Lead acid start battery and LiFePO4 service/house batteries - how to charge both from the alternator or generator.

We are often asked if there is a way to keep a lead acid start battery and install LiFePO4 batteries for the house and charge them from the same alternator. The answer is generally yes. Lead acid or AGM batteries should never be combined with LiFePO4 batteries.  These are totally different battery technologies and they are not compatible.  Thus, a battery combiner is not an option.  Here are two alternatives for charging both battery banks from a single alternator.

A DC-DC charger enables two battery banks to be charged from a single alternator while being isolated from each other.  The Victron Orion DC-DC charger is an excellent option.  It enables the start battery to be charged directly from the alternator and at the same time charge the LiFePO4 house battery bank with a different charging profile.  The advantages of this approach include:
– Multiple batteries of different types can be charged independently.
– Multiple battery charging profiles.
– Easy to install and monitor
– Protects the alternator
– Multiple Orion units can be wired in parallel for higher power charging.

The Orion is available in two types; isolated and non-isolated.  The isolated unit is designed for unit in boats or vehicles with battery banks that do not have a common ground.  The non-isolated unit is designed for boats or vehicles that have a common ground.  Most boats are wired with a common negative ground throughout so a non-isolated Orion unit is appropriate for most marine applications.

Below is a wiring diagram for a single Orion DC-DC charger with two battery banks:​

A battery isolator enables the charging of multiple battery banks while keeping them isolated from each other.  The Victron ARGO FET Battery Isolator enables the charging of three battery banks.  The disadvantage of this approach is that there is a single charging profile applied to all three battery banks.  Thus, if the start battery has a significantly different charging profile from the LiFePO4 battery bank, this may not be a good option.
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Advantage – multiple batteries can be charged independently.
Disadvantage – only one battery charging profile.