Lithium Battery Design for Catamaran

Discussion in 'OnBoard Electronics & Controls' started by SailBella, Sep 2, 2020.

  1. SailBella
    Joined: Sep 2020
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    Location: Devon

    SailBella Junior Member

    Firstly, thanks so much for spending the time to write such a comprehensive and helpful response.

    I think we are going to go with your recommendation 1 and keep the 30A shore charger.

    We effectively have 3 scenarios:

    On passage motoring - we can charge from the alternator Port and Starboard Engine and LifePo4 (via Battery to Battery Charger)
    On passage sailing or at anchor - we can charge the LifePo4 from the solar. We can charge the Port and Starboard engine via separate small solar panels, small battery to battery charger or even a car charger from the inverter. And we can always turn the engine on to charge all batteries.
    In marina or on hard - Leave LifePo4 disconnected and leave at 50%. Plug in shore power which will maintain the engine batteries and provide AC. Then for DC distribution we can:

    a) Charger with custom profiles that can charge both lead acid and Lithium.
    b) Leave existing charger in place to charge lead acid AND buy another charger that can charge lithium.
    c) Buy regulated DC power supply to run DC distribution (bi-passing the lithium batteries)
    d) Turn on the existing charger for charging the engine batteries and AC distribution. Keep Battery to battery charger off and run DC side as we would sailing via solar.

    Have I understood correctly?
  2. SailBella
    Joined: Sep 2020
    Posts: 11
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    Location: Devon

    SailBella Junior Member

    I will try and re-draw the diagrams as you suggested - it takes a long time in Illustrator. Thanks for your recommendations.
  3. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    You got it more or less. Scenarios:

    Motoring: alternator charghing staring battery and you can also charge lithiums via B2B charger in adition to main solar. To make that happen you use the combining switch to add the B2B charger to one or both alternators or charge directly with smart regulators.

    Sailing or anchored: main solar charges the lithiums. Starters are charged either via separate small solar or separate small B2B charger (basicly keeps the lead acid on float and healthy).

    Marina or on hard: lithiums disconnected from any loads and chargers, resting at 50%. DC house loads taken over by 30A (you must count your A needs and choose appropriately) DC source (charger or power supply) plugged into AC, and main solar (if the controllers allow, otherwise solar is covered or open voltage). Starting batteries are mantained either via separate solar or B2B charger.

    The last scenario depends on what you actually end up fitting, and involves leaving the marina with full lithiums. Some time before you depart you can:
    1. Switch on main solar charging.
    2. If you have a charger capable of charghing lithiums, switch that on.
    3. If you have a "dumb" charger or DC source you use it to power the B2B lithium charger (if fitted) or use manual control (regulated DC source).

    In other words, you have two systems:
    1. the lead acid starters, are always charghing (mostly on float), either from the alternators, small dedicated solar panels or B2B charger. They do not need a dedicated AC charger connection when in the marina. If you have solar panels they always work, and if you have a B2B charger that will get it's power from whatever runs the DC loads when plugged in. You can of course keep a small AC charger just for them as backup.

    2. lithium house battery. It has several modes:
    a) in use, running the boats DC loads, main solar connected. This is the normal state underway, anchored and while staying in a marina for a short term.
    b) charghing from alternator (via regulators or B2B charger). This happens when you decide is necessary (no sun for days) or from opportunistic motoring.
    c) in storage, loads and charghers disconnected. This is only for long stays in a marina or on the hard.
    d) charghing from AC, with whatever you fitted.

    I know it's funny, but with lithiums and big solar you normally arrive at a marina with full batteries and instead of powering up the charger you disconnect solar and stay on battery to run them down to 50%. Then, instead of switching on the charger to bring them back up, you use it to run the DC loads and disconnect the battery. You charge them only before you leave. It's a totally different rutine you must adapt to. I predict you will hardly ever charge the lithium from AC or disconnect it from the boat. Solar does not stop working when in the marina and loads go down since you don't use navigation and inverters anymore. I am pretty sure that if your AC battery charger breaks, you will not miss it. You may not even notice unless you prepare a long term marina stay and don't want to cycle the lithium.
  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _


    Two things for you to research:

    1.) At a bare minimum a high temperature alarm on your battery.
    ( you may want to consider an auto shut-off if the alarm is not silenced within a short period. )

    And 2.) The nature of lithium battery fires. Be prepared.
  5. SailBella
    Joined: Sep 2020
    Posts: 11
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    Location: Devon

    SailBella Junior Member

    I have re-drawn the schematics taking in your advice and it has made it so much easier to understand. I have put all three systems on the same drawing but separated them all on the drawings. I have also referenced every component, which has been incredibly useful in calculating what I need to buy and what I can re-cycle.

    For charging the lead acid, my preference would be a small solar panel, but they are quite expensive out here in French Polynesia.

    I was thinking for the time being of feeding the existing shore charger with the inverter from the LifePo4 system to charge the port battery and then use the battery switch (BG) to charge both engine batteries (with switch BF in the off position).

    Or we have a spare inverter/charger that could charge the lead acid batteries and then we could pass on the old shore power unit.

    Or for weight reasons, we could get rid of the old shore charger and use a cheap light car charger like the BUDDYGO-Intelligent-Automatic-Maintainer on Amazon and use the inverter.

    Am thinking of taking a crossover cable from positive LifePo4 to lead acid starter battery to allow us to use the LifePo4 to start the engine with a switch with the handle removed. Although that might be a bit belt and braces seeing as we have two starter batteries already.

    You mentioned on the old diagram that Sw 4 needs to be wired for 1000A - I was thinking of using 300A switches for this - where does the 1000A come from?
  6. SailBella
    Joined: Sep 2020
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    Location: Devon

    SailBella Junior Member

    Here is the latest circuit diagram.

    Attached Files:

  7. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Well, you haven't said what size batteries those start batteries are - group 8D or group 27? Anyhow, the switches or contactors need to be "special duty" for high inrush motor starting and battery combining. And trying to start a port engine from a starboard battery on a catamaran requires some serious wire. I'd use at least 3/0 for a group 31 battery. When using an off-1-all-2 battery switch, the switches and wire need to carry a heavy rating because of the potential for high battery-to-battery inrush. I don't know what the maximum dumping current would be for a start battery pair, but the MCA rating would seem to be a safe bet. Perko makes a heavy duty switch series that has up to 450 amp continuous and 1200 amp intermittent rating, and also has your alternator field disconnect built in to the switch. Single off-on switches can have a lower rating if you remember to not have the starter engaged when you flip the switch:D

    Wire- Part 1: Choosing the Correct Wire Size for a DC Circuit - Blue Sea Systems

    Last edited: Sep 10, 2020
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