Tweet
Last night I ran my batteries down all evening to give them a workout; they normally run in float with my grid-tied hybrid system. It is a 48V 410 Ah battery bank that consists of eight Rolls S6-460AGM batteries.They are AGM, so the only SOC indication I get during usage is the Outback FlexNet DC, which is a simple Coulomb counter that assumes a 97% charge efficiency.
I disconnected the batteries from everything at around 10 PM, with a 76% SOC reading from the FlexNet after 97 Ah had been pulled out of the batteries. That's about what you would expect: 24% of 410 Ah is 98.4 Ah.
On this cold morning, about 10 hours later, with the batteries at -12 degrees C, I measured the following voltages across each battery:
1: 6.17
2: 6.18
3: 6.21
4: 6.19
5: 6.17
6: 6.19
7: 6.15
8: 6.19
This averages out to 6.181 V.
According to this white paper from Discover Energy Corp., the Voc of a 100% full battery is 2.125V at -10 degrees C as compared to 2.150V. Rolls says that the Voc of their AGMs is between 6.24V and 6.31V, presumably at 25 degrees C. I scaled my average measurement up by 1.8% to get an adjusted average of 6.25V.
Thus it looks like my SOC may indeed have been 76% although probably more like 70%. The range of Voc for an SOC of 70% is between 6.21V and 6.28V, with 6.25V right in the middle.
That makes sense; another graph provided by Rolls says that the battery capacity last night was only about 72% of the room-temperature rating, or 295 Ah. So discharging 97 Ah would have left me with an SOC of 67%.
Disconnecting the battery from everything overnight is a pain, and you can't measure specific gravity in a sealed battery. So it's reassuring to see my Coulomb counter giving me a fairly decent indication of SOC.
Question: Is there anything to be concerned about with the slightly lower Voc of battery #7? These are brand new, in float service for about a month. This is only their second discharge down to around 75%.
I disconnected the batteries from everything at around 10 PM, with a 76% SOC reading from the FlexNet after 97 Ah had been pulled out of the batteries. That's about what you would expect: 24% of 410 Ah is 98.4 Ah.
On this cold morning, about 10 hours later, with the batteries at -12 degrees C, I measured the following voltages across each battery:
1: 6.17
2: 6.18
3: 6.21
4: 6.19
5: 6.17
6: 6.19
7: 6.15
8: 6.19
This averages out to 6.181 V.
According to this white paper from Discover Energy Corp., the Voc of a 100% full battery is 2.125V at -10 degrees C as compared to 2.150V. Rolls says that the Voc of their AGMs is between 6.24V and 6.31V, presumably at 25 degrees C. I scaled my average measurement up by 1.8% to get an adjusted average of 6.25V.
Thus it looks like my SOC may indeed have been 76% although probably more like 70%. The range of Voc for an SOC of 70% is between 6.21V and 6.28V, with 6.25V right in the middle.
That makes sense; another graph provided by Rolls says that the battery capacity last night was only about 72% of the room-temperature rating, or 295 Ah. So discharging 97 Ah would have left me with an SOC of 67%.
Disconnecting the battery from everything overnight is a pain, and you can't measure specific gravity in a sealed battery. So it's reassuring to see my Coulomb counter giving me a fairly decent indication of SOC.
Question: Is there anything to be concerned about with the slightly lower Voc of battery #7? These are brand new, in float service for about a month. This is only their second discharge down to around 75%.
Comment