A/B switch for removing shut from battery circuit

[patrickstickler]

I've been looking for a particular kind of switch but without success, and am hoping someone might be able to point me in the right direction.

I have a battery monitor for my house bank (2 x 110AH AGMs) which has a shunt rated at max 100A. This is in addition to a 55AH flooded starter battery.

I'd like to install a switching solution that will allow me to use the house bank for starting in the case the starter battery goes flat, but my engine draws just under 400A so with the shunt installed in the house battery ground lead, I need a way to bypass the shunt.

I've been trying to find a simple A/B switch (not A/B/Both/Off) switch that I could put in the house bank ground lead to take the shunt out of the circuit, and another A/B switch to connect the engine positive lead to either the starter battery or house bank, but all I can find are either On/Off switches or A/B/Both/Off switches.

My dream switch would be a single switch that would allow me to choose either starter battery to engine + shunt in circuit, or then house bank to engine + shunt out of circuit, with a single throw of the switch between either option ;-)

I've seen some switches with the correct functionality, but for AC shorepower installations, and doubt they'd handle 400+A DC.

Any suggestions?

[patrickstickler]

Would it be possible to protect the shunt by switching in a short across the shunt (i.e. "path of least resistance"). E.g.

Normal usage, shunt inline:

                                       Gnd
                                        |
            |-------[Off]--/   /--------|
            |                           |
           (-)----- Shunt --------------|
House Bank (+)---------(+) FuseBox (-)--|
                                        |
                  [A]---(+) Engine (-)--|
                   |                    |
Starting Batt (+)--|                    |
              (-)-----------------------|

Emergency start, shunt shorted:

                                        Gnd
                                         |
             |-------[On]----------------|
             |                           |
            (-)----- Shunt --------------|
 House Bank (+)---------(+) FuseBox (-)--|
                    |                    |
                   [b]---(+) Engine (-)--|
                                         |
Starting Batt  (+)                       |
               (-)-----------------------|

So, normally, engine switch (A/B/Both/Off) is on A (starter battery) and shunt bypass switch is off.

For emergency start, engine switch is on B (house battery) and shunt bypass switch on, providing direct path to ground around shunt.

???

[GordMay]

To take the 100A [strike]Shunt[/strike] & Ammeter out of the circuit, you could wire a simple DPST “off-on” toggle switch (Blue Sea Systems #8287) in series with the two little shunt wires. Switch “on” shunt in cct; switch “off” shunt isolated. Ammeter shunts typically only draw 50 milliAmps at full scale, so switch ampacity won’t be an issue.

[patrickstickler]

Originally Posted by GordMay

To take the 100A Shunt & Ammeter out of the circuit, you could wire a simple DPST “off-on” toggle switch (Blue Sea Systems #8287) in series with the two little shunt wires. Switch “on” shunt in cct; switch “off” shunt isolated. Ammeter shunts typically only draw 50 milliAmps at full scale, so switch ampacity won’t be an issue.

My understanding is that it's not so much a matter of taking the ammeter out of the circuit, but taking the entire shunt out of the circuit. I.e., even if you disconnected the low current ammeter wires from the shunt, you'd still be running ~400A through a shunt that is rated at 100A max, and the docs state that doing so can permanently alter the characteristics of the shunt (i.e. screw it up so that the ammeter doesn't get the correct current sampling).

So I need a way to take the shunt itself out of the circuit (in a way that doesn't involve tools ;-)

[patrickstickler]

I've worked out a circuit that would do what I need to do with three Blue Sea 6010 switches, but it's not terribly elegant.

I could make things alot simpler if just shorting across the shunt would do the trick.

[patrickstickler]

Originally Posted by patrickstickler

I've worked out a circuit that would do what I need to do with three Blue Sea 6010 switches, but it's not terribly elegant.

I could make things alot simpler if just shorting across the shunt would do the trick.

An A/B switch would be even better.

[sailingdog]

You could wire a single pole, double throw (SPDT or A/B) switch in the circuit, and use that to take the shunt off line when you need to start using the house bank. The common terminal of the switch would go to the batteries, and one pole would go to the shunt and the other to the shunt bypass.

Setup with both switches in A for normal use:

.............................................Gnd
..............................................|
............|-------------[shunt bypass]------|
............. ................................|
...........(a)------------- Shunt ------------|
............|................................ |
House Bank (+)---------(+) FuseBox (-)--------|
..............................................|
...............--------{a]---(+) Engine (-)---|
...............|..............................|
Starting Batt (+) ........................... |
..............(-)-----------------------------|


Setup with both switches in B for bypass use:
.............................................Gnd
..............................................|
............|-------------[shunt bypass]------|
............| ................................|
...........(b)......------- Shunt ------------|
............|................................ |
House Bank (+)---------(+) FuseBox (-)--------|
........................|.....................|
.......................{b]---(+) Engine (-)---|
..............................................|
Starting Batt (+) ........................... |
..............(-)-----------------------------|

If you had a single, heavy duty double pole double throw switch, it could do it automatically, and prevent any risk of frying the shunt by accident.

[Bene505]

Is having a dead starting battery so much of an issue that your really want to put a dedicated switch in place to do a bypass?

I'd attack the problem in two ways 1) focus $ and effort on the problem rather than the symptom. 2) Have a plan B. For one thing you won't have to buy as many starting batteries over time.

1) Attack the real problem. Find the source of the drain on the engine battery, using a multi-meter. Then, to help keep the starting battery charged, wire a small solar cell to support the engine battery. A small one will not need a voltage controller. It will maintain the voltage, and will even provide a small bit of charging. Think about a cell that is about 1/2 square foot in size. They sell them for this purpose, and I've seen sailboats with a solar cell built-in for this purpose.

2) For plan B, get a piece of wire that you can use as a jumper cable. Better yet, get a jumper cable. Wherever you go, you can build good karma, jumping your slip-neighbor's batteries or loaning the jumper cables to someone with a dead car battery. You may even get a beer out of it.

[Since for me, a dead engine battery happens before a trip and not usually during, there's no need to rush anything with a large amount of current all at once. I sometimes use a 5 foot long piece of normal speaker wire that I happen to have. It won't help if the engine battery is truly dead, but after waiting awhile, it get's the starter battery up to the point where it can start the diesel. The wire is long enough to not get hot. That is critical with this approach -- if it gets hot, you need a longer wire. If I need something faster, I'll rig a thicker cable for a jump. That's my method and it might not work for everyone. So it's IMHO.]

IMHO, attack the real problem and have a plan B. You'll keep the system's complexity down and have a plan B that you can apply to other uses.

[GordMay]

Originally Posted by patrickstickler

My understanding is that it's not so much a matter of taking the ammeter out of the circuit, but taking the entire shunt out of the circuit. I.e., even if you disconnected the low current ammeter wires from the shunt, you'd still be running ~400A through a shunt that is rated at 100A max, and the docs state that doing so can permanently alter the characteristics of the shunt ...

Patrick is entirely correct. I apologise for my misleading & inaccurate post.

[Maine Sail]

Sorry for being a contradictory view point but I would personally take a different approach. I would spend the money and buy a true battery monitor like the Victron BVM-600 (LINK). This monitor comes standard with a 500 amp shunt for $194.65. The Xantrex Link Lite is also another option.

By the time you're done buying battery cables, DPDT or DPST, switches or high amp battery switches, and building a by pass for a 100 amp shunt, you could have yourself a beautiful new battery monitor that will give you lots more information and for not a lot more money than a by-pass.

Out with the old and in with the new would be my suggestion. I've yet to meet any boater, though I'm sure they are out there, who's installed a battery monitor and properly calibrated it that regretted it.