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Old 22-12-2021, 15:48   #1
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Join Date: Aug 2017
Location: Lake St. Clair, Michigan
Boat: Beneteau Oceanis 46.1
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Charging needs for start and bow thruster batteries

I'm adding solar and lithium to my sailboat in the spring, leaving me with a lithium house bank, an AGM starter battery, and an AGM bow thruster battery.

The LFP house bank will be charged by the solar MPPT as well as 2x 40amp chargers while at shore power.

The starter battery and bow thruster battery will be charged by my 125amp 'dumb' alternator, split between the batteries using a battery isolator.

I'm wondering if the alternator will be sufficient to rely on to charge the starter/BT batteries, or if I should add additional charging sources. I currently do mostly 'day sailing', so limited engine use (and also limited bow thruster use). I'm concerned that I will end up with my starter/BT batteries not being topped up often, and problems to the AGM batteries by doing this/risk they won't be charged when needed.

Alternatives would seem to be:
  1. use one of the 40amp chargers to charge the starter/BT batteries when on shore power, and add a DC-DC charger to send surplus current from this charger to the Lithium house bank. I would expect the DC-DC charger to get lots of use in this case while at shore power, until the house bank gets full. Another benefit of this option would be that I could run the engine, which would then 'turn on' the DC-DC charger to charge the house lithium bank, if I was ever in a pinch and needed to charge the house bank, but had no sun or shore power.
  2. keep the 2 40a chargers supplying the lithium house bank, and add a DC-DC charger from the house bank to the starter/BT batteries. DC -DC charger would 'turn on' based on high voltage sensing at lithium bank - so only when getting MPPT charging or at shore power. I would need to add an isolator (or other method of 'splitting' the charge from the DC-DC charger in order to split the charge between the house/BT batteries?).
Option 2 would seem most efficient/less additional drain of DC-DC charger running most of the time.

I was wondering if anyone can weigh in with thoughts on these options?

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Old 22-12-2021, 22:10   #2
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Location: canada
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Re: Charging needs for start and bow thruster batteries

If you are running the engine once a month I don’t think I yiu need it. If the boat is sitting for months At a time then you probably want more charging on them To maintain them.

The engine and bow battery will basicly never be drained. Self discharge on agm is very slow.

You could add a little 2 bank 20a charger to them to top off in shore.
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Old 22-12-2021, 22:41   #3
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Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
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Re: Charging needs for start and bow thruster batteries

Seems an ideal situation for use of a pair of Echo-chargers. Once hooked up, no attention is needed... they just keep your start/thruster batts topped up from the house bank. Simple and very effective.

Jim and Ann s/v Insatiable II, house-sitting ashore for the winter (and it is weird living ashore!)
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