Not to derail the thread but can you tell me more on your set up? I also have a LIFEPO4 "house" battery and not sure if my current standard charger will work, (seems to be for now but I`d rather not kill the battery) I also have a ACR

Dan,

By all means…. Derail away. This is a discussion forum. I’ll gather the info as you do. All good.

Thanks. Brad.

Guys,

FWIW, I think I’m good. Just put two solid days on our boat, and the charging system seems to be keeping up just fine. The batteries metered at ~12.7v after both days of running. The volt
meter just lies.

Thanks. Brad.

Grock says the safest bet is to use a DC-DC charger and I don`t need the ACR I put on .
Seems like I over designed it.

I just bought a lithium battery for my street car. Was told NOT TO RUN a regular battery charger not designed for lithium battery. It will damage the battery. A regular charger lowers the voltage as the battery is charged. A lithium charger merits raising the voltage. With lithium charger i bought, it did in fact keep raising the voltage as the battery is charged. When the battery was fully charged, it was at 14.4v. That was info from feather lite battery tech. After sitting the battery went down to 13.6v.

I have been using these battery chargers in both of my boats. You can set each bank to the type of battery being charged. It does a good job on the lithium battery in my bass boat that runs the electronics. Charges it to 13.8V. I also have one for the 36V lithium for my trolling motor, it charges to 38V

Dan,

How would you plan to charge both batteries while underway without the ACR?

One of the interesting side effects/benefits I wouldn't have thought of until it was presented to me is that, with an ACR, there is no need for a tandem battery charger for basic charging on the trailer; the ACR will automatically close the circuit and charge both. For conditioning, I will need to trip the breaker in the ACR bridge circuit, or the tandem charger won't recognize separate batteries.

Thanks. Brad.

Brad,
The engine charges the starting battery, the DC-DC charges the LIFEPO4 battery basically on its own circuit from the starting battery .
I thought using the ACR would help if the DC-DC charger started to drain the charging battery when engines aren`t running but Grock does not like combining the 2 different voltages and the ACR would combine both.
I solved that by getting a DC-DC charger that will automatically shut off once the incoming voltage (from starting battery) falls below a certain threshold.... via bluetooth control.

Dan,

Gotcha. It's a "one or the other" scenario.

Thanks. Brad.

Sorry, I was out of town and missed this.. It seems like you have something similar based on the previous responses, but here's my "plan".. not in the boat yet, but its the plan..
I will have a standard 12V lead-acid start battery and a 12V LiFEPO4 house battery. I have my new high-er output alternator (105A) going directly to the start battery. I then have a Victron 50A Orion XS DC-DC charger between the start battery and lithium battery. The input is hooked up to the start battery, and output is hooked up to the lithium. It is configurable for different battery chemistries, so mine is setup to charge lithium per the battery recommended parameters. This is a one way charger, so it can only connect input to output and not the other way. Like yours, it by default will turn on when it detects a high enough voltage, however, I'm going to be setting mine up to only turn on when the key is in the "ON" position with a 30 second delay. I'm doing this because I plan to hook up a small solar panel to keep my start battery topped off when I'm away from the boat and I don't want the solar charger to trick the DC-DC charger into thinking the engine is running and sucking power from the start battery. I will also have a Victron inverter setup to charge the lithium battery only from shore power.
I have a express cruiser, so I plan to maybe spend a night or so at anchor, so overkill for a go-fast boat, but the same logic still applies to a start-house battery setup for you guys if you go to lithium, just minus the big inverter.

The biggest downside to the setup I have is I'm limited to 50A of charging, but I'm okay with that because thats about 50% of my alternators rated output and there will be some base running load, so maybe 60% loaded?? I feel much more comfortable with that than trying to pull 100% of its rated output the whole time my boat is running trying to charge the lithium battery.. The biggest killer of alternators is heat, and the fastest way to heat it up is running at full output and low engine rpm (low speed cruising) because the fan is not doing its job...
If you want your alternator to last as long as possible, run the engine wide open and limit its output to something less than 100%... If you want to go above and beyond, I plan to have 1 of my 2 engine compartment inlet vent hoses running right next to the alternator, so the fresh intake air is hitting the alternator when the blower is running, which I usually just leave on when the engine is on.

Thats probably way more info than you wanted, but thats my plan lol