I got the below message from a guy trying to figure out where to wire his DC refrigeration and how to incorporate a battery monitor.
"I recently purchased a boat with a (company left blank) installed. It is wired through an on/off switch directly to the house battery bank, and is not being monitored by the Blue Sea digital battery monitor about 15' away. I found the owner's manual for the XXXX where they clearly state that the 12v negative connection was to be made directly to the battery/bank negative terminal, and NOT to the load side of an ammeter shunt. There was some reference made to the voltage drop across the shunt which could cause the system to cycle more frequently or draw more current. Does this sound correct to you? I would really like to monitor the current consumed by the refrigeration."
I guess we have to take owners manuals with a grain of salt too. I come across a number of them with questionable advice. Sad really...
So let's look at the oddity of this one.....
These battery monitor shunts are rated for a max of a 50mV drop, 0.05V, but this is at 500A of load current! All shunts do is measure a millivolt drop and transpose that to current in the display. They are really voltage drop readers but on a very small scale voltage drop. 50mV is so small (0.05V @ 500A) it does not even matter a gnats hair to a compressor. Heck the battery cable on most boats is not even rated for 500A continuous but the shunts that ship with most battery monitors are...
There is no way that at 3.5A - 5.0A of current that mV drop matters one iota. Even doubling those numbers for the in-rush does not matter if your wire has been properly sized. At the low end of the shunts rating, pulling 3.5A - 5.0A, you're looking at a 1 - 2 mV drop or 0.001V - 0.002V drop....
Making sure your wiring can get the voltage to your compressor is far more important than worrying about a 0.001V - 0.002V drop in that circuit caused by the shunt, at compressor loads.. Heck a poor crimp connection can easily give you a worse voltage drop than a shunt at 500A...
If you started at 12V and put a 500A load on that shunt you'd be at 11.95V. Even at 500A this is inconsequential to what you'd see in wire losses at 500A or even at just 5A with some of the wire runs I've seen..
At a normal fridge compressor load you'll likely be at 12V - .001V = 11.999V (.008% volt drop) and at 2mV 12V - .002V = 11.998V (.017% voltage drop)...
Drawing the battery too deeply will cause far more voltage related issues than properly running the fridge through the shunt so you know not to over draw the bank. Without a battery monitor it is far more possible to have a low voltage situation than by using one. The battery monitor is the friend of this manufacturer, not the enemy.
We can also look at this another way. A 14' round trip wire run from the compressor to the bank based on 12V with:
14GA wire @ 8A results in a voltage drop of 2.42% / VD = .29V
12GA wire @ 8A results in a voltage drop of 1.53% / VD = .183V
10GA wire @ 8A results in a voltage drop of 0.96% / VD = .115V
8 GA wire @ 8A results in a voltage drop of 0.6% / VD = .072V
6 GA wire @ 8A results in a voltage drop of 0.38% / VD = .046V
4 GA wire @ 8A results in a voltage drop of 0.24% / VD = .029V
2 GA wire @ 8A results in a voltage drop of 0.15%/ VD = .018V
1 GA wire @ 8A results in a voltage drop of 0.12% / VD = .014V
1/0 GA wire @ 8A results in a voltage drop of 0.09% / VD = .011V
2/0 GA wire @ 8A results in a voltage drop of 0.07% / VD = .009V
3/0 GA wire @ 8A results in a voltage drop of 0.06% / VD = .007V
4/0 GA wire @ 8A results in a voltage drop of 0.05% / VD = .006V
Can you see the absurdity of these claims? You'd need approximately 1/0 wire on a 14' circuit run for the compressor, if the volt drop they claim in the manual matters, actually mattered. I have yet to see any 12V DC fridge with a 14' circuit length wired with 1/0 battery cable and they certainly don't provide the terminals for connecting to 1/0 wire. The terminals are sized for #10 wire and if you need to geed it larger wire you need to step it down using external bus bars and short jumpers of 10GA. All these extra connections can potentially result in more voltage drop than you'll see adding a battery monitor shunt into the systems neg wiring.
In this day and age it has become clear that we need to take much of the "tech support" with a grain of salt but manuals can often be misleading or poorly written too..
The battery monitor is a friend to a fridge compressor not an enemy but I guess nobody ran the actual numbers before hitting "print" on the manual...
Sorry so blunt I've not had my coffee.....
"I recently purchased a boat with a (company left blank) installed. It is wired through an on/off switch directly to the house battery bank, and is not being monitored by the Blue Sea digital battery monitor about 15' away. I found the owner's manual for the XXXX where they clearly state that the 12v negative connection was to be made directly to the battery/bank negative terminal, and NOT to the load side of an ammeter shunt. There was some reference made to the voltage drop across the shunt which could cause the system to cycle more frequently or draw more current. Does this sound correct to you? I would really like to monitor the current consumed by the refrigeration."
I guess we have to take owners manuals with a grain of salt too. I come across a number of them with questionable advice. Sad really...
So let's look at the oddity of this one.....
These battery monitor shunts are rated for a max of a 50mV drop, 0.05V, but this is at 500A of load current! All shunts do is measure a millivolt drop and transpose that to current in the display. They are really voltage drop readers but on a very small scale voltage drop. 50mV is so small (0.05V @ 500A) it does not even matter a gnats hair to a compressor. Heck the battery cable on most boats is not even rated for 500A continuous but the shunts that ship with most battery monitors are...
There is no way that at 3.5A - 5.0A of current that mV drop matters one iota. Even doubling those numbers for the in-rush does not matter if your wire has been properly sized. At the low end of the shunts rating, pulling 3.5A - 5.0A, you're looking at a 1 - 2 mV drop or 0.001V - 0.002V drop....
Making sure your wiring can get the voltage to your compressor is far more important than worrying about a 0.001V - 0.002V drop in that circuit caused by the shunt, at compressor loads.. Heck a poor crimp connection can easily give you a worse voltage drop than a shunt at 500A...
If you started at 12V and put a 500A load on that shunt you'd be at 11.95V. Even at 500A this is inconsequential to what you'd see in wire losses at 500A or even at just 5A with some of the wire runs I've seen..
At a normal fridge compressor load you'll likely be at 12V - .001V = 11.999V (.008% volt drop) and at 2mV 12V - .002V = 11.998V (.017% voltage drop)...
Drawing the battery too deeply will cause far more voltage related issues than properly running the fridge through the shunt so you know not to over draw the bank. Without a battery monitor it is far more possible to have a low voltage situation than by using one. The battery monitor is the friend of this manufacturer, not the enemy.
We can also look at this another way. A 14' round trip wire run from the compressor to the bank based on 12V with:
14GA wire @ 8A results in a voltage drop of 2.42% / VD = .29V
12GA wire @ 8A results in a voltage drop of 1.53% / VD = .183V
10GA wire @ 8A results in a voltage drop of 0.96% / VD = .115V
8 GA wire @ 8A results in a voltage drop of 0.6% / VD = .072V
6 GA wire @ 8A results in a voltage drop of 0.38% / VD = .046V
4 GA wire @ 8A results in a voltage drop of 0.24% / VD = .029V
2 GA wire @ 8A results in a voltage drop of 0.15%/ VD = .018V
1 GA wire @ 8A results in a voltage drop of 0.12% / VD = .014V
1/0 GA wire @ 8A results in a voltage drop of 0.09% / VD = .011V
2/0 GA wire @ 8A results in a voltage drop of 0.07% / VD = .009V
3/0 GA wire @ 8A results in a voltage drop of 0.06% / VD = .007V
4/0 GA wire @ 8A results in a voltage drop of 0.05% / VD = .006V
Can you see the absurdity of these claims? You'd need approximately 1/0 wire on a 14' circuit run for the compressor, if the volt drop they claim in the manual matters, actually mattered. I have yet to see any 12V DC fridge with a 14' circuit length wired with 1/0 battery cable and they certainly don't provide the terminals for connecting to 1/0 wire. The terminals are sized for #10 wire and if you need to geed it larger wire you need to step it down using external bus bars and short jumpers of 10GA. All these extra connections can potentially result in more voltage drop than you'll see adding a battery monitor shunt into the systems neg wiring.
In this day and age it has become clear that we need to take much of the "tech support" with a grain of salt but manuals can often be misleading or poorly written too..
The battery monitor is a friend to a fridge compressor not an enemy but I guess nobody ran the actual numbers before hitting "print" on the manual...
Sorry so blunt I've not had my coffee.....
