — over 1000W. This setup would keep pace with the overall power usage quite handily, but the spike would not go down well at all.
posted by breakfast burrito at 4:12 PM on June 13, 2022
posted by breakfast burrito at 4:12 PM on June 13, 2022
Even if it worked, your inverter isn't designed for continuous duty and will die after not long, taking down your freezer. Buy a UPS if you want to do this exactly this way, but I keep an inverter and power it up only when the power goes out.
posted by flimflam at 7:08 PM on June 13, 2022
posted by flimflam at 7:08 PM on June 13, 2022
If power outages are relatively rare, you might want to consider buying some used telecom AGM batteries. These are batteries that ATT and other companies purchase, install in their data centers, and almost never get used. They rotate them out after a few years, and you can often buy them used at great discount. A few years ago I purhcased 2 x 260AH 12V batteries for about $100 each, to serve as a backup for my server rack.
posted by soylent00FF00 at 7:10 PM on June 13, 2022
posted by soylent00FF00 at 7:10 PM on June 13, 2022
Other than the inefficiency of the inverter, this should work, right? The reason I want to do it is so that if there was an outage while I was in the office, the freezer wouldn't start warming up and then have to work harder off the battery to stay cool.
Freezers are relatively well insulated; your freezer will certainly stay frozen during the eight or so hours you are away at work, so it wouldn't be a problem to just keep the battery on the charger and only connect it to the freezer if needed. The freezer will run a little longer to cool back down, but it won't work harder overall (in fact, it will work slightly less hard) and it won't draw a larger current.
What you're proposing will actually have two inefficiencies, one for the charger (AC to DC) and another for the inverter (DC back to AC). Not the end of the world, but definitely into significant territory.
If you want something that does exactly what you want, a UPS does exactly this. You could attach your freezer to a UPS with a suitable current capacity to meet the start up draw of your freezer, then switch over to the larger battery once you get home if needed.
posted by ssg at 7:12 PM on June 13, 2022 [1 favorite]
Freezers are relatively well insulated; your freezer will certainly stay frozen during the eight or so hours you are away at work, so it wouldn't be a problem to just keep the battery on the charger and only connect it to the freezer if needed. The freezer will run a little longer to cool back down, but it won't work harder overall (in fact, it will work slightly less hard) and it won't draw a larger current.
What you're proposing will actually have two inefficiencies, one for the charger (AC to DC) and another for the inverter (DC back to AC). Not the end of the world, but definitely into significant territory.
If you want something that does exactly what you want, a UPS does exactly this. You could attach your freezer to a UPS with a suitable current capacity to meet the start up draw of your freezer, then switch over to the larger battery once you get home if needed.
posted by ssg at 7:12 PM on June 13, 2022 [1 favorite]
This thread is closed to new comments.
But -- I'm also concerned you're going to "pop" something in either the inverter or the charger, because most freezer/fridge compressors draw a HUGE amount of current when they kick in -- I'm not sure the inverter's 700W is high enough for the two seconds that the compressor needs to fire up, or that the charger can meet that demand either.
However: freezers, as long as you don't open them, are very efficient in keeping their insides below the temperature danger zone; ice is also a good store of 'cold', so if your existing freezer isn't packed full, adding a couple bags of ice to help weather some power outages may help.
Edit: They do make UPS-es for fridges for medical reasons, but they're about as expensive as a generator and probably won't run all day while you're at work. That scale, too -- a 12v car battery probably isn't going to hold that much energy to keep the freezer going all day either.
posted by AzraelBrown at 11:42 AM on June 13, 2022