Are These Batteries Suitable for Backup Power?

[Mr onetwo]

I have a chance to buy some 12 volt 175 Ah AGM telecom batteries for $100 each and was looking for some advice.These are 6 years old and used in a powergrid backup facility by one of the utilities. I want to set up a 5kw bank to go with my Onan 4.5kw generator and an Aims 5000w 12v to 240 modified sine wave inverter. This inverter will only run my well pump. I plan on a much smaller pure sine wave inverter to run the rest of the house....probably 2000w. I have not done a test to find the total load yet. I want to add a few solar pv's next year. How many batteries should I get ?Better ways to do this?

 

[snoobler]

https://www.cdtechno.com/pdf/lit/12_1068_0812.pdf

Sure.

The concern becomes the capacity they have remaining after 6 years of standby use and the current you plan to draw from them.

5000/12 = 417A - a perfect example of the absurdity of a 5000W 12V inverter.

Each battery is 12*175 2.1kW - assuming they meet rated (they don't). Let's assume they're in spectacular shape at 75%, so about 1.5kWh each.

You'd need 4.

The question becomes how much of that 5kW inverter will you actually be using? Let's say 2500W.

2500W/12 = 208A.

Divided 4 ways, that's 52A/battery, which is about 0.4C at a reduced cap of 125Ah.

SWAGging from the chart, that's likely only 2 hours of run time.

 

[Hedges]

Aims 5000w 12v to 240 modified sine wave inverter. This inverter will only run my well pump.

There is a good chance that inexpensive high frequency "modified sine wave" (I call them "modified square wave") inverter won't start the well pump. If it does start, the harmonics at 180 Hz, 300 Hz, etc. will cause additional heating of the pump motor.

Better to get a higher quality low frequency pure sine wave inverter. It will weight between 100 and 200 pounds, cost $1500 to $5000, and will supply all your power needs, well pump and otherwise.

You need to get the specs of your well pump or measure running current and starting current. Running current is just measured with an ammeter. Starting current I measure with an AC current probe and oscilloscope.

From specs, multiply label watts or amps by 5x to get starting current. If you can't find that and can't measure starting current (which lasts 1/10th of second), measure running current with an ammeter and multiply by 10x (my guess.) That's how much surge current the inverter needs to be able to produce for at least one second. (not for less than 100 milliseconds)

The Sunny Island inverter I use may or may not start your pump, depending on how large it is. Sunny Island is 120V 6kW with 11kW surge for 5 seconds. Two of them produce 120/240V at double the power.

There are several other brands of inverter with similar capability.

Another thing about these better inverters is they have an AC input as well. Some can pass through power from your generator, add power from batteries as needed to start the pump, charge batteries while generator is running.

 

[snoobler]

Good point. I saw "AIMS" and just figured one of their LF behemoths.

 

[Trev2323]

https://www.cdtechno.com/pdf/lit/12_1068_0812.pdf

Sure.

The concern becomes the capacity they have remaining after 6 years of standby use and the current you plan to draw from them.

5000/12 = 417A - a perfect example of the absurdity of a 5000W 12V inverter.

Each battery is 12*175 2.1kW - assuming they meet rated (they don't). Let's assume they're in spectacular shape at 75%, so about 1.5kWh each.

You'd need 4.

The question becomes how much of that 5kW inverter will you actually be using? Let's say 2500W.

2500W/12 = 208A.

Divided 4 ways, that's 52A/battery, which is about 0.4C at a reduced cap of 125Ah.

SWAGging from the chart, that's likely only 2 hours of run time.

I didnt understand ANYTHING you said but I SURE am GLAD to have you here. THANK YOU for taking time and helping others like me out