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zach-lustig avatar image
zach-lustig asked

PV System without Battery

I have read through some threads regarding systems without batteries, but have found nothing conclusive. I have Victron SmartSolar 75/15 and I think they have updated its construction/firmware since this question was last asked. I want to record exactly how much power my panel is producing throughout the day through VE.Direct. I have another panel set up with VE.Direct so I know how to log the data, however I want the PV power to be consumed by a resistive load (heater) so I can record the power. I tried to connect the heater to the load output on the controller, and change the controller settings to load always on and charger off, and thought this would work. However, apparently the load can only draw current if the charger mode is switched to on. When the charger mode is on and nothing is connected to the battery terminals on the controller, the load output was pulsating current because the system didn't know what voltage to charge at because it is an open circuit. Then I thought I could put the heater on the battery terminals to trick it into thinking it was charging a battery but it is actually powering the heater. This idea is working, however the voltage is about 9.5V despite me setting it manually to 12.5V for both absorption and float voltages (just modified the LiFePO4 default voltages and turned off temp compensation and low temp cutoff). I am happy with how it is running but I am not sure if the controller is attenuating the power output because it thinks the battery is "dead" or something, hence the low voltage. I am going to compare its power output to the system with a battery to see if the power is reduced at all, but I figured I would ask the community first.

And yes, I am okay with the system only logging data during the day, as the controller needs power to log the data.

Any insight is appreciated, it is entirely possible I am overcomplicating things.

MPPT ControllersMPPT SmartSolarbattery
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wkirby avatar image
wkirby answered ·

As I'm sure you understand, this MPPT is designed to charge a battery and you are using it outside of it's design parameters if it is not connected to a battery.

So, if you don't want to connect a battery and you prefer to use a resistor, then you should expect the MPPT to behave differently from what it is designed to do.
A resistor has a fixed resistance (within reason) whereas a battery does not. A battery internal resistance changes as it charges, that is why the Voltage increases.
If you connect a fixed resistor and it causes the Voltage to dip to 9.5V with a certain ammount of current (you did not mention the output current when the Voltage was 9.5V), then the Voltage cannot increase unless the current increases. If the current flowing at that time when it was 9.5V is 15A, then it will not reach the target Voltage of 12.5V unless the resistance is increased.
The Voltages that you program into the device are target Voltages, these Voltages can only be reached if there is enough current (power) to do so.

You did not say how much PV power you have connected.
The output Voltage and current when using a fixed resistor is going to fluctuate quite a lot, you may even get to a point in bad weather where the MPPT does not start or is starts and stops a lot because there is not enough power to keep the MPPT running.
A battery connected can keep the MPPT running even in poor weather conditions.

I understand what you are trying to do, but ultimately this product is supposed to be used with a battery connected.

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zach-lustig avatar image zach-lustig commented ·

I have connected a single 90W panel, and yes you are correct, the voltage and current were fluctuating quite a bit. I have not run into the issue of the system powering off during the daytime yet though. The current was not close to 15A luckily, it was closer to 7A even in direct sunlight (max power). I see what you are saying about the internal resistance of a battery, that explains it.

However, I did set up the other system with a battery and an identical panel right next to the other system's panel, and the PV power was exactly the same, even with intermittent clouds. This inspired confidence in my system, so I think it will work for this application. Thank you for your response!

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