new to this site, i have a trailer issue with particular problems with current being pulled from trailer by the tractor charge cord as this system is straight 2ga cable from charge cord to batteries on the lift gate the high current draw when cranking will blow the 100 amp fuse every time, to remedy this issue i would like to limit current to 40 amps at the parallel wired batteries i think 20 amps for charging each should be sufficient to maintain them while in use, alternatively i would need to convert the 5 trailers with same issue to a 12 volt-12 volt charger to remove the high current draw issue, does anyone have a solution like an inline bolt in limiter that would do what i require?

 

OK, to the TS: What I think that I see is that the current draw from the tractor battery connections encounters a high enough resistance that it draws excess current from the more distant battery pack in the trailer. THAT INDICATES A PROBLEM WITH THE LOCAL BATTERY PACK CONNECTION. So the best choice is to do an experiment by disconnecting the trailer battery pack temporarily, and locating the poor connection in the tractor battery pack, or in the wiring to the local battery pack. Consider that is a much shorter connection. It might also be a failing battery in the local battery array.
Repairing the local problem is a better choice than "putting a band-aid over the bullet hole".

But if there is actually a good reason to not try to fix the local battery system, a somple series resistor of about 1/3 to1/2 ohm will limit the current in both directions

 

OK, to the TS: What I think that I see is that the current draw from the tractor battery connections encounters a high enough resistance that it draws excess current from the more distant battery pack in the trailer. THAT INDICATES A PROBLEM WITH THE LOCAL BATTERY PACK CONNECTION. So the best choice is to do an experiment by disconnecting the trailer battery pack temporarily, and locating the poor connection in the tractor battery pack, or in the wiring to the local battery pack. Consider that is a much shorter connection. It might also be a failing battery in the local battery array.
Repairing the local problem is a better choice than "putting a band-aid over the bullet hole".

But if there is actually a good reason to not try to fix the local battery system, a somple series resistor of about 1/3 to1/2 ohm will limit the current in both directions

The problem is the tractor is brand new literally off the production line so when really cold the engine draws every amp it can muster to start it now knowing this is have drivers remove the trailer charge cords before starting so that eliminates that issue problem comes when they plug cord back in and the trailer batteries(which also are brand new) have been sitting at 5-9 degrees for 2 days when the cord makes connection the batteries pull 120 amps peak enough to blow the bus fuses , we are having this issue every year with the same trailers gor same reason the company doesnt want to modify them at costs over $500 so i am trying to idiot proof these units so they just dont pull the high amps under these conditions to me easiest way is put 12 volt-12volt charge unit in remote box it only pulls 25 amp but that idea was shot down so i need alternative

 

OK, that is a rather different sort of problem. My suggestion would be suitable for an older poorly maintained truck. New trucks in good condition are not supposed to have those problems. So my advice was way off.
A power resistor could limit the current but it will produce a lot of heat. (That is what power resistors do). Another possibility Is a bank of lights, perhaps 20 incandescent bulbs that would each normally draw an amp or two. THAT could limit the charging current to the trailer batteries. It will still generate heat but not start a fire.

The obvious challenge is to reduce the charging current to "the batteries in back" but not excessivly much. Hard charging is not good for batteries, ever.

A current limiting switching regulator could do it also, but for that application it will not be a simple easy task. It would need to chop the power into very narrow pulses that would be ten to 20 amps, effectively. That will be far from simple and even farther from rugged.

Really, the charger could be an adjustable switching regulated variable voltage 12 volt powered six volt supply. Feed the rear batteries six volts, and when the current drops, raise the voltage to 8 volts, and then when the current drops, raise it higher again. That supply's design is a bit more than I have the resources to produce presently, especially for an OTR big truck.But it could be a 20 or so amp switcher supply in a reasonably sized package.