I want to build a "portable" dual battery/power bank setup.

[Kevin B.]

I have a 105 amp hour marine battery I want to rig up as a poor man's dual battery/power bank similar to one of these. I don't want to dedicate it to just one truck, I want to move it from rig to rig or into the boat or into the house and whatnot. But I'd like to be able to charge it from the rig's alternator while we're on the go, in sort of a temporary dual battery setup.

I'd like to get a solenoid mounted in the box with the battery, something like this maybe? And then run cables w/connectors from under the hood into the cargo area. The alternative would be to install a solenoid under the hood of each rig, which means more solenoids, but maybe that's the better way to do it?

And then I'm unclear about wire sizing. I have this chart that I'm looking at, but should I size my wires off my alternator output or the solenoid capacity?

 

[Unimog]

Lots of tradeoffs with where to put the solenoid. I will say that charging a battery is very dependent on voltage at the battery. If it is not going to be charged at the same rate as the vehicle battery, then you really need a second charger that runs off the alternator and has the ability to step the voltage up when needed. The other big issue stems from the same issue of charging effectiveness being a factor of voltage. If you aren't going to have a separate charger that can step the voltage up, I would run the wiring to the battery two or three times as large as it has to be to carry the amperage. This is just to reduce the voltage drop that is so significant at 12 volts.

 

[J-mobzz]

I dont have much feedback other than I have that exact same solenoid you linked in two of my vehicles. One with the Solenoid in the engine compartment near the main battery and one with it in the rear of the vehicle next to the AUX battery. I didn't put much thought into any of it but they both seem to work just fine and the Aux batteries stay charged. A couple weeks ago my fridge shut off due to low battery voltage on my aux system. Vehicle still fired right up and aux batteries where back to full charge after driving it around some.

 

[J-mobzz]

Actually this is the kit I used. https://www.amazon.com/gp/product/B07YHNFLHK/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

 

[Kevin B.]

Actually this is the kit I used. https://www.amazon.com/gp/product/B07YHNFLHK/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

So to @Unimog's point, maybe I'll step that 7ga up to 4ga or so.

No other problems with that setup though? I like that those kits don't need a trigger like a traditional solenoid.

 

[J-mobzz]

One of them has been in for a year and a half with no issues the other has only been a few months. But so far so good

 

[Corban_White]

Is your goal to be able to start the vehicle with the portable box if your accessories drain the vehicle battery? Because the solenoid in the box will just disconnect the portable battery if it starts to get low. Or are you planning to run the accessories off the portable battery and just want to disconnect it from the main battery when it gets low? And then reconnect when the vehicle is running to charge it?

 

[nnnnnate]

@J-mobzz Do you have dedicated AUX batteries in each vehicle?

I'm wondering the same thing Corban, what are the accessories going to be connected? I guess the issue with having them connected to the AUX battery is that when its not in the vehicle none of them work.

 

[J-mobzz]

@J-mobzz Do you have dedicated AUX batteries in each vehicle?

I'm wondering the same thing Corban, what are the accessories going to be connected? I guess the issue with having them connected to the AUX battery is that when its not in the vehicle none of them work.

My setups are not portable. I have Aux battery systems in both vehicles. This is mainly for running a fridge that’s only used when I need it but the few hundred dollars it costs to add a 100 amp/hr deep cycle battery and solenoid is worth the peace of mind to me to not be left stranded when out alone.

Side note is that while the aux battery isn’t used for starting because it’s disconnected when not running I also have a small lithium jump pack in all my vehicles that actually does trigger the solenoid when hooked up to the main battery because it’s output voltage is over the threshold of the solenoid allowing the aux battery to help with cranking along with the jumper pack.

 

[Kevin B.]

Or are you planning to run the accessories off the portable battery and just want to disconnect it from the main battery when it gets low? And then reconnect when the vehicle is running to charge it?

Mostly this. I have a little jump pack, and if I needed to jump off the portable battery I could always use jumper cables.

 

[Kevin B.]

I'm wondering the same thing Corban, what are the accessories going to be connected?

You know, just the necessities - fridge, camp lights, tent fan, margarita blender, massage chair, that kinda thing.

The goal is not so much "dual battery that moves from rig to rig" as much as "power pack that I can recharge on the fly".

 

[Corban_White]

If "power pack that I can recharge on the fly" is the goal then I would skip the solenoid and battery cables all together. Install a small DC DC charger (like this)

into the box with the battery and wire it to a male cigarette lighter plug. Throw in a couple of these (or similar) for your devices:

Then it moves from vehicle to vehicle with your devices and the only thing you have to do is plug it in to the vehicles cigarette lighter to charge.

 

[johngottfredson]

I’ve got an extra one of these I used in my mobile office van for aux power. Turn key, portable. I was going to list on ksl this morning actually. Pm if interested? National Luna portable power pack I picked up from Paul at Equipt, with deep cycle battery and 1000w inverter attached. I hate having to cobble stuff together because A) I’ll do it wrong, and B) see “A”. So this was awesome for me
John

 

[Kevin B.]

View attachment 129402
I’ve got an extra one of these I used in my mobile office van for aux power. Turn key, portable. I was going to list on ksl this morning actually. Pm if interested? National Luna portable power pack I picked up from Paul at Equipt, with deep cycle battery and 1000w inverter attached. I hate having to cobble stuff together because A) I’ll do it wrong, and B) see “A”. So this was awesome for me
John

PM incoming. That power pack was what got me thinking about building my own.

 

[johngottfredson]