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james avatar image
james asked

Help with ac coupled battery sizing

Hi, I would appreciate some help on minimum battery sizing on a AC coupled system(linked to grid and generator - no feed back is allowed), we also have 4 x 250/100 Victron MPPT's contributing to the micro-grid system. I have recently taken over this system and it was sized by someone else. I am pretty sure the batteries quoted are not the correct minimum battery size.


There are three 10kva Quattro's, 1 x Fronius Symo 15kva 3phase, the above MPPT's and 28.8 kwp pv power. 14.4kwp is on the fronius and the rest split between the MPPT's. Am i right in thinking that the minimum battery size for this system would be approximately 92kwh? The batteries we are using are Blue Nova lithium batteries. Any help and advice would be appreciated. Thanks.

AC PV Couplingbattery capacitydesign
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Murray van Graan avatar image
Murray van Graan answered ·

Hi James. If the BlueNova battery is connected to a Venus device via CanBus, DVCC enabled and all connected equipment is relatively up to date with their firmware, the BlueNova will be able to control the maximum charge current to it. Theoretically, a relatively small battery would do. That being said, you want to size your battery so that the maximum potential PV charge does not exceed the battery limits. Your 28Kwp PV can charge a BlueNova at roughly 500A at absorption voltage of 56.2V. Therefore, the smallest BlueNova that can handle a continuous 500A charge is a 40K. Further to that, you also have to keep in mind the backup time required by the client in the event of a grid failure. In my opinion, I wouldn’t size that system with a BlueNova battery less than 65K, as a minimum.

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james avatar image james commented ·

Ok thanks very much that is helpful. The client won't have enough money to upgrade to that size battery i reckon(there are two systems similarly sized so they would need two of them!). We might need to take out the Fronius and some panels(on both systems) and just use the inverters with Victron mppts, using ESS and work on a way of managing loads. Or maybe we can use fewer panels and still utilise the fronius and the mppts - but with fewer panels on each component and still manage loads a bit more carefully. They can work toward more batteries at a later stage. Do you think this might be a better compromise? Thanks

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Murray van Graan avatar image Murray van Graan james commented ·

Hi James, if you use ESS - and enable DVCC and subsequently the current limit, you could manage it just fine without having to remove anything. Also make sure the battery is communicating via the CanBus.

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tommas avatar image
tommas answered ·

https://www.victronenergy.com/live/ac_coupling:start

Did you read this document already? See 3.2 for battery sizing

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