Charge dead truck battery (LA) from house battery (LiFePO4)

[Jim_VA]

Recently I was in a very remote area when my "check guages" light came on indicating the alternator wasn't charging. It eventually came back on (could be a bad ground?). I had a similar situation last year in central Idaho but in that case my alternator actually did die -- fortunately 5 miles before the only town for many miles and with an hour of daylight left. It got me to thinking about options should the truck batteries ever drain down. My house battery is a 200aH LiFePO4. I've been looking to the internet for answers but so far nothing related to this type of situation comes up. How could I safely recharge my truck batteries enough to get me back to civilization? Any thoughts?

[Grit_dog]

Never hurts to be prepared. Murphy’s law and superstition basically dictate the need for what you’re prepared with will somehow be less necessary than the stuff you arent prepared for…..lol.
The key is knowing when to say when, with spare parts, or with a vehicle that is too old or unreliable to fit your needs/abilities or lifestyle.

[Jim_VA]

Thanks for the input folks. I was just trying to sort out the practicality of charging from one type of battery to the other. I suspect the BMS won't allow it and if it did, the rate of charge would be more than a depleted LA battery could handle. I already keep tools plus a spare belt and water pump among other things. Why not throw in an alternator, I suppose?

[MrWizard]

A spare alternator is less expensive than replacing damaged batteries, has the advantage of being on hand, just install it and be on your way again, a preemptive strick, be prepared and have it ready , insurance like carrying a spare tire and engine belts

[Grit_dog]

Only 1 guy suggested that it may not jump start the truck. And he may be right since the OP likely has a diesel since he said batteries, plural.
However the OP is preparing for another bad alternator. In which case a jump start would not help him at all. And the equalization of 1 - 200ah battery into 2 ea 100+ ah lead acid batteries wouldn’t get him far unless it is a 30 year old mechanical injection diesel and he can drive with no other or few things using any power.
There’s a whole bunch of hokey riggin including a jump start that “may” get him down the road a little ways, but none of that matters if one is “preparing” to handle that issue.
Fck, just pack a spare alternator and a jump pack or cables or both. No need to get fancy. Just bring the part you’re worried will break. And if you’re not totally unobservant you’ll see when it quits before you use up much of the battery reserve.

The weird tangents that get explored or entertained on this forum are quite often completely baffling….

[wa8yxm]

Boomerweps wrote:
You could use jumper cables to parallel your house battery with your vehicle battery and let it charge your vehicle battery BUT LiFePO4 batteries are not made for starting heavy motors, like an engine starting motor. The power surge could cause the BMS to shut down the house battery, then you’re really screwed.
Most auto parts stores will test your battery and alternator because they want to sell parts. Let them test it.

LiFePO4 batteries can be made for high current or .. Well I have a small (10 amp hour capacity) that has started 3 cars with dead batteries so far (Strangely never my car).

I used to use a 27 amp hour AGM for jump starts.. Not always successful.

Normally I use a 73 amp hour maintenence free to start my car (The car's battery)

I have a 12 amp hour LiFePO4 I normally pull up to 25 amps off of with no problem.

A 200 AH LiFePO4 should have no problem at all jumping a car.
The resting voltage of the LiFePO4 is a bit low for charging a 12 volt Lead Acid. the Charging voltage is on the high side for that job but about what GM sets their voltage regulators at (Based on owning GM vehicles and reading the voltage on a digital meter)

[Grit_dog]

pianotuna wrote:
Connect an inverter to the house battery. Use the inverter to power a battery charger.

Or

Use a dc to DC charger between the house bank and the starter battery.

These are just to get you to a repair facility.

And good luck getting enough juice loaded into the truck batteries to even start it much less run it for any distance, with either method.
You’re a self proclaimed battery guru…maybe I’m wrong but what makes you think this is viable?

And the OP is thinking about 1 potential failure mode that “could” strand him in the boondocks, when the reality of that failure happening again with proper maintenance and quality parts is VERY slim.
If worried enough to start a conversation about it, there is a LONG list of other things to consider to “bulletproof” his setup.

[pianotuna]

Connect an inverter to the house battery. Use the inverter to power a battery charger.

Or

Use a dc to DC charger between the house bank and the starter battery.

These are just to get you to a repair facility.

[valhalla360]

Two issues:
- Starting the truck: a simple jump pack should do it unless the truck battery is completely dead.
- Running the truck: Assuming the alternator is dead, the truck will run with the ignition system powered by the battery for a while. Assuming 12v lithium, in principal, you should be able to run wires to parallel to the truck battery and extend the run time until both truck and house batteries are dead.

Best solution is find the fault in the truck and repair it.

[S_Davis]

I have a manual battery switch to bypass my 50 amp DC/DC charger and parallel my 560ah LifeP04 with the two truck batteries. Have not had to use it but there if needed. I also use 250amp BMS’s on my batteries so I can pull up to 500amps before the BMS trips.

[3_tons]

Boomerweps wrote:
You could use jumper cables to parallel your house battery with your vehicle battery and let it charge your vehicle battery BUT LiFePO4 batteries are not made for starting heavy motors, like an engine starting motor. The power surge could cause the BMS to shut down the house battery, then you’re really screwed.
Most auto parts stores will test your battery and alternator because they want to sell parts. Let them test it.

X2

[Boomerweps]

You could use jumper cables to parallel your house battery with your vehicle battery and let it charge your vehicle battery BUT LiFePO4 batteries are not made for starting heavy motors, like an engine starting motor. The power surge could cause the BMS to shut down the house battery, then you’re really screwed.
Most auto parts stores will test your battery and alternator because they want to sell parts. Let them test it.

[Grit_dog]

Bobbo wrote:
If you have a built in generator in your RV, you could carry a small 120v battery charger. Otherwise, you can carry a battery powered jump starter like this one:

Jump Starter

How would a jump pack or a jump start help a dead alternator?
(It won’t).
The only logical choice here is your other suggestion. Fire up a genny and fully charge the starting battery(s) and then drive to town, or drive til they’re dead and repeat the process.
Not a logical option to try to “charge” batteries from another battery. The even if it works somewhat, they will just equalize and best case you have 2 - half dead batteries. (Or 2 - 3/4 dead batteries in the truck and 1 half dead bigger battery in the camper. At that point save the camper battery so you can keep the lights on and beer cold for a while, while you’re camping on the side of the road. Or swap that big camper battery into the truck somehow and hope you get to town before the truck dies again AND the beer gets warm!)

BUT, here’s a crazy thought, ditch the (likely) cheap reman alternator you put in last year. Either find (not easy or cheap) a quality NEW alternator, or have the current one fully rebuilt at a local shop. Unless the reman was already a totally old and cooked alt, bearings, brushes and diodes and she’s good as new.
To explain further, remans are (I’ve learned) largely horrible because they are not “rebuilt”. They get whatever broke replaced and sent back out in a new box. And they virtually never get new diodes, which is likely (almost certainly) the cause of intermittent charging.

So in short, spend no time on concocting an elaborate band-aid and just fix the actual problem correctly.

[Bobbo]

If you have a built in generator in your RV, you could carry a small 120v battery charger. Otherwise, you can carry a battery powered jump starter like this one:

Jump Starter