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Old 20-11-2008, 11:01   #1
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Battery Switch Question

I have a Yanmar 2GM20F with 2 batteries, 1 house deep cycle, and 1 starting. There is a separate battery switch for each battery - just connected or disconnected, not one of those on/off/both. I charge them with the engine only normally and just do day trips or short vacations along the coast anchored out. My theory is - when sailing or at anchor for the night, disconnect the starting battery after I have shut the engine off. When starting the engine, connect the starting battery and disconnect the house battery. Then with the engine running, go ahead and connect the house battery while leaving the starting battery connected.

I think since the alternator is always connected to at least 1 battery, it should be OK to connect the 2nd while running?
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Old 20-11-2008, 11:10   #2
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That works. I like the simplicity of your system. When the engine is running charge both the House and the Start. When the engine is off run on just the House. No potentially unreliable battery combiners or battery isolators necessary.

Just remember to never disconnect a battery when the engine is running. A collapsing magnetic field in the alternator can cause a voltage spike which can do damage to various electronics. You can buy a "zap stopper" in case you forget, but still I would not rely on it. Always stop the engine first.
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Old 20-11-2008, 11:16   #3
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Probably OK.

Actually, the use of two ON-OFF switches rather than a 1-2-BOTH-OFF switch is preferred by some knowledgeable marine electricians.

How are the batteries wired now? Is the house battery exclusively for the house load, and the start battery exclusively for the engine? If so, how does the alternator charge both batteries?

Once we know how your boat is set up now, we can better advise you.

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Old 20-11-2008, 11:24   #4
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I think what he means is there is an on-off for the start and another on-off for the house. Having the house on charges the house combined with having the start on charges the start. Turning either off, disconnects from the alternator. A simply elegant setup.

house batt <---> on/off switch<--->alternator<---> on/off switch<--->start batt
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Old 20-11-2008, 11:36   #5
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Maybe. Not sure I'd call it elegant, if wired that way though. Consider, e.g., what would happen if both switches were left "ON" and the house battery was significantly discharged (and the alternator output connected directly to both batteries).

Could also be that the switches just disconnect the loads (i.e., the starter and the house load). Could be an isolator dividing the alternator's output to each batt.

Could be a lot of things...that's why I asked him exactly how his boat is presently wired. Have seen all kinds of crazy -- and a few not-so-crazy -- schemes :-)

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Old 20-11-2008, 16:47   #6
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I agree, that is definitely a downside...forgetting to turn the switch for the start battery to off after shutting down the engine. Its hard to determine if ones memory, combiners or isolators are more reliable.
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Old 20-11-2008, 17:06   #7
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:-) I try to avoid all three.

The little devices designed to maintain a separate battery automatically are what I favor. The EchoCharge by Xantrex and the DuoCharge by Balmar are two examples. I've had the former on my boat for 4 years now and it's been flawless. Have installed a bunch of EchoCharges and DuoCharges for customers. Both work very well.

I like these better than battery combiners because they don't "combine" the batteries (as you would manually with a switch). Rather, they sense a charging voltage and, when present, bleed off some of the charge to keep a second battery charged.

In this fashion, you simply route ALL onboard charging sources (battery charger, generator, alternator, solar panels, wind generator, etc.) to the house battery bank, and use the EchoCharge or DuoCharge to maintain the starting battery (which requires very little charging to keep it topped up).

No need to switch anything or remember anything, and you don't lie awake nights wondering what's going on when you "combine" a topped-up battery with a much larger depleted battery bank :-)

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Old 20-11-2008, 20:05   #8
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I have the echo charge and my smaller start batt is always topped up... by the big brother house bank.

My alternator and other charging sources charges ONLY the house bank. W

I use a Blue Seas switch which has separate switches for the start and house bank. The house bank switch is a 100 amp breaker and the start batt switch is a 250 amp IIRC rotary switch. There is a combine switch on the panel as well which essentially lets me use the house bank to start with, though I never have.

The problem with any parallel battery wiring is that a bad battery can suck down the good one. An one two both does parallel the two banks.

And how does the charging source determine which battery in a parallel set up is weaker. If one bank is quite charged it will mask the discharge of the weaker battery.

My start batt does one thing - crank the starter for a few seconds. I have removed all associated power loads such as engine instrumentation and the engine ventilation from the start bank. The blower was a vary large continual drain when motoring, larger than the echo could match so the start would slowly discharge from motoring. So now start batt has a very limited purpose - crank the starter.

This system has worked well.

One thing that is important about multiple charging sources. You need to have a charge controller which can manage them or use them separately, ie turning off the least powerful charging source. So if you have a small solar cell and it's a bright day and you start your engine, and the alternator is looking to replenish the system, if it sees an elevated voltage from the solar trickle it will "think" the batts are quite topped up while they might be fairly discharged, but on trickle charge. Alternator would politely defer! Solution is to kill the solar charge when you use the poweful alternator.
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Old 20-11-2008, 23:34   #9
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House Bank, Starting Bank, each has on off switch and an emergency combiner switch if needed, alternator has smart reg that has voltage sensor on battery side of Diode splitter so it compensates automatically for voltage drop, leave both batterries turned on all the time, simple cheap and almost idiot proof
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