Good morning, i have question about the mid point voltage deviation on 48 V System (4 batteries victron lead carbon 12V/160 Ah) with 3 battery balancers & BMV installed at the begin.
Its normal after 3 years operation the bmv show fault on mid point voltage 4,7 % and how fix it ?
Thanks
It depends on many factors and usage.
For a hard working system with lead batteries and discharges below 50%, 3 years and 5% deviation is quite OK.
Thank you very much. the system is seasonal, it works demandingly for about 2 months in the winter and 3 months in the summer. the largest load it powered is a 9000 btu inverter air condition. Does it play a role that the batteries never discharge during the rest of the time? (only powered by a router and a small fridge).
should I do some deep charging and discharging?
On the contrary.
Lead acid batteries like to be kept as close as possible to 100%. As few as possible deep discharges.
It has to do with the chemistry and the lead sulfate deposits that accumulates when they are not fully charged immediately after discharge.
Or left alone for long period of times without recharging/topping them up. This is why there is that option for re-bulk or periodic equalization.
Install battery balancers
Rotate your bank.
Use 12 chargers to charge each battery up separately as part of regular maintenance if necessary.
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He already said to us in the first post: “…4 batteries…with 3 battery balancers…”. 
I saw. But clearly they aren’t keeping up. So add balancers… ?
Batteries have aged. So there will be more need