Diode Battery Isolators vs Relay/MOSFET Isolators

[Kosmofreeze]

I just learned why I can't get a full charge on my batteries while charging off my alternator. I'm pretty sure it's the voltage drop from my diode type battery isolator. Best I see when charging from the alternator is about 65% ... per my Victron battery monitor. I guess I could try to increase the output voltage of the alternator or switch to a more efficient isolator ... or, do nothing?

 

[dingbat]

I just learned why I can't get a full charge on my batteries while charging off my alternator. I'm pretty sure it's the voltage drop from my diode type battery isolator. Best I see when charging from the alternator is about 65% ... per my Victron battery monitor. I guess I could try to increase the output voltage of the alternator or switch to a more efficient isolator ... or, do nothing?

Far better off with a automatic charge relay (ACR)

 

[Chris1956]

Gee, the voltage drop of the diode is about 1 volt DC. Alternators often charge at 14+Vdc. I would think you should be able to get more than 65% charge. Something is wrong.

You might check the diodes in the VR of the alternator.

 

[boscoe99]

What voltage are you seeing at the battery when the isolator is allowing charge current to reach the battery?

Even with the voltage drop across the diode the voltage should be higher than the resting battery voltage, allowing the battery to be charged. It will just take longer.

 

[Scott Danforth]

Did voltage drop should be 0.7volts

Most diode style isolators also have a dedicated sense terminal for the alternator

 

[alldodge]

My 1994 Formula started with an isolator and the Alternator output wire was disconnected from at the starter and the ALT. Formula ran a new wire to the isolator and from it they ran to each Bat. This way the sense wire remained connected to the start Bat via the regular harness

Do agree ACR is best

 

[Kosmofreeze]

What voltage are you seeing at the battery when the isolator is allowing charge current to reach the battery?

Even with the voltage drop across the diode the voltage should be higher than the resting battery voltage, allowing the battery to be charged. It will just take longer.

I see about 13.5V at the battery when charging off the alternator. When using my AC (shore power or genny) charger, I see just about 14.1V at the battery. It looks like my alternator voltage regulator is set at about 14.2V. (14.2-0.7 diode drop=13.5V ... just like the text book says!)

So ... while I never seem to be able to "top-off" the batteries when charging off the alternator, I think it is only because the SYSTEM only provides a maintenance (or float) level charge voltage ... i.e. very slow (and safe) rate of charge. My typical scenario is - run the house battery down to about 50% after an overnight on the hook. Run around for a few hours before running the house battery down again ... rinse and repeat. If I ran the boat for more than a couple hours, I'd probably see more than 65% charge off the alternator.

After digesting the good advise here and understanding the inherent limitations of the diode isolator - I think every thing is working as designed. Now I understand the quirks. If I was putting in a new SYSTEM, I'd use a more efficient isolator.

Just like my wife ... not perfect and a little quirky, but I think I'll keep her. Too much work and expense to swap them out now.

 

[boscoe99]

Less voltage results in less current flow. Less current flow results in a longer time to charge the battery. Just like it takes longer to fill a 50 gallon drum at 1 gallon a minute versus 2 gallons a minute. But given enough time it will fully charge the battery.