How to make a cheap isolated dual-battery setup for $50

Looking at diagram is the solenoid on a red continues power if so I really would have a fire extinguisher on hand. But what do I know worked on things from fuel-powered MAXXIS to Mack semi's 1.5 hp outboards to ships with 300-ton crankshafts for over 30 years.
Joined yesterday.... welcome to the forum... what is a “red continues power”? Are you asking a question? Question marks help clarify that.
 
Of course what he's saying is that a solenoid presents a fire risk. Of course it might- anything added to the system presents a new possible source of failure, from a poor quality part to a poor installation. And even the factory system carries a risk of fire (Ford cruise controls, anything made by Ferrari, etc.) But the real questions for this expert are 1) Why would a continuous duty solenoid rated at 500 amps present an unacceptable or unusual risk of fire? and 2) How to improve the parts or design? (And don't cop out by saying "leave it stock.")
 
Of course what he's saying is that a solenoid presents a fire risk. Of course it might- anything added to the system presents a new possible source of failure, from a poor quality part to a poor installation. And even the factory system carries a risk of fire (Ford cruise controls, anything made by Ferrari, etc.) But the real questions for this expert are 1) Why would a continuous duty solenoid rated at 500 amps present an unacceptable or unusual risk of fire? and 2) How to improve the parts or design? (And don't cop out by saying "leave it stock.")
He might have also been assuming the power to switch the solenoid was on constant hot? Who knows except for him, and it was a drive by posting by someone who only joined hours earlier and from we will probably never hear from again.
 

dwh

Tail-End Charlie
Looking at diagram is the solenoid on a red continues power if so I really would have a fire extinguisher on hand. But what do I know worked on things from fuel-powered MAXXIS to Mack semi's 1.5 hp outboards to ships with 300-ton crankshafts for over 30 years.
No, it's on an ignition hot. Clarified in later diagrams after Dave partially sobered up.
 
Sorry if it’s been covered here, but isn’t it an option to connect a second “house” battery to your vehicles trailer aux 12v line to charge up just like a trailer battery. It should have a pre installed fuse and relay to isolate it from starting battery.
 
Sorry if it’s been covered here, but isn’t it an option to connect a second “house” battery to your vehicles trailer aux 12v line to charge up just like a trailer battery. It should have a pre installed fuse and relay to isolate it from starting battery.
Yes, I've done it for years. I have a deep cycle battery in a box that I move from vehicle to vehicle, where ever the fridge is going. I've wired a 7-pin trailer plug to the charge circuit only (2 wires). The plug goes into the trailer socket next to the hitch, and through the back hatch. When the engine is running, the battery charges. When off, it doesn't. I've also installed a volt meter into the box to keep track of the battery charge, a bunch of accessory ports, and a 800w 110v inverter on the lid. It's a bit ghetto what with the wire pinched in the door, but it works just fine. If I really had my act together, I'd wire a permanent socket off of the same trailer battery charge circuit into an access panel inside the vehicle.

Some will say that the 8-10 gauge wire and 40 amp fuse won't allow full charging or fast charging, and they may be right. But like I said, I've used it on and off for years and it works.
 
When you combine your start and house batteries, due to your start being fully discharged (otherwise known as self-jumping), isn't the current flowing between them going to be crazy high? Is that bad?
 

dwh

Tail-End Charlie
When you combine your start and house batteries, due to your start being fully discharged (otherwise known as self-jumping), isn't the current flowing between them going to be crazy high? Is that bad?
No. The resistance curve of a lead-acid battery is low in the center, high on both ends. Meaning that the resistance is higher as the battery approaches full, and also higher as it approaches dead.

So connecting a full battery to a dead battery doesn't result in a big surge or a heavy amp flow.


However, after you do that and then engage the starter, the starter will draw what it needs from the aux battery.
 
No. The resistance curve of a lead-acid battery is low in the center, high on both ends. Meaning that the resistance is higher as the battery approaches full, and also higher as it approaches dead.

So connecting a full battery to a dead battery doesn't result in a big surge or a heavy amp flow.

However, after you do that and then engage the starter, the starter will draw what it needs from the aux battery.
Thanks very much for that explanation, clears things up for me.

A.
 
Yes, I've done it for years. I have a deep cycle battery in a box that I move from vehicle to vehicle, where ever the fridge is going. I've wired a 7-pin trailer plug to the charge circuit only (2 wires). The plug goes into the trailer socket next to the hitch, and through the back hatch. When the engine is running, the battery charges. When off, it doesn't. I've also installed a volt meter into the box to keep track of the battery charge, a bunch of accessory ports, and a 800w 110v inverter on the lid. It's a bit ghetto what with the wire pinched in the door, but it works just fine. If I really had my act together, I'd wire a permanent socket off of the same trailer battery charge circuit into an access panel inside the vehicle.

Some will say that the 8-10 gauge wire and 40 amp fuse won't allow full charging or fast charging, and they may be right. But like I said, I've used it on and off for years and it works.
When you combine your start and house batteries, due to your start being fully discharged (otherwise known as self-jumping), isn't the current flowing between them going to be crazy high? Is that bad?
I have a similar setup to power my fridge. 10ga wires running to the rear cargo compartment, connected to a battery box which in turn powers the fridge. I initially had the same concern about the current spike on startup and thought I would need to limit the current, but after some testing I've found there's not much to worry about. Even when the fridge shuts down from the low voltage cutoff, the resting voltage of the aux battery is still about 11.2V. Normally I drive everyday so it shouldn't get down that far, but this last weekend I was on a road trip and didn't drive much for a few days, so it did hit the low voltage cutoff. At engine startup, the initial current to the depleted aux battery is less than 12A, and will drop fairly quickly to <9A. Worst case, if I start the engine while the fridge is running and the battery is right above the cutoff voltage, in theory it should draw about 18A max, but in reality I haven't seen more than a 13A draw.

I have the rear circuit running on a relay triggered by the accessory circuit, so power doesn't start flowing until after the engine is started, so that hopefully prevents the current backfeeding into the starter. As a preventative measure, there are 3 fuses inline (30A under the hood, another 30A at the aux battery, and an 80A circuit breaker powering the fuse box) to prevent problems in case either battery get discharged below the minimum voltage.
 
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I got the idea for my setup from our camping trailer. We had two 6v golf cart batteries in series to power the trailer, charged either by shore power or the trailer battery charge circuit. It worked fine there, so I just adapted the idea to a portable battery box. Maybe it doesn't charge as well as a properly engineered dual battery system (I'm no diesel mechanic) but I haven't seen any downside, and the portability between two cars is a bonus. It seems like the hard work of engineering a engine-on-only charge circuit has already been done, so why not take advantage of it.
 
How do I size the wire and fuses for my RAM 4500 with dual 380 amp alternators? Each one is 190 amp. I want to charge the house batteries (2 6v 220Ah each) in my truck camper.

Guestimating I'll have about a 50' run (round trip).

I have no idea how much 'need' the house batteries will require. I'm assuming I'll require #1AWG or #1/0AWG wire for that length, so do I just use 175 amp fuse for #1 and a 200 fuse for #1/0? Or do I need bigger wire due to the alternator output? Or will #2 AWG work fine instead?

I'd like to use Anderson connectors....SB175 or SB350?
 
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