What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Diminishing voltage?

RV8iator

Well Known Member
Benefactor
I noticed a few flights back that upon start and early into the flight my system voltage was 14.2, right where it should be. As the flight time increased, the voltage decreased down to 13.8. If I loaded it up by turning everything on it would hold at 13.7. I’ve changed the alternator (ND 14870(14824)) twice and they all 3 can’t have the same problem. I’ve cleaned the main ground strap from the engine to airframe and other ground wires with no affect. All connections have been pulled and wiggled and can’t find anything loose. Next I’m going to check resistance of the main feed from the alternator to see if it may be internally corroded. If that is not it, I’m stumped. Where should I look next?
 
Connect a temporary multi-meter to the system. This will tell you if your installed voltage meter is the issue - as in the reading changes with time/temp. If both change, then look for something else changing with temp - like high resistance connections.

Carl
 
Is the belt tight? A slipping alternator belt can drop the voltage; The greater the load, the greater the slippage. I would expect that slipping would increase as heat increases. Do you have an ammeter tied to the alt output? If so, what is happening to the current when the voltage drops?

Resistance in the system should create a high voltage from the Alt, not a low one, if the alt gets it voltage sense from the B lead. If it has a separate voltage sense terminal, then resistance on the B lead can cause a low voltage on the main buss. Easy to test. With engine off and master on, measure voltage at the battery and then voltage at the B lead on the alternator.

Larry
 
Last edited:
I would suspect this behavior is coming from the voltage regulator. Where is yours mounted (internal?). Some regulators include temperature compensation (as temperature goes up ideal charging voltage goes down) so it’s possible the regulator is behaving normally (although running warm). Is your battery fully charged at the end of these flights?
 
Battery

Why do you want the voltage to stay so high? If you have a gel battery, it should not stay that high for very long, but come down, at least according to some stuff I've read on the Internet - and now I'm an expert! :D


http://www.chargingchargers.com/tutorials/charging.html

Three Stage Battery Charging
The BULK stage involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger), and voltage increases. The properly sized charger will give the battery as much current as it will accept up to charger capacity (25% of battery capacity in amp hours), and not raise a wet battery over 125° F, or an AGM or GEL (valve regulated) battery over 100° F.
The ABSORPTION stage (the remaining 20%, approximately) has the charger holding the voltage at the charger's absorption voltage (between 14.1 VDC and 14.8 VDC, depending on charger set points) and decreasing the current until the battery is fully charged. Some charger manufacturers call this absorption stage an equalization stage. We don't agree with this use of the term. If the battery won't hold a charge, or the current does not drop after the expected recharge time, the battery may have some permanent sulphation.
The FLOAT stage is where the charge voltage is reduced to between 13.0 VDC and 13.8 VDC and held constant, while the current is reduced to less than 1% of battery capacity. This mode can be used to maintain a fully charged battery indefinitely.

Can you see how much current you are pulling from the alternator? That might give you some clues.
 
Why do you want the voltage to stay so high? If you have a gel battery, it should not stay that high for very long, but come down, at least according to some stuff I've read on the Internet - and now I'm an expert! :D
http://www.chargingchargers.com/tutorials/charging.html
.

I'm pretty sure that Odyssey batteries recommends 14.2 to 14.5 for charging voltage.

If you go to the Technical manual they recommend 14.7 for a max charge rate.
I use 14.6 -14.7 personally at a set point for the B&C reg.

RAPID CHARGING OF ODYSSEY®
BATTERIES
All ODYSSEY batteries can be quickly charged. Figure 7
on the next page shows their exceptional fast charge
characteristics at a constant 14.7V for three levels of inrush
current. These current levels are similar to the output
currents of modern automotive alternators. Table 6 and
Figure 7 show the capacity returned as a function of the
magnitude of the inrush3 current.

Standard internal combustion engine alternators with an
output voltage of 14.2V can also charge these batteries.
The inrush current does not need to be limited under
constant voltage charge. However, because the typical
alternator voltage is only 14.2V instead of 14.7V, the
charge times will be longer than those shown in Table 5.

Table 5 shows 10A and 20A times as:
PC680 2.7 hours 1.5 hours
 
Last edited:
Thanks guys for all the comments.
I have EarthX batteries. The amps look like they always have. Up to 35-45 immediately after start then slowly drops to 5 to 10 depending on lights, fuel pump etc. usage. All that looks like it always has. I’ve been running the ND alternators for 3000 hours so I have a pretty good baseline of how they have performed.
Belt is correct tension with no indication of slippage, black dust, noise..
Since 3 alternators are all behaving exactly the same I’ve pretty much ruled them out.
I’m on a trip for a few days but will measure voltage at batt and b lead when I het back.
I’m wondering if the internal regulators may be different than the ones I got 5 years ago and before. Could they be set to drop voltage when amp load goes down like auto’s do? Just a thought. None of my cars ever maintain over 14 volts after the batt is fully charged.
I’ll let you know if I find a smoking gun.
And..
Can I hurt anything running it like this?
PS. I put the proper earthX charger on both batteries about twice a month nad let them do there thing for a couple days each.
 
Last edited:
Do you have an avionics switch? I the same problem, same alternator, same exchanges. Not until the avionics switch completely failed; toggle came out of the switch breaker in my hand, did I discover the true culprit. Changed the avionics switch breaker to higher amperage and problem solved. Hope this helps, Dan from Reno
 
Do you have an avionics switch? I the same problem, same alternator, same exchanges. Not until the avionics switch completely failed; toggle came out of the switch breaker in my hand, did I discover the true culprit. Changed the avionics switch breaker to higher amperage and problem solved. Hope this helps, Dan from Reno

Good thought but no. I have one EFIS wired to main bus, and 2 backups to the stby bus. My 650 has it’s own switch.
 
Is the belt tight? A slipping alternator belt can drop the voltage; The greater the load, the greater the slippage. I would expect that slipping would increase as heat increases. Do you have an ammeter tied to the alt output? If so, what is happening to the current when the voltage drops?

Resistance in the system should create a high voltage from the Alt, not a low one, if the alt gets it voltage sense from the B lead. If it has a separate voltage sense terminal, then resistance on the B lead can cause a low voltage on the main buss. Easy to test. With engine off and master on, measure voltage at the battery and then voltage at the B lead on the alternator.

Larry

The belt shrinks when it gets hotter, it has a negative coefficient of thermal expansion. Slip will still exist but somewhat self compensating until it wears excessively.
 
I'm pretty sure that Odyssey batteries recommends 14.2 to 14.5 for charging voltage.

If you go to the Technical manual they recommend 14.7 for a max charge rate.
I use 14.6 -14.7 personally at a set point for the B&C reg.

...
Thanks Walt. I still have a lot to learn about batteries.

Looking at my graphs on Savvy for voltage and current with my PP alternator and EarthX I see very consistent 14.0 +/- .2 volts after the first minute where it re-charges the battery. I have not used the external charger in about a year, but I fly every 3-7 days.

EarthX has some charging information here: https://earthxbatteries.com/faqs

EarthXFAQs said:
How do I know if my vehicle charging system is working ok?
Connect a voltmeter to the battery and start your engine. At idle, you should get a reading of 13-13.3V. Increase the RPM’s to around 3,000 and for 5 minutes monitor the voltage which should increase and maintain a voltage of 13.8 – 14.6 volts . If your charging system does not output this voltage, there may be a problem with the charging system.

What they describe above is what I see with my PP alternator.
 
I’m wondering if the internal regulators may be different than the ones I got 5 years ago and before. Could they be set to drop voltage when amp load goes down like auto’s do? Just a thought. None of my cars ever maintain over 14 volts after the batt is fully charged.
I’ll let you know if I find a smoking gun.
And..
Can I hurt anything running it like this?
PS. I put the proper earthX charger on both batteries about twice a month nad let them do there thing for a couple days each.

Decades ago, it was common for 13.8 or 14 volts as a set point for regulators. However, over the last 2 decades, most companies have gone to 14.2 +/- as a standard and all of my cars float at this level. The fact that it used to run at 14.2 is a good indication that is the set point of the regulator in your alternator design. It is possible some aftermarket rebuilders are using set points of 13.8. Pretty sure you will eventually find the smoking gun.

You won't hurt anything by running at 13.8. However, your % charge rate on your battery may drop from 95 ->90% Batteries in alternator applications rarely reach a 100% charge state. Battery damage from sulphation typically doesn't happen above 80% charge state.
 
Last edited:
The belt shrinks when it gets hotter, it has a negative coefficient of thermal expansion. Slip will still exist but somewhat self compensating until it wears excessively.

Interesting; Didn't know that. Thanks.
 
Back
Top