Shottkey diode voltage drop ?

[Larry DeCamp]

There are often comments about voltage drop when using a shottkey to charge and isolate an ebuss battery. Typical forward voltage drop is .5Volts.

If the alternater puts out 14.2V and the battery holds 13.3V or 12.8V, won?t they fully charge despite the forward voltage drop ?

[Carl Froehlich]

Originally Posted by Larry DeCamp

There are often comments about voltage drop when using a shottkey to charge and isolate an ebuss battery. Typical forward voltage drop is .5Volts.

If the alternater puts out 14.2V and the battery holds 13.3V or 12.8V, won?t they fully charge despite the forward voltage drop ?

Depending on current draw and specific diode, you may have more than a 0.5vdc drop across the diode.

Typical Odyssey batteries need 14.1 to 14.7vdc to fully charge after a deep discharge. Float charge voltage could be around 13.3vdc depending on the battery. Your 14.2vdc alternator and 0.5vdc diode voltage drop gives you 13.7vdc charging voltage, or perhaps a little less.

The 12.8vdc full charge no load terminal voltage is just that, no load. For charging you have to overcome the battery internal resistance, thus the reason why charging voltage is alway more than fully charged no load battery terminal voltage.

If this is a small backup battery you should be fine. I would never consider a diode in line for charging a standard battery (e.g. PC680). If your design requires this I recommend you reconsider.

Carl

[Mich48041]

Anything above 13 could eventually charge a 12 volt battery, but could take many days to do so. In order to recharge a battery in reasonable amount of time, the bare minimum voltage should be at least 13.8 for a slightly discharged battery. For a heavily discharged battery to be recharged in a reasonable amount of time, 14.2 to 14.4 volts are recommended. The voltage drop across a Schottky diode varies with the current. It could be 1 volt while conducting heavy current.
There are over 500 RV-12s flying with a small PM alternator and Ducati regulator putting out 13.8 volts. However, the Rotax usually starts very quickly without running the battery down. To a casual observer, it might seem as if the engine suddenly starts without even cranking.
The answer to your question is, "it depends".