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Old 12-09-2020, 10:58 PM   #1
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Bad Idea for Jumping off House Battery?

My van does not have a switch that allows me to connect the house battery to the starting battery in the case where I find myself in the boondocks with a dead starting battery. I have the stock SMB installed Sure Power 1602 battery isolator.

In the quest for simplicity and not spending a bunch of money on something I will hopefully never need, I am wondering if I can make a jumper wire that I can connect to the house battery side of my battery isolator on one end and the other end connects to the positive side of the battery terminal. The wire would only need to be 5 feet or so and I would put clips on either side so that they can clamp on the terminals. The wire could just coil up in a compartment and hopefully never get used.

Is this a reasonable idea?
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Old 12-10-2020, 01:11 AM   #2
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To jump start a dead engine battery, the connecting bus wire going from the house battery to the starting battery needs to be a large enough to pack the necessary amps. SMB typically sized this wire much smaller for the lower amp charging the alternator puts out. So unless you upgrade the bus, it can lead to melting the wire especially during a long crank.


A couple thoughts:



Why not just carry one of those jump start lithium battery units? At least with those, they hold they're own charge. Another issue is if you kill the starting battery by morning, the chances are your house battery would be somewhat low as well. These jump starters such as the antigravity battery jump packs do work well. Even the large ones are compact for storage.



If you have solar, I say to get rid of the one way isolator and replace it with a bidirectional separator such as a Blue Sea 7622 but kind of expensive and you still have to deal with high amp cranks. But with solar, you just sit around for a few hours as the battery charges. A bypass switch could be used to jump around the isolator but not a preferred method IMO.



Do you have an inverter? You could get a large enough inverter to power a small automotive low amp charger and use that with an extension cord to charge up the starting battery.



I have a large inverter and I always carry one of those 2/10/50 amp chargers, mainly to jump start others (I don't want their problem to become mine) but it could be used to charge your own vehicle I guess.


(edit)
I forgot about these. Slow but might do in a pinch. Most of us have 12 volt outlets powered off the house battery.

https://www.homedepot.com/p/Wagan-Te...E&gclsrc=aw.ds
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Old 12-10-2020, 09:00 AM   #3
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20' jumper cables?
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Old 12-10-2020, 11:44 AM   #4
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I looked long and hard at what it would take to configure my electrical system to allow a connection between the (lithium) house battery and SLA battery for jump starts, and instead I took the two-fold approach outlined above: 20ft jumper cables and a Lithium jump start pack. Perhaps less convenient, but from a K.I.S.S. perspective you can't beat jumper cables (plus you should probably have a set to help others out anyhow). The lithium jump packs these days are simple, affordable, functional, and take up little space.
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Old 12-10-2020, 11:10 PM   #5
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Yes, I already have jumper cables and a small jump starter brick. I am more worried about being in the boonies with a dead starting battery with no one around for miles. The hot wire idea is just a fail-safe back-up idea. I was thinking that I would just make the jumper connection and let it sit without starting to put some charge in the starter battery and remove the jumper before starting it so I don't put the high amp draw through the jumper wire. I do have 240 watts of portable solar that runs through a Victron mppt but don't have the bi-directional isolator. So I could have the solar plugged in and charging my house 4D battery while the jumper wire is connected so I don't drain my house battery. Again, this is just a fail-safe. I also have a Jakery 500 solar generator and a small jump starter brick as jump starting/charging options but thought a jump wire was an interesting concept that takes no storage room and can stay tucked away in case of emergency.

I was thinking a short six foot 10 gauge wire with clips on the end would work since it is short there is not much voltage drop and there wouldn't be a lot of amps going through because I am only charging and not starting the engine while the jumper is connected.

Is my logic flawed here?
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Old 12-11-2020, 12:19 PM   #6
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No that is possible but you can just jumper over the isolator with a one foot jumper as well or is it too difficult to reach? I would recommend at good DC battery switch to kill any chance of a spark. Wire size, you just need to understand the limitations.

I was up in Yosemite and left my lights on my truck. After a short hike we returned to a almost dead battery. There was only one vehicle there and asked if I could get a jump. No cables but I had a coat hanger. We pushed each vehicles bumper together and used the hanger for the positive. Left it on for a few minutes while the guy ran his vehicle to let the battery pump up a bit and started my truck Needless to say, that coat hanger got hot. but it did the job.

Keeping it simple is a good concept but it should be a safe solution as well. Seems like the simplest solution would be that power plug charger I linked to (provided they work as advertised). No chance of spark, no probing around in the engine compartment or the house battery.


I don't have a choice...getting to my house battery terminals is not an option in my build so jumper cables are almost useless to self start my rig.
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Old 12-11-2020, 12:37 PM   #7
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While you may not be able to crank directly off a smaller-gauge wire, or off a lithium battery with a low current limit, what you *can* do is wait for the charged battery to partially charge the discharged one over that connection. I've had to do this before with undersized jumper cables. It's time consuming but beats being stranded.
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Old 12-11-2020, 01:29 PM   #8
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Quote:
Originally Posted by daveb View Post
No that is possible but you can just jumper over the isolator with a one foot jumper as well or is it too difficult to reach?

..That's what I've got....about a 9" 6ga battery cable that I can just put across the two posts of the isolator.....


If I'm stuck with a dead starting battery my plan is to jumper the isolator and relax for 15-20 minutes while the house batteries charge the starter battery thru my perhaps less than adequate battery cables connecting the banks together. I've run 2ga (not 2-0)cables so they may start a 5.4L without having to wait.


I store the small jumper cable in the passenger door pocket..cheap and simple.
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Old 12-11-2020, 01:50 PM   #9
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I've got an older build that just has a contactor connecting the two batteries when the ignition is on, so all I really need to do in that situation is turn the ignition on and wait a bit. So far it hasn't come up, though. I've depleted the house battery a few times but never the starting battery. About the only thing I use the starting battery for while parked is the stereo. (Well, half the stereo -- the subwoofer amp is connected to the house battery, because that avoided running another wire.)
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Old 12-11-2020, 03:25 PM   #10
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I split a length of jumper cable with Anderson connectors and hardwired one side to my house battery. Eventually I'll mount that half of the connection to the side door's stepwell for convenience instead of having to crawl underneath to connect. It's already saved me once when I left the lights on. Good thing about this setup is that I can jump myself, someone else without opening the hood, and I've also wired my air compressor with an Anderson connector for quick setup.
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