I have a trailer trawler that needs new house batteries. Presently both house and start are AGM. I want to replace the house with 2 LiFEPO4 100 ah units. The boat has twin Honda outboards and the alternators are either 40a according to my manual or 50-55a when searched online. In the attached schematic the blue lines are what I think is the way to deal with shore power, solar and alternators so they play nice. Both engines run at the same time so my thought is to add a 30a DC-DC charger as shown. The only question in my mind is weather to combine both start batteries to the input of the charger or just designate one start battery to feed the charger. When on shore power the inverter charger used to charge all batteries but I will isolate the house batteries with the new layout and charge them via DC-DC charger from the start batteries. When at anchor my concern is draining start batteries so would it make sense to put an A / B selector switch on the alternator outputs to the charger so I can choose which alternator/battery will feed the charger?
Proposed schematic
- Thread starter Old Salt
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12VoltInstalls
life passes by too quickly to not live in freedom
I have a 19’ Starcraft remodel that will have an anemic charging potential from the (older) Johnson 90 2-stroke. Like you I will have LiFePo house power but use an isolated Grp27 battery for a starting battery. I may not run a second starting battery, however.
For what you are describing I assume no solar? On 18-25’ (trailerables) vessels like a ‘trawler” (or my Starcraft which will have a “lobster” cabin like a maine striped bass boat) it should not be to difficult to locate 200- or 300W of solar on the roof. While 300W is not huge power it will provide a wildhat guess of ~15A+ charging for the house batteries.
You ‘could’ use a DC2DC charger or other lowbuck method to charge the startup battery but realistically? It should never be discharged as 200A of LiFePo should tun bilge pumps, coffeemaker, vhf, sonar, nav lights, or whatever without issue so the starting battery should be 100% all the time.
If you want engine charging you could utilize an ignition-sense circuit that only connects when a) motors are running and b) when voltage is above “x.”
Keeping the starting battery “holy” is what I’d do. The capacity of LiFePo essentially negates the need to have switched battery banks imho, although if I were planning for blue water salt sailing or Great Lakes home waters I might still do a second starting battery.
SparkyGage
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I can't speak to specs but I'm not understanding the design philosophy. I can't picture a scenario where you would want the start batteries feeding the house batteries other than some medical emergency.
- The start batteries are on the essential circuit: Starting, pumps, navigation and essential lighting.
- The house batteries are on the convenience circuit: Lighting, Galley and AC loads. And as a backup to the essential circuit.
Underway or at anchor, the house batteries should pull from the essential circuit ONLY if the alternators are operating and not from the starter batteries, going so far as adding an isolation switch (again, baring some medical emergency). On shore power, I would think I would want a couple of inexpensive tender chargers for the starter batteries connected directly to the terminals and keep the prosine charger/inverter on the house batteries.
Combine all this with the fact that you are mixing chemistries, I think the isolation of the charging and load circuits is just as important for reliable operation.
If I'm missing something please correct me. I'm not a sailor, I just play one on the internet.
- The start batteries are on the essential circuit: Starting, pumps, navigation and essential lighting.
- The house batteries are on the convenience circuit: Lighting, Galley and AC loads. And as a backup to the essential circuit.
Underway or at anchor, the house batteries should pull from the essential circuit ONLY if the alternators are operating and not from the starter batteries, going so far as adding an isolation switch (again, baring some medical emergency). On shore power, I would think I would want a couple of inexpensive tender chargers for the starter batteries connected directly to the terminals and keep the prosine charger/inverter on the house batteries.
Combine all this with the fact that you are mixing chemistries, I think the isolation of the charging and load circuits is just as important for reliable operation.
If I'm missing something please correct me. I'm not a sailor, I just play one on the internet.
12VoltInstalls
life passes by too quickly to not live in freedom
While the motor is running the start batteries are primary charge recipients. So technically he misspoke re: ‘feeding the house batteries’ because with a progressive charging isolator or a simple switch it is ‘the same circuit’
SparkyGage
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(*EDIT* I see now that the ACR DOES protect during discharge as well as starting. I hadn't found that in the spec when I typed the comment below)
Ok. I wasn't sure because the diagram looked like the starting and house batteries exist parallel at the bus and could try to "charge level" each other EXCEPT when starting (that ACR looks like it only disconnects for starting, but not for ignition off discharging, so even with the motors are off, the starter batteries are supplying the house circuit.)
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Sorry for the delay. I was out making the final few $ before the snow flies which it did today. I guess what I'm trying to figure out is the 3 charging sources working together. When plugged into shore power my thought is to use the Prosine charger to maintain the the AGM which in turn feeds the DC-DC charger to lithium. I have 2 100w panels in series feeding the Victron 75/15 MPPT that are connected to the house bank. When I do the conversion I can re-program that. I have to keep the house batteries in the circuit on shore power because I'm running a Dometic fridge/ freezer that runs > 50% of the time. Based on the comments I think I need to add a switch on the DC-DC to isolate start batteries when on the hook.
Well after 3-4 weeks of educating and thinking I remembered the site of the MTOA technical writer and after reading this article two points jumped out at me; 1. the insurance issue and 2. the age thing. So I have decided to replace current AGM's with another set.
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12VoltInstalls
life passes by too quickly to not live in freedom
Just as an fyi ABYC has guidelines on this.
In real situations, practical installations, I have done the starting battery as holy unto itself. No lighting, bilge, or radios- just the starter and motor keyswitch harness “see” the starting battery. Everything else I put on the ‘house’ battery with a provision to place the VHF or nav lights on the starting battery while underway.
On my own small 14’ closed bow I was running a Suzuki that pull-started on the first pull with a dead battery and used a starting battery ~four times the necessary capacity; the VHF, GPS, Sonar, and nav lights were on that battery, while an electric trolling motor was on a grp31 deep cycle. I only did this to save weight in the ol’ Niagara because I’d proven the pull-starting scenario. A simple blue sea A/off/B took care of the emergency scenario switching.
Most boats I installed in were 18-19loa and the motors were generally too big to pull start reliably- those generally had the holy starting battery with a second battery for lights, VHF etc. and didn’t require a third because they had gasoline trolling motors.
If doing blue water or Great Lakes I’d recommend two “holy” isolated starting batteries with a third for ‘house’
The idea is to be lifesaving conscious because water is cruel and fickle.
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