March wrote:In which case, again, if you measure the amps between the the two negative poles of the two batteries, will you get 200 Amps, or even 100 as Rich is suggesting? I kind of doubt it--the Amps will be needed to turn the engine, not to melt the cable--or is that dumb to assume?
Unless there is another ground return path available, all of the current will need to be returned to the negative terminal. without a complete circuit, the needed amperage will never reach the starter.[/quote]
March wrote:Aha! That makes sense now. Thanks, Rich. How about the other questions? Wouldn't the battery fuse blow before the 8 gauge cable melts?
I am not aware of too many starting circuits that have fuses. Yours may be different, but I would carry a lot of spares. The initial surge current on starters increases with age and can easily go to 2X-3X original. Even just a momentarily hanging starter would instantly overload a fuse that is properly rated to protect the wiring.
Thanks, guys. That makes perfect sense now. With the fuse thing--I do not have the manual here, but I seem to remember that Xantrex requires fuses on the positive cable from the battery. But following the diagram, it looks like the fuse is meant for the "secondary" circuit to the separator (the one that accepts the skinny ground wire--thank goodness that issue is clear now) not the full amp load that goes from the battery to the engine. But when the batteries are combined, wouldn't the current flow through that wire too, regardless?
We are waiting on a Voltage Sensitive Relay (VSR) to arrive for our dual battery setup, the unit we purchase is easy to install, move the positive lead from the outboard to the VSR terminal, move the cranking battery positive lead to the battery 1 terminal, add the second battery positive lead to the to battery 2 terminal. In operation the cranking battery is charged first, then the second battery bank is connected allowing it to charge. If you have a fridge connected to the second battery there is no chance of the cranking battery going flat.
The cable size we have installed is 10mm diameter conductor, 4.5mm diameter conductor is the minimum recommended in a similar system by the Australian caravan industry.
I have a KISS system for now with a starter battery, house battery and switch, but no combiner/isolator. I keep them separate, but wish to have the house battery available for backup, in case I run the starter battery down. Here's my simple schematic...
You can see the ground jumper between the two batteries. If the backup battery is selected, all cranking amps have to run thru the ground shunt between the two batteries. That's why I'd spend a couple of bucks and make sure it up to the task.
I have a Guest dual 6A/6A charger (16102) with shorepower hookup. Each "channel" is isolated from the other. If I'm away on a weekend or such, I can charge it from the engine's alternater using the battery switch - if I had to. I've never had to. I'm usually not too far from a marina at any given time.
Bendix 1.1 kW starter loaded amperage draw is 174 Amps
The AWG 8 wire wont act as a fuse, since even 200 amps for a few seconds of cranking wont dissipate enough heat to come near the melting point of the wire (you would need the heat capacity and composition of the wire to calculate this exactly, but I dont even need a SWAG to know it wont melt.)
Still, Mike is right, for a couple of bucks I would replace with heavier gauge.
One should be able to take a cue from the size of the OEM cable going from battery bank to motor. That AWG or larger is a good choice for motor support. Anywhere else, for me the total amp peak under all possible combinations of simultaneous equipment draws, plus total length of circuit is the safest consideration.
Now I understand why my Mercury 50 hp on my 26X sometimes hesitates when I first start cranking it despite the fact the size 27 starting and house batteries were fully charged and even when I've connected them in parallel via a combining switch. The original dealer used 4 AWG from the factory battery location to the motor. Without measuring it, it must be at least a 20' round trip. Using Currie's chart and a conservative a 150 amp starter load, I need to replace it with 2/0AWG cable. That's some hefty cable!!
Shoulda mentioned that.