I'm running 2 200Ah Victron Smart Battery in my system.
I'm charging then to 70% but someone told me i need to charge them up to 100% to balance the cells now and then, like ones every month.
Is that needed?
Yes, that is absolutely needed. At least once a month if there are only small loads/discharges and almost daily on heavy discharges. This to balance all cells accurately. At 70% there is not enough voltage to balance the cells.
See also the manual "How to ensure that the battery remains balanced": https://www.victronenergy.com/media/pg/Lithium_Battery_Smart/en/operation.html#UUID-bd5c63a2-ff2f-7d25-896b-182910e8dad5
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