Kill switch for starter batteries

[David from Ontario]

I have a 2004 Thomas built bus with two starter batteries in parallel as shown in the picture. I think there may be a parasitic draw that drains the batteries over a few days. They’re hard to diagnose and repair. So I’ve started to disconnect the black negative lead on the batteries when I won’t be driving the bus for a while. But it’s inconvenient to open the battery bay and use a socket wrench to do that each time, so I want to add a kill switch inside the bus.

I’ve been disconnecting the thick black cable (Red arrow in the picture points to it. The other end of that thick black cable is bolted to the bus frame.) But there is also a pair of skinny black wires connected to the battery ground (Black arrow points to them. I don’t know where the other ends go.)

Question: am I right that I should be disconnecting both the thick black wire and the thin black wires from the battery ground via the kill switch? If so, does that mean it’s OK to connect all those black wires together? (i.e. is there a technical reason why the skinny black wires are bolted to the negative pole of one battery, and the thick ground wires is bolted to the negative pole of other battery?)

As you can see from the label (circled in yellow) the batteries offer 950 Cold Cranking Amps each, for a total of 1900 Cold Cranking Amps. I’m buying the kill switch from the Thomas dealer and the parts guy assures me it is beefy enough for this load. I’ll be running 4/0 AWG cables to the switch. I’ll also need to add in the circuit a power distribution block bus bar.

Question: how beefy does that distribution bar need to be? The one linked below is rated at 250 Amps. Is that sufficient?
https://www.amazon.ca/gp/product/B09YCTBCVG/

Thanks.

[rossvtaylor]

Hello, David from the Great White North!

It makes no difference to which terminals those ground wires are connected. And good job getting a quality disconnect switch and 4/0 cable. I don't see that you need that bus bar/distribution block, though, unless I'm missing something.

I would disconnect the large black cable from the batteries and connect that ring terminal to one side of your disconnect. Move the smaller black wires to that same disconnect stud, stacked on top of the big cable ring terminal. Then have a cable made (with the 4/0) to run from the battery to the "input" side of the disconnect switch. There's not really an input/output side on most of those...I just mean the opposite stud on the switch.

I hope that makes sense!

[flattracker]

You may wish to consider installing a battery switch. Some Crown buses came with a Cole Hersey battery switch. It would require adding a wire to connect between the batteries and the bus. It would be possible to even selectively connect/disconnect each battery that way. There are currently several on eBay.

[jimmythomas]

My 2007 Thomas HDX has a factory installed battery disconnect switch (1500 amp rating @ 12 volts! ) on the positive battery cable between the batteries and starter.
It's a 5 inch square chunk with a big lever handle and 2 battery cable connections.
Nothing available on ebay or amazone comes close.

[Rucker]

Originally Posted by David from Ontario

I have a 2004 Thomas built bus with two starter batteries in parallel as shown in the picture. I think there may be a parasitic draw that drains the batteries over a few days. They’re hard to diagnose and repair. So I’ve started to disconnect the black negative lead on the batteries when I won’t be driving the bus for a while. But it’s inconvenient to open the battery bay and use a socket wrench to do that each time, so I want to add a kill switch inside the bus.

I’ve been disconnecting the thick black cable (Red arrow in the picture points to it. The other end of that thick black cable is bolted to the bus frame.) But there is also a pair of skinny black wires connected to the battery ground (Black arrow points to them. I don’t know where the other ends go.)

Question: am I right that I should be disconnecting both the thick black wire and the thin black wires from the battery ground via the kill switch? If so, does that mean it’s OK to connect all those black wires together? (i.e. is there a technical reason why the skinny black wires are bolted to the negative pole of one battery, and the thick ground wires is bolted to the negative pole of other battery?)

As you can see from the label (circled in yellow) the batteries offer 950 Cold Cranking Amps each, for a total of 1900 Cold Cranking Amps. I’m buying the kill switch from the Thomas dealer and the parts guy assures me it is beefy enough for this load. I’ll be running 4/0 AWG cables to the switch. I’ll also need to add in the circuit a power distribution block bus bar.

Question: how beefy does that distribution bar need to be? The one linked below is rated at 250 Amps. Is that sufficient?
https://www.amazon.ca/gp/product/B09YCTBCVG/

Thanks.

Great question-the kind I like.

I looked around but Google was not particularly helpful. Best I could find is larger engines tend to crank at about 230 amps, so you're probably in the right range. That busbar is rated above that, so that should be fine as well.

For empirical data, put a DC clamp amp meter on the wire and crank it. Better if you have a max feature on the meter, that will give you a pretty good measure (note that colder temps may increase amp draw...)

[rossvtaylor]

Originally Posted by Rucker

Great question-the kind I like.

I looked around but Google was not particularly helpful. Best I could find is larger engines tend to crank at about 230 amps, so you're probably in the right range. That busbar is rated above that, so that should be fine as well.

It was definitely a good question...and I love that there are folks here who will then go search for more info. This is definitely a thinking and helpful group.

I still don't see that he (David, the OP) needs a bus bar do do what he's planning, unless he intended to add more circuits. If you (again, David) do plan to use the 250A-rated bus bar keep in mind that rating is most likely for current through the bar and studs. If you stack your existing heavy cable on one stud (bolt) along with the new cable to the battery, then clamp those tightly with the nut, that connection isn't actually going through the bus bar. A good connection between those two ring terminals won't be a source of high resistance (and resulting heat) and the current's not actually flowing through the bus bar at all. So, even though Rucker's info points to a typical draw lower than 250A it probably wouldn't matter even if you exceeded that. I believe the 250A limit would only apply to things connected to other studs and flowing through the bus bar assembly.

[David from Ontario]

Thanks for all the helpful responses. I mentioned the bus bar because I also have a DC to DC converter that will allow me to charge the house batteries from the alternator when the bus us running. I plan to connect cables to both poles of the starter batteries and run the other ends to the bus interior behind the driver's seat. Using the bus bars I will connect both the kill switch and the DC to DC converter. To ensure adequate capacity I'm thinking of making bus bars from some 1/4 inch steel bar I have and bolts spot welded to the bar. For insulation I was going to mount the bus bars on pieces of 2x4.

[Iceni John]

Cole-Hersee's M-750 battery disconnect switch is rated at 500A intermittent and 310A continuous, and is configured Off-1-2-Both. I have one (actually the older M-705) which is still working fine after more than three decades. One thing to consider: what is the maximum horsepower of the starter motor itself? My Delco Remy 42MT starter is rated at up to 10.5 HP, which equates to more than 600 amps! I know it's unlikely to ever draw that much current, and if it did it would be only very briefly. Blue Sea also makes the HD3002 or 3003, a heavy-duty Off-1-2-Both switch with a 500A continuous rating for large diesel engines in boats. If my Cole-Hersee switch ever karks, I'll probably replace it with a Blue Sea HD. Just don't buy a Perko!

John