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Can anyone think why either of these wouldn’t work with the D3?

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Ask me how I know this. I managed to make a melted blob out of a 200 amp relay.

mzplcg wrote:

^^^ Exactly the right thing to do.

100 amp relays (even the constant duty ones) are not rated highly enough to cope with inrush current when first powered up.

Imagine the situation where your starter battery is fine but your 2nd (leisure) battery is 75% depleted. Regardless of the alternator output, when you start the engine and the relay connects both batteries together there will be an inrush current of at least several hundred amps. Ask me how I know this. I managed to make a melted blob out of a 200 amp relay.

Blew a 150A megafuse doing much the same

nighthawk wrote:

Current can be limited by the thickness of the cable IIRC. In fact the Traxide system is rated to 80Amps (hence the SC-80 name) and uses 13mm2 cable.

Erm, no. The thickness of the cable will dictate the amount of current it can carry safely, i.e. just before it catches fire. The Traxide limits current with electronics, not a thin cable.

But using a thinner cable in the mistaken belief that it will limit the current passed is a recipe for a fire. You have to either limit the current electronically, or beef up the cables to take the maximum needed in the application at hand.

It goes without saying that every installation should be fused appropriately.

mzplcg wrote:

nighthawk wrote:

Current can be limited by the thickness of the cable IIRC. In fact the Traxide system is rated to 80Amps (hence the SC-80 name) and uses 13mm2 cable.

Erm, no. The thickness of the cable will dictate the amount of current it can carry safely, i.e. just before it catches fire. The Traxide limits current with electronics, not a thin cable.

But using a thinner cable in the mistaken belief that it will limit the current passed is a recipe for a fire. You have to either limit the current electronically, or beef up the cables to take the maximum needed in the application at hand.

It goes without saying that every installation should be fused appropriately.

I don't agree. What you are talking about is for a resistive load (ie. an appliance... a CONSUMER of power). In case of cars, consumers are engine starter, radio, that fridge in the back, and an inverter). Batteries are a SOURCE of power. They don't consume current. You can NEVER meet the maximum needed current flow between 2 batteries. If one is full and the other is empty, a few thousand amps will flow if, and only if, you supply a cable as thick as one coming right out of a power station between the batteries. And good luck finding a relay for that

The same principle applies for jump leads. Have you ever seen jump leads melting? I haven't, unless you count some idiot trying to crank the engine of the car with the dead battery directly off the jump leads (the starter is a consumer of power) There are 100A jump leads, 400A jump leads, and 800A jump leads. You can use any jump lead to connect any 2 batteries. The only difference between them is the amount of time it takes for the good battery to replenish the dead battery sufficiently. This is why it is always recommended to connect the jump leads to the dead battery and WAIT A FEW MINUTES before cranking the engine of the car with the dead battery. If you use 100A jump leads you might have to wait, say, 5 minutes . 400A leads... 2 minutes, 800A leads... 1 minute will be more than enough.

Think of batteries as large tanks of water and appliances as dishwasher or washing machine. You can connect a tiny pipe between the tanks of water and things will still work. The 2 tanks will struggle to share the load and if one tank was empty and one was full it will take AGES to level things out, whereas if you put a nice think pipe between them things things will work much better. However you cannot connect a tiny pipe between the tank and the dishwasher as it will starve the dishwasher of the water flow it needs. It needs more water than the pipe can supply. This water/power analogy is used VERY OFTEN when explaining dual battery systems.

The actual calculations about what wire size can carry what current, voltage drop due to length of cable, voltage drop due to temperature, etc etc is what I really didn't feel like guessing or playing around with, hence me going for the Traxide kit. I'm pretty sure there's no electronic limiting in the Traxide. Not sure it's even possible to limit such currents using electronics. Pretty sure anything attenuating such high currents will need VERY BIG heatsinks to dissipate that kind of power, heatsinks much bigger than the tiny traxide box