Am I overcharging my battery?

DakotaHawk · #1 12-13-2011, 06:28 PM

The no-load voltage on my PC680 has been trending down over the past year, and it's becoming noticeable when I attempt to start my engine on a cold day. The cold cranking amps just aren't what they used to be.

Another thread about voltage drop across diodes got me thinking about how my system is set up.

My electrical system is based on Nuckoll's electrical schematic for a single engine/single alternator/single battery system. In a nutshell, during normal ops, power goes from

the Battery
to the Battery Contactor
to the Main Bus
to a Diode
to the Essential Bus

During abnormal ops, power goes from

the Battery
through a "Essential Bus" Switch
to the Essential Bus

So here's the problem... Currently, I have my Alt Field Ckt Bkr and my Alternator Voltage Sense both come from my Essential Bus. But my Essential Bus voltage runs about 1.4volts lower than my Main Bus and my Battery voltage (due to the diode). So when the voltage regulator senses voltage, it's seeing a voltage that is 1.4 volts too low, and begins overcharging my battery in an attempt to get my Essential Bus up to 14.4 volts.

So now I'm wondering. Am I frying my battery? Should I swap the Alt Field and the Alternator Voltage Sense over to the Main Bus?

kevinh · #2 12-13-2011, 06:43 PM

I think you are. You want the voltage sense for the regulator to see the main bus voltage (not after the the >0.6V drop of the diode between the main and essential).

Moving the alternator field current over to the main bus is also a good thing, because when on the ebus you don't want to be wasting current to power the alternator that is presumably doing you no good. (If the alternator was working you presumably wouldn't be running from the ebus)

Sparky · #3 12-13-2011, 07:19 PM

I agree with kevinh. Why have your Alt Field and Alt Sense wired to the essential bus when the whole reason for the essential bus is in case of alternator failure? You could be overcharging the battery because of the voltage drop across the essential bus diode, as well as "wasting" current from the essential bus (powering the alternator field) if the alternator fails.

So my answer is: "Yes, you should."

erich weaver · #4 12-13-2011, 08:18 PM

I noticed you say you have a 1.4V drop. Are you sure? Should only be about 0.7, as previously noted. Check your numbers and if it's 1.4, you have a poor connection somewhere.

Erich

kevinh · #5 12-13-2011, 08:19 PM

Quote:

Originally Posted by erich weaver

I noticed you say you have a 1.4V drop. Are you sure? Should only be about 0.7, as previously noted. Check your numbers and if it's 1.4, you have a poor connection somewhere.

Actually depending on the diode used and the current draw, the drop can be higher than the standard 0.6V.

Sparky · #6 12-13-2011, 08:26 PM

1.4 vdc drop is not unusual with a silicon diode in a fairly high current situation such as this application. A much lower drop (around 0.3 vdc) can be obtained with a Schottky diode set up. Search the archives for details.

Walt · #7 12-13-2011, 08:37 PM

Bottom line is as long as you have the correct voltage at the battery (14.4 is ideal) then you're fine, if you have 15.5 at the battery thats bad.

erich weaver · #8 12-13-2011, 08:54 PM

Quote:

Originally Posted by Sparky

1.4 vdc drop is not unusual with a silicon diode in a fairly high current situation such as this application. A much lower drop (around 0.3 vdc) can be obtained with a Schottky diode set up. Search the archives for details.

Ok, I get it, but not sure why you think the original poster was referring to a high current situation. I certainly haven't seen that big of a voltage drop between my main buss and e buss.

Best to check what's happening at the battery, as suggested below.

Erich

DakotaHawk · #9 12-13-2011, 10:06 PM

Quote:

Originally Posted by erich weaver

I noticed you say you have a 1.4V drop. Are you sure? Should only be about 0.7, as previously noted. Check your numbers and if it's 1.4, you have a poor connection somewhere.

Erich

I incorrectly assumed that my bridge rectifier meant that the current flowed across two diodes - hence 2 X 0.7 = 1.4V. Turns out that the current only flows across one diode so the theoretic voltage drop is only 0.7V.

Wayne Gillispie · #10 12-14-2011, 07:12 AM

Scott,

Max charge voltage for Odyssey batteries is 15.0 V. If your alt is supplying 15.1 V to your main bus that could be part of the problem. While building I was using an AGM charger purchased at local auto parts store. I was charging my aux bat pc680 at 14.7-16.1 V. When I checked OCV 12 hours later it was always showing 75% SOC.

I purchased a 12A Odyssey charger online for $30 more than the wrong charger. After 5-6 rechargings the SOC was back up to 100%.

The other thing can be cold weather related. Capacity drops with temperature. A good charger, charge port and some preheating will help.