I am installing 2x 48v 100ah victron lithium batteries. To make it a 48v 200ah set up, but because of space restrictions I went for the multiplus 48/500/6 charger. It has now been suggested that this charger may be too slow for the battery set up, (200 ah) and as a result the batteries maybe damaged. Does anyone have an opinion on this? I have not switched it on yet as I am waiting for cables etc so I could contemplate a larger (with difficulty) charger rather than ruin the batteries.
They will charge very slowly. Which will not damage them.
But, in the meantime, if other loads are present, the batteries may not charge at all, or even discharge.
Discharging too much will damage them.
So the charger should be bigger than the load, plus some for charging.
Kirchhoff’s laws 101… 
Thanks Alex, it’s a boat that will be sitting and has no other draw on the batteries as these are for the motor only.
As long as in an unit of time (let’s say one day) the total energy used for charging is bigger than the energy used by the motor & co, you’ll be fine.
Just make sure that from time to time, the battery’s cells reach the end voltage (3.50V - 3.55V), for a proper balance.
I think it will be ok given how this boat is used. But it’s a good point re the balancing…. So to on occasion use it till safe cut off and do a full recharge…. Hmm, may take a while. Maybe I should re consider the bigger charger.