What is this device connected to alternator?

[sv_sharky]

Hey all, hoping to figure this one out quickly...I installed a new regulator and l just noticed this thing that is in between the alternator B+ terminal akd the battery. It seems to pass 5 volts or so when the engine isn't running, and the full battery voltage when it is?

Thanks

[Wolfe10]

That is a diode-based battery isolator.


OK (maybe) if there is a regulator with external sense wire on the battery side of the isolator as there is about a .7 VDC drop across the isolator which is turned into HEAT (hence the fins to dissipate the heat).


I much prefer a manual switch which has zero voltage drop and is less likely to fail.

[sv_sharky]

Originally Posted by Wolfe10

That is a diode-based battery isolator.


OK (maybe) if there is a regulator with external sense wire on the battery side of the isolator as there is about a .7 VDC drop across the isolator which is turned into HEAT (hence the fins to dissipate the heat).


I much prefer a manual switch which has zero voltage drop and is less likely to fail.

much appreciated!

Assuming these generally fail open?

[GordMay]

The most common failure mode(s), on a Schottky diode, is/are ‘closed’ (dead short) or ‘semi-closed’ (< 10-20Ω).
But, I think Bill Trayfors (RIP) said* it well: ”Diode isolator devices as used on boats have several potential modes of failure...”
* ➥ https://www.cruisersforum.com/forums...tml#post900881

[sv_sharky]

Thank you! Seems like there's a large voltage drop. Next up is 1-2-B switch and dc to dc to dc charger. Alternator direct to house

[Wolfe10]

Originally Posted by sv_sharky

Thank you! Seems like there's a large voltage drop. Next up is 1-2-B switch and dc to dc to dc charger. Alternator direct to house

If you have two battery banks and you are going to wire the alternator directly to house, all you need is a marine ON-OFF switch (one lug to house, the other to auxiliary).


Yes, if Li or other battery technology with low internal resistance, the DC to DC charger or other means of controlling alternator load is needed.

[donradcliffe]

Originally Posted by Wolfe10

If you have two battery banks and you are going to wire the alternator directly to house, all you need is a marine ON-OFF switch (one lug to house, the other to auxiliary).


Yes, if Li or other battery technology with low internal resistance, the DC to DC charger or other means of controlling alternator load is needed.

If you use a manual switch to isolate the starter battery, the first time you forget to isolate it and the motor won't start the next day, you will replace it with voltage sensitive relay, which is the modern isolator with no voltage drop.

The need to add a DC-to-DC charger for lithium batteries is debatable.