This topic is for a low alternator charge current (e.g. 100 Amp fuse installed and only charging at 50 Amps not 90 Amps) on the Smart BMS 12/200 or BMS CL12/100 or older alternator current limiting BMS versions.
The BMS has a fuse, which will have a known resistance being used as a shunt to generate a voltage drop to measure the alternator current. They also then limit the current to a maximum of approximately 90% of the fuse value.
The first check is that you are using a Victron branded Mega fuse in the BMS because this will be the type of fuse that the BMS is calibrated for.
The main cause of the undercharging is low voltage being delivered by the alternator. By its nature, the BMS is introducing some voltage drop into the alternator charge circuit in terms of electronics to control the charge current, the fuse and extra wiring connections. If the alternator only charges at 70 Amps without the BMS when connected directly to the batteries, then with the BMS the charge current will be lower EVEN with a 125A fuse installed.
It should be noted that the BMS does not have a voltage boost function incorporated, therefore, if your alternator has too low a voltage then it will not be able to charge at the expected rate.
If you have an older vehicle then the alternator may only charge at 13.8V or 14.0V to keep a lead acid starter battery at float conditions. The alternator can not charge lithium batteries at 14.2V and an Orion DC to DC charger with voltage boost is required, or if possible, the voltage regulator in the alternator me be able to be changed to boost the regulated voltage to 14.5 or 14.6V if the vehicle systems can accept this.
For modern vehicles with “Smart” alternators and some measure of regenerative braking the vehicles ECU (Engine Control Unit) often runs the alternator at very low voltages to keep room in the battery for the regenerative braking and it is only under braking that the voltage increases to 14.5V or thereabouts. Again, an Orion DC to DC charge is required to boost the alternator voltage above the lithium battery charge voltage and the BMS on its own cannot make the alternator charge the battery. In some vehicles it may be possible to have the ECU reprogrammed to always run at a high enough voltage.