I have a Catamaran with two motors, batteries, alternators, starters, etc.
At the dashboard all comes together and each motor has its own controls.
I can run each motor independently of the other.
I have a dashboard light which is connected to the starter battery of one motor.
Sometimes I only go with one motor. If it is the right, the light works as expected.
But when I only run with the left motor, the dashboard light stays dark because it is connected to
the starter battery of the right motor.

I wonder if it is safe, to connecting both batteries in parallel to
the dashboard light, dividing them by two diodes. (see atm)
I know when 2 batteries of different levels of charge are connected in parallel together, big currents can flow.
My question is, if the 3A max current is meant to the current which flows from + to - through the diodes
or if it also can only stop 3A from - to +.

VESPUCCI

 

Yes, that will work.

 

The scheme is good. What it does is that whichever circuit is selected tp provide power to the light reverse biases the opposite diode, turning it off. Also note that much less expensive diodes could do the job as well and cost a whole lot less. (Unless you have a lot of those 3 amp 1000 volt diodes with no other use in your stock.) I don't know the required current, if it is much less than one amp, a 1N4001 will be entirely adequate.

 

That will work, but there is one subtle point to keep in mind.

The dash light will draw current from whichever battery has the higher voltage.

If you are running from one battery, that battery will drain and quickly make the other battery, the one not in use, the battery with the higher voltage. This means that your dash light will be drawing from the battery that is not in use most of the time.

The result is that you might run off of one motor, thinking you've got the other motor available in reserve. But, then when you go to use it, discover that your lamp has drained too much charge off of that battery.

If that is a concern, you would be better off to put in a simple switch that allows you to choose which battery your dash lamp is drawing from so that you can be sure that it is drawing from the motor in use (and that is therefore being charged to offset the lamp's draw).

 

If you are running from one battery, that battery will drain and quickly make the other battery, the one not in use, the battery with the higher voltage. This means that your dash light will be drawing from the battery that is not in use most of the time.

What will actually happen is that, at some point, the two battery voltages become equal. After that it will take half the current from each battery.
And if you leave it long enough, neither battery will have enough charge to start either motor.
Have you considered a split-charge relay, so that whenever one alternator is running, it charges both batteries?

 

Thanks all for the help. I am aware that I will use the starter battery for the lamp and risk to empty it. I will always have one motor running with its alternator, so if the lamp always uses the battery with higher voltage, it will use the battery from which motor is running and charging. That sounds perfect.
It is only a small LED and draws less than 1A, so I am confident that the starter battery even can handle this for some time. For the worst case scenario, I can bridge the two batteries, so a start will be always possible, if one motor runs.
For me it was only important, that I don't stress small wires and connected diodes.
Thanks!
VESPUCCI

 

I would only add a fuse at the diode’s output (cathode). Batteries have a very large short circuit current.

 

Does your dashboard (led) light indicate state of charge of the battery, or does it simply indicate that the battery is producing some voltage? If it is the later, I would say that it isn't worthwhile to have it connected to both batteries.

 

I would only add a fuse at the diode’s output (cathode). Batteries have a very large short circuit current.

Sure, i will do.

 

Does your dashboard (led) light indicate state of charge of the battery, or does it simply indicate that the battery is producing some voltage? If it is the later, I would say that it isn't worthwhile to have it connected to both batteries.

It is just a instruments backlight.

 

It is just a instruments backlight.

I'm sorry, I didn't read your starting post carefully enough. How much current does the backlight draw? My guess is that it draws much less than 3 amps. So where does the requirement for sustaining 3 amps of current come from?

 

Using diodes to keep batteries from backfeeding each other works but that voltage drop can be a pain, Schottky diodes help but MOSFET-based solutions are way more efficient.

 

Consider the load: " I have a dashboard light which is connected to the starter battery of one motor. " Probably, no matter what, that is a small light and most probably a small voltage drop will not be noticed. Nor would such a drop matter. Most "dasboard lights" operate at less than maximum.

 

My guess is that the existing wiring includes a SPDT (single pole double throw) switch with the pole connected to the positive wire of the dashboard lights and 1st and 2nd throws connected to respectively 1st and 2nd + posts of the two batteries. So the question is where you would insert your network of diodes in that wiring. Perhaps your plan is to bypass the existing wiring. The best option would be to repair the existing wiring if possible I would think.

(Edit) Do you have a wiring diagram for your boat? Do you know for a fact that it is the battery for the right- hand engine only that is intended to power back lighting? That is a possible arrangement, but I think that it is equally possible that the wiring that I describe above is what is supposed to be in place.

 

I'm sorry, I didn't read your starting post carefully enough. How much current does the backlight draw? My guess is that it draws much less than 3 amps. So where does the requirement for sustaining 3 amps of current come from?

That's right. I just choose the 3 Amps because I have those diodes laying around...