Dual battery reflections

[t00manyuserz]

So I have a dual battery setup where the secondary battery cannot be used for cranking, but is there for aux stuff in the car. Today, I discharged the main battery to 8V (someone left the door open for a few days, and I think the lights ate it...) and I tried cranking the car with the aux battery and booster cables (booster battery > main battery > rest of car), but it was a complete dud - no crank. Tried two other batteries, and zero success. I had to manually charge the main battery to 12V before the car would even consider cranking (and then it cranked and ran just fine).

This made me wonder. My 2-battery setup is just via one of those $30 relays that monitors power to the main battery and charges the aux when main is fully charged; it is not designed to be cranked from. I didn't like this idea, though, that I couldn't start the car without the battery being charged because that completely defeats the purpose of having one - it's basically a glorified cell phone charger more than anything in this case.

Would having a "proper" relay setup (i.e. not $30 but one of those $300+ ones) that actually allows cranking from the secondary battery have worked? I feel like the only way the car could've started is if the primary battery was completely out of the system, cranked from secondary, then re-loop primary battery into the system to begin charging. Is that how the more expensive relays work?

 

[COS80]

My system has a switch, like this, which allows the batteries to be put in parallel, eliminating the entire isolation circuit and allowing self-jumping. It might be worth finding something similar from another brand, because Bluesea's customer service is non-existent.

 

[NeverFinis]

I do have a 2nd automotive battery, planning to wire it in with one of those $300 isolators, which appears to be discounted now. It's on my todo list.

I recently needed to jump my starting battery due to a loose connection from the alternator. I also have a house battery that has a DC-DC charger with a reverse charge function. Hit the button on that, and truck started right up. Whether it would have worked with a deeply discharged starting battery? Don't know.

 

[Comet]

I have the RedArc kit with a BlueSea over ride for the winch and air compressor. House runs the fridge. Has been swapped through three vehicles and seems flawless to this point.

 

[Comet]

Also added a power pole at the back just in case someone is behind me in need of a jump. It was in my 80 previously just to keep on topic.

 

[250below10]

I have a system similar to @NeverFinis
But tbh, I still carry one of those Noco Boost jump starters, and this would be my first go to in a scenario like yours.

 

[NeverFinis]

I have a system similar to @NeverFinis
But tbh, I still carry one of those Norco Boost jump starters, and this would be my first go to in a scenario like yours.

I have a GOOLOO GT4000S in my tailgate storage that I completely forgot about until I got home. It has jumped a ton of vehicles, including a 6.5L diesel.

 

[250below10]

Better to have many ways to do the job than to have none right?

 

[Douglas S]

I have dual batteries as well...have for years with the ability to combine and self-jump.

If I'm being completely honest I think they are verging on or likely already are obsolete given the lithium power stations and jumper packs that are basically ubiquitous now. If I was starting my build today I'm not sure I'd bother.

 

[Shoredreamer]

So I have a dual battery setup where the secondary battery cannot be used for cranking, but is there for aux stuff in the car. Today, I discharged the main battery to 8V (someone left the door open for a few days, and I think the lights ate it...) and I tried cranking the car with the aux battery and booster cables (booster battery > main battery > rest of car), but it was a complete dud - no crank. Tried two other batteries, and zero success. I had to manually charge the main battery to 12V before the car would even consider cranking (and then it cranked and ran just fine).

This made me wonder. My 2-battery setup is just via one of those $30 relays that monitors power to the main battery and charges the aux when main is fully charged; it is not designed to be cranked from. I didn't like this idea, though, that I couldn't start the car without the battery being charged because that completely defeats the purpose of having one - it's basically a glorified cell phone charger more than anything in this case.

Would having a "proper" relay setup (i.e. not $30 but one of those $300+ ones) that actually allows cranking from the secondary battery have worked? I feel like the only way the car could've started is if the primary battery was completely out of the system, cranked from secondary, then re-loop primary battery into the system to begin charging. Is that how the more expensive relays work?

What was the resting voltage of your aux battery at the time of attempting to self jump your truck? What brand charger or relay do you have that charges your second battery?

If you want to build a setup with the ability to self jump your truck with a dead start battery you will want to some large cables from each battery + to a high amp solenoid or switch. Separate in normal mode and connect when you need to self jump.