You can charge your batteries from your vehicles alternator. The alternator delivers power to all the vehicles electrical components and keeps the starter battery charged. The vehicles alternator may deliver more than 180 amps. Let’s say that 80 amps are needed for the vehicle that leaves a surplus of 100 amps. Those 100 amps can be used to charge your lithium batteries.
To charge from the alternator you need an Isolator, Charging relay, or Battery to Battery charger. These devices combine the starter and lithium batteries together.
A BIM, or battery isolation manager monitors voltage and connects batteries when needed. It will connect for 15 minutes, disconnect for 20 minutes, and repeat this cycle until the coach battery is charged.
If the coach battery resting voltage exceeds 13.4 V then the BIM will disconnect. A resting voltage greater than 13.4 V indicates a fully charged battery. The BIM will disconnect if the alternator voltage exceeds 14.4 V. This protects the coach battery from over charging. If the BIM did not disconnect it could continue to put a load on the vehicles alternator – reducing the life of the alternator, or causing it to fail.
A Battery to Battery charger isolates the starter battery from the house (lithium) batteries. It also controls the current and voltage taken from the starter battery. This insures that you do not harm (or discharge) the starter battery or alternator. Based on my research a battery to battery charger is safer for the vehicles alternator! So that’s what I chose for my van build.
I selected the 40 AMP DC-DC from Renogy. It was simple to setup and so far is simply working! https://amzn.to/3C3QUIy