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Adding an auxiliary LIFEPO4 battery, charged off the alternator, to a vehicle 1.0

A schematic drawing showing how an Auxiliary 12 volt, 100ah LIFEPO4 battery can be added to an car, boat or truck.
Included is low temp charging protection via a common electronic thermostat control board. Isolation is provided via a smart isolator. This prevents the LIFEPO4 battery from discharging into the lead acid starting battery and prioritizes the starting battery for charging. This setup is proven to work well.
Since a smart thermostat is included, it would be easy to add a heating pad to warm the battery in low temps, to allow charging below a 32 degree ambient temperature. All of this can fit into a common 12 volt marine battery box which is widely available for about $15.
Feedback on this schematic is welcome.

Added 5/2/24:
User "Mercruiser" and I had a conversation regarding this setup. He asked how is the current limited to the Lifepo4 battery. The answer is that it is only limited by the 30amp fuse and the demands of the battery. (Refer to the circuit) What I have found, is that with a 12v, 100ah battery, the typical charging current in this setup is no more than 25 amps. That usually only occurs when the vehicle is first started, after the vehicle has been off for some time so the Lithium battery has been partially discharged. It then quickly drops to something below 20 amps. (I used a DC clamp on ammeter to measure the currents) If your Lithium battery is deeply discharged, it is possible that it could blow the 30 amp fuse. I recommend that you carry spare fuses. or use a 30 amp breaker instead. This has not been a problem for me since the load I have connected to the Lithium Battery (a BougeRV refrigerator) has an automatic low voltage cutoff. The refrigerator won't run below a certain voltage. That prevents the battery from becoming deeply discharged.
Notice that I used a 100ah battery in this circuit. If you decide you need more Lifepo4 battery capacity and you obtain a 12 volt, 200ah battery, this circuit may not work for you. In my installation, the battery draws less than 25 amps during initial charging with a 100ah battery. However you can bet that if you have a 200ah battery or two 100ah batteries in parallel, that the initial charging current will be approximately 2x what I have measured. I would expect an initial charging current of about 50 amps.
You can get around that buy upsizing the cabling and the fuse from the vehicle battery to the auxiliary batteries. Your power relay may also need an upgrade. For 200ah of battery, I would go with a 60 amp fuse or circuit breaker and a 6 gauge wire.

User "Mercruiser" was concerned that this circuit could "blow up" your alternator. That shouldn't be possible anymore so than if you hooked up 360 watts (30amps) of high output lights or a high performance sound system. Make sure that you use a 30 amp fuse. It is required to protect the 10 gauge wires running to the Lifepo4 battery and the associated circuitry.
If you want to connect 200ah of auxiliary battery then you need to make sure your alternator is up to the task. Many newer alternators are rated for 140 amps and more. Many truck alternators are rated for over 200amps. If your car has a heated back window , heated mirrors, seats, etc, you can bet that you have a sizeable alternator as those are power hogs.

Also, user "Mercruiser" was wondering why the Lifepo4 battery was not fused. In this circuit the feed to the LiFepo4 battery is fused. When you add loads to the Lifepo4 battery, you need to add fusing for each load that is connected. I think that should be obvious, but perhaps not everyone knows that. (Please read this carefully) The wiring for an attached load must be sized for the connected load. And the fuse must be sized to protect the wire that runs to the load. Yes, the fuse protects the wire, not the load. The reason being that if the wire was to short to ground, you want the fuse to blow before the wire gets hot. That keeps everything safe.

Anyway, if you want to do things like this I recommend that you have a good DC clamp on Ammeter.
The Klein CL800 is a good unit. It is a very versatile device. There are other good ones out there as well.
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Dave911
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Author claims "This setup is proven to work well". Where is the data to substantiate this claim? Schematic uses a battery isolator from an unknown company instead of a proper DC-DC charger from a reputable vendor. My concern is that someone might burn up their alternator if they follow this schematic.
D
Dave911
This is a very simple circuit. Not much to go wrong. I am a degreed Mechanical/Electrical engineer. Yes, both. 40+ years of experience. If its good enough for me, it might be good enough for you. If you want to verify it, assemble it and check it out yourself. If you can improve on it, let me know and I may update the drawing. No rocket science here. I looked for an existing wiring diagram and found nothing, which is pretty crazy considering how common this should be. What kind of data would you recognize as being substantial?
This setup is literally in my car, running my refrigerator right now.
If you don't like that isolator, buy a different one. The Victron Argofet units look ok, but they aren't intelligent.
Note that this circuit is tied to the existing car start battery. If you are not comfortable working with 12 volt power, don't do this.
If you short out your car battery very bad things can happen. If you short out the Lifepo4 battery bad things can happen. Don't do that!
Also, if you disconnect your car start battery while the engine is running, you will blow your alternator. But you don't need this circuit to do that.
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