Off-grid solar system design

I saw many off-grid solar system designs are like this:

[PV modules] =(DC)= [charge controller] =(DC)= [batteries]
|(DC)|                     |(DC)|
[DC outlets]               [inverter]
|(AC)|
[AC outlets]


The inverter use DC from the batteries. Will this cause the batteries charging and discharging at the same time, and shortening the batteries life?

Can the inverter connect to charge controller, and use DC from the PV modules without consuming batteries if PV modules provide enough power? The idea is just like this:

[PV modules] =(DC)= [charge controller] =(DC)= [batteries]
/(DC)/     \(DC)\
[DC outlets]  [inverter]
|(AC)|
[AC outlets]


How do I integrate other power sources like the electrical grid (plugs), an electric generator, or a car engine into this design? Just connect them to the charge controller?

              [electrical grid]
|(AC)|
[inverter]  [car engine]
\(DC)\     /(DC)/
[PV modules] =(DC)= [charge controller] =(DC)= [batteries]
/(DC)/     \(DC)\
[DC outlets]  [inverter]
|(AC)|
[AC outlets]

• It might surprise you to learn that there is a schematic editor built in to this site. It's not perfect, but it generally produces results more readable than ASCII-Art. – brhans May 27 '19 at 22:46
• @brhans, thanks for your information. I'll try to learn that. – 李岡諭 May 27 '19 at 23:50

2 Answers

The batteries don't charge and discharge at the same time. If they were charging and discharging at the same time, the current would be flowing both ways on the same wire at the same time, which doesn't make sense.

The batteries make up the difference between the power the inverter wants and the power the charge controller is providing. When it's very sunny, the extra power available charges the batteries. When it's less sunny, the batteries make up the extra power needed. When it's not sunny, all the power comes from the batteries.

They cannot work like this unless the BMS senses battery current separate from the rest of the system ( in addition to other current sensors. ) unless they are flooded cells where CV is OK.

how a battery behaves in parallel to the charger with a shared load but with a voltage difference