What size battery disconnect switch?

[Guest Jeff_Miller]

I want to replace the battery disconnect switch that came with my car and I'm wondering if anybody has any guidance as to the capacity it should be.

My car is a 1936 Special still running on 6 volts. From the shop manual I see that the max charging rate is less than 20 amps and that the starter lists 375 amps at "running torque" and 575 amps at "stall torque". I think the max charging rate should be an indication as to the continuous service requirements while the starter amperage should indicate the surge needs.

Looking at the Flaming River little switch I see it is rated for 100 amps continuous service and 500 amps for 10 second surges but these are quoted at 12v/24v. But since I already know that 6 volt systems need bigger wires then 12 volt systems I wonder if I can compare this 100amp/500amp service to what I need. It seems like this switch might be big enough to run the car but would it melt when I apply the starter?

Ok, so being naive and knowing just enough to get in trouble, if I were to apply ohms law I=V/R and assume R is a constant then it seems that to be safe I would need a switch that is twice the rated 12 volt amperage when used in a 6 volt system. That is, if the numbers that I got from my service manual are a guide I would need about 40 amps continuous service but 1000 amps to run my starter.

Anybody out there have any opinions on this?

I have another question as well. The battery on my car is under the seat so getting to it is a pain. The previous switch was a though hole switch mounted on the firewall but that resulted in a very ugly installation and hole where I no longer want one. I like the Flaming River switches that I could mount to the firewall in the engine bay or I could reuse or go with another through hole switch and move it to the foot panel so that I don't have to mess up the new firewall liner I'm about to install. However a nicer solution might be to find a remote disconnect and be able to mount a little switch someplace less conspicuous; the only issue is that these seem to be 12 volt systems and I don't think they would work in a 6 volt system.

So I guess I'll solicit opinions from others as to what they like.

Thanks,

Jeff

[Joseph P. Indusi]

I am not familiar with the Flaming River switch, but the surge current is in the correct range for a 6 volt car. I would buy the best disconnect switch that I could afford and then use 1(0) gauge wire to connect to it and the battery and starter. Also use a suitably heavy ground cable. Be sure all the connections are tight with bright clean metal at all connection points.

Joe, BCA 33493

[Daves1940Buick56S]

OK, a couple of things here. First, the power limits of the switch are strictly related to current, or amperage. The power dissipated by the switch is I^2 * R, or (amps * amps) * resistance. Notice no volts in that equation. (There is also likely a voltage limit on the switch due to arc-over or insulator failure, but it's likely a lot higher than 12 VDC). So the 500 A intermittant limit holds no matter the voltage.

Next, look at the limits spec'd in the manual. A stall torque limit is when the starter armature is held fast in place and not allowed to turn. This would be the max the starter motor can draw, and is in effect the max current draw the battery needs to supply since the other current required when starting in trivial in comparison. So for you that is 575 A.

Now, the switch lists 500 A for 10 sec so you can interpolate the correct value for 575 A (remember it's current squared) and then add a safety factor since the switch parts may not respond in a linear fashion near failure limits. So I would say about 6 secs. But the reality is that 6 secs at stall would likely severely damage your starter so it's not very likely you would have more than a sec or 2 of that current once you realized it wasn't turning over and killed it.

For the running value of 375 A you're still going to have a time limit with the switch. Again, interpolating the values, I would not crank it for more than 15 sec. But we know that that is an awfully long time to crank for these old cars so I would doubt you would be up against that limit anyway.

Lastly, with increased resistances due to ancient wiring connections, etc in the circuitry and components I would doubt that you would be able to actually pull the 375 A / 575 A limits anyway.

[Guest Jeff_Miller]

Well that makes a lot more sense than the gibberish I got from flaming river tech support when I asked about using it with a 6 volt system and got back:

"Unfortunately we can not guarantee proper performance of those switches, which are designed for higher amp applications, on a lower amp vehicle. We just don’t have the data to definitively say whether they will function properly or not."

Perhaps he really meant volt instead of amp but even still, it appears they really don't know what they are selling.

I'm thinking I'll go with the biggest switch I can find (amperage wise) even if I don't need it in the hopes that if nothing else it would result in less loss going to the starter.

Thanks for all the input,

Jeff

[Guest Jeff_Miller]

I'm still trying to get the rundown on this but this looks really interesting.

I was looking around for disconnect switches when I ran across this remote disconnect: Batteries Products

I like it because I thought I could have easy access and still hide the switch someplace that isn't so obvious. I also thought that if I used the keyed version that I could eliminate the ugly key that some previous owner installed and replace it with the disconnect key possibly hidden out of site. I would then run all power through the car via the original column mounted switch and you can't believe how much joy that would give me.

This switch is 12V so I had some concerns but I liked the notion that it only needed 12v when it was flipping the switch. Hmm... I have seen voltage doublers and I thought if it only needs momentary power I should be able to use a doubler with my 6v system to supply 12v to enable/disable the switch. The trouble is that some of those switches can cost quite a bit. Hmm.... ebay to the rescue

Classic Car 6V to 12V Voltage Converter Power Power | eBay

OK, I'm still not ready to toss $125 at a solution without knowing more so I call watson's to ask about the switch and whether I could use 6 volts and temporary 12v to switch. You could see the blank stare across the phone line. Sigh... no joy.

So I'm still not ready to give up on this idea and I keep searching the web for a 6 volt remote battery disconnect solution when I run across what appears to be the guts of the watsons system at Search Results | Cole Hersee - Littelfuse Cole Hersee doesn't list any kind of burst amperage for the solenoid but they do have a technical note on using the solenoid as a battery disconnect. I'm trying to reach them now to see if they can supply a burst number but since it looks like the watson implementation and the continuous amperage is the same 110A I'm betting it will have the 750A burst that watsons advertises.

If they do, I managed to find the solenoid for sale for anywhere from $41 to $60. At that price it feels like an experiment waiting to happen

Anybody think I'm crazy or know something I don't? I'll report back if I get some good news.

Jeff

A

[Mark Shaw]

Check out this thread...

http://forums.aaca.org/f120/battery-disconnect-switch-220873.html

[Guest Jeff_Miller]

Yup, I've researched that thread and many other threads related to battery disconnects and they only seem to cement in my mind that I want to continue to use one.

It seems many people like the green knob but that isn't a good solution for me because my battery is under the seat and it would be extremely inconvenient to get to it. I currently have one on the firewall but when it was installed by some previous owner it got installed by butchering the firewall and the insulator in the cab. I'm replacing the insulator now so I'd like to not repeat that ugly installation.

Knifes, or other manually switched disconnects like the one I have could be mounted someplace else. I read a thread about putting it on the kick panel but my current device is too big to fit there and I suspect other manual disconnect switches wouldn't fit either. If I have to give up on the remote disconnect I'll get a switch that will mount on the firewall where the old one was but I'll pick a model that leaves the key in the engine bay instead of through the firewall. The flaming river big switch comes to mind.

In my search I ran across the remote disconnects and really liked the idea. The thread you pointed me to has one mention of a "Bill Hersh" possibly selling one. I did various searches and on other threads I saw a reference to a Bill Hersh in New Jersey. More searching and the best result is a Bill Hirsch Auto Restoration (http://www.hirschauto.com/) but he only has the green knob listed so that was a dead end.

The more I look at the Painless, Watson's, and Keep It Clean remote disconnects the more it looks like they are all using the 24200 latching relay from Cole Hersee and the more it convinces me that I should be able to build my own for less. I really don't want to mess around with making something that could end up torching my car so I am still patiently waiting for some final numbers from Cole Hersee but I suspect it will be very capable of handling surge power for my starter and the 110A continuous is significantly more than I'll actually need to run the car.

Jeff

[Guest Jeff_Miller]

OK, some not so good news followed by some better news.

It turns out the Cole Hersee 24200 latching solenoid only requires 1 amp to latch which is great because getting a 6 to 12 volt doubler that will pass 1 amp of current is pretty simple and cheap. Unfortunately, the 24200 can only surge up to about 200 amps and they didn't indicate for how many seconds of this current it could handle. The other bad thing is that the solenoid is not sealed and so not a good idea to use anywhere except in a protected cab.

However, my research brought me to the Intellitec 01-00055-002 latching solenoid. This guy has a continuous current rating of 100 amps and a surge of 500 amps for up to 30 seconds. If that isn't enough power its big brother 00-00507-512 brings it up to 200 amps continuous and 1200 amps surge. These are both sealed and safe in the elements. The 100A is about the same price as the Cole Hersee but the big boy gets kind of pricey; its a good thing that the little one should meet my needs. Ok, so here is the catch; these solenoids take 3 amps to latch so now I'm looking at a better voltage doubler to go from 6 to 12 volts or I use an external battery to do the latching.

Still researching.

Jeff

[Daves1940Buick56S]

One thing as you research the doublers - the latch coil is an inductive load so you may have to ensure the doubler can handle the kickback when the current is removed. You may have to put a diode across it. I assume the 3A is just to activate the latch and is not continuously flowing to hold the latch closed.

Cheers, Dave

Edited: not a pure inductive load, brain lock on my part. Lots of resistance also.

Edited February 15, 2013 by Daves1940Buick56S (see edit history)