Quote Originally Posted by Die SwartKat View Post
Ok, I am no expert but I have done two of my own systems before.

Also, my setup does not include the smart pass.

So if I understand you correctly then:

- 1st Aux will be in the engine bay;
- 2nd Aux is in vehicle behind front seat;
- These two will be connected in parallel with thick cabling;
- The Ctek D250S will then be connected to the 2nd Aux in the vehicle with the heat probe at its + terminal

If this is correct, then I cant see a problem apart from only one possible one. Might be a very small one.

My thinking is this, and the experts must please assist here or correct me if I'm wrong.

The D250S monitors the heat of the 2nd Aux (which should be very happy inside the vehicle) and constantly adjusts its charging current to ensure it is optimally charged.

But, "half" of this happy battery is in the engine bay in the form of the 1st Aux. The D250S wont know this though. It sees only one battery.

A battery that gets warm charges less well and slower. Well this on its own might not be a problem (batteries are in engine compartments all the time), the D250S might be merrily charging away on the cooler 2nd Aux, while the 1st Aux is taking strain as a result of this higher charging rate whilst it is much hotter and actually requires a slower rate of charging.

Something could even go wrong with the 1st Aux and this may affect your 2nd Aux and you might find that the 2nd Aux also will not charge well.

If the 1st Aux is completely buggered for some or other reason and you don't know this, it might even cause your 2nd Aux to heat up. Guessing here and I could be completely wrong, but this is the only small issue you might have.

Ideally speaking those two batteries should at least be in the same environment so that their ambient temperature is the same.

Ps. Also (but this is just me) I would not want a battery inside the vehicle with me. If something goes wrong in the way it charges and it heats up, there will be excessive gassing which is released into the vehicle which is a potential fire hazard and no doubt poisoness. Also, if not properly secured and tied down, it will become a lethal projectile in the event of a crash or roll over.
You are 100% correct in your analysis. In addition the D250S ideally needs to "see" the same cable length to each battery.