DF250 Alternator current output

[Favourite Child]

I recently upgraded my batteries as follows:
Lead-acid starter battery with the motor and 2 Victron DC-DC 18A chargers connected, nothing else. One charger charges the house batteries (200AH lithium) which runs all my electronics and the second charger, charges the electric trolling motor batteries (400AH lithium). I sized the chargers on the assumption that the motor outputs max of 54A with 38A at 1000rpm. Note I used DC-DC chargers to ensure the lithium batteries did not damage the alternator through a direct connection
My problem:
I have found that with one charger turned on it all works fine. When I turn on the second charger and it tries to deliver 18A as well the voltage on the starter battery drops to as low as 12.4V from 13.5v+ and both chargers are unable to deliver their full 18A. It appears that the motor is not putting out sufficient amps even at 4000rpm when it should be delivering 50A+.
Has anyone got ideas why this is happening? TIA

[briscoe]

Could it be that the chargers are demanding more amps than 18 each?

[Favourite Child]

Good point briscoe, I checked the specs and they are 87% efficient meaning likely pulling 21A for 18A output. I have also been informed separately the motor draws 12-15A when running which I did not consider at all. Add these up and I'm above the 50A output. Might just have to operate 1 charger at a time or downsize one of them unless someone knows if you can upsize the alternator on the DF250?

[briscoe]

Well, the chargers cost is a drop in the bucket of the big scheme. I a. still too scared to look at the prices of 3 - 200 amh lithium batteries. I sometimes wish I had put the 24v trolling motor in. But, when the 36v system will run my 20 footer all day long I remember why I chose it.