DC-DC Wiring - RV Solenoid - LiFePO4 House Battery

[zipsonic]

This is somewhat of a continuation of

Emergency starting

I just ordered my LifePO4 batteries for my motorhome. I realize these are not starting batteries but I just realiezed that in an emergency I can use my house batteries to start my engine. My questions are 1.) Would these start my batteries if absolutely needed. 2.) Would it damage the batteries...

diysolarforum.com

Understanding that the Solenoid automatically connects the house battery to the alternator upon startup, it would seem that in order to facilitate charging the house battery with the alternator and protect the alternator, you'd need to put the DC-DC converter between the solenoid and the house battery. Is this correct?

Also, it would appear that if you did that, you'd lose the ability to join the house batteries to the chassis battery for an emergency jump using that same solenoid. Is that also correct?

Going on the logic that solenoids fail, it seems like it would be better to remove the ability of the alternator to charge the house battery entirely, especially if you have alternate methods of charging the battery (solar and electric generator). In this setup, would you just remove the leads to the battery from the solenoid? Or does that create an "open" connection capable of frying the alternator?

What would be the best way, given this setup, to wire this?

 

[snoobler]

This is somewhat of a continuation of

Emergency starting

I just ordered my LifePO4 batteries for my motorhome. I realize these are not starting batteries but I just realiezed that in an emergency I can use my house batteries to start my engine. My questions are 1.) Would these start my batteries if absolutely needed. 2.) Would it damage the batteries...

diysolarforum.com

(1) Understanding that the Solenoid automatically connects the house battery to the alternator upon startup, it would seem that in order to facilitate charging the house battery with the alternator and protect the alternator, you'd need to put the DC-DC converter between the solenoid and the house battery. Is this correct?

(2) Also, it would appear that if you did that, you'd lose the ability to join the house batteries to the chassis battery for an emergency jump using that same solenoid. Is that also correct?

(3) Going on the logic that solenoids fail, it seems like it would be better to remove the ability of the alternator to charge the house battery entirely, especially if you have alternate methods of charging the battery (solar and electric generator). (4) In this setup, would you just remove the leads to the battery from the solenoid? Or does that create an "open" connection capable of frying the alternator?

What would be the best way, given this setup, to wire this?

Hello again.

(1) Yep
(2) Yep
(3) Yep
(4) Nope. Just disable the solenoid by disconnecting the leads that power the solenoid coil. Solenoid will be permanently open circuit.

Mine has failed. I replaced it once. It failed 6 hours later. I said eff it. I just use a cheap jumper cable clamp to connect the two terminals. I spend 99.9% of the time parked, so I'm just floating all the batteries together.

 

[zipsonic]

Thanks for the info. I think what I'm going to do is put a connector between the solenoid control wires so that if I need to connect them to start the engine, I can do that, then just disconnect them after the engine starts. Do you see any downsides to this?

Your solution is pretty elegant too. Even if I don't float the batteries together, I could just use that same jumper cable to jump start the RV if necessary.

 

[snoobler]

My solution is not elegant. It is ugly, but it functions.

I think your solution would work.

Honestly, floating the chassis battery with the off-grid system is the best way to not need jumping with the coach batteries. IMHO, you should just disable the solenoid when driving.

Note that there is usually a fuse associated with the solenoid operation. Removing that fuse would disable the solenoid.