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System is up and running. Funcitonal, EXCEPT... it seems like the DC to DC output isn't high enough to trigger the ACR to combine the batteries for parallel charging from the DC to DC. There are some DIP switches that can boost the DC to DC charging voltage a bit, I'll try that. Otherwise I'm going to go to a primary/reserve setup, where the 1st lead-acid battery that's connected directly to the DC to DC will be the primary, with the 2nd lead acid as backup only, to start the motor if the LION and the 1st lead acid battery are both discharged. Not exactly what I was hoping for but still much better than the original.

I just finished the install of the Suzuki SMG4 multifunction gauge and harness. Oh, man, I hope I didn't mess anything up, there were a LOT of wires from the tach and trim and troll mode switches to disconnect. The gauge fit in the tach hole (which was badly placed by the original owner), and switching the motor to DDC output was relatively easy. The gauge was easy to initialize, with the single complication that the default communication protocol was wrong (has to be Kline not Can2). Now, engine data shows up on my SeaTalkNG network, so I have things like trim, oil temp, RPM and fuel consumption available on the plotter and on my phone. The gauge has inputs for two fuel senders: I'd love to connect those but I am not sure how to make holes in my tanks without dropping plastic shavings in the fuel. Does one just take the tanks out, drain them, flip them over and drill upwards from below with a vacuum running?

Pics of the ACR and all of its connections would help.

Generically, the ACR will not combine the batteries until a set voltage for a set time has been seen. It will disconnect if the voltage drops below another set point.

Boy this all sounds a bit complicated for me to add Li batteries…

We’ve talked about this a couple times with regard to electrical (house) power for extended cruising.
After looking at several different configurations and options the one that made the most functional and versatility sense for us would be to just buy a pre-packaged type Li battery/inverter package. It comes with a case and handles for safely moving it around. They are also set up to take a 12 VDC input charging supply which it then converts via solid-state circuits and have solar electrical connections.
An up side is that the various systems are also reasonably convertible to Solar input.

Best Regards,
Over Easy

The 'existing system' in our 26X was a new Interstate lead-acid starter battery connected to the motor and it's gauges, while the lights/VHF were disconnected.

I've added a 100Ah LifePO4 house battery next to it, a 130W flexible solar panel on the hatch and a 30A DC to DC MPPT solar charge controller. So far they've existed as separate systems, the Nissan's 11A alternator charges the lead acid battery while it's running and the solar panel charges the house LifePO4 battery via the MPPT controller. It's been fine charging phones/iPad, powering a Starlink Mini and running lights throughout the evening.

The DC to DC converter has settings to manage a lead acid starter battery and a LifePO4 house battery using a solar array as well as a secondary input, such as a dedicated charging unit or discrete alternator out. The plan is to connect the lead acid battery as well so that both batteries can be charged via the converter, while their levels are monitored via the Bluetooth app.

This will let us recharge the house LifePO4 via the alternator on cloudy days while always prioritizing the starter battery stays at a sufficient charge level. That is if it all works as designed, this is the unit: https://hqsolarpower.com/12v-24v-30a-bu ... with-mppt/

tuxonpup,

If the 130W panel is able to keep your battery charged then the alternator will probably work pretty much the same once you get everything hooked up and working. Both the panel and the alternator are putting out about the same power. That's fine if you are already using the engine to move the boat but an outboard motor is a very expensive generator if all you are using it for is to charge the battery.

I don't know how long it takes to charge your battery with the solar panel but that's about the same amount of time you will need to run your outboard to charge on cloudy days.

Outboard manufacturers consider 100 hours to be an average year's run time. Just something to think about.

Now that we have one trip under our belts with the LiFEPO4 battery, I should update this idea in case anyone follows my footsteps.

What I discovered is that the system mostly works as planned, but with one particular problem. When leaving the boat on shore power, so that both my normal single-bank AC charger w/ACR and my 2A AC Li charger have power, is that the fridge and bilge pump circuits draw more than 2A. I thought that the single-bank AC charger w/ACR would handle those loads, but it turns out that the DC to DC output provides higher voltage than the AC charger, so the current gets drawn from the LiFEPO4 battery BEFORE it gets drawn from my AC charger. This led to discharging the LiFEPO4 battery while hooked up on shore power.

Two possible solutions: change to a 10A AC Li charger (or any one that exceeds the storage loads of the boat), or turn off the switch on the D+ contact of the DC to DC charger while the boat is plugged in to shore power.

I think I'm going to add the 10A Li charger, that way I don't need to muck around with turning the D+ on and off. 10A is more than my house will draw, and actually more than my 6A single-bank AC charger.

AI seems to approve:
With a unidirectional DC – DC converter (Li → Batt 2) and D+ tied to the Li-ion positive, here’s exactly what happens—and why it does what you want:
- Li-ion sits at ~14.2 V after your 2 A AC charger tops it.
- D+ “sense” sees that > threshold (usually ~11–12 V), so the DC – DC wakes and discharges the Li-ion into Batt 2.
- All house loads on Batt 2 (or “Both”) then pull first from that DC – DC source (i.e. your Li-ion bank) until the Li-ion voltage falls back to the sense cutoff.
- Once Li-ion drops near ~11 V, D+ falls below its turn-off point and the DC – DC shuts down—leaving your now-lower Batt 2 voltage to be carried by the two AGMs.

It actually suggested an interesting but problematic improvement:
- Use a relay instead of a toggle switch
• Pick a 12 V automotive relay (30 A or so), coil powered by the AC charger’s 12 V output.
• Wire the relay’s switch contacts in series with your DC–DC’s D+ lead.
• Result: as soon as the AC charger goes live, the relay pulls in and opens the D+ circuit automatically—no manual flip needed


AI is great, but the suggestion wouldn't work and the AI needed a prompt to add the information below, after I suggested that the relay would defeat the purpose of using the LiFEPO4 battery energy FIRST, as the relay would open the D+ circuit any time that the house batteries were charged. If my AC charger has a remote-on terminal then a relay would work, I think.
- Use the charger’s remote-on terminal (or its 12 V “load” output)
• Most single-bank smart chargers provide either a “remote on/off” terminal or a small 12 V output that only goes hot when AC power is present and the unit is in float.
• Wire your relay coil to that terminal (through a 1 A fuse)—so the coil only energizes when the charger is actually plugged in and functioning.

Starscream,

I don't remember the size of your lithium bank but I thought you said you either had or were thinking about a pair of 135Ah batteries. If I'm remembering wrong just modify the math as required.

LiFePO4 batteries are usually charged at .2C to 1C so for a 270Ah bank that would suggest a 54A to 270A charger. You can get by with less but if you are actually using 270Ah while you away from shore power a 50A charger is going to take more than 5 hours to recharge your house bank. It will take even longer if you have other loads on it.

Be Free wrote: ↑Mon Jun 30, 2025 12:53 pm Starscream,

I don't remember the size of your lithium bank but I thought you said you either had or were thinking about a pair of 135Ah batteries. If I'm remembering wrong just modify the math as required.

LiFePO4 batteries are usually charged at .2C to 1C so for a 270Ah bank that would suggest a 54A to 270A charger. You can get by with less but if you are actually using 270Ah while you away from shore power a 50A charger is going to take more than 5 hours to recharge your house bank. It will take even longer if you have other loads on it.

I have two 80AH lead-acid batteries with the 1-2-both selector, which are linked with an ACR, charged by a 6A AC shore-powered charger wired on battery #1. These start the motor and are charged by the motor alternator. Then, I added a 100AH LiFEPO4 battery and DC to DC charger wired to LA battery #2, and a 2A AC lithium charger also wired to shore power. Then, I have 100W of solar which sends power through a small selector switch to either the LiFEPO4 or battery #1.

I have to do some more testing and observing, but I'm not 100% sure the DC to DC charger creates enough output voltage to trigger the ACR to connect the LA batteries. When I initially installed it, it did not, but I changed the DIPs to get a higher output voltage from the DC to DC. It's been a busy spring and I haven't been on the boat much, but should have some time shortly.

I did all that because I didn't want to splurge on the two big LiFEPo4 motor-start-capable batteries that would have given me 270AH with just two batteries, but I'm kinda regretting that now. Maybe next year's budget.

One thing I knew but didn't really realize until this last trip with the whole fam and now girlfriends as well, is how much freakin' energy charging 6 cellphones overnight takes: could easily be 25AH overnight. That and watching the F1 grand-prix and part of the 24 hours of Lemans and a movie on the TV completely drained the lithium and one LA battery over the course of a 3-day trip a couple of weeks ago. Mast was down though, shading the solar panels so I couldn't get much more than a couple of amps out of them.

Starscream wrote: ↑Mon Jun 30, 2025 1:54 pm I have two 80AH lead-acid batteries with the 1-2-both selector, which are linked with an ACR, charged by a 6A AC shore-powered charger wired on battery #1. These start the motor and are charged by the motor alternator. Then, I added a 100AH LiFEPO4 battery and DC to DC charger wired to LA battery #2, and a 2A AC lithium charger also wired to shore power. Then, I have 100W of solar which sends power through a small selector switch to either the LiFEPO4 or battery #1.

I have to do some more testing and observing, but I'm not 100% sure the DC to DC charger creates enough output voltage to trigger the ACR to connect the LA batteries. When I initially installed it, it did not, but I changed the DIPs to get a higher output voltage from the DC to DC. It's been a busy spring and I haven't been on the boat much, but should have some time shortly.

I did all that because I didn't want to splurge on the two big LiFEPo4 motor-start-capable batteries that would have given me 270AH with just two batteries, but I'm kinda regretting that now. Maybe next year's budget.

One thing I knew but didn't really realize until this last trip with the whole fam and now girlfriends as well, is how much freakin' energy charging 6 cellphones overnight takes: could easily be 25AH overnight. That and watching the F1 grand-prix and part of the 24 hours of Lemans and a movie on the TV completely drained the lithium and one LA battery over the course of a 3-day trip a couple of weeks ago. Mast was down though, shading the solar panels so I couldn't get much more than a couple of amps out of them.

TLDR: Your lead charger is marginal. You need more lithium charge capacity. Consider turning your dc/dc charger around and use to charge your lithium battery. Revise your power budget. Use lead for start; lithium for house.
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Got it.
You have one 80Ah LA battery that can be charged from the engine (27A), shore power (6A), or solar (about the same).
The second 80Ah LA battery can be charged from the ACR or the DC-DC charger.
There is a 100Ah LiFePO4 than can be charged from shore power (2A) or solar (6A).

You have learned an important truth: anything with a screen uses a lot of power. Just wait until you see what heating elements, compressors, and motors do to your power budget.

Your experience reminds me of the adage* that "no plan survives contact with the enemy". The key is to adapt after that jarring contact with reality and come up with the next version that we hope will work better.

Your charging needs are around 60Ah per day in excess of current charge sources based on "draining" your 100Ah lithium and one 80Ah lead battery over 3 days.

Rule of thumb: lead acid should charge at around 0.2C - 0.3C. You have two 80Ah batteries that are effectively charging from the same AC charger so you should aim for around 16A to 24A of shore power charger for the lead acid batteries. The ACR does not add any appreciable losses so you don't need to factor it in.

I would not recommend using lithium to start your outboard. It's possible but I don't think it's a good idea (see my setup at the end). Lead makes for a great start battery; lithium makes for a great house battery. Given the current prices for lithium batteries, any deviation in either direction is sub-optimal in my opinion.

Rule of thumb: lithium should charge 0.2C to 1C. Your 100Ah lithium battery needs more than a 2A charger (.02C). Consider a minimum of a 20A and up to a 100A charger (more if you plan to add more lithium in the future). The only charge sources you had for your lithium battery were shore power (2A) and a maximum of 30Ah from the solar. Best case you would have put 30Ah back into the battery leaving a 70Ah deficit to be made up in your slip. 70Ah/2A is 35 hours to recharge your battery (ignoring any losses). I don't think that's going to work.

Your lithium battery has no path to be charged from the engine. Once both of your lead batteries are charged and you are running the engine you have no way of getting any of that excess power to the lithium battery. That is potentially 27A that is 100% going to waste. If you can get that into the lithium battery you will cut down on how long the charger has to run at the slip.

Bottom line: you might have barely enough capacity to keep your lead batteries charged on shore power but 2 to 3 times more would be better. Your lithium is going to need a much larger charger. If it is capable of charging a lithium battery, consider turning your dc-dc charger around and letting the #2 battery charge the lithium when the engine is running. When you are on shore power each battery type should have it's own ac powered charge source.

You need a power budget to design this correctly. You not only need to know how many Amp hours you need but when you need them. For me, and I suspect for you as well, I use more power at night than I do during the day. My house bank is designed to deliver what I need over a 24 hour period and my charge sources are designed to replace any remaining deficit in around 2 hours. That's the longest I want to listen to a generator run in the morning before I raise anchor.

For my boat, I have a single lead start battery hard-wired to the engine (can't blow diodes that way). All house loads are connected to my lithium house bank. There is no way any house load can kill my start battery. No matter what I leave running at night, my engine will start in the morning and the engine will quickly recharge the start battery.

I also have a dc-dc charger but it is sourced from the start battery and charges the lithium bank. That way my engine can charge my lithium after it has brought the start battery back to 100%. In a pinch I've been known to motor sail when wind or weather is not cooperating and I hate the idea of missing out on 12 hours or more of "excess" charging capacity in those conditions. Even running a bit over idle speed and well under hull speed there are a lot of Amps coming out of that alternator. It's a shame to waste them.

Finally, for those days when I'm just sailing or just anchored out I have the AC powered lithium charger. My budget predicts I'll have a 60-70Ah deficit by morning so a 50A lithium charger makes short work of getting everything back to 100%. It runs off my shore power wiring and does not care if I'm at home, in a slip, or have the generator feeding the shore power connection.


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* "Kein Operationsplan reicht mit einiger Sicherheit über das erste Zusammentreffen mit der feindlichen Hauptmacht hinaus." Prussian Field Marshal Helmuth von Moltke

Bill, thank you so much for your well thought out replies. Your comments are spot-on.

My current layout isn't ideal, but it was born from the triple goal of 1)adding available energy to our trips 2) maintaining my "forget it" setup that doesn't require switching batteries for the season and 3) minimizing impact to the original 2-battery system (being easily reversible if/when I go to dual lithium start batteries and require no modifications to the pre-existing system).

My optimal setup is the two 135 AH Lithium motor-start-capable batteries, with 220W of solar. I spent this year's boat budget on AIS and EPIRB and MOB and watermaker so I decided to cheap out here with the 100AH third battery and DC to DC. I also wanted to do it because it was very different.

Basically my first 100AH on a trip come from the Lithium battery: after that it only recharges on solar. This works for us because Bear Hunt primarily moves under motor power on our lakes and rivers (we have a geography that is really anti-sailing: whenever we move, we only have one choice of heading, and 50% of the time we have to head into a significant current). When we move under power the LA batteries are quickly charged by the alternator and all the solar will dump into the lithium battery. If we moved under sail for long periods, the lithium battery would quickly drop out of the equation, and we'd switch the solar output to the LA batteries to avoid the inefficiencies of the DC to DC. I'm hoping that the switch from 100W of solar to 220W of solar will mean that the lithium battery will contribute continuously on our Bahamas trip, where 220W will not only power our instruments under sail but have plenty leftover to store in the lithium.

We're going to do a 3 week trip in midsummer with this system, and if it's not effective (i.e. noticeably better than the pre-existing system) then I'll bite the bullet and scrap it and buy the 270 AH Lithium start-battery system. At least the switch to the 270 AH system will be fast and easy because of point 3 above.

Thanks for the kind words.

I must have misunderstood something in your setup. How does the engine charge the lithium battery?

I still have strong reservations about a lithium start battery. Matching a lithium battery to an outboard alternator can be tricky. Make sure that they are going to play well together. When a lithium battery decides it's through charging it can abruptly take itself out of the circuit. Some alternators don't like that. If you have a "smart" alternator that can communicate with the battery management system then you should be ok. My alternator is not smart and would almost certainly blow its diodes or overheat trying to deal directly with my lithium batteries.

I fully understand the iterative nature of battery changes. When I bought Simple Interest the house/start bank consisted of two worn out "dual purpose" batteries. They could barely keep the anchor light on overnight. I never left the dock without a jumper pack. I actually used that setup for a couple of years but I never went out for more than a couple of days at a time and was very frugal with power at night.

The next version was four deep cycle golf cart batteries hooked up in a series/parallel configuration. That worked great as a house bank and they were more than capable of cranking a 40 hp outboard - right up to the morning they didn't. That was the same morning that I found out that the jumper pack (see above) had also become very tired over the last five years and it wouldn't start the engine either. I'm pretty sure it actually sucked power out of the main bank when I hooked it up!

Now I won't say that you can't pull start a 40hp Honda, in fact the manual says you can. I will say that I couldn't do it that morning. I finally gave up, raised the anchor, and sailed the last 20 miles home counting on those tired, old batteries to give me one last start after being left with no load for the last 5 or 6 hours. I turned into the channel and gave it a shot. The engine cranked up and I started planning for version 3.

While I was researching and saving up started carrying a small generator. It allowed me to recharge the batteries as needed so I knew the engine would always start. A new jumper pack would have also helped but I figured the generator gave me more options.

That brings me to the version 3. It was going to be a close duplicate of version 2 but then a funny thing happened. The price of lithium batteries dropped and their capacity went up. The long-term costs for lithium dropped significantly below a similar sized lead acid configuration.

The new design meant that some of the old parts were no longer needed. My three bank charger and ACR are sitting on a shelf waiting for a new application. A new, large lithium charger and a dc-dc charger were a large part of the budget. All new cables, fuses, and gauges took quite a while to source and install but in the end I had the system I described in my last post. I left the LA start battery in the mix to protect my diodes and to be absolutely sure that nothing on the house circuit could kill it.

So far I've not come close to killing my house bank even on multi-day trips. If necessary, running the generator while I'm preparing breakfast has been enough to get everything back to 100%. Adding solar to the mix is the next step.

Be Free wrote: ↑Tue Jul 01, 2025 1:41 pm
I must have misunderstood something in your setup. How does the engine charge the lithium battery?

It doesn't. The Lithium battery only gets charged via solar or shorepower. I didn't want to connect it to my alternator.

I have the new 10A lithium charger here on the table beside me, will install it tomorrow. With that in place, the fridge can stay powered by the Lithium battery charger when at the marina, even if the ACR doesn't connect the batteries because of low DC to DC output voltage.

These are the batteries that I'm thinking about upgrading to: https://dakotalithium.com/product/dl-pl ... rformance/

It doesn't specifically say that the BMS shutoff issue is solved, but it does say that they're recommended for marine outboards and cars. If I understand correctly, the sudden-disconnect alternator blow-up issue can be solved by adding a DC to DC charger between the alternator and the batteries. So it looks like the upgrade will have to include the price of at least a 30A DC to DC charger and associated wiring, which will eat into some of the weight savings and almost certainly push the upgrade to next year's budget.


In the most convoluted of ways, it occurs to me that I could add a switch to be able to connect the Lithium charger to my 120V inverter output and charge the Lithium via alternator that way I imagine that would also act as a buffer to avoid an alternator blow-up in the event of a BMS shutdown. (Not gonna do this, I just find it funny how circular this could get).

Outboard <--> lead acid start battery --> dc-dc charger --> lithium house bank.
Cheap(ish), safe, easy.

Be Free wrote: ↑Wed Jul 02, 2025 8:33 am Outboard <--> lead acid start battery --> dc-dc charger --> lithium house bank.
Cheap(ish), safe, easy.

This is what our configuration ends up as, just with the addition that our DC-DC converter has an integrated MPPT solar controller as well.

tuxonpup wrote: ↑Wed Jul 02, 2025 12:37 pm

Be Free wrote: ↑Wed Jul 02, 2025 8:33 am Outboard <--> lead acid start battery --> dc-dc charger --> lithium house bank.
Cheap(ish), safe, easy.

This is what our configuration ends up as, just with the addition that our DC-DC converter has an integrated MPPT solar controller as well.

Ya, woulda been so easy except I love not having to ever change my battery selector and that's part of what drove this convoluted project. It's set and forget for the first 180AH of house load, and I always have a full lead-acid battery ready to start the motor in the morning with another 80AH in reserve.

Plus, I have a bit of no-start PTSD and I want 2 batteries capable of starting my motor, plus the lithium jump-pack for good measure. Apparently you can pull-start a 90HP as well: I mean, Suzuki and Etec both give you a pull cord. I tried it on the old Etec90 once...that's a big nope.