Stand-Alone (off-grid) PV System

[binaryninja]

I am a bit confused about stand-alone pv systems diagrams. Many of them show something similar to the attached pic. If the system is designed in this manner, how would the battery be able to charge and discharge at the same time? It can't just wait until the battery is depleted then start charging because that would disconnect the load. How is it possible to provide the load and keep the battery voltage up? If you have any knowledge or experience please help me!
THANKS!

 

[betwixt]

The idea is the PV array provides more power than the inverter requires and the excess is stored in the battery. When the PV voltage drops, the stored energy in the battery is released to keep the inverter running.
You can just connect the battery and inverter across the PV but you run the risk of boiling the battery if the current is allowed to go too high. It's best to use some additional control system to regulate the charge if it becomes necessary. A dump load is the normal method used, it diverts excess current to a load resistor (usually a heater element) to protect the battery but converting solar power to heat is extremely un-green!
You usually find an MPPT between the PV and battery to hold the PV voltage up for as long as possible as it gets dark.

Brian.

 

[binaryninja]

Thanks. I guess the diagram threw me off a bit. It just looked like the battery was single-handily providing the power to the loads, even during the day, and the PVs purpose was only providing a charge to the battery. I've been doing a lot of research on this since the post and it seems like there are a few variations of connections. One was connected so that the battery voltage supplied the inverter, that means if the user needed AC power then it would be discharging the battery (even during the day). What is the correct way to supply the inverter DC input?

 

[betwixt]

There is no 'correct' way, there are so many different systems and environmental situations and each has its own optimal solution.
I would suggest the simplest way to do it would be to share the PV power between the load and charging the battery until full charge was reached then switch the battery out of circuit until needed. That way you have as much stored as possible but still use it 'live' while you can. The best place to look for options is another forum that specializes in this sort of thing, try looking at Fieldlines.com: The Otherpower discussion board - Index in the solar power section. Please read back through recent posts as your question is asked and answered frequently and sometimes annoys the regular contributors!

Brian.

 

[ptinarwo]

A stand alone PV system is designed in such a way that the power from the solar panels (solar panel current * number of hours the battery is charged) should be always be greater than your load requirements. this will mean the battery is charged to its full capacity and the inverter and all DC loads will draw power from the battery. but its always safe to have a charge/discharge control within you system.

 

[binaryninja]

ptinarwo, you say

the inverter and all DC loads will draw power from the battery

. From what you're saying it seems like there is no difference between day and night. When will the battery get charged? Is the PV current going into the battery while at the same time the load are drawing current from the battery? I always thought of it as during the day you use as much power from the sun as possible only drawing from the battery when needed. Any other time the battery is being trickle charged or kept at close to full capacity for when the sun goes down or when there isn't enough sun. This way at night you have a full battery to draw from. Then in the morning or next day the battery gets charged back up from the PV.
Is there something wrong with my theory of operation? Is this not the normal way of doing it? Or is there a better way?
THANKS!