Depth of Discharge under load

[SunEagle]

Just to keep the language strictly correct, what you mean is $1 per kWh delivered based on amortizing the cost over the battery life in cycles, whether the energy was originally generated by PV or taken from the grid.

You are correct. Thanks for the clarification.

[sdold]

How is discharging the batteries 40% raising the wear and tear on the batteries?

What you're doing is akin to driving a car on a trip and forgetting about the cost of filling the tank down the road. As the others have said, the number of cycles you'll get from a battery depends (among other things) on how deeply you discharge the battery in those cycles. Here's an example of a 12V 95AH battery that we use that costs around $500. The mfg expects about 800 cycles at 40% DOD, or 38AH. That times 800 cycles is 30,400 AH expected from the battery in its lifetime. That's 364.8 KWH or $1.37 per kWh. At only 10% DOD it's better at about $0.62 per kWh. And that's just the cost of the battery in ideal service, not cut short by temp or poor maintenance or putting them in parallel, and not including all of the other equipment you need to buy and maintain. Try to find data for your specific battery and see how it works out, but I bet you'll never see an ROI.

XE95 cycle chart.jpg

[inetdog]

I am not sure how you did the math to get an ROI of 5 years for those batteries.

Based on every calculation I have done on FLA type batteries (depending on the cost and quality) I get close to $1 for them to generate a kWh which IMO is much more than 30 cents/kWh.

Just to keep the language strictly correct, what you mean is $1 per kWh delivered based on amortizing the cost over the battery life in cycles, whether the energy was originally generated by PV or taken from the grid.

[SunEagle]

Sunkin, why would I raise the voltage? How is discharging the batteries 40% raising the wear and tear on the batteries? As far as throwing a lot of money away, The reason I did this, was instead of an emergency generator. At current power generation I save about $300 - $400 on my electric bill. I live in Massachusetts and pay 30 cents per kwh from the power company. It'll be paid off in 5 years or so.

I am not sure how you did the math to get an ROI of 5 years for those batteries.

Based on every calculation I have done on FLA type batteries (depending on the cost and quality) I get close to $1 for them to generate a kWh which IMO is much more than 30 cents/kWh.

[Mike90250]

Batteries have a cycle life and a calendar life

After 5,000 cycles (or whatever your spec is) they are worn out

After 10 years, they are dead, used or not

Sunkin, why would I raise the voltage? How is discharging the batteries 40% raising the wear and tear on the batteries? As far as throwing a lot of money away, The reason I did this, was instead of an emergency generator. At current power generation I save about $300 - $400 on my electric bill. I live in Massachusetts and pay 30 cents per kwh from the power company. It'll be paid off in 5 years or so.

[Sunking]

Raise the voltage, not lower it.

To be honest you really have no need for the solar, and throwing away a lot of money. Not to mention putting a lot of wear and tear on your batteries that need not be done. .

Understood. I want the system to last, so your advice sounds good. I'll leave it alone, and add battery capacity. Thanks for the advise

[Mike90250]

Because of Mr Perkert's law, https://en.wikipedia.org/wiki/Peukert%27s_law
measurements on a live system will always be less than optimal. You can come up with a closer estimate by making a trial run, with a known load on the inverter and when you reach the level you want, you set the LVD there. BUT what is a 50% discharge today , is next month a 55% discharge as the battery ages and capacity walks down. You will be discharging deeper and deeper, accelrating the death of the bank. Perhaps Li batteries won't age as fast, and they have less of a Peukert effect than lead acid

I've created an off grid Solar system which powers select loads.

The solar Panel's are connected to a Charge controller, which is connected to a 24v, 450ah battery bank. An inverter is connected directly to the battery bank, and a relay is on the load output on the charge controller, so when the charge controller goes into low voltage disconnect, the inverter shuts off. There's an automatic transfer switch that puts the load back on the Grid once in LVD. There's an online double conversion UPS, between the inverter and the load. This setup seems to work great. When the Inverter cut's out the UPS steps in for the few seconds for the relay to switch.

The charge controller LVD is set to 22.1v. The average load is 300 - 400 watts. I've noticed under this load, after the batteries have normalized for a couple hours, they're at 24.48v or about 60%. Can/Should I try lowering the LVD? so it discharges down to 30% or 40%? I'm only seeing about 1.5 - 2.0 KWH of power drained from the battery until LVD.

Thanks in advance.