GXMnow
Solar Wizard
- Joined
- Jul 17, 2020
- Messages
- 2,738
If your charge current and load current are from the same device, you have no choice, you have to use a common port BMS. This is my case with an inverter/charger being the only thing connected to the battery bank.
If you have a separate charge controller and inverter, then you have to decide which is better. For most systems, I agree, the common port is still better, and I will try to explain why.
Imagine it is near noon. The solar panels are producing a lot of power. The charge controller is pushing that power to the battery. But you have a large inverter, and you decide to turn on the microwave, make some toast, dry your hair, and vacuum the floor. The inverter is pulling 3,000 watts while the solar panels are making 3,000 watts. Where is all the power coming from and going to?
With a common port BMS, the BMS sees almost no power at all. The current coming in from the solar charge controller is going straight to the inverter and powering the loads. In this situation, the MOSFETs or relay for the BMS does not need to carry any current.
Now we do the same thing with a separate port BMS setup. ALL of the current from the solar charge controller has to go through the BMS over charge cut off switch. And all the inverter current has to then also go through the BMS over discharge cutoff switch. In a MOSFET based BMS, you now have 2 banks of FETs, both carrying 3,000 watts of power.
With the common port setup, the BMS will never see as much power. Even if the system was charging at noon, and discharging at midnight, you still only have the power going through one half at a time. And as both run at the same time, the BMS only has to deal with the difference in power. The separate ports has to deal with the sum of the charge and discharge power.
My only separate port BMS units are on my jump start pack. I only use a small 5 amp wall pack to charge it. I was going to go separate port for my e-bike batteries, but it turned out to be just as easy to just plug the charger into the same connector that goes to the speed controller on the bike. I never want them both in at the same time anyways. If I ever do add a solar panel and charge controller on the bike, I can still "Y" cable it between the battery pack and speed controller.
If you have a separate charge controller and inverter, then you have to decide which is better. For most systems, I agree, the common port is still better, and I will try to explain why.
Imagine it is near noon. The solar panels are producing a lot of power. The charge controller is pushing that power to the battery. But you have a large inverter, and you decide to turn on the microwave, make some toast, dry your hair, and vacuum the floor. The inverter is pulling 3,000 watts while the solar panels are making 3,000 watts. Where is all the power coming from and going to?
With a common port BMS, the BMS sees almost no power at all. The current coming in from the solar charge controller is going straight to the inverter and powering the loads. In this situation, the MOSFETs or relay for the BMS does not need to carry any current.
Now we do the same thing with a separate port BMS setup. ALL of the current from the solar charge controller has to go through the BMS over charge cut off switch. And all the inverter current has to then also go through the BMS over discharge cutoff switch. In a MOSFET based BMS, you now have 2 banks of FETs, both carrying 3,000 watts of power.
With the common port setup, the BMS will never see as much power. Even if the system was charging at noon, and discharging at midnight, you still only have the power going through one half at a time. And as both run at the same time, the BMS only has to deal with the difference in power. The separate ports has to deal with the sum of the charge and discharge power.
My only separate port BMS units are on my jump start pack. I only use a small 5 amp wall pack to charge it. I was going to go separate port for my e-bike batteries, but it turned out to be just as easy to just plug the charger into the same connector that goes to the speed controller on the bike. I never want them both in at the same time anyways. If I ever do add a solar panel and charge controller on the bike, I can still "Y" cable it between the battery pack and speed controller.