optimum battery size

[Portsolar]

How big should the battery capacity be relative to your daily usage? A bigger battery reduces the amount of charging and discharging, and may have a longer life, but is it worth the extra cost?

 

[johnb2713]

How big should the battery capacity be relative to your daily usage? A bigger battery reduces the amount of charging and discharging, and may have a longer life, but is it worth the extra cost?

A bigger battery wont reduce the amount of charging / discharging that you do, that's set by the amount of energy you use. If you consume 10 kWh in a day you need to charge slightly more to recharge the battery. The same is true no matter what battery size you have. With batteries bigger = better, the load applied compared to capacity 'C' becomes a much smaller figure and therefore helps battery life, reduces heating etc. With bigger batteries you're less likely to runout during peak periods, I run my system totally on off peak electricity.

 

[Portsolar]

Thanks, I meant what you said, I think. I have a new system and noticed via the app. that the battery discharges to 15% during the night (supplying the fridge etc.) then gets charged to 20% by the grid. Maybe one or two cycles of this each night. This seems like a poor set-up given that nothing is 100% efficient. A tweak to the set-up may be possible.

 

[bladerunnerpv]

Sounds odd, perhaps you could post some graphs from the monitoring app to help understand what's happening

 

[johnb2713]

Thanks, I meant what you said, I think. I have a new system and noticed via the app. that the battery discharges to 15% during the night (supplying the fridge etc.) then gets charged to 20% by the grid. Maybe one or two cycles of this each night. This seems like a poor set-up given that nothing is 100% efficient. A tweak to the set-up may be possible.

Do you have a tariff that gives you off peak energy at a lower cost eg Octopus GO? If so I would adjust the charging schedule to charge your battery to 90% for the full duration of the off peak time (for GO that would be 00:30 to 04:30). The house loads would all be supplied at the off peak price and your storage battery would be charged to 90%. At 04:30 your storage battery would be supplying your house until such time solar takes over or the battery becomes discharged.

 

[Portsolar]

Thank you both for your interest. I don't have any off-peak tariff. I have experimented with the charge and discharge schedule, but its not useful at present. Attached is a typical plot from a recent cloudy day, showing the discharge and recharge from 15 to 20% in purple. There was very little load that day, the app doesn't show a load of less than 100W, goodness knows why not. It just managed to get to 21% with a bit of sun!
This evening I have set the limits, called ForceCharge SOC and Overdischg SOC to 15 and 16%. The system won't let me set them equal to each other, even though the Solis manual says that can be done. That might stop recharge from the grid unless the battery self-discharged (unlikely for a day or so). The manual also gives a contradictory description of Overdischg SOC (para. 6.6.6.2). The option to prevent recharge from grid is overridden when ForceCharge SOC is reached. Sorry to ramble on!

 

[monkey5]

Are you forcing it to discharge or just using the battery for general use? How big a battery have you got? Might be worth changing to octopus and charging in. Cheaper rate.

 

[johnb2713]

Thank you both for your interest. I don't have any off-peak tariff. I have experimented with the charge and discharge schedule, but its not useful at present. Attached is a typical plot from a recent cloudy day, showing the discharge and recharge from 15 to 20% in purple. There was very little load that day, the app doesn't show a load of less than 100W, goodness knows why not. It just managed to get to 21% with a bit of sun!
This evening I have set the limits, called ForceCharge SOC and Overdischg SOC to 15 and 16%. The system won't let me set them equal to each other, even though the Solis manual says that can be done. That might stop recharge from the grid unless the battery self-discharged (unlikely for a day or so). The manual also gives a contradictory description of Overdischg SOC (para. 6.6.6.2). The option to prevent recharge from grid is overridden when ForceCharge SOC is reached. Sorry to ramble on!View attachment 16475

With some of the Octopus tariffs available this all becomes a no brainer. Export any excess you have at 15p kWh, import off peak as low as 7.5p kWh.
I can't honestly see the logic in running the system how you are.

 

[Oldman22]

Thank you both for your interest. I don't have any off-peak tariff. I have experimented with the charge and discharge schedule, but its not useful at present. Attached is a typical plot from a recent cloudy day, showing the discharge and recharge from 15 to 20% in purple. There was very little load that day, the app doesn't show a load of less than 100W, goodness knows why not. It just managed to get to 21% with a bit of sun!
This evening I have set the limits, called ForceCharge SOC and Overdischg SOC to 15 and 16%. The system won't let me set them equal to each other, even though the Solis manual says that can be done. That might stop recharge from the grid unless the battery self-discharged (unlikely for a day or so). The manual also gives a contradictory description of Overdischg SOC (para. 6.6.6.2). The option to prevent recharge from grid is overridden when ForceCharge SOC is reached. Sorry to ramble on!View attachment 16475

I have a Solis inverter and mine would go into loop mode and repeatedly self charge with the % set too close between force and over soc. At 1% difference yours is imo much too close.
I settled for Overdischarge SOC which has a range of 6-40% at 15% and Forcedischarge SOC which has a range of 4-15% I set at 6%

 

[Portsolar]

I have a Solis inverter and mine would go into loop mode and repeatedly self charge with the % set too close between force and over soc. At 1% difference yours is imo much too close.
I settled for Overdischarge SOC which has a range of 6-40% at 15% and Forcedischarge SOC which has a range of 4-15% I set at 6%

OK, thanks.

 

[Portsolar]

I have now set the levels at 10% and 15%. Surprisingly the battery now discharges to 11% and stays there overnight without being recharged from the grid. There's some logic to it somewhere!

 

[Oldman22]

I have now set the levels at 10% and 15%. Surprisingly the battery now discharges to 11% and stays there overnight without being recharged from the grid. There's some logic to it somewhere!

I think if you were to use a tool like PbmsTools to view what's going on with your batteries overnight, you would see that stopping discharge at 15% isn't absolute and that there will be maybe 1-2% further discharge almost within minutes of the 15% cut off point, then further discharge due maybe to cell imbalance and Inverter overheads taking you down to 11% by morning.
Pretty much any percentage below 15% Overdischarge SOC triggered a cycle of charge/discharge cascading for me with 3 x 5kWh batteries.