Battery isolation ideas

[curtis73]

New-to-me 06 Express AWD. Main goal is work van with overlanding/boondock camping for long-ish periods. I more or less have the summers off, so I tend to throw a mattress in the back and just go all the places for weeks at a time.

Take a look at this. If it makes sense to you, critique it and let me know what I'm missing.



The basic idea is to have rear camp batteries that isolate from the starting battery. Using the 3-pot switch to control the solenoid means that I can isolate always, connect always, or have it connect when there is oil pressure (engine running). Basically make an on/off/auto system. Run the camp batteries dead and they'll charge once you start the engine, or if you have a dead starting battery you can flip the switch to ground and jump start from the rear batteries. This also isolates the 12v charge circuit for the trailer in case there is a travel trailer involved, so that the trailer can't suck the starting battery dry.

1) I do realize that I may have to control the solenoid via relay depending on which solenoid configuration I go with. Thoughts?
2) I have seen a lot of these systems installed without a fuse at the aux battery end of things. Even installed one myself. I just can't help but worry that if it isn't there and something happens in the 18 ft of heavy cable between the front and rear of the van, that could be a really bad day without a fuse, am I right?
3) looking at the solenoid wiring... my concern is the ability to energize the solenoid if the starting battery gets low. What kind of voltage threshold will a solenoid require to energize?

I'm just kind of generally sharing thoughts so that any of you can point out potential things I'm going to mess up

 

[Guri]

Yes, that it's a cool idea, but this is the commercial version in Amazon, Probably is more cost effective.

 

[DaveInDenver]

Voltage to reliably actuate a solenoid varies alot between manufacturer and style of construction. There is no one size fits all answer.

Just to put numbers to one for example, a typical Bosch p/n 0332019150 40A automotive type has a pull-in voltage of ≤ 8 V and a drop out of 1 to 5 volts. So the indeterminate region is at least 5 to 8 volts where you can't say what it'll do. Might pick one, might chatter. It depends on current (it needs 140mA) through the coil so the control circuit factors in and they'll usually pull in lower than their rating.

 

[DiploStrat]

Why would you go to all of this effort? (Oil pressure sensor?)

-- Simple ignition controlled relay with override switch, or, better,
-- Intelligent relay, like the popular Blue Sea ACR? (Which can also incorporate a manual overrride.)

Intelligent relay has the benefit that a charger on your camper battery (e.g. solar) will also maintain your starter battery, and, likewise, a shore charger on the starter battery will do the same for the camper.

N.B. Needing a battery disconnect to "be sure that you can still start the truck" is actually a sign of an undercapacity camper battery. Size your battery for your loads and required autonomy. Use the isolation merely as a backup.

KISS!

 

[Clrussell]

Yes, that it's a cool idea, but this is the commercial version in Amazon, Probably is more cost effective.

View attachment 661693

I’ve been running this isolator about a week and no issues yet.

 

[curtis73]

Why would you go to all of this effort? (Oil pressure sensor?)

-- Simple ignition controlled relay with override switch, or, better,
-- Intelligent relay, like the popular Blue Sea ACR? (Which can also incorporate a manual overrride.)

Intelligent relay has the benefit that a charger on your camper battery (e.g. solar) will also maintain your starter battery, and, likewise, a shore charger on the starter battery will do the same for the camper.

N.B. Needing a battery disconnect to "be sure that you can still start the truck" is actually a sign of an undercapacity camper battery. Size your battery for your loads and required autonomy. Use the isolation merely as a backup.

KISS!

Because the oil pressure sensor keeps the solenoid off until after the engine starts. If I have a dead camp battery and the solenoid energizes when I turn on the switch, I'm sending 100A (depending on the battery) to the camp batt before I even energize the starter. It's one extra precaution in case I find myself in the middle of nowhere with a failing/weak starter battery. It simply gives me more control.

And it's not complex at all. I'm simply grounding the solenoid at an oil pressure switch instead of chassis. The pressure switch is already installed on the engine from the factory.

 

[curtis73]

Yes, that it's a cool idea, but this is the commercial version in Amazon, Probably is more cost effective.

View attachment 661693

That sure looks like a great thing. Worth a shot. Even if it fails it looks like something that is easy to bypass in the boonies if needed.

 

[DiploStrat]

... If I have a dead camp battery and the solenoid energizes when I turn on the switch, I'm sending 100A (depending on the battery) to the camp batt before I even energize the starter.

Actually, you are not. See, if nowhere else: http://www.smartgauge.co.uk/nosurge2.html

Nothing wrong with doing this, but, if it were a real world issue, you can be sure everyone would be doing it. Bit of a cure for which there is no disease. If you wish, some models of Blue Sea ACR can be wired to stay open during cranking, but this is to eliminate any danger of voltage spikes. I\in the real world, a voltage sensing relay is going to open the instant that the starter hits, due to voltage drop, so it is really a moot point.

In the end, however, battery isolation is like a fuse; a last ditch defense. You need to be sure that both of your batteries are properly charged and properly sized. If you are running either one down regularly, it will not last long.

 

[WSS]

https://www.hellroaring.com/products.php

I run this one in my Jeep:

https://www.hellroaring.com/bic95300B.php

 

[VanWaLife]

I've been reworking parts of my system. Whatever device / wiring you end up selecting, my critique would be to go through a carwash or a rainstorm before selecting locations for devices / terminations / etc., then choose locations or use standoffs to ensure everything stays relatively dry. Also have a plan for when stuff fails that maybe just involves reconfiguring spade connectors (or whatever you're using) instead of cutting, stripping, and crimping. There are probably many right ways to achieve your goal, but the details will determine whether your solution is robust or failure prone.

 

[curtis73]

I've been reworking parts of my system. Whatever device / wiring you end up selecting, my critique would be to go through a carwash or a rainstorm before selecting locations for devices / terminations / etc., then choose locations or use standoffs to ensure everything stays relatively dry. Also have a plan for when stuff fails that maybe just involves reconfiguring spade connectors (or whatever you're using) instead of cutting, stripping, and crimping. There are probably many right ways to achieve your goal, but the details will determine whether your solution is robust or failure prone.

I usually build robustly and then it's still failure prone

 

[skyfree]

I don't know what kind of batteries you are thinking about, but if you go LiFePO4 (Lithium), you can forego the isolator completely and use a DC-DC charger that turns on only when the ignition is on. Use one of those maxi-fuse inline fuse holders close to where it connects to the truck battery and you are done.

 

[Andrew_S]

https://www.victronenergy.com/dc-dc-converters/orion-tr-smart

I went down this road a few months ago.
Spend the extra money & get the above offering. Totally programmable & easy to install.