I would like to extend my storage with an additional battery. Is there a possibility to add a second battery (different brand/size) with a second MP II or is there another solution?
Not easily. If the batteries are matched in terms of voltage, then it is possible to operate the 2 sets in parallel. MPII units can only be operated in parallel with a common battery, so that would not be a solution. You could operate the systems independently though, but would be faced by different soc in the 2 systems - however, you could use the larger system to feed the ac in of the 2nd multi, and set the 2nd multi to ignore AC in, so it would use it's own battery first, then switch its load to the original multi once the battery goes low.
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