Adding 2nd Battery, Charging Relay, and Switch

hoowaa01

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Jul 11, 2018
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I am adding a 2nd battery to my 2003 Supra SS. I purchased the Blue Sea Systems SI-ACR and Dual Circuit Plus battery switch. Attached is the diagram I am using. I am looking for guidance on the following:

1. How does my diagram look? Did I miss anything? Anything I should set up differently?
2. Am I using the proper gauge wire throughout?
3. What type of fuse and fuse bar should I use for the SI-ACR. (Links to specific products would be appreciated)
4. What kind of negative bus bar should I use? (Links to specific products would be appreciated)

Thanks

https://www.iboats.com/ideabook/2nd-battery/128260
 
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bruceb58

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Mar 5, 2006
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That all looks good. Problem with most boats is separating the house from engine. Often, those circuits are commingled in the fuse panel. May need a second fuse panel that has just engine circuits like ignition.
 

dingbat

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Nov 20, 2001
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I would question the need for 175A fuses on the charge circuits unless you have a 175A alternator.

I have 35A charge system. Installed in-line, water proof fuse holders with 40A ATC fuses.
 

Silverbullet555

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Mar 13, 2011
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When I did mine I used the existing negative bus bar. You might have one already that you can use. I used 1/0 I think because I had some for the stereo install. Otherwise I would have used the same size battery cable that was already there.

I didn't separate the existing wiring and added new stuff/stereo to the new battery. I would like to separate some of the circuits. Might be a good winter project.

I run pretty large capacity batteries so even with the starting battery having some drain, it's not a big deal. The amps are the biggest draw and the house battery supports them all day
 

sam am I

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Jun 26, 2013
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I would question the need for 175A fuses on the charge circuits unless you have a 175A alternator.

I have 35A charge system. Installed in-line, water proof fuse holders with 40A ATC fuses.

It's more or less the high instantaneous (di/dt) inrush of current flowing from one battery to the other battery that requires this size of fuse installed inline between the batts and ACR.....It's fast (usually) BUT, can be big and slower(last minutes, not seconds), depending on how low the house batt got of course, so that's why the manu suggests 175 amps..

Happens obviously when the ACR is paralleling a full'ish charged engine battery with a more dis-charged'ish house battery.........Also can/does happen with our Perko "Off, 1, both 2" switches when we switch from "1" to "both" for example, same difference as this is also paralleling the two batts but, the typical Perko O1B2 switch can handle 300+ continuous and 800+ instantaneous amps, the relays in the VSR/ACR can't, so they fuse them in the event the inrush is too long and too big for the relays to handle..

Most of the time however, exceedingly low voltage on the house batt (or engine, dual sensing type) is usually a reflection (prediction?) that at combine time, the inrush is going to be too great and typically is too slow, and since the ACR is always monitoring the batt voltage/s , it won't combine to begin with in this case BUT..........weird things can happen, things break and don't always go as planned as you know and that is I guess why we fuses things I reckon.
 
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dingbat

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It's more or less the high instantaneous (di/dt) inrush of current flowing from one battery to the other battery that requires this size of fuse installed inline between the batts and ACR.....It's fast (usually) BUT, can be big and slower(last minutes, not seconds), depending on how low the house batt got of course, so that's why the manu suggests 175 amps..

Happens obviously when the ACR is paralleling a full'ish charged engine battery with a more dis-charged'ish house battery........
At no time are the batteries combined thru the ACR unregulated.

The ACR is dual sensing (charge and discharge). It uses a very narrow control voltage to regulate/limit the flow of current between the batteries to the output of the alternator.

Per the installation manual, the ACR lines should be fused to a minimum 150% of your charge circuit output.
 

sam am I

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At no time are the batteries combined thru the ACR unregulated.

I would agree if you're referring to "regulation" in this case as "ACR continually is monitoring the batt's voltages"


The ACR is dual sensing (charge and discharge). It uses a very narrow control voltage to regulate/limit the flow of current between the batteries to the output of the alternator.

dual sensing = "senses charge source on either battery bank" as per manu, discharge has no bearing.

Yes, when the current demanded from a/the batt/s from the alt (charge source)is too much, the alternator (6,20, 40, 100, etc amp) output voltage sags BUT, lead acid, AGM, Lithium batteries can, as you know instantaneously supply 1000's of amps for short periods with little or no voltage sag.

No sag below ACR detection level = No open relay, bluesea's uses 210 intermittent amps max (120 continuous amp) relay's.....If the house is 10V (low threshold "reset" combine set point...bluesea) and the engine batt is 13.6 for example and you jumper (combine) the two together, your jumping essentially a 0 SOC batt with a 100 SOC batt.....Measure that series battery current sometime when you get a chance, I've seen too many times too small of same size wires fired ONLY between the batts and but not to the alt (6/20/40 amp alt for example) and/or open unrated fuses between the two batts.

Per the installation manual, the ACR lines should be fused to a minimum 150% of your charge circuit output.

Bluesea recommends a minimum of 75-90 amp fuses installed between the two batts with ANY alt less than or equal to 60 amps.....i.e., a 75-90 fuse is recommended even for a 6 alt for example then. Its the potential current shared between the batts that dictates that minimum fuse rating, obviously not a 6 amp alt.
 
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sam am I

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*under rated, not "unrated"
*disconnection level, not "detection"
 
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