# What happens to excess solar energy at midday with SoC at 90%?

Posts: 2Registered Users
Assuming in your offgrid system you have a separate inverter and a separate MPPT charge controller (which pushes out about 2400W = 50A at 48V) what happens to the excess solar energy if let's say by 11am, the batteries are 90% full and there is a total load of 800W (100W DC & 700W AC).

The load draws power from where; charge controller, batteries or both? Batteries are flooded lead acid batteries of 48V 400AH
The extra current being drawn by the load is obviously 'seen' by the batteries? Won't this higher current harm the batteries?
We assume there is no diversion load
Perhaps, the batteries only takes what it needs and ignores the extra current? Please can any experts/professionals explain what really happens here?

Thanks

Short Answer, as long as there is enough sun (and everything is configure/working correctly). The charge controller supplies the energy needed to run the AC inverter and whatever charging current is needed by the battery bank.

Say the charge controller is set to hold 58.0 volts (absorb state charging). If the inverter tries to take a little more current and drop the voltage to 57.6 volts, the charge controller brings the voltage back up to 58.0 volts.

Details do matter--For example, the AC inverter does not draw "DC--Direct Current in a steady flow... The inverter actually draws current in a "sine squared" waveform (assuming single phase AC inverter). The inverter draws peak current when the AC wave form is at "peak voltage. And near zero current when the AC waveform is near zero volts.

And charge controllers may not always output pure DC current--They may output a square wave current. The battery then "averages" the voltage (supplying current during "peak current load" or absorbing current during peak charging "PWM"--pulse width modulation-- current, and supply current when the load is at peak current).

The idea is to keep the "micro cycling" voltage above the true discharging voltage of the battery (i.e. for a 12 volt battery... The "micro cycling" voltage range is ~13.6 to 14.0 volts (as an example of "float charging").

The other question--where does the solar energy go when not all is taken from the solar pane? It actually flows "backwards" through the solar cells (they are just "diodes") and do heat up the panel (panels under load are a bit "cooler" than panels with no loads).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 78Registered Users ✭✭
Any functioning system will often have excess energy that is never used.  That is the reason diversion to something like heating water is very cost effective. I have a system I built that monitors battery voltage and diverts power to loads of opportunity. Having these loads only operate in full sun reduces the amount of battery storage required. I get all my hot water from excess solar.  Having more panels is often a smarter move than having more batteries.
• Posts: 27Registered Users ✭✭
I was wondering the same thing, as well as the other side of the question:  What happens when the battery needs bulk charging but there is not enough power in the system to do so? It seems that in an off grid system this would happen pretty much every day (early in the morning).  I presume that the MPPT charge controller provides as much current as possible.

1.2KW off grid system; 2 strings of 2ea 305W 60 cell panels on a redneck ground mount;  MNPV3 combiner feeds a MN Classic 150 located 100' away;  12V 460AH FLA battery bank powers a cabin-wide 12V DC system as well as a Cotek 700W PSW inverter; Honda EU2000i  and IOTA 55A charger bridge cloudy days and a Champion 3800W generator for short duration, power hungry appliances.

Just like a car electrical system. You can run lights, radio, etc. With the engine running or not.

Off grid per systems are going to not use all the energy they could generate during the summer, but during winter bad weather, you have to conserve power and or use your generator.

It is the nature of the beast.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 27Registered Users ✭✭
Thank you @BB.  I wasn't very clear with my question, I was wondering what the charge controller does when it knows the battery requires bulk charging but there is not enough power to do so.

1.2KW off grid system; 2 strings of 2ea 305W 60 cell panels on a redneck ground mount;  MNPV3 combiner feeds a MN Classic 150 located 100' away;  12V 460AH FLA battery bank powers a cabin-wide 12V DC system as well as a Cotek 700W PSW inverter; Honda EU2000i  and IOTA 55A charger bridge cloudy days and a Champion 3800W generator for short duration, power hungry appliances.

• Posts: 2Registered Users
Thanks Bill, NANO and Scooby. I have the option of getting exactly that setup or go for a hybrid inverter which comprises AC charger, MPPT charger controller & inverter in one unit. Here, it is clearly started that, excess solar energy is passed onto the load so no real need for diversion load, just to use more load around that time.
The reason why I am not keen on it is because it does not come with temperature sensor to monitor the temperature of the batteries and adjust the charging voltage according to the batteries' temperature.

@ScoobyMike, the charging current is always low in the morning but I intend to increase (possibly double the number of panels) so that, from early in the morning, I am already getting my desired current. I will set the current limit on the MPPT charge controller.
If there is not enough sun, then the battery slowly continues to discharge.

The solar+battery supply current to the load.

When available solar current exceeds the load current, then the battery starts to charge.

Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Posts: 2,228Solar Expert ✭✭✭✭✭
Every system is different, learning ones particular system is key to achieving a ballance, a conservative approach is best adding opportunity loads whilst monitoring charging requirements, thereby assuring the batteries needs are the priority. This learning curve takes at least a year to get it somewhat accurate due to seasonal variations in solar availability, which is more pronounced in higher latitudes.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank

• Posts: 5,090Solar Expert ✭✭✭✭
@ScoobyMike, the charging current is always low in the morning but I intend to increase (possibly double the number of panels) so that, from early in the morning, I am already getting my desired current. I will set the current limit on the MPPT charge controller.

You really need to be sure you do not over-panel your CC as there is  the possibility of damaging it and voiding the warranty

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep