Where to install low voltage disconnect device?

[Ben4195]

Hi folks, there was a recent discussion about batteries and refrigerators and how to prevent the fridge from damaging the batteries in the event the marina's power went out for an extended period of time. I can't remember if it was on this site, or another site, but it got me thinking...

A few years ago I installed a Dometic(?) CU-100 compressor and other necessary gear to have a functioning fridge, and it improved the quality of life significantly. When I bought the boat, the fridge didn't work. However, the original owner had installed an auxiliary battery with a separate Sentry brand battery charger. There is a switch on the panel to switch from the regular house batteries to this smaller, Group 27 auxiliary battery. The idea being that when at dock and connect to shore power, the switch is set so that the fridge is powered from the Group 27 battery, and if the marina loses power, then the $100 battery is ruined instead of the $1,000 battery bank.

This has worked fine since I had installed the new fridge, but the Sentry battery charger seems to be failing. The charger is no longer keeping the Group 27 battery topped off when the boat is connected to shore power. So I'm thinking I have to get a new charger for this Group 27 battery.

But then I read the thread about a low voltage disconnect (never knew there was such a thing), and someone recommended the Blue Sea m-LVD. This makes much more sense to me. And before anyone mentions this, I did read the manual on the CU-100, and it DOES cut off the voltage at a certain point, but it's something like 10.4 volts, which is way too low of course, and there doesn't seem a way to adjust it.

Anyway, I looked at the schematic for the m-LVD, and it appears to go on the positive cable between the battery switch and the panel. But I have two, thick house battery cables, as shown in the attachment (you can sorta see the "house" label on the cables on the left side of the image). How would a person install the m-LVD in this setup?

I'm not the most handy person, and don't have years and years of experience with boat systems such as this, so it may be a simple matter, but I am mystified.

My house batteries are two 4D batteries.

Any insight is greatly appreciated!

 

[Stu Jackson]

Why not get a VSR to charge the Grp 27 from the house bank and keep the concept. Why do you need the/a separate charger?

IIRC, don't the Dometic fridges come with their own lv cutoff?

 

[Don S/V ILLusion]

If crimping and splicing in new battery cables (#6 or larger depending on load) seems intimidating, you can buy ready made cables of any length at some marine stores or online.

 

[Ben4195]

Hi Stu, I wouldn't get the additional charger if I can install the LVD. Sorry I didn't make that clear. If I could install the LVD I could do away with the Group 27 altogether. And according to the Dometic manual, it DOES have a lv cutoff, but at 10.4 volts. Which makes no sense to me. It's my understanding you would want the cutoff to be 12.1 volts, and there's nothing in the Dometic literature that suggests a way to increase the cutoff from 10.4 to 12.1.

 

[Ben4195]

Thanks Don, would I then connect both of those house battery cables to the LVD? It just seems like they might be too thick to BOTH connect.

 

[Blitz]

I don't think the Blue Sea m-LVD instructions really state that you large cables need to go through it, only the cable size required to carry the loads on the other side of the device such as your refrig.

Sailor solutions has had a device like this for years as well. Although I purchased one I never hooked it up but seemed straight forward in the hook up as well:
https://www.sailorssolutions.com/?page=ProductDetails&Item=LVA01

For your reference; according to the manual the Dometic CU-100 drops out at 10.0 Volts and restarts 11.1 volts.

 

[Don S/V ILLusion]

Thanks Don, would I then connect both of those house battery cables to the LVD? It just seems like they might be too thick to BOTH connect.

I've never tried the Blu Seas LVD but if the loads aren't that big and the run is small, you might get away with a #8 or #10 wire size (caution - load dependent).

 

[LeslieTroyer]

I installed the LVD ~2’ under my panel. I moved the wire off the CB and put on load side of theLVD line went to the CB. Lott’s of other wires to switch and alarm.

Works well it turns off fridg @ 60% charge.

 

[Ben4195]

Thanks guys, and thanks for the reference Blitz, I remember CU-100 automatic cutoff was too low to be practical so am not surprised I didn't retain that detail.

Maybe I just need to hire a professional to install this, because I'm not understanding some of this. Leslie, are you saying you installed the LVD on your 12V panel instead of near the battery switch? And what is the "CB," I'm terrible with acronyms.

So stupid question, but Leslie's comment illuminated a light bulb, can I simply put this LVD in the wire just before the fridge switch on the panel? That power line will be much smaller, of course, and maybe my assumption that I had to put the LVD right at the battery switch was mistaken. If that's the case, I'll have to open the access panel to the 12V switch panel and see what's in there. I didn't think to take a picture of that.

Thanks again!!

 

[jviss]

Ben, some good advice on this thread. CB = Circuit Breaker.

The way the LVD is wired is on the load side of the fridge circuit breaker, between the breaker and the load fridge).

So, your LVD looks like the load now, and it drives the fridge. You don't need to increase the wire gauge unless it was too small to start off. It is likely 10AWG. With a 5A load, which is what this fridge is, you can go up to a 60' round trip with 10AWG wire and stay within a 3% voltage drop. In any event, make sure all of your connections are clean and tight.

 

[LeslieTroyer]

Ben you don’t need it by the batteries. My fridge is wired with 12ga so that’s what I used from the fridge circuit breaker to the line side of the LVD. It’s not on the panel, but below it, with the override switch on the panel and alarm, behind.

I could have placed it by the compressor and put the switch & alarm in the cabin by the fridge.

Les

 

[Ben4195]

Oh okay, I think I'm understanding this now. Thanks so much Les, Jviss, and everyone else!

 

[Blitz]

You don't need to increase the wire gauge unless it was too small to start off. It is likely 10AWG. With a 5A load, which is what this fridge is, you can go up to a 60' round trip with 10AWG wire and stay within a 3% voltage drop.

For what it's worth, the refrigerator manual for installation from Adler/Barbour limits the round trip length for #10 AWG to 11 to 17 feet so 60 feet might be a bit of a stretch.
From the manual:
# 14 is 4 feet and under
#12 is 5 to 10 feet
#10 is 11 to 17 feet
#8 is 18 to 27 feet
#6 is 28 to 35 feet.

It indicates as well that the terminal block on the refrigerator junction will accept #10 wire maximum and if heavier wire is needed you should terminate it a a heavy terminal strip and run "#10 AWG from there to the condensing unit".

 

[jssailem]

if heavier wire is needed you should terminate it a a heavy terminal strip and run "#10 AWG from there to the condensing unit".

Blitz. That makes sense. But you will need to add a Fuse for the #10AWG wire some where in the circuit. It is likely you will have a fuse for the #6 AWG within 7 inches of the battery. But when you drop it to #10 that fuse will not be correct.

The fuse protects the wire.

 

[Ben4195]

You guys are amazing. Love the information. Now if only one of you would make a road trip to Sandusky to watch over my shoulder while I install this...

 

[jssailem]

Now if only one of you would make a road trip to Sandusky

Be careful what you wish for.

 

[Don S/V ILLusion]

Or to complicate the discussion but there is a reason to place the LVD close/er to the batteries if it’s possible other loads could be left on while the boat is unattended.