I'm looking for some feedback on my plan to use a second old inverter as a second AC battery charger to half generator run time in the winter by.
This post became long with all the details so I've tried to break it down logically.
Current system
5.2kw solar panels,
428ah 48v rolls battery bank (soon to be upgraded to roughly 856ah)
Conext SW4048 inverter/charger
8.1kw starting 6.5kw continuous propane generator
Very loww generator usage in summer, will be near zero usage after battery upgrade. Daily generator usage Dec - Feb.
Background why I have this opportunity
My inverter recently died and I replaced it with the same model to get up and running right away. Upon further investigation the old one still works fine however the L1 terminal for power out from the inverter is burnt up due to a loose connection. It was a prewired panel and I never checked the connection from the factory was tight, it lasted a couple years but finally burnt up under a long duration large load. I'm trying to see if I can get a replacement board with the terminals so it can be a spare going forward.
The SW series no longer supports stacking so I cannot set it up as a stacked inverter unfortunately however its rare I need more then the 4kw anyways so that's not a big deal. But regardless if I can get a new board I would like to use this as a second DC charger since that will not require using the melted AC out terminal only the AC in terminals.
Charging Current calculations
The 45amps DC charging from my current inverter only load the generator up to 2200-2700watts (battery voltage dependant) so adding a second 45 amp charger will load the generator up to max 5400 watts plus house load which is still within its continuous rating.
The batteries only care about total charge current having a second AC charger is no different then having solar panels with their own charge controller. As long as I don't exceed the recommended charge rate of the batteries it should be fine.
Charging rate is recommended 10% maximum 20%, so 42.8 amps recommended and 85amps max. I plan to double its size so 85 amps will be the recommended charge current and if I only have the single charger I will be charging them at too slow a rate and the generator will need to run for way longer then necessary.
My solar system when it hits peak is around the 85 amps so thats not a problem. I never run the generator when we have good sun so I don't need to worry about exceeding charge current by combining solar and generator power.
Coordinating charging
Since the inverters wont be stacked they will not know about each other. To coordinate charging my plan is to set the battery voltage setpoints on the charging only inverter slightly higher then the active inverter. That way the active inverter starts ramping down charging when the battery hits absorb while leaving the charging only inverter at the full 45amps.
Managing Generator Load
45 amps charging at max 60amps 2700watts, which is 11.25 amps at 240 AC. Add a little for inverter losses say it pulls 13 amps. That leaves 27 amps for the main inverter, its already on a 30 amp breaker however I will set the AC breaker setting to 27 in the inverter so it should never trigger the breaker on the generator.
Glaring problem with this setup is the house load, if I load up the generator close to its capacity with the house load the second charger will exceed the continuous rating however if I turn down the AC breaker setting on the inverter down then each time a motor startup such as the water pump kicks in the inverter will kick off the generator even though the generator can handle the startup current.
I will monitor both inverters from my PLC and total the total generator wattage. If the combined continuous wattage exceeds the 6500 watts for more then a couple seconds (motor startups) I can automatically disable or scale back charging on one of the inverters until the load drops back to an acceptable level. I can also monitor the total DC amps going into the battery so if I run the generator on a marginal sun day but there is still some solar production I can automatically change the max charge current set point on the inverter to hit the 85 amp until I have my new battery bank.
Wiring details if anyone interested
My plan is to wire up the DC terminals from the second inverter to the same buss bar as the running inverter. This way its on the correct side of the shunt and power in and out of the batteries is still tracked by my wizbangjr. I'll put it on a 50amp DC breaker since the max charging current is 45 amps. The main inverter is on a 100 amp breaker but since this one wont be inverting that's not needed.
The generator breaker is 40amps, it comes into the panel onto a bus bar then through a 30 amp breaker to the existing inverter. I'll add a 15 amp breaker and feed that into the AC in on the second inverter.
This post became long with all the details so I've tried to break it down logically.
Current system
5.2kw solar panels,
428ah 48v rolls battery bank (soon to be upgraded to roughly 856ah)
Conext SW4048 inverter/charger
8.1kw starting 6.5kw continuous propane generator
Very loww generator usage in summer, will be near zero usage after battery upgrade. Daily generator usage Dec - Feb.
Background why I have this opportunity
My inverter recently died and I replaced it with the same model to get up and running right away. Upon further investigation the old one still works fine however the L1 terminal for power out from the inverter is burnt up due to a loose connection. It was a prewired panel and I never checked the connection from the factory was tight, it lasted a couple years but finally burnt up under a long duration large load. I'm trying to see if I can get a replacement board with the terminals so it can be a spare going forward.
The SW series no longer supports stacking so I cannot set it up as a stacked inverter unfortunately however its rare I need more then the 4kw anyways so that's not a big deal. But regardless if I can get a new board I would like to use this as a second DC charger since that will not require using the melted AC out terminal only the AC in terminals.
Charging Current calculations
The 45amps DC charging from my current inverter only load the generator up to 2200-2700watts (battery voltage dependant) so adding a second 45 amp charger will load the generator up to max 5400 watts plus house load which is still within its continuous rating.
The batteries only care about total charge current having a second AC charger is no different then having solar panels with their own charge controller. As long as I don't exceed the recommended charge rate of the batteries it should be fine.
Charging rate is recommended 10% maximum 20%, so 42.8 amps recommended and 85amps max. I plan to double its size so 85 amps will be the recommended charge current and if I only have the single charger I will be charging them at too slow a rate and the generator will need to run for way longer then necessary.
My solar system when it hits peak is around the 85 amps so thats not a problem. I never run the generator when we have good sun so I don't need to worry about exceeding charge current by combining solar and generator power.
Coordinating charging
Since the inverters wont be stacked they will not know about each other. To coordinate charging my plan is to set the battery voltage setpoints on the charging only inverter slightly higher then the active inverter. That way the active inverter starts ramping down charging when the battery hits absorb while leaving the charging only inverter at the full 45amps.
Managing Generator Load
45 amps charging at max 60amps 2700watts, which is 11.25 amps at 240 AC. Add a little for inverter losses say it pulls 13 amps. That leaves 27 amps for the main inverter, its already on a 30 amp breaker however I will set the AC breaker setting to 27 in the inverter so it should never trigger the breaker on the generator.
Glaring problem with this setup is the house load, if I load up the generator close to its capacity with the house load the second charger will exceed the continuous rating however if I turn down the AC breaker setting on the inverter down then each time a motor startup such as the water pump kicks in the inverter will kick off the generator even though the generator can handle the startup current.
I will monitor both inverters from my PLC and total the total generator wattage. If the combined continuous wattage exceeds the 6500 watts for more then a couple seconds (motor startups) I can automatically disable or scale back charging on one of the inverters until the load drops back to an acceptable level. I can also monitor the total DC amps going into the battery so if I run the generator on a marginal sun day but there is still some solar production I can automatically change the max charge current set point on the inverter to hit the 85 amp until I have my new battery bank.
Wiring details if anyone interested
My plan is to wire up the DC terminals from the second inverter to the same buss bar as the running inverter. This way its on the correct side of the shunt and power in and out of the batteries is still tracked by my wizbangjr. I'll put it on a 50amp DC breaker since the max charging current is 45 amps. The main inverter is on a 100 amp breaker but since this one wont be inverting that's not needed.
The generator breaker is 40amps, it comes into the panel onto a bus bar then through a 30 amp breaker to the existing inverter. I'll add a 15 amp breaker and feed that into the AC in on the second inverter.
