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Old 07-27-2009, 08:31 PM   #1

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Default 12V DC wiring for 2 battery system with battery switch

OK, I've seen a number of posts dancing around the subject, but decided rather than hi-jakcing a thread I'll start a new one, so my apologies for anyone who feels this has been discussed to death.

I have a 1997 2800 SCR with 2 batteries connected to a two bank battery switch. The whole thing started when I found the orginal old group 24 flooded batteries were dead when I bought this boat. I upgraded to group 27 flooded, but the boat had a cheap 2 bank battery charger on it that cooked the group 27's (Now I beleive I understand what happened to the batteries I replaced).

I bought a new battery charger and decided to go with group 31 AGM batteries, (which the new charger supports). While hooking up the new battery charger and batteries I found a few things which concerned me:

1) There are devices connected to a dedicated battery which I cannot control which battery is being drawn upon and no means to easily swtich the device from one battery to the other.

2) I have no means of turning these devices off with the battery switch in the event of fuel vapors or electrical fire

3) There are mutiple ring connectors on a single battery terminal on both batteries.

Here is how things are currently configured:

Battery 1 connects to Batetery 1 on the battery swtich
Battery 2 connects to Battery 2 on the battery switch
Starter is connected tothe FEED on the Battery switch

Charger is a 3 bank charger connected to Battery 1 & Battery 2 on the swtich. The third bank is not currently being used.

1) Can I install a Power Post of Postive Bus bar and simply move the connections from the battery terminals to the Power Post or Positive Bus Bar? Couldn't I then run a line from the feed on the switch to the positive bus bar/Power Post to isolate everything using the switch?

2) Is there a poblem running the legs from the charger to tehe Battery 1 & 2 connectiosn on the switch, or should the charger be connected directly to the battery terminals?

3) How many connections should be on a single terminal on the batteries, switch or Power Post?

4) Do I need to leave the battery swtich set to BOTH in order to allow the engine to recharge both batteries while running?

I would think that by doing something along these lines EVERYTHING would run off of either Battery 1 or battery 2. I would like to start and run teh boat on battery 1 and then flip to battery 2 when on an morring or on anchor to protect teh battery used to start.


Thanks in advance,


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Old 07-28-2009, 04:45 AM   #2
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Default Re: 12V DC wiring for 2 battery system with battery switch

I would add up all of the load and see if the switch can handle it. There are instances where you want a high load device direct from a battery, such as a windlass.

I believe ABYC is 2 connections per battery post.

Yes, you must be in BOTH to have alternator charge both but be very careful to never switch through off while engine is running.

I run everything off batt 1 and only switch to batt 2 to start engine when batt 1 is down too low.

When I spend a weekend on hook, I prefer to run a genny to charge batts, run A/C, etc rather than running engine. I've also installed a monitor in cabin so I always know my battery conditions.

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Old 07-28-2009, 07:52 PM   #3

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Default Re: 12V DC wiring for 2 battery system with battery switch

OK, so the Starter is connected to the FEED on the switch, would there be much that would draw more than the starter? I don't have a windlass. I supposed I could pick up a Heavy Duty Battery Switch. In the example proposed, if you connect directly to a battery, then you can no longer isolate that device, since shutting the battery swtich off would still allow DC to be sent from the battery to the device. If you swtich the battery switch to BOTH, then your putting the batteries in parallel and still routing DC through the switch to the device.

Isn't that correct?

Would it make more sense to install a Heavy Duty Switch, and run a cable from the Feed post on the switch to Poswer Post and relocate the various items on the powerpost? What kinds of problems would this potenially cause? IS there a resistance issue to contend with?
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Old 07-29-2009, 03:55 PM   #4
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Default Re: 12V DC wiring for 2 battery system with battery switch

You have the correct idea, I ran my windlass direct to a battery because I don't consider it critical and if in BOTH then it has access to other battery.

Rather than worry about resistance, I look at it differently in planning on total load and distance. Ancor has a chart on its website you can use as a guide. My wiring between switch and each battery is sized to handle total load. The feed side of my switch goes to a distribution bar, which everything else is terminated on.

I also added a couple more fused links as protection in my rewire.

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Old 07-29-2009, 07:11 PM   #5
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Default Re: 12V DC wiring for 2 battery system with battery switch

My Switch is 150 amps continuously rated but obviously when the starter is cranked it pulls a lot more than that, but because its such a short time its fine unless you have a large continuous current draw I think you will be fine
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