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.

[Steve-in-SEA]

Everyone - thanks for the advice!

My initial post above was just about adding an LFP battery as a house battery (only), turning what was a single AGM start+house boat into a start AGM + house LHP boat, but without battery combiner-type functionality for the reasons stated here. The crazy diagram is my solution to keeping it simple for the operator and avoiding some edge cases I'm concerned about.

That aside: I did for a while go down the path of simply replacing the 100Ah AGM battery with a start-capable LHP battery... from what I could tell, I could shave 50+ pounds, save some space, benefit from a better discharge capability, avoid new wires, etc.

BUT I kept running into advice to the contrary... For example:

[Marco Flamingo]

I didn't go with lithium "start" batteries. They are about 6x the cost of simply using sufficient "regular" LFP batteries to do the job. My reasoning is that electrons don't know if they are being used to start a motor. There simply needs to be enough electrons.

The mathematics on my trawler are different than my C-Dory, but it might help to see my reasoning. I pulled out big lead start and house batteries and replaced with four 280Ah LFP batteries. The manufacturer said that they are rated for 200A continuous (each), so I have at least 800A to start my 80hp diesel. But here is where things get difficult to evaluate. My batteries have Bluetooth and I can look at the BMS and see what the settings are. 200A continuous, okay, but 800A for 6 seconds and 1,120A for several milliseconds. The last is the short circuit protection. The 800A for 6 seconds isn't mentioned anywhere by the manufacturer.

Old online posts said that my vintage diesel used just over 500 amps to start. In theory, I can start with one battery (and I have). What surprised me was that my multimeter showed the starter draw at under 400A. The reason is that "over 500A" is with lead batteries that sag down to 10V when starting. LFP doesn't do that. So when calculating how many amps do I need to start, be aware that the online number will be given for lead. One will likely only need 75% of that with LFP. And no reason not to go well above that. My batteries can put out a jolt of 3,200A for several seconds. I only need 400 for a fraction of a second.

Which brings up the issue of fusing all that juice (that I won't go into here)

My understanding of LFP admonition "NOT TO BE USED AS A START BATTERY" means 1) based on the battery's traditional size it is unlikely that 2) a single battery will be sufficient to start the motor. A Group 21 sized LFP battery (100Ah) that fits in the vehicle's battery tray might not start the car. But will five 100Ah LFP batteries start your car? Yep. And you have saved money over buying the supposed lithium 100Ah "start" battery (plus getting an additional 400Ah). So the first issue is whether your boat has room for more than one battery.

Plus, there is an additional benefit to having more than one LFP. That is reducing the fear of a BMS shutting down and a voltage spike damaging your electronics. Various websites have asked for people to report having this problem. Radio silence. But we know that turning the 1-2-Both switch to Off can do this. As will disconnecting battery cables when the engine is running. However, it is a bit like a pilot accidentally pulling the "Ejection Seat" lever. It can happen in theory but actually doesn't happen that often. And you can't really blame the lever.

With more than one LFP battery online, it is almost impossible to tell if a battery has "checked out" from some default condition. A cell overvolt issue (the most common) shuts down a single battery while the cells balance and then comes back online. With a sophisticated Bluetooth BMS, one can watch this happen to a single battery (and I have with my LFPs). Kind of a non-issue, but then I have 4 LFP so the chance of every BMS shutting down simultaneously is extremely remote. Much more so than accidentally pulling the Ejection Seat lever or the 1-2-Both switch to Off.

I also have a Sterling "protection module" that senses a battery disconnect (or "batteries disconnect" for a bank) and absorbs the spike. A Youtube video shows this when the alternator is putting out 150A. No damage and the alt comes back online when the BMSs open again. Sort of a belt and suspenders given that I have 4 LFP batteries. And much simpler electronics than the various gadgets used to keep a lead battery onboard and trying to get them to play nice.

Whether one would be comfortable with a "bank" of only 2 LFP, I can't say. Three would be better. I also don't know what is available to control the voltage output of an outboard or if the Sterling protection module can be used for additional insurance. I do know that getting rid of lead can simplify things to the point where the research is worth pursuing.

The issue of having a perfect LFP charging profile from an outboard is also something of a red herring when talking about a sufficiently large bank of LFP. With lead, one needs to get the battery charged back up quickly to slow the eventual degradation. LFP isn't harmed by charging to only 80% and sitting there for a month if need be. With a trailerable boat, it is very likely that one will get to a shore charger every once in a while and simply use an inexpensive LFP charger. Get to 100% once a month and balance the cells. If when out and about you only get to 90% (to stay away from possible cell overvoltage issues when charging), you will still have 5x the available amp hours over lead.

Anyway, that is some of the general reasoning I used to get rid of lead. So far, so good.

Mark

[thataway]

Good reasoning and execution, Mark.

[Steve-in-SEA]

Mark - many thanks for posting that detailed background/reasoning! Your design process/decision tree all makes sense.

I'm guessing this is the Sterling device you mentioned?
https://www.sterling-power-usa.com/SterlingPower12voltalternatorprotectiondevice.aspx

Fascinating topic. As I try to educate myself about this, it seems there are several things to consider:

- CCA & related specs
- "load dump" scenario
- LHP pulling too much current from alternator
- Expected charge profile (LHP-designed charger vs what an outboard alternator puts out)

Alas... I couldn't find it explicit evidence, but apparently Suzuki doesn't (warranty) support the use of LHP for their outboards. Again, like other aspects of this topic (and the internet in general! there's lots of anecdata but few links to manufacturers' documentation. Maybe I'll talk to my mechanic/installer...

My current 100Ah AGM, which currently powers start & house loads, is at least 8+ years old. It *seems* healthy, but I carry a compact "jump start" battery just in case. The AGM is kept on a West Marine trickle charger when stored. Maybe it'll last until my Suzuki warranty expires?

THANKS AGAIN - Steve

PS: Supercapacitors! That's the answer to the "load dump" scenario!!

[Tom Hruby]

There is one issue with using Li batteries for starting that has not been mentioned. If you are using them for starting I assume that they are also being charged by an alternator on the motor. This is where it gets tricky. Most alternators are designed to charge lead batteries at a certain voltage and then provide a "Float" current. Most Li batteries are not designed to accommodate the charging profile of an alternator. In reading up on this issue I have seen comments to the effect that the maximum voltage provided by an alternator is too low to fully charge Li batteries;2) Li batteries should not be "floated" where a low current is constantly run through the battery when it is full; 3) Li batteries can accept high currents even when close to full, and this has an impact on the internal electronics of some alternators.

There are Li batteries that are designed for starting and alternators that can charge Li batteries but one needs to be specific when sourcing these items.

[thataway]

Basic protection of the alternator, especially in inboard boats and RV's. The Li battery can take the entire output of an alternator, and allow it to overheat. That must be protected. Some "fixes" allow charging for 15 minute and then 15 minutes no charging to cool off. Some use heat sensor on the alternator case. Many ways to skin the cat.

I think, the easiest is the DC to DC charger off the engine start battery is often the cheapest and best solution.