GXMnow
Solar Wizard
- Joined
- Jul 17, 2020
- Messages
- 2,673
I have read that paper about the degradation of cells. Most of the capacity loss in the testing occurs due to holding the cells at a high voltage. So there is a big trade off between stuffing in more energy and how many cycles you can get. Based on what I have read on the NMC cells I have my absorption voltage dialed down to 57.2 volts for the 14S pack, 4.08 volts per cell, and my CV time is short, just 10 minutes where the current falls from 25 amps to just 8 amps and terminates the charge cycle. The battery only sits at that state of charge for about an hour or two, depending on how long the solar output is running the house before it starts discharging again. I chose the voltage and the shorter absorb cycle to extend the number of cycles. The low charge current also helps battery life. The SEI growth graphs are "normalized" to the maximum growth for a full charge cycle. So it becomes a bit misleading. The RATIO looks worse at 1/2 charge when charging slow, but the total SEI growth when charged at the slower rate is actually less. So if you are doing a partial charge, the faster rate looks better, but for a full cycle, the slower rate is better. This becomes a big issue in a car when you do many partial cycles due to regen braking. This does not happen in a solar storage system. Here is the chart for the actual SEI layer growth over a full charge at the different rates.
Table I.
Percentage of SEI growth from DST driving cycle Charging Rate
Pinson and Safari and Ramadass,
Bazant Delacourt et al.
1C 6.32% 8.48% 7.54%
C/4 3.07% 6.09% 5.67%
C/8 1.62% 3.60% 3.31%
As you can easily see, in all driving cycles with different drivers, the total SEI layer growth is still far less at the lower charging rates. Almost 1/4 for Pinson and Bazant driving their cycles, and still less than half for the others. Still trying to figure out how well this data translates to a cycle a day low rate solar storage. Going from 50% to 90% at 0.1 C rate and only 10 minutes of absorb a day is being very gentle on the batteries. I could basically eliminate the absorb time, and it would not cost me any run time, but at these low rates, it does not seem to add much to the degradation. My system is at about 280 cycles already. What I am doing is about the same as very lightly driving a Chevy Bolt about 100-130 miles every day, and then charging it on a level 1 charger to just under 90%. That would be driving 42,000 miles a year. That is certainly a lot more than an average driver, but this would all be light, fairly steady cruising, and no regen or sudden acceleration. My largest burst current is only hitting 40 amps per string (80 amps total) where the Bolt can hit 500 amps under acceleration on a single string. The Bolt is 96S3P compared to my 14S6P. Yesterday, the absorb charge only put about 2 amp hours (about 150 watt hours) into the battery. That is really nothing. So I will try setting the absorb time to minimum and see if I notice any loss in my run time down to 51 volts. If it nets me a hundred more cycles, then great. At less than a year in, the cells are obviously still acting like brand new.
Table I.
Percentage of SEI growth from DST driving cycle Charging Rate
Pinson and Safari and Ramadass,
Bazant Delacourt et al.
1C 6.32% 8.48% 7.54%
C/4 3.07% 6.09% 5.67%
C/8 1.62% 3.60% 3.31%
As you can easily see, in all driving cycles with different drivers, the total SEI layer growth is still far less at the lower charging rates. Almost 1/4 for Pinson and Bazant driving their cycles, and still less than half for the others. Still trying to figure out how well this data translates to a cycle a day low rate solar storage. Going from 50% to 90% at 0.1 C rate and only 10 minutes of absorb a day is being very gentle on the batteries. I could basically eliminate the absorb time, and it would not cost me any run time, but at these low rates, it does not seem to add much to the degradation. My system is at about 280 cycles already. What I am doing is about the same as very lightly driving a Chevy Bolt about 100-130 miles every day, and then charging it on a level 1 charger to just under 90%. That would be driving 42,000 miles a year. That is certainly a lot more than an average driver, but this would all be light, fairly steady cruising, and no regen or sudden acceleration. My largest burst current is only hitting 40 amps per string (80 amps total) where the Bolt can hit 500 amps under acceleration on a single string. The Bolt is 96S3P compared to my 14S6P. Yesterday, the absorb charge only put about 2 amp hours (about 150 watt hours) into the battery. That is really nothing. So I will try setting the absorb time to minimum and see if I notice any loss in my run time down to 51 volts. If it nets me a hundred more cycles, then great. At less than a year in, the cells are obviously still acting like brand new.