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Old 03-05-2020, 06:33 AM   #1
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Question for all toads requiring a negative battery disconnect.

I posted on another thread a question regarding the Chevy Spark but later realized that it applies to all toads requiring a negative battery disconnect. The problem is not disconnecting the negative battery cable - that in itself would be easy. It is because I also want to provide the car's battery with a charge line from the RV.

If I just disconnect the carís negative battery cable and attach a + charge line to the car's positive battery terminal from the RV and then attach a separate negative line from the RV directly to the negative terminal of the car's battery, I am not only charging the battery and powering the braking system, I am also providing the car with a 12v +/- source from the RV through the + charge line and the metal to metal negative through the towbar (RVís chassis ground to car's chassis ground). The RV's battery/alternator is now providing the 12 volts +/- to the car's electrical system instead of the carís battery.

I see two alternatives:
1. Disconnect the Spark's positive battery terminal instead of the negative. This would work but of course this goes against best practices.
2. Put an auxiliary battery (i.e. Group 27) in a box on the floor of the Spark to power the braking system with the charge line from the RV going directly to the auxiliary battery.

Does anyone have any other suggestions?


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Old 03-05-2020, 07:19 AM   #2
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The question is why do you need to disconnect the negative terminal? The Spark is a pretty simple car without a bunch of sophisticated electronics. Either add a charge line and leave the battery connected or don't add a charge line, disconnect the battery and wire in an always on 12V outlet for the brake.
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Old 03-05-2020, 08:53 PM   #3
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If, for example, your toad is equipped with circuits you don't want energized, but you want to charge the toad battery to ensure your braking system has enough power without drawing the toad battery flat, it's easy enough to arrange the wiring to do this. I have a negative battery disconnect on my Wrangler that clamps to the negative post. It has an integral knife blade switch that you simply open to disconnect the battery. Power to all the systems such as radio, headlights, whatever, are connected to the end of the disconnect such that opening the knife blade keeps them from getting any power. Meanwhile, the thing you want energized, your braking system, is connected directly to the two battery posts, along with your positive and negative charge wire. You definitely want both a positive and negative wire from the tow vehicle for this to work.
Some much smarter forum members can probably explain this better, and maybe even point out a simpler way to manage things, but this is what finally let me keep my invisibrake energy hog working on long tow days with no dead battery.
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Old 03-06-2020, 05:37 AM   #4
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Quote:
Originally Posted by Dark Sky View Post
If, for example, your toad is equipped with circuits you don't want energized, but you want to charge the toad battery to ensure your braking system has enough power without drawing the toad battery flat, it's easy enough to arrange the wiring to do this. I have a negative battery disconnect on my Wrangler that clamps to the negative post. It has an integral knife blade switch that you simply open to disconnect the battery. Power to all the systems such as radio, headlights, whatever, are connected to the end of the disconnect such that opening the knife blade keeps them from getting any power. Meanwhile, the thing you want energized, your braking system, is connected directly to the two battery posts, along with your positive and negative charge wire. You definitely want both a positive and negative wire from the tow vehicle for this to work.
Some much smarter forum members can probably explain this better, and maybe even point out a simpler way to manage things, but this is what finally let me keep my invisibrake energy hog working on long tow days with no dead battery.
This is what I did works fine for 25000 miles. Opening the knife switch disconnects the car electrical while allowing the battery to be charged as described above.
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Old 03-06-2020, 05:59 AM   #5
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Yes, that photo shows exactly what I did. Thanks for posting!
And, fwiw, my next toad does not have much room at the negative terminal at all. The car is a 2020 Ford Edge. Another member here set up their Edge with the disconnect on the positive post and it's working for them. Not perfect since there's more likelihood of accidentally making sparks, but in the case of the newer Edge it works.
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Old 03-06-2020, 06:02 AM   #6
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^^^^^^^Ditto
Did same on the Jeep except negative post.
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Old 03-07-2020, 06:01 AM   #7
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Thanks for responding luvlabs!

Quote:
Originally Posted by luvlabs View Post
The question is why do you need to disconnect the negative terminal? The Spark is a pretty simple car without a bunch of sophisticated electronics. Either add a charge line and leave the battery connected or don't add a charge line, disconnect the battery and wire in an always on 12V outlet for the brake.

The Spark has electric assisted steering. While there is some disagreement among various posters as to what system needs to be powered down, the symptom itself is not in disagreement. The steering tries to center itself when cornering. Chevrolet has issued a bulletin for those Sparks with an owner's manual that doesn't require disconnecting the negative battery cable. I don't know all the years involved, but my 2020 owner's manual requires the battery disconnect.
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Old 03-07-2020, 06:21 AM   #8
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Thanks for the reply georgelesley!

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Originally Posted by georgelesley View Post
This is what I did works fine for 25000 miles. Opening the knife switch disconnects the car electrical while allowing the battery to be charged as described above.

This was exactly the procedure I was going to follow. However, Some say it is the ABS system while others the Anti-skid system. No firm answer yet. Regardless, if the car's battery is not disconnected, the electric assisted steering tries to center itself when cornering. Obviously, a + charge line from the RV to the car's battery supplies positive power to the car's battery. Then it occurred to me that the RV's chassis ground supplies the negative through the metal to metal contact via the towbar to the car's chassis. This negative is from the same source (RV battery/alternator) as the + charge line. The problem: regardless of which system needs to be powered down, if that system is not electrically isolated from the car's chassis ground, the system will have both + and - minus power from the RV battery/alternator.


Some Spark owners claim that they do not disconnect the battery and have not had any problems. I don't want to take that chance (Goodby warranty).
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Old 03-08-2020, 09:48 AM   #9
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With the knife switch opened, I have not seen any circuits 'completed' by grounding from the bus. If you're really concerned, you could try towing without a charge line. A lot depends on how much juice your braking system needs. For example, if you use one of the air brake systems, your braking system won't take any power at all (I believe, someone correct me if I'm wrong please.) Your power requirement would depend on what you do for brake and tail lights.
Another option would be to install your battery isolation switch on the positive post. Not preferable, but that would certainly guarantee that your toad electrical systems are not getting energized, as long as you wire your charge line directly to the battery and not to the 'isolated' size of your switch.
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Old 03-08-2020, 11:04 AM   #10
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I agree!

Quote:
Originally Posted by Dark Sky View Post
With the knife switch opened, I have not seen any circuits 'completed' by grounding from the bus. If you're really concerned, you could try towing without a charge line. A lot depends on how much juice your braking system needs. For example, if you use one of the air brake systems, your braking system won't take any power at all (I believe, someone correct me if I'm wrong please.) Your power requirement would depend on what you do for brake and tail lights.
Another option would be to install your battery isolation switch on the positive post. Not preferable, but that would certainly guarantee that your toad electrical systems are not getting energized, as long as you wire your charge line directly to the battery and not to the 'isolated' size of your switch.

My brake system will be electric and I would be afraid to not use a charge line. Thanks to a lot of input from this forum, I have narrowed down my choices to either a disconnect at the positive terminal or a portable deep cycle battery in the toad. Walmart has a Group 24 deep cycle for $74.00 and I have a spare battery box.



The positive disconnect would be less work at hookup but would require a lot of insulation as there is a complicated connection to the battery - not just a single cable but a box with three wires connected to a plate at the bottom (see photos). For the extra battery, I would have to run four wires through the firewall. The extra battery would be charged by the RV (with a diode to prevent back flow) and provide 12V power for the brake system and brake away. The extra battery is much more cumbersome as I would still need to disconnect the negative cable. My MH will be in storage until April and I still have time to ponder my choices.
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Old 03-08-2020, 07:50 PM   #11
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I understand your thinking. The risk is that your Spark could be damaged if certain systems are energized while towing, even with your negative post isolated by a switch. Your battery looks like an isolation switch on the negative post will be very easy to install. When the time comes, you might consider installing a knife gate switch on your negative post and doing some testing. You could run the positive and negative charge lines as temporary jumpers and with your Spark hooked up to the motorhome with your towbar you could check whether any current is getting to the systems of concern. With the knife switch thrown on my Wrangler, everything is as dead as a farm wagon. No detectable power to lights, horn, door locks, nothing. The Spark might be different, but you could test for power at the fuse box for the circuits that put your car at risk. If successful, you could save the hassle and expense of an extra battery, and you could avoid the somewhat messy positive post disconnect. Just an idea.....
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Old 03-09-2020, 06:24 AM   #12
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Thank you Dark Sky!

Quote:
Originally Posted by Dark Sky View Post
I understand your thinking. The risk is that your Spark could be damaged if certain systems are energized while towing, even with your negative post isolated by a switch. Your battery looks like an isolation switch on the negative post will be very easy to install. When the time comes, you might consider installing a knife gate switch on your negative post and doing some testing. You could run the positive and negative charge lines as temporary jumpers and with your Spark hooked up to the motorhome with your towbar you could check whether any current is getting to the systems of concern.
That is great idea! As I stated in a previous post, my MH will be in storage until the middle of April. I had anticipated getting the Spark ready prior to that. I believe that the best strategy is to wait until I can verify that no systems become energized when the charge line from the RV and the towbar are in place. Thank you Dark Sky!
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Old 03-09-2020, 05:22 PM   #13
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Roadmaster makes a battery disconnect that disconnects the positive side of the battery. My battery is set up the same way as yours. Just remove the positive post and put the disconnect solenoid between the two. You'll need to add a short 6" cable.

What's nice about the disconnect, it uses a momentary switch that mounts inside the car.
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Old 03-22-2020, 01:58 AM   #14
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I disconnect the positive cable and tuck it down beside the battery on my 2018 Spark. It literally takes less than a minute. I keep a spare 10 mm socket on a 1/4" rachet on the driver's floorboard while towing so that I'm never looking for a tool. A single nut has to be removed. You flip back the red plastic cover, undo the nut, and the multi wire connector comes off the battery in one piece. Put the nut back on the post so that you dont lose it.

I used to keep the rachet in the hatch of the Spark but you cant open the hatch with the battery disconnected, at least not on the basic LS model, even with the key.
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