Possible usage of Argofet to charge one battery from another battery

DIY
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Hi Folks,

I am looking for some technical answers to my question.

A friend of mine recently bought a sail boat and wanted to change his dead AGM service batteries with LiFePO4 battery system. I am a mechnical engineer and he wanted me to assist him in selection and installation of proper system.

Boat has 80A main engine alternator (with Cristec Bat. Iso.), 2xCristec 40A AC/DC charger (running in parallel), 700W solar panel with VE SmartSolar 100/50 MPPT, Mastervolt ACMaster 2000W inverter.

With the help of local Victron Energy dealer we choose the following system.

3x LiFePO4 Battery 12.8V/200Ah NG

1x smallBMS NG

1x Smart BatteryProtect 12/24V-220A (for all loads)

1x Smart BatteryProtect 12/24V-100A (for AC/DC charger)

1x Orion XS 12/12-50A DC-DC battery charger (to regulate alternator charing)

1x Smartsolar charge controller MPPT 100/50 (overcharge protection via direct non-inverting remote on-off cable thru BMS)

+proper fuses where needed.

So far so good… Now we want to use the excess solar (when sailing or moored) power to top-up main engine battery, genset battery and additional bow thruster batteries, but all must be kept as isolated.

I read enough datasheet and documentation and came up some possible alternatives.

  1. Orion-Tr Smart DC-DC charger: Will be connected to LiFePO4 battery bank. But, I need one charger for each battery which means usage of 3. Advantage is each battery can be perfectly charged independently of their SOC.

  2. Cyrix Battery Combiners: Will be connected to LiFePO4 battery bank. Same as DC-DC charger, I need 3 of them. Unlike Orion, charge current can not be regulated, it is a simple relay operates according to voltage.

  3. ARGOFET Battery Isolators: Will be connected to LiFePO4 battery bank. One is enough to charge 3 different isolated battery. Like Cyrix Argofet does not regulate charging current. Charging takes place as long as LiFePo4 voltage is higher than any/all other battery. Therefore first service batteries have to be full (by solar) than others can be charged.

I am in favor of the third alternative (simpler and less expensive) yet dealer did not recommed that without sharing enough technical reasons.

My basic electrical knowledge says current flow from high potential to low potential. Isolator by its design will not allow any current flowing back to LiFePO4 battery. Thus, service battery can charge independently other 3 battery if service battery voltage is higher than the others.

So, when I check LiFePO4 battery absorbtion voltage is 14,2V, float voltage is 13,5V. Typical fully charged flooded lead-acid OC voltage is 12,8V, that my drops to 12,2V when it is %50 charged. Thus, LiFePO4 battery bank can charge the others easily.
Although charging current can not be regulated, typical engine driven alternator voltage is almost the same potential mostly without current control (new generations may have but old ones not) Thus FLA battery should handle current due to 14,2V LiFePO4 voltage.

Only one worst case scenario tickles my mind. During engine cranking there will be large voltage drop due to large current drawn by starter motor. In that case, current flowing thru argofet could be high. But I can protect the system via amp rated circuit breakers or somehow disable system when ignition key is on.

Question, Finally :)))

Is there a danger/risk directly charging FLA/AGM battery from LiFePO4 battery via Argofet device? (explaination is highly appreciated)

Tuna Saracoglu/Turkiye