DC-DC - RV Solenoid - DIY LiFePO4 House Battery

[zipsonic]

I posted this in the DIY LiFePO4 Forum, but thought it might be better here.

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?

 

[ZebraPainter]

I plan on using a DC-DC charger that is well within the capability of my alternator. It only works when the van is running and alternator can’t accidentally get connected to a deeply discharged house battery.

 

[eXodus]

The solenoid is has usually a few small cables which control it. One of them is a circuit from the engine - that switches is ON as soon as it's running.
The other one is the jump start switch on the dashboard.

You can just disconnect the engine circuit and call it a day. You still keep the ability to jump-start yourself.

The DC-DC charge controller you can wired to both leads of the solenoid - so basically "around" it.

But with lithium - for the case of jumping - hit the switch and wait - at least like 30 second to a minute - so the house battery bank is charging your dead starter battery - those things are usually discharged and not dead and even a half dead lead acid batteries delivers double the current as a lithium which is limited by the BMS.

 

[nosys70]

it depends on the size of the battery.
the C rate is the ruler, so you can check what is the maximum C rate discharge your battery can sustain.
LFP has low C rate, usually between 2 and 5.
if you have a 280aH battery , that's not a problem, because 2C is already more than needed .
if you have a 100Ah battery and a big motor to start, you could go as high as 600A, that is a bit dangerous.
Let alone that if you battery has integrated BMS or fuse , it could not let you do it.
Lead acid (FLA) are made to deliver huge current, like 10C without damage.

 

[eXodus]

it depends on the size of the battery.
the C rate is the ruler, so you can check what is the maximum C rate discharge your battery can sustain.
LFP has low C rate, usually between 2 and 5.
if you have a 280aH battery , that's not a problem, because 2C is already more than needed .
if you have a 100Ah battery and a big motor to start, you could go as high as 600A, that is a bit dangerous.
Lead acid (FLA) are made to deliver huge current, like 10C without damage.

It's an RV is probably is a bigger engine.

It's not that Lithium could not deliver that current, but depending on your battery is setup - either the BMS shuts it off or just vanishes in smoke.
Many BMS are sized at ONE C

A big block starter - pulls around 300-400A on a good day. But a starter battery usually fails during cold nights - when that thing may double easy.
Worse if you got a Diesel Engine - on cold days - sometimes 800-1000A - that's why they usually have two starter batteries. OR 24V systems.

 

[curiouscarbon]

Someone please correct me if I’m wrong, but could you not charge the lead acid starter battery with a low power solar charge controller? Boost house battery voltage and input into solar charge controller PV, maybe add new solenoid between positive house battery and positive solar charge controller.

This sounds silly to me even as I write it, but it makes sense in theory.

LFP house battery (12v?) -> solenoid switch -> boost (12v to 24v) -> solar charge controller (24v to starter voltage) -> starter battery

My thought was that this way, no huge surge current needed, and you can toggle the charge with a solenoid separate from the alternator LFP link. eg try to crank, fail, enable LFP to starter charging solenoid for 30min, try to crank again sort of thing.

Kind regards

 

[curiouscarbon]

Found in the other thread: https://www.renogy.com/dcc50s-12v-50a-dc-dc-on-board-battery-charger-with-mppt/

That thing is really expensive, Personally was thinking of an epever mppt or small victron mppt.