Battery discharging beyond energy need

I am experimenting to see what size battery would be needed to move off grid for a sample load. PV system set approximately equal to 100% of annual load. Battery set to a very high energy and power level (250 kWh / 50 kW), and can charge and discharge in any hour from PV. Behavior as expected through spring months (battery slowly charges from excess energy), but on a relatively high use day in spring, the battery basically drains 200 kWh of stored energy in 5 hours against a load of maybe 25 kWh in that same time. The battery then builds up again, only to discharge completely on another high load day. Unclear why battery state is falling faster than the energy need would imply.

Hello,

For the "Battery" case in your file, it looks like May 19 is an example of your observation. On the Battery Storage input page, the manual dispatch option is set to "PV meets load before charging battery" which results in the battery fully discharging more frequently than it would with the "PV charges battery before meeting load" option. Does changing that option result in behavior more like what you expect?

You might also want to consider changing the battery's initial state of charge 100% under "Charge Limits & Priority" to get more use out of the battery in January.

Best regards,
Paul.

Paul,

I think the concern that I have is that the battery is discharging well beyond the load that is needed to be met, and since the battery cannot export to the grid, I don't know where the energy is going. Most days it works fine - battery discharges say 5 kWh to meet 5 kWh of load (ignoring losses). But on some days, the battery discharges hundreds of kWh against a load of 1/10 as much. So the balance of energy is not adding up. I am not suggesting that this simulation is the optimal configuration, but the math on the load and discharge should still add up.

Thanks,
Kevin

Hi Kevin,

Sorry for the delay. I believe there is an issue with the DC-connected battery, probably iterating trying to meet multiple constraints and then ending the iteration without meeting all of them. I will investigate. In the meantime, I'd recommend changing to an AC connected battery, which is more straightforward for our model to handle.

Best,
Nick

Over the last few months, I've worked on redesigning the DC-connected battery model, and believe I have solved this issue, which will be available in the next version of SAM. I'll send out a beta when we have it.

Best,
Nick