My 18Kpv does not top balance, nor even fully charge, my battery bank.
This is a whole house backup system consisting of one 18Kpv and six LL-S in an EG4 rack. No PV. System includes a Chargeverter permanently connected to the rack bus with a 120VAC generator input.
Battery bank and inverter system has been installed for about a month now. I have performed two backup discharge tests, pulling the SOC to 20% and then restoring grid power and letting the inverter fully charge the bank. So all batteries have been cycled to 20% and back to 100% at least twice. In addition to that, looking at the data history it appears the inverter periodically enters a charge cycle and brings the battery bank back to 100%.
Recently I installed the Chargeverter and performed a charging test. Battery bank at the start of the test was at 98% SOC (five batteries at 100% and one at 88%, which itself is an issue for another post). Chargeverter was configured for a modest 26 A of charging current, because my use case is to use a small 1600 watt 120VAC generator only as a last resort for a grid outage lasting more than two days. I ran the Chargeverter until the battery voltage finally ramped up to 56.0 VDC (the set charging voltage). All batteries were charging equally at about 4A each. It took a total of six hours of charging at a constant ~24 A, 4 A at each battery, before the current started to slowly drop and the voltage began to slowly rise. That’s a total of 144 Ah, including 24 Ah into each battery which started at 100 Ah, and 8,064 Wh of charging energy, or 24% of the total capacity of this battery bank. It took two more hours of charging during the ramp up period before all six batteries ceased charging and entered standby. I estimate another 15 to 20 Ah went into the bank during that final two hours for a total charge of 164 Ah or 9,184 Wh, which is 30% of the capacity of this battery bank that was showing 98% SOC at the start of the charging test.
So clearly the 18Kpv has not been bringing these batteries to a full charge and has thus never reset the BMS 100% reference point to something closer to full. It appears the 18Kpv stops charging immediately upon seeing a 100% SOC reported by the battery BMS and does not continue applying a charge voltage past that point, so there’s no opportunity for performing an equalization charge or to top-balance the battery bank. Looking at the data history for an inverter charging cycle, I never see the vBat data parameter go higher than 54.5 before the inverter has stopped charging.
The inverter is operating in closed loop, and the charging control is set for SOC rather than voltage, as recommended in the manual. AC Charging is enabled, Power Backup is enabled, and Stop Charging setting is 100% SOC.
Questions
- What charging voltage is the 18Kpv using? There is no adjustable setting for this value and I can’t locate an absolute value anywhere in the manual or inverter specifications.
- What settings should be changed to cause the inverter to fully charge the battery bank? Should the charge control setting be changed from SOC to Voltage, despite the manual’s recommendation for closed loop operation to use SOC?
- If setting inverter to Volt charge control, are the control setting values for Start & Stop referenced to the vBat or the Vbat_Inv data parameter? For Start operation this isn’t a critical question since the two data values track close to each other during standby and discharge, but for Stop this is an important question. Note that vBat never reaches the charging voltage at the end of the charge cycle, while Vbat_Inv does. The difference is more than 0.5 volt.
- For the LL-S battery, how long should a top-balance charge voltage be applied when manually charging the battery? In other words, during charging, once the battery has stopped accepting charge current and has entered Standby, and the battery voltage has reached the charging voltage, how much longer should the charging voltage be applied if doing this manually using the Chargeverter in open loop? One hour? Stop charging immediately? Looking for a best-practice number here for battery health and longevity. Note that once the battery first enters Standby at the end of the charging cycle, it will then randomly alternate between Standby / Charging for another hour and continue accepting a significant amount of additional charging current. My question above is for after this period, when the battery enters and remains in Standby.
- What value should be used for the inverter Lithium Brand setting for the LL-S battery? The out of box value was 2, Signature Solar advised to set this to 1, and the 18Kpv manual states to use 0. It is currently set to 1.
- What value should be used for the System Charge SOC Limit setting? The out of box value was 101%, which I changed to 100% to match the Stop AC Charge SOC setting.
- What is the difference between the Power Backup setting (under the Application section) and the Battery Backup Mode setting under the Charge section? How are these two settings related to the “Start Battery Backup” function button on the monitor page?
- Does the LL-S BMS adjust and reset it's calculated SOC value over time? Does it move its 100% reference point upward whenever it measures additional charging current flowing into a battery that was already at 100%?
