Using Batteries for EPS

[Not Aerosmith]

My batteries are not work horses. I live on the grid in Panama and there is no time of use penalty or savings. Electricity is expensive in Panama and more than most US states, so the solar power system has significantly reduced my electric bill. Currently, I seem to be producing more electricity than I am using. I imagined that will change when I buy an electric car.

We do have frequent power outages, usually < hour, usually not over 6 hours and probably only about 15 hours/month in total. Usually the longer power outages is where the power is shutdown during the day for maintenance from 9-4pm. My main reasons besides saving on utility cost is to prepare for an extended outage, since there seems to be so many events occurring around the world with hurricanes, tornados, tsunami, earthquakes, floods, fire, extreme heat, volcanos, storms, lightning, wars, riots, etc.

I have installed a whole house backup system using (2) Luxpower 12K hybrid inverters, (2) GSL 52V 14.3K 280Ah 16 cells powerwall batteries and 28.6K solar panels. I am not expecting 24/7 power, but I would like for my family to have some power during an event.

System was installed in June so I am still learning about batteries that were installed as an EPS (emergency power system). I found this chart for Lifepo4 or LFP batteries.

GSL battery requires DOD 80% max, so that is not an issue and probably would not occur more than once a month anyway without an event. Most power outages are less 30% DOD. The chart is based on 0.5C which for my battery is 140A and my batteries are rated at 150A continuous charge/discharge current. I have set my batteries at 100A or only 0.35C. I understand that a lower C reduces degradation from heat from charging and extends the battery life.

Luxpower inverters has a mandatory SOC=90 upper limit when on-grid power, so my batteries are discharging to 90%

During the day I can use Charge First and my solar panels will keep the batteries at full charge from about 8AM till 7PM. During the day my BMS floats the batteries from 100% to about 98%. After 5PM my solar production drops significantly, but it is enough to qualify as being not fully on-grid. Once fully on-grid my batteries will discharge to 90% and float at 90% until recharged in the morning by the solar panels to 100%. At night my EPS will have less charge, but should be adequate. This is acceptable to me, since batteries always fully charge can cause problems like cell balancing, electrons migration, etc., which over time can also cause battery degradation.

AT 8:54 we just loss power and my alarm is sounding. Not too bad, the grid was back in only 2 minutes.

According to the chart that I attached, DOD=80% increases number of cycle to 6500 cycles and DOD=100% reduces battery number of cycles to 3500 cycles. This graph is only an example and not for all LFP batteries, you need to get the proper graph for each battery. It only illustrates the concept that less DOD increases the number of cycles. My normal DOD would be >10% and <30%,

I do want to extend my battery life to the warranty period of 15 years, so that I have batteries should a disaster hit Panama. These are my first batteries, so I know nothing about batteries. What little knowledge I have is not from experience, but is limited to DIY members sharing their knowledge, internet research and yes, I have read the battery manual.

Have I missed something? I do tend to overthink things.

 

[DIYrich]

The number of cycles is Full Cycles (100%-0%-100%). Let's say you are going from 100%-30% every day, that is 70%, so you could get 3,500/.75=4,666 partial cycles, or 12.7 years.

Since the 100-90% range is not doing you any good, I would charge to 90%, and hold that. Once a week, charge to 100% to maintain top balance. Then you may get 5,000/.75=6,666 partial cycles or 18 years.

If you don't have time of use rates, then charge to 90%, hold that, and only go to battery when grid is down. I assume you have net metering.

 

[Not Aerosmith]

The number of cycles is Full Cycles (100%-0%-100%). Let's say you are going from 100%-30% every day, that is 70%, so you could get 3,500/.75=4,666 partial cycles, or 12.7 years.

Since the 100-90% range is not doing you any good, I would charge to 90%, and hold that. Once a week, charge to 100% to maintain top balance. Then you may get 5,000/.75=6,666 partial cycles or 18 years.

If you don't have time of use rates, then charge to 90%, hold that, and only go to battery when grid is down. I assume you have net metering.

I appreciate the response. Again I have no experience with solar batteries. The FPE graph in my post above was to demonstrate that >DOD reduces battery life. Since my batteries are only for EPS, I was just trying to verify that maintaining SOC=100% to SOC=88% is not going to damage my batteries. I understood that LFP batteries don't have memory where your capacity is reduces based on typical usage. I had read that LFP don't like to always be at 100% which can causes internal issues for the batteries.

GSL graph indicates DOD=90% for 3500 cycles and DOD=85% for 6000 cycles. DOD=80% is over 8500 cycles according to GSL graph. Does GSL graph for batteries changes your expectations for my battery life? I have included some of the specifications on batteries below.



I have Luxpower inverters and they are programmable for daily events not programmable for a weekly events. I am 77 years old, so I don't think in my absence that I can expect my family to maintain a weekly routine to balance the cells. I am just trying to get the correct setting for batteries being used as EPS and that I don't damage them nor shorten their lifespan.

Having the batteries at 100% during solar time does have a benefit in that most extended power outages seem to occur between 9-4PM when maintenance is being performed on the grid. When fully on-grid Luxpower limits batteries SOC to 90%. These parameters are the maximum settings, I have the option to use less, but would have less EPS power available.

 

[Not Aerosmith]

Deleted a post that was meant for another thread.