Need advice on a battery backup system

I want to build a simple battery backup to run a small chest freezer and a small refrigerator during power failures. Nothing fancy or automatic – I will manually unplug the units from the wall and plug into the inverter when needed. I understand the basics OK, capacities for batteries and inverter and charger. Deep cycle batteries seem to be called for, but I’m not concerned with having hundreds of charge/discharge cycles per year like with a solar panel system. If the system gets cycled three or four times a year that would be about what I expect.

So what I’m looking for is advice on brands. Who makes good inverters/batteries/chargers? And can I save some money on batteries if I don't need hundreds of cycles? And of course any other good advice that comes to mind, I’ll be glad to hear it.

I think there is a concern for proper ventilation of the batteries.

Do you know how many KW the two appliances use daily on average?

My 1st guess would be ~1 KW for each, or 2 KW total. Since a battery can't be drawn down more than half way without damaging it, that means if you want to run the appliances on batteries for 1 day you need 4 KW of storage. That is 4,000 Watts.

If you have a 24 volt inverter you will need at least 4,000/24 = 167 Amp hours of deep cycle battery storage at 24 volts.

When wired in series batteries maintain the same amp hours and their voltage sums, so if you can find 2 x 12V deep cycle batteries with ratings of 167 amp hours each (or more) and a 24 volt inverter you are all set, and you would just wire the batteries in series to the inverter and plug the appliances into the inverter.

For parallel wiring the voltage stays constant and the amperage goes up. So you can wire 4 x 12V batteries of 84 AH each in a series/parallel configuration to get 24V and 168 AH. 24V x 168 AH = 4,032 Watts, of which 2,016 are available to you daily without doing excessive damage to the batteries.

In general you can draw lead/acid batteries down half way roughly 300 times before they will show signs of needing to be replaced, and/or before they outright fail. When a 24V battery is half empty it will read 24.2 Volts after sitting idle with zero draw for about 4 hours. When fully charged it will read 25.4 to 25.5 volts when similarly left to idle for 4 hours with no use. If it reads less than 24.2 volts it has been drawn down below 50% and it is in the danger zone.

When freshly charged, a 24 volt battery will read 26+ volts (perhaps up to around 26.7 volts), but this is excess voltage that withers (vaporizes) away after about 4 hours of non use, as the 24V battery can only hold onto (or retain) a maximum of 25.46 volts when fully charged.

A charger needs to deliver at least 27 volts in order to overcome resistance and push electricity into a 24V battery. A 20 amp hour x 24V charger will take a minimum of ~4.17 hours to charge a 167 AH 24V battery that is half empty.

4.17 hours x 24V x 20 amps/hour = 2,002 Watts = 2KW

The ultimate deep cycle for just such a project would be the 6 volt golf cart batteries...most warehouse clubs have them at reasonable price.

At the very least your inverter will need to output 1,500 Watts, and even at that it may trip and fail if both refrigerating appliances compressors kick on at roughly the same time. To assure that the inverter never accidentally trips due to both compressors ramping up simultaneously get a 24V x 2,000 Watt model with a surge rating of 4,000 Watts.

Here's how I know. I have a 1,500 Watt rated inverter with a 3,000 Watt momentary surge rating, and when I had both my refrigerator and freezer (~1.0 to 1.2 KWH usage per average day for each) running on it I saw occasional trip failures that required an inverter reset. Perhaps 3 or 4 "trips" over the course of a year.

That plus my 1,100 Watt microwave is borderline with it and trips it often. It turns out that although its magnetron draws only 1,100 Watts, the rotating platter and digital display draw at least about another 400 to 500 Watts. I can go perhaps a minute at full power and get about 50% success. At less than full power i get closer to zero success. I think the magnetron always draws at 1,100 Watts, and it merely cycles on and off to simulate a less than full power output. This repetitive on/off trips my inverter.

franco b wrote: ↑

Thu. Apr. 02, 2020 8:15 pm

I think there is a concern for proper ventilation of the batteries.

I would like to use AGM batteries that supposedly have minimal venting.

lsayre wrote: ↑

Thu. Apr. 02, 2020 8:43 pm

Do you know how many KW the two appliances use daily on average?

Larry, thanks for all the calculations. They pretty much agree with mine. But I didn’t know all that stuff about battery voltage vs. discharge.

McGiever wrote: ↑

Thu. Apr. 02, 2020 10:15 pm

The ultimate deep cycle for just such a project would be the 6 volt golf cart batteries...most warehouse clubs have them at reasonable price.

I’ll check my local Sam’s Club. I didn’t realize they carried golf cart batteries. And maybe the Walmart tire & battery department.

Any recommendations on brand names for inverters? I can’t believe they are all created equal.

I forgot: When a 24 volt battery reads only 23 volts (actually 22.94) on your voltage meter it is empty (fully discharged).

rberq wrote: ↑

Fri. Apr. 03, 2020 9:06 am

Any recommendations on brand names for inverters? I can’t believe they are all created equal.

A number of the better brands trace their manufacturing origin back to Cotek. I.E., under the skin they are Cotek inverters. Search for "Cotek SP-2000-124".

You could also wire 4 x 12V & 84AH batteries in straight series for 48V, and get a 2,000 Watt 48V Inverter. "Cotek SP-2000-148"

48V x 84AH = 4,032 Watts of storage (of which 2,016 Watts are available to you)

Same price, and it requires wires of lower gauge, so you save money in wiring it.

Double everything I previously mentioned as to voltage and you will have the same for 48V. Such as: 48.4V = half discharged.

The greater the AH rating of the batteries, the longer it will keep your refrigerator and freezer running. 84 AH is for 24 hours of use.

Battery lessons are usually expensive lessons...

Series batteries preferred over any paralleled...hence the 6 volt preference being recommended as opposed to not using 12 volt, in those cases.
Paralleled batteries are problematic to performance and to be charged in balance and kept always in balance.

Sorry golf cart batteries do not come in AGM. However, standard lead acid batteries only off gas heavily during a heavy deep recharge cycle...slow and steady charge is not so much.

Big size inverter will never make up for under-sized battery...

BTW: 8 X 6volt series = 48volt.
if you build it they will come...

I know it was desired by the OP to be fully manual as for plugging and unplugging, but for others there are options that do exist...
There are couple ways to consider to go about the inverter...plain inverter, inverter/charger or inverter/charger w/ built-in auto transfer switch for load(s).

As for 2 appliance motor starts and kicking the inverter overload, a added time delay relay wired in can hold 2nd start-up off for a few seconds until the first has leveled off...

I thought about making my own auto transfer switch once (to run my boilers circulator) by utilizing a DPDT relay, but I'm not sure if it would be considered legal to use it.

lsayre wrote: ↑

Fri. Apr. 03, 2020 11:06 am

I thought about making my own auto transfer switch once (to run my boilers circulator) by utilizing a DPDT relay, but I'm not sure if it would be considered legal to use it.

DIY is fine if everything is designed, sized and fused proper so as to not burn your house down.

It does take a pretty good relay coil to remain "latched" and energized (read hot) 24/7 waiting for a random power outage...

McGiever wrote: ↑

Fri. Apr. 03, 2020 11:13 am

DIY is fine if everything is designed, sized and fused proper so as to not burn your house down.

Does my diagram appear correct as to wiring (see attached PDF above)? Should the relay be SS or old fashioned electro/mechanical?

lsayre wrote: ↑

Fri. Apr. 03, 2020 11:15 am

Does my diagram appear correct as to wiring (see attached PDF above)? Should the relay be SS or old fashioned electro/mechanical?

No need of solid-state relay (SSR).
Circuity is fine...typically the neutral is not switched, but wouldn't really matter.
But as I stated above hardware needs to be way more than just in the correct circuitry...

Relay is for "pilot duty" as in switching control circuits...Contactor or Starter is for "load switching" as in start and stop loads.
Contactor is a big relay, starter a contactor for high surge motor starts and incorporates over-load protection for the specific motor size in use.
Relays can start/control a contactor or starter.