# question

## Li-Ion battery bank for PV AC Coupled

Dear All,

I have a grid-connected installation where there is about 10kWp of PV.

I wonder what size Li-Ion battery bank should I use.

According to Victron blog:

https://www.victronenergy.com/live/ac_coupling:start?_ga=2.137144624.1420485591.1544172331-1021279916.1539680797#lithium_batteries

I should use:

100Ah 48V bank for each 1,5kWp.

670Ah 48V battery bank.

Calculating that to kWh it is about 32kWh battery bank.

Then I should use 9x3,84kWh BYD LVS modules.

Is my calculation okay?

P.S.

Why cannot I use less capacity, for example, 20kWh (it allows me to charge my batteries with max 0,5C)?

What other criteria are taken in recommendations to give such huge battery banks?

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Hello @TechSol,

as far my calculator works correct, your calculation is correct. I think this "rule" is according to the fact, when in an 10 kWp system a huge supplier switches off an ESS system has to get rid off the energy in the batteries. Assuming a bigger PV system would have bigger energy users, perhaps there is the keyhole effect. But I am sure you will get information of the profs ,here.;-)

I am writing, because I use a quite similar system but with 6 kWp PV and 8 pieces Pylontech with 2.500 Wh (max, usable 2.200 Wh) so this according to the rules and I wn ated to give you some information I found out.. I found out, that the capacity is some how small for the daily usage. For example during a good sunny day, the batteries are loaded from 40 to 100% SOC until noon or early afternoon. And now what to do with rest of the energy? Or the other way round you have to use more electricity. Okay when the batteries are bigger it will go one or might be two day longer until your batteries are full very early during the day. So it depends if your are allowed to feed the energy into grid or not. When not, you will be enforced to do something to reduce PV power when Batteries are full. With fronius it's easy to solve. Otherwise you have to think about.

We had in the past 10 days very bad day, no sun at all, so I wished my batteries would be bigger, when more sun was before available.

I think there are so many factors and things you have to respect, that to give an simple answer with YES or No is not possible.

Perhaps it is possible to learn something with this starting disuccsion.

Wish you all the best, kind regard from Munich

DayAndNight

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dayandnight ·

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You could always connect lithium batteries with built in BMS's. I use DCS and EnergyPlus batteries in both DC connect and DC/AC connect systems. These batteries do not require and data communication between them and the inverter system/s. They can be used as direct replacements for wet cell batteris.
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