What would you do

[PSGB]

I’m installing a 12v lithium house bank “LiTime drop in style” from AGM. I plan to upgrade my alternator and add an external regulator. Just considering the alternator connection at this point, there are a few options as I see it, all with pros/cons wrt charge efficiency and preserving the alternator.
(1) use a FET battery isolator for the AGM starter and LiFePO4 house. Main con is a single charge profile from the alternator.
(2) alternator to AGM with dc-dc charger to house bank, possibly 2 in parallel to boost the amps. Main con is limited amps going to the house bank.
(3) alternator to house bank with a Balmar APM or similar APD then a dc-dc to the AGM.

I’m new to all of this and trying to get it straight in my head before I start considering the modules/parts I’ll need.
Thanks.

[Wotname]

Assuming the AGM is the start battery, then I would choose 3.

Recharging the start battery normally only requires a few amps for a few minutes. Keeping the house bank charged requires real effort.

Thus I would have all the smart stuff doing the heavy work connected directly to house and use a simple dc-dc charger to do the simple easy job of keeping the start battery happy.

[smac999]

all 3 will work. number 3 will have the best charging but the risk of damaging the alt if all the battery bms shut off at the same time. which is pretty unlikly. and if it does you just hope the apm works. if you are offshore and rely on the alt for power then carry a spare one.

2 is the safest but least efficient.

[misssherry]

My setup is similar to #3 except there is no dc-dc converter. Instead I have a small Victron charger to keep the start battery happy. The charger plugs into the ac circuit that is powered by the inverter when away from shore power.

[Bowdrie]

PSGB, given the choices you listed I "might" flip a coin between #2 and #3,
and hope it came out #3.
How's that for being decisive.
Have no idea of your boat/engine, or accessibility/space around the engine.
That said, another choice perhaps worthy to ponder is a second alternator.
I went that route, the big alternator for the house batts and the stock unit for only the start batts.
The little stock alternator keeps its normal internal regulator, the big one uses an external device.
Each with total isolation from each other, each in their own world.
It has been a peaceful/no stress arrangement for many years and needs no additional devices to turn on/off/moderating/controlling voltages/current between house and start batts.

[wholybee]

I choose Number 1.
Set the charge profile for LFP. It'll work for the AGM.
Number 2 is a no go because of the slow charge rate for the LFP. But would otherwise work fine.
Number 3 would work fine, but you should also add an APD. The DCDC will not be enough to protect against a load dump. With number 1, the APD isn't needed because the FET isolator keeps the AGM connected during a load dump.

[NorthCoastJoe]

The alternator power should go to the largest battery(s) then split off from there to the smaller one. So #3 wins.

[CaptainRivet]

Originally Posted by PSGB

I’m installing a 12v lithium house bank “LiTime drop in style” from AGM. I plan to upgrade my alternator and add an external regulator. Just considering the alternator connection at this point, there are a few options as I see it, all with pros/cons wrt charge efficiency and preserving the alternator.
(1) use a FET battery isolator for the AGM starter and LiFePO4 house. Main con is a single charge profile from the alternator.
(2) alternator to AGM with dc-dc charger to house bank, possibly 2 in parallel to boost the amps. Main con is limited amps going to the house bank.
(3) alternator to house bank with a Balmar APM or similar APD then a dc-dc to the AGM.

I’m new to all of this and trying to get it straight in my head before I start considering the modules/parts I’ll need.
Thanks.

See you wanna have Li-time LFP so you are on a budget build.
To give you best advise it depends on your alternator and also engine as eg newer Volvo have the D+ system which makes it quite a hassle to convert to external regulation.
What is your alternator model, whats its current rating and how is it driven, V-Belt single or double, serpentine belt? Which engine do you have?
Eg if you have single V and an 80A alt from 2000 or older, the max it can deliver is around 35A as its a dumb car alternator without internal temp regulation which can do max 50% of its rating and the single V-belt limiting additionally, here the way to go is 3) with a 30A DC2DC, not worth going external regulation and 1) your alt will go up in smoke as it cannot temp regulate itself.

To 1) only works with an internally temp regulated Alternator or externally with temp probe...it won't go full flat out but still a high load on the alternator and without temp regulation at low RPM it will go up in smoke. The one charge profil doesn’t matter at all as you charge your LFP only to 98%, the rest absorption phase is done by solar anyhow. The only con its not fool proof, you need to check max voltage at the terminal of the LFP when AGM is in float and voltage could be to high...in this case you just use a longer cable or less diameter between argofet and LFP to cause a voltage drop so the max volatge is max 14.6V at the terminal of the LFP.