Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
Good Day everyone,
This is a topic that seems to be generally neglected by not being mentioned or even considered in many posts. This is unfortunate because components such as Solar Charger Controllers, Inverter/Chargers need to "know" the precise voltages being dealt with. With Lead Acid / AGM batteries there is a bit of elbow room such as it is but with Lithium Based batteries, accurate voltage sensing is essential.
This is not a difficult process to do but as equipment varies a great deal on how they are configured and what options they have, I cannot address individual components as such, you will have to refer to the manuals for your particular equipment.
! You will require an accurate DVOM (Digital Volt Ohm Meter) or DMM (Digital Multi-Meter) to accomplish this task.
Simple Steps: Do this when there is no charging from the SCC, best just after sundown, so there is no solar activity.
Example using basic numbers to Keep It Simple:
Assume the Battery reads 24.0 VDC, the SCC reads 23.75 VDC and the Inverter/Charger sees 23.60 VDC.
IF the desired CHARGING cutoff is 24.0 VDC, then the SCC would have to be corrected for the 0.25V shortfall in readings, so it would be programmed to cutoff @ 24.25 VDC. The Inverter Charger "Charge cutoff" would then also have to be corrected to 24.40 VDC to compensate for the 0.40 VDC difference.
Same for LOW Volt Disconnect !
The Inverter will have it's own LVD setting and this is really important. While 0.40V is not a big difference, it can be if you want to keep with a very specific range and with Lithium .40V at the bottom edge CAN BE SIGNIFICANT ! So you would have to Correct the voltage the Inverter/Charger sees so that it cuts off exactly at the voltage specified "at the battery terminal end". So IF you want the LVD to kick on when the cells reach 2.75VDC ea / 22.0 VDC for the 24V pack/bank the LVD setting will have to be adjusted to 22.40. This way when the Inverter/Charger sees 22.40 Volts it cuts off as the actual batteries are at 22.0VDC. 21.60 VDC = 2.70v per cell.(uncorrected) * REMEMBER, that below 2.80V per cell the voltage drops very fast as you in the "bottom 20%" of cell capacity.
Don't make the BMS do the work it shouldn't do...
The BMS of course will cut off for High Volt, Low Volt etc but this is not it's job, those are "safety" features to protect you batteries and are more or less the "fail safe mechanism", as such they should not be doing that work as a matter of normal operations. This is really the job of the SCC & Inverter/Charger to manage on an ongoing basis. Continually using the BMS to do this lifting can actually affect the BMS negatively and even cause burn outs on FET based systems with repeated "abuse" being shifted to the BMS, it is not what they are designed to do.
I expect that some will want to dig into minutia and details... This is just a basic overview and I am writing this while still working on Coffee #1 so I'm sure typo's and some minor details may be left out. This is also quite GENERIC because different equipment handles settings & configurations in their own way and that makes it pretty difficult (read impossible) to address all the variables.
I hope this helps
Steve
This is a topic that seems to be generally neglected by not being mentioned or even considered in many posts. This is unfortunate because components such as Solar Charger Controllers, Inverter/Chargers need to "know" the precise voltages being dealt with. With Lead Acid / AGM batteries there is a bit of elbow room such as it is but with Lithium Based batteries, accurate voltage sensing is essential.
This is not a difficult process to do but as equipment varies a great deal on how they are configured and what options they have, I cannot address individual components as such, you will have to refer to the manuals for your particular equipment.
! You will require an accurate DVOM (Digital Volt Ohm Meter) or DMM (Digital Multi-Meter) to accomplish this task.
Simple Steps: Do this when there is no charging from the SCC, best just after sundown, so there is no solar activity.
- Ensure your batteries are charged and "at rest" meaning no loads or charging for 1 hour.
- The SCC, Inverter/Charger connected and ON, as well if you have a Buck Converter / Step Down converter have that on BUT NO LOAD.
- Take a voltage reading @ Battery Terminal (if only one pack) or at BUS Terminals if multiple packs in parallel. Test "after" the BMS as the BMS is on the "batt side". NOTE the Voltage as ##.## volts (IE 28.92vdc or 14.86vdc)
- Measure the Voltage at the Inverter/Charger DC Input Terminals and again note it.
- Next measure the voltage at the SCC "Battery Terminals" (not the solar input terminals) NB: SCC should not be getting any sun, no input. NOTE the Voltage seen..
Example using basic numbers to Keep It Simple:
Assume the Battery reads 24.0 VDC, the SCC reads 23.75 VDC and the Inverter/Charger sees 23.60 VDC.
IF the desired CHARGING cutoff is 24.0 VDC, then the SCC would have to be corrected for the 0.25V shortfall in readings, so it would be programmed to cutoff @ 24.25 VDC. The Inverter Charger "Charge cutoff" would then also have to be corrected to 24.40 VDC to compensate for the 0.40 VDC difference.
Same for LOW Volt Disconnect !
The Inverter will have it's own LVD setting and this is really important. While 0.40V is not a big difference, it can be if you want to keep with a very specific range and with Lithium .40V at the bottom edge CAN BE SIGNIFICANT ! So you would have to Correct the voltage the Inverter/Charger sees so that it cuts off exactly at the voltage specified "at the battery terminal end". So IF you want the LVD to kick on when the cells reach 2.75VDC ea / 22.0 VDC for the 24V pack/bank the LVD setting will have to be adjusted to 22.40. This way when the Inverter/Charger sees 22.40 Volts it cuts off as the actual batteries are at 22.0VDC. 21.60 VDC = 2.70v per cell.(uncorrected) * REMEMBER, that below 2.80V per cell the voltage drops very fast as you in the "bottom 20%" of cell capacity.
Don't make the BMS do the work it shouldn't do...
The BMS of course will cut off for High Volt, Low Volt etc but this is not it's job, those are "safety" features to protect you batteries and are more or less the "fail safe mechanism", as such they should not be doing that work as a matter of normal operations. This is really the job of the SCC & Inverter/Charger to manage on an ongoing basis. Continually using the BMS to do this lifting can actually affect the BMS negatively and even cause burn outs on FET based systems with repeated "abuse" being shifted to the BMS, it is not what they are designed to do.
I expect that some will want to dig into minutia and details... This is just a basic overview and I am writing this while still working on Coffee #1 so I'm sure typo's and some minor details may be left out. This is also quite GENERIC because different equipment handles settings & configurations in their own way and that makes it pretty difficult (read impossible) to address all the variables.
I hope this helps
Steve
