LifePo4 and Return on Investment - Basic Calculations

[Solana]

Hi guys, just installed an hybrid solar system 2 weeks ago and trying to figure out the return on investment, lifespan, depth of discharge etc.

We live on a small tropical island that is run by diesel generators and we experience 4 to 5 power interruptions (1h to 4h) per week. So the battery is a needed backup. I drain it in baby steps currently every night to make sure it has enough charge left to get us through a grid power interruption during the night.

Meritsun LiFePO4 Battery 200Ah (10KWH)
Price: 200,000 Pesos
KWH Price: 11.23 Pesos
Operating Temperature: 30 to 35 Celsius (86F to 95F)
Humidity: 60 to 80%
Cycles: 6,000 (Manufacturer)
Warranty: 10 Years
Drain it daily to: 75% (SOC 25%) or use 7.5 KWH for easy calculation.

7.5 KWH x 11.23 Pesos = 84.225 Pesos

200,000 Pesos / 84.225 Pesos = 2375 Battery Cycles
2375 Battery Cycles / 30 days = 79 Months
79 Months / 12 Months = 6.58 Years

My conclusion: The battery paid for itself in 6 and a half years when drained down to 75% every day and it will have 3.625 Cycles left and 3.5 Years in warranty.

My big question is: Should I go easy on with cycling the battery and use it purely as a backup, or does it make more sense to use it heavily to get a quick ROI?

What are your thoughts and opinions?

Thanks
Solana

 

[sunshine_eggo]

My big question is: Should I go easy on with cycling the battery and use it purely as a backup, or does it make more sense to use it heavily to get a quick ROI?

I wouldn't hesitate to use it as much as possible down to 20% SoC.

My only concern would be to retain enough capacity to weather the 4 hour outages.

 

[GXMnow]

Your math looks pretty good. But from what I have read, the batteries see the most stress when charging up to the top. So I have only been charging my cells up to 90% state of charge, and then only running them down to 50% state of charge. The return on investment is a little slower, but the batteries should last longer. I have only been running my batteries 2 years, and they still measure like new, so I don't have any hard evidence yet. But on older tech lead acid batteries, they have proven that running twice as much battery can more than double the life of the system so it ends up cheaper in the long run before the cells need to be replaced. Your LFP cells already have a much longer cycle life, so using 75% may be just fine. My cells are only rated for 3,000 full cycles, I am hoping to stretch that out to over 5,000 40% cycles.

 

[Pim57]

To get the most cycles out of your LFP don’t charge fully up to 100%. If you have the possibility to adjust the BMS or charger, you can lower the OverVoltage or End Of Charge setting from 3.65 V to 3.45 V per cel.

That will charge the LFP to 98-99%.

 

[Solana]

My only concern would be to retain enough capacity to weather the 4 hour outages.

Luckily the inverter can control the discharge at certain levels and hours of the day, its a great feature that i wouldn't miss.

To get the most cycles out of your LFP don’t charge fully up to 100%. If you have the possibility to adjust the BMS or charger, you can lower the OverVoltage or End Of Charge setting from 3.65 V to 3.45 V per cel.

That will charge the LFP to 98-99%.

Thanks, will have a look in the settings in a bit if its possible.

I have only been running my batteries 2 years, and they still measure like new, so I don't have any hard evidence yet.

How to measure? Lets keep in touch if u want to exchange some info over the years.

 

[GXMnow]

How to measure? Lets keep in touch if u want to exchange some info over the years.

My Chevy Bolt cells are Li NMC chemistry. That does not hold near as flat of voltage as LFP cells do. This actually allows me to get a rough idea of the cells internal state of charge just from the voltage when the cells are at rest. So far, I have measured the resting cell voltage after a day of discharge running my house. I then compare that from the discharge charts for NMC cells and compare it with the amount the BMS says was discharged from the cells, and also the energy charge and discharge data from the Schneider XW-Pro inverter/charger. I know it is not perfect, but it does show me the cells have not lost much yet. To get a true battery capacity, I would need to fully cycle the cells at a fixed steady rate and measure the watt hours. I can split my battery bank and run on just half of it while testing the other half, so I may do this after a while to see how they are truly holding up. I certainly don't want to just throw away 18 kilowatt hours though, so I am thinking of actually having the inverter run the house off grid as a load test. If I do it with just one half of the battery bank, it will still take about a full day. Then I can switch back to the other battery bank to keep running the house until the sun comes back to recharge the first tested bank. I probably won't run a test like that until fall, when I don't need to run the central A/C again.

 

[AntronX]

Assuming you got solar and no net metering then yes. Your battery cost $1000/kWh. Your grid cost $0.57/kWh. 6000 partial cycles will normalize to 4050 "full" cycles. $1000/4050=0.247/kWh battery cost. Over 16 year period your battery will pay for itself and should save you $13,395 in grid cost assuming your solar if free. Keep it around 25C in cool airconditioned space and it should last you over 10 years. Heat shortens LFP battery life.