Depth of Discharge under load

[Guest]

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.

[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

[Guest]

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

[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. .

[Guest]

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.

[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

[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.

[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.

[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

[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.

[Guest]

I see where the confusion is. I did not pay even close to $500 each per batteries. I payed closer to $100 per each 12v 90 ah batteries brand new. It was part of a liquidation. I also payed only $100 per each 325 watt panel, also new, part of the same liquidation. I guess my upfront costs are not as much as you would typically pay for the system.

10 x batteries $1000
8 x 325 watt panels $800
2 x MPPT Charge controllers $300
1 x 1000 watt Pure Sign wave inverter $100

Total $2200. I guess It's technically somewhere between 5 - 7 years, but during that time, if we have a prolonged power outage, I can still run the hot water baseboard furnace, and select loads, which was my objective, over simply purchasing a generator.

[Guest]

But I do now understand the DOD better thank you for explaining why i should not go any further. Would you suggest only using 30% of the battery or even less?

[SunEagle]

I see where the confusion is. I did not pay even close to $500 each per batteries. I payed closer to $100 per each 12v 90 ah batteries brand new. It was part of a liquidation. I also payed only $100 per each 325 watt panel, also new, part of the same liquidation. I guess my upfront costs are not as much as you would typically pay for the system.

10 x batteries $1000
8 x 325 watt panels $800
2 x MPPT Charge controllers $300
1 x 1000 watt Pure Sign wave inverter $100

Total $2200. I guess It's technically somewhere between 5 - 7 years, but during that time, if we have a prolonged power outage, I can still run the hot water baseboard furnace, and select loads, which was my objective, over simply purchasing a generator.

So you have a 24v 450Ah battery system using 10 - 12V batteries wired in 5 parallel pairs and you expect to get more than 5 years out of them. Good luck with that.

[Guest]

SunEagle, I'm not sure where all the negativity is coming from? I'm sorry if I've offended you. I'm just trying to ask a question and get an answer. Nothing I've said was meant to be negative. This has been a fun project for me creating an emergency power system, but using that solar power too. What would the expected lifespan of this battery setup be? If i'm doing something wrong, I'd like to fix it if possible before problems arise, or modify how I use it so it lasts longer.

Thanks in advance for any assistance,

[Sunking]

At current power generation I save about $300 - $400 on my electric bill. .

You must be smoking some really good stuff, or drinking really bad moonshine. You are not saving any money, Whatever you take off grid will cost you 5 to 10 times more than buying it from the POCO. In 5 years you will be on your 3rd set of batteries. Just in battery cost alone you are looking at $1 to $2 per Kwh used. so much for that 30-cents per KLwh the POCO charges you is a bargain compared to how deep th ebattery guy is going to get into your pockets. He will be able to take a nice weekend vacation off you.