Do I care about battery voltage or bus voltage?

[claycookiemonster]

The Skyview Engine Monitoring System (EMS) has a built in lead to connect to "Battery Voltage (Voltmeter Input)" according to the installation manual. I wasn't sure what that meant, but a quick email from Dynon said it was intended to connect to the battery to read it's voltage (hint: RTFQ), but I could also connect it to the bus and read bus voltage instead rather than string a long 22 ga lead all the way back to an aft mounted battery.

But it occurred to me that battery voltage may not be exactly the same as bus voltage. Which do I really care about? If I could only display one, which should I choose? Will one of those readings be available in another way, so I should pick the other to monitor?

Curious.

 

[rv8ch]

Bus voltage is helpful to know if your alternator is working. If you set the alert correctly, somewhere around 13 volts, you will know if your alternator is offline or not producing enough power to keep things running and charge your battery.

The exact number to use for the alert is slightly higher than the normal resting battery voltage of your battery. Can't recall off the top of my head what number I used for my EarthX, but it's easy to configure.

 

[dmattmul]

Battery type

The Skyview Engine Monitoring System (EMS) has a built in lead to connect to "Battery Voltage (Voltmeter Input)" according to the installation manual. I wasn't sure what that meant, but a quick email from Dynon said it was intended to connect to the battery to read it's voltage (hint: RTFQ), but I could also connect it to the bus and read bus voltage instead rather than string a long 22 ga lead all the way back to an aft mounted battery.

But it occurred to me that battery voltage may not be exactly the same as bus voltage. Which do I really care about? If I could only display one, which should I choose? Will one of those readings be available in another way, so I should pick the other to monitor?

Curious.

Not sure if you are using a EarthX battery but if so, you have an indication of the battery voltage through the EarthX alarm system. It's a rough gauge but there. Also, if you haven't run an extra wire to the battery for this function easier to do now that later. My voltage sense is on the bus. It will read less than the battery as diodes, wire length and other magic but the important part you will understand what is "normal" in flight and set the appropriate alarms. I'm a believer in also monitoring amp outputs of your alternator(s). It's true you can tell if your alternator is working correctly from voltage but also good to have a second reference point.

 

[Bavafa]

I suppose it depends on your electrical system design. But in general, if you connect it to the battery or main bus, you will read the battery voltage before the start of the engine or alternator comes on-line. Once the alternator comes on-line it reads what the alternator provides unless it is not healthy.

 

[RV6_flyer]

My Dynon Engine Monitor reads bus voltage. Battery voltage is 0.4 V higher than bus voltage in my airplane.

My opinion, you need one voltage. IF you measure during build / test flying that there is a delta, and memorize what the delta is, that should be good enough. In an case, it is good enough for me.

 

[DanH]

Connect to the main bus. Before start, master on, you'll see battery voltage. After start, you'll see alternator output voltage, typically described as bus voltage.

 

[Carl Froehlich]

Connect to the main bus. Before start, master on, you'll see battery voltage. After start, you'll see alternator output voltage, typically described as bus voltage.

+1

Carl

 

[PCHunt]

Connect to the main bus. Before start, master on, you'll see battery voltage. After start, you'll see alternator output voltage, typically described as bus voltage.

Doing it this way, as Dan correctly suggests, will offer some troubleshooting advantages, as well.

Let's say, as an example, that one day you go to start up your plane, and very observantly, you notice that the "bus" voltage is 11.7 volts, instead of your normal 12.6 volts. (I'm making these numbers up, for the purpose of discussion.)

So, wisely, you decide to investigate. Is the battery low/flat, or going bad? To check that, you put your shop voltmeter tester directly on the battery terminals, and the reading is 12.6.

You now know that the battery is good, and that there is likely a wiring issue between the battery and the "bus". Probably a loose or bad connection.

OTOH, if your test of the battery directly on the terminals reads 11.7 volts, you can surmise that the battery is either bad, or hasn't been charged in quite a while.

This is the advantage of having the voltmeter attached to the "bus" instead of directly to the battery.

 

[vic syracuse]

I like it connected to the bus, as everything you have running is connected to the bus, except perhaps for some electronic ignition systems which are connected directly to the battery.

Then, during initial engine runs with a fully charged battery, use a meter to determine battery voltage with everything running and adjust your alternator regulator for the appropriate battery charging voltage. Then note the bus voltage reading. That’s what is should be for subsequent flights.

If it changes outside of a reasonable range, say .5-2 volts, it is time to start chasing it down.

The problem with connecting it to the buss and adjusting for bus voltage only could cause you to be overcharging the battery, especially if there is a long run between the alternator and the battery. You’ll keep changing batteries as it slowly destroys them, not realizing you are causing the problem.

Vic

 

[Richard Connell]

I like it connected to the bus, as everything you have running is connected to the bus, except perhaps for some electronic ignition systems which are connected directly to the battery.

Then, during initial engine runs with a fully charged battery, use a meter to determine battery voltage with everything running and adjust your alternator regulator for the appropriate battery charging voltage. Then note the bus voltage reading. That’s what is should be for subsequent flights.

If it changes outside of a reasonable range, say .5-2 volts, it is time to start chasing it down.

The problem with connecting it to the buss and adjusting for bus voltage only could cause you to be overcharging the battery, especially if there is a long run between the alternator and the battery. You’ll keep changing batteries as it slowly destroys them, not realizing you are causing the problem.

Vic

Agree with everyone here and the bus voltage has the most diagnostic value.
One question though- on the 10 the run from the alternator to the battery is quite long as Vic says. My bus voltage is measured much closer to the alternator than the battery (at the firewall) so I’d just always assumed that the voltage the battery sees in flight is lower than bus. By how much I’m not sure but I could probably calculate it or measure it ( although that would be a little tricky and require a few sets of hands)
Am I missing something?

 

[Carl Froehlich]

Agree with everyone here and the bus voltage has the most diagnostic value.
One question though- on the 10 the run from the alternator to the battery is quite long as Vic says. My bus voltage is measured much closer to the alternator than the battery (at the firewall) so I’d just always assumed that the voltage the battery sees in flight is lower than bus. By how much I’m not sure but I could probably calculate it or measure it ( although that would be a little tricky and require a few sets of hands)
Am I missing something?

The difference is in the grass.

For the standard RV-10 battery(s) install (aft of the baggage compartment) you have a #2 wire going from the battery(s) to the starter solenoid on the firewall. After engine start this wire sees a relatively low current flow - so little voltage drop from the battery to the main buss (engine off) or from the main buss to the battery (engine running and battery recovered from engine start draw).

Do not confuse battery open terminal voltage to main buss voltage. The battery(s) internal resistance will cause terminal voltage to be below open terminal voltage even with a small load.

Carl

 

[RV6_flyer]

Agree with everyone here and the bus voltage has the most diagnostic value.
One question though- on the 10 the run from the alternator to the battery is quite long as Vic says. My bus voltage is measured much closer to the alternator than the battery (at the firewall) so I’d just always assumed that the voltage the battery sees in flight is lower than bus. By how much I’m not sure but I could probably calculate it or measure it ( although that would be a little tricky and require a few sets of hands)
Am I missing something?

My Dynon Engine Monitor reads voltage LOWER than it actually is. Use a good quality multimeter to determine how much.

My Fluke DMM reads 0.4V higher than what the Dynon Engine Monitor does and there is NO way to calibrate the Dynon.

 

[gasman]

Looking for a voltage drop? After checking battery connections and airframe ground, then check the load side of your master solenoid.

All big connections.... battery, master sol, starter sol, starter lug, buss, and all grounds should be sealed with DE-OX.. https://havetotravelincognito.top/p...4478&msclkid=8dbcc4395bd214cb3f0da1bddbd2dd2c

The contacts in the master solenoid will continue to deteriorate as it ages. Easily found with a tester.

The contact surface of the solenoid can be altered if the stud happens to rotate even slightly during installation. Use a wrench on the installed nut to hold it in place when installing the lead.

 

[Richard Connell]

My Dynon Engine Monitor reads voltage LOWER than it actually is. Use a good quality multimeter to determine how much.

My Fluke DMM reads 0.4V higher than what the Dynon Engine Monitor does and there is NO way to calibrate the Dynon.

Im surprised the voltage meter is inaccurate by that much. That significantly reduces its usefulness.
But I don’t have a Dynon.

With a Garmin GEA you can measure the voltage the engine monitor box is using (possibly influenced by the dioded input) or a specific voltage pin. Is there something similar going on with the Dynon? 0.4 is the right sort of drop for a Schottky diode.

I can see a situation where you might see the box seeing 14.3 but the bus voltage is actually 14.7 if you were using the input power source rather than the dedicated voltage input pin. Maybe Garmin or Dynon can clarify.

IMHO it’s worth understanding what’s going on. My 2 B&Cs are rock steady at all times so I have my CAS set super tight at 0.2 either side of 14.7 (or is it?)

There may well be a measurable voltage drop at high recharge current to the battery in a 10 immediately after start, but im happy that it’s probably very small once they have recovered from that so the float voltage is probably ok.
I do have a recharge port- which goes directly to the battery terminals- easily accessible in the baggage compartment so I’ll rig something up to test when I get round to it.

 

[dmn056]

I might be missing something here, but I don't see that it matters exactly where you measure the voltage along the heavy wire from the battery to the bus or starter contactor.

Using AC43.13-1B data, for a 2AWG lead with a 20ft round trip from aft battery to somewhere near the firewall, the resistance is about 3.9 mOhm and the 0.5V voltage drop allowed on a 14V system will only be exceeded if you put more than 90A through it.

During normal operations (other than engine cranking) it would take a very large current draw, in either direction, for the precise location of the voltage measurement to make a difference. What am I missing?

 

[GalinHdz]

During normal operations (other than engine cranking) it would take a very large current draw, in either direction, for the precise location of the voltage measurement to make a difference. What am I missing?

You are missing nothing at all.

Unless you are in a calibration laboratory under controlled conditions and testing extremely precise equipment, a 0.4v difference in voltage is totally irrelevant, especially in our installations.

"Don't sweat the small stuff..." - Gen Colin Powell