Adding a LHP4 house battery to a small 1 AGM battery boat...

[Steve-in-SEA]

Hi folks - Hoping to share some ideas and also learn from the rest of you 😉

I have a C-Dory 16 Angler (BEHEMOTH), used mostly for cruising around and exploring here (Seattle / San Juans - day trips, 1-2 day overnights).

Currently, there's just one 100Ah AGM battery for the 50HP main and a light house load: VHF, Chartplotter, USB, etc.

Given how lightweight/small/powerful/capable LiFePo4 batteries are these days, I've been toying with the idea of adding one as a House battery (30-50Ah). I probably wouldn't do this if AGM was my only option (weight, size).

But mixing battery chemistries adds complexities due to differences in charging requirements, so I'd need a LHP4-capable DC-DC charger (example: Victron Orion XS). (Added bonus: the charger will isolate house loads from the starting circuit.)

I puzzled through various use cases:

U1: Boat in storage
U2: Boat in storage with external charger (for both batteries)
U3: Boat cruising (engine charging both batteries, house loads energized)
U4: Boat at anchor (engine off, house loads energized)
U5: Start Battery failed
U6: House Battery failed
U7: Charger failed
... and I ended up with 4 switches (and fuses as appropriate), 3 of which are new:

Engine (S1): connects Start Battery to Engine (existing)
House (S2): supplies power from S1 to Charger input
House (S3): connects Charger output to House Battery
House (S4): connects House Battery to Loads
While I realize that the Charger represents minimal drain when quiescent, I really want to be able to disconnect it from the batteries in some cases.

But this quest for conservative flexibility brings a lot of complexity for a 16' boat. I'm not sure I'd remember to flip all 4 switches the right ways at all the right times 😉

So I came up with an approach where there are just 2 switches... the expected Engine (S1) switch and a single on-off-on style 4 pole toggle switch that combines S2-S3-S4 into one. Let's call it the "House Power" switch:

UP position - Charging Only: connects Start Battery power to the Charger's input (S2) and connects the Charger's output to the House Battery (S3). House Loads are isolated.
MIDDLE position - All Off: Charger, House Battery, and House Loads are all isolated from each other
DOWN position - Charging and House Loads: similar to UP, but House Loads are also connected to the House Battery (S4).
Mapping this to the "typical" use cases above:

U1: Boat in storage -- Engine (S1): Off, House Power: MIDDLE (All off)
U2: Boat in storage with external charger -- Engine (S1): Off, House Power: UP (External charging power flows to Start Battery, and then through Victron charger to House Battery)
U3: Boat cruising -- Engine (S1): On, House Power: DOWN (Engine running, House Battery charging and House Loads energized)
U4: Boat at anchor (engine off, house loads energized) - Engine (S1): Off, House Power: Down* (House Battery not charging, and House Loads energized)
For failure use cases:

U5: Start Battery failure: bring along an automotive "Jump Start" battery to get the 50HP outboard going...
U6: House Battery failure: disconnect the House Battery (via XT60 connectors), switch the Victron Charger to "Power Supply" mode, "House Power" switch to UP or DOWN. Alternatively: swap the House Battery with the 20Ah LHP4 trolling motor battery.
U7: Charger failure: if House Loads are needed: Pull Charger's input/output fuses, and switch "House Power" switch to DOWN; otherwise MIDDLE for "All off"
So, when I'm ready to cruise, I flip the Engine switch (S1) to ON (as I do today) and "House Power" switch to DOWN. When the boat is on the hard, the Engine switch is OFF and "House Power" is UP (if external charging) or MIDDLE (All off). Simple enough that I should be able to remember it 😉

I found a toggle switch that fits the bill: Alcoswitch 8-6437630-2; it supports an on-off-on DT configuration over 4 poles. In the description above, I'm using only 3 poles. I could use the 4th pole for indicator LEDs near the switch, to visually reinforce what mode the switch is in.

The switch is intended for industrial use, and is IP67-rated... but it's not advertised as "marine rated". It would be mounted near the steering wheel, away from the engine and portable gas tanks. I realize that marine/ABYC best practices may require careful prep of the terminal connections.

(Here's a schematic)

If you've read this far: thank you, and would be very appreciative of any feedback or advice...!


*PS: Or UP, since the Victron by default looks for alternator voltage levels to commence charging. If the engine is off, the Victron won't charge (by default).

[Steve-in-SEA]

[thataway]

Way overkill for a 16, unless you are going to curise to AK....
You have overthought the switching. You cannot / should not start your engines from a 50 amp hour LiFePO4 battery.

If I was putting a Li battery on any boat, I would just charge thru the DC to DC charger. Put the battery charger on the Engine start battery, and then the DC to DC to the LI. No switch necessary.

You can have an on/off switch for each battery. I would avoid the on/1/2/all, because you don't want to directly combine the AGM and Li. The BMS on the Li would shut it down most likely.

I didn't pay much attention to your fusing of the LI, but that is very important.

The diagams are beautiful.

[Steve-in-SEA]

[robhwa]

I agree with Bob that a DC/DC charger between the starting and house battery is the way to go. I have had good luck with the Renogy DC/DC with solar input (charges from solar and/or alternator). I have this setup for two trucks/campers (RAM 1500/4-wheel camper, RAM3500/Alaskan camper) and the C-Dory 22. One advantage of this combined charger is if you do add solar, it will charge the starting battery after the house battery is full. If working properly, you will always have a full starting and house charge if sitting in sun. The Suzuki "alternator" is rated to output 40A, I think. Make sure and match the max charging amps of the DC/DC (I do this with the app over bluetooth) to about half of the rated output. Your LiFePO4 can take lots of amps that can toast an alternator. I haven't had this happen, but a buddy did burn up two alternators after switching to LiFePO4 for his trawler before he figured it out.

[Marco Flamingo]

Not sure how far down the road you are, but here's my experience with LFP batteries aboard. I still have lead acid on my C-Dory, but have gone to lithium on my trawler. All lithium, house and start.

I'll premise this by saying that about 65% of what you read about LFP doesn't apply to your application on a boat. LFP "info" comes from those using the batteries in golf carts, solar systems, RVs, etc., so take that into consideration when deciding if LFP can do what you need.

I think that the main admonition that LFP can't be used as a start battery comes from the automotive world, probably the most common application for most people. Auto batteries have traditional sizes and for a long time LFP mimicked those sizes. Group 27 was a common size for light truck, and with lead acid, big enough to turn over a V8. But the LFP that size were likely 100Ah with a 100A discharge max. Hence the sticker on the LFP battery "not to be used as a start battery." Why? Internals that couldn't support >100A, both light gauge wiring and a BMS set to shut down at >100A.

Things are changing in the LFP world. One thing is that LFP prices have come down. Now it's easy to get a 200Ah LFP for $300. Two for $600. They will weigh less than Group 27 lead but with way more usable amps. And on a boat, you are not generally limited by the size of the vehicle's battery tray.

Each BMS would likely be rated at 200A continuous and able to discharge +300A for several seconds. That's +600A. For a 50hp outboard you might need 300A for several milliseconds. So a 1-2-Both switch could be set to both for starting, running, and then switch to one battery at anchor. Even if one is run dead, the other will start a 50hp outboard in a pinch. The "problem" will be that you have way more amp hours than you need. Oh, darn.

That leaves the issue of how to charge with an outboard. The issue with LFP is that they can take as much as the outboard can give, thus overtaxing the outboard. On my inboard boat, I have an external regulator that takes care of that. I haven't looked into what is available for an outboard, but my guess is that it would be much simpler than the mind-bending schematic you have figured out. Somebody has likely already figured this out. It's just that it isn't the golf cart or solar people and research will be required.

Mark

[thataway]

There are specific LFP batteries which are "certified" for engine start. (As I suspect Marco uses) I would not hesitate to use one of those for an ourboart. However in this case we don't know what brand, and even in the same "brand" there are specific batteries which are rated with the internal BMS to handle the starting load. I bought a volt meter to measure instintanous peak current, just to test that out--an never got a chance to find out; selling first the C Dory and now the RV.

The other issue is attatching directly the LFP battery to an AGM or FLA battery, is the voltage differential, and potential damage.

Based on those factors, I would advise that if you have a LFP battery rated for starting, that you not combine it with the AGM or FLA, but use it directly, as I suspect has been done in Marco's trawler, where the start battery is LIFEPO4.

I choose a Sterling DC to DC charger in my C Dory, because at the time, it was the only one where you could easily program the amount of charging current flow between the Lead battery and Li battery. This accomidates the specific need of the outboard's demand and charging the LIFEPO4 battery. For example an outboard which max output is 44 amps, you may want to use 25 amps to charge the Li batteries, and allowing 19 amps for the house needs: (running the outboard, and any electronics not attatched to the LI battery)

In the setup I use in my SUV, I have a 100 amp hour Li battery (which runs a chest refer/freezer when traveling). It only needs 18 amps charging and uses the "Victron Orion" for that. Currently there are more DC to DC chargers available which allow adjustment via blue tooth or a connection to their respective control panel to determine the amount of current sent to the Li battery vs the needs of the vehicle, without taxing the alternator.

I have now added an emergency bank of (cheap~ $200/100 amps) batteries, solar Panels, and Inverter to run my freezer, Refrigerators, some LED lights and charge electronics, in case of a prolonged loss of power--such as we may have with hurricanes. So far that has not been needed, since I have a natural gas 18 KW generator--but if I lose the natural gas, then a fuel source becomes an issue. These only rellie on mains power (a 50 amp Li Fe PO4 charger) and Solar (1200 watts) no alternators.