G3X Aux Battery

When I start up my -7A, the first switch on is the Aux Battery which powers the G3X on the Power2 pins through a diode. Even with the direct voltage reading of the Aux Battery at 12.6 volts, the voltage reading through the diode on G3X is only 11.9 volts or so, which is enough to keep the G3X powered until the Power1 pins provide power through the avionics bus after the alternator is running. When starting without the Aux Battery, the main battery powers up the G3X which then drops out as the voltage sags while the starter turns the engine. Then the G3X reboots after the engine is running, but there is a minute or two of no engine information before the boot up is complete.

The alternator recharges the main battery directly at 14.3 volts, but the Aux Battery only sees about 13.7 volts charge rate through the diode. In practice, this setup works ok, but it seems that the Aux Battery doesn't really get enough charge to get topped up. Depending on how much I'm flying, I'll remove the Aux Battery to give it a full charge. This was especially true with the EarthX 12 Aux Battery I just removed. It would directly read 13.5 volts, 12.9 volts through the diode, but didn't have any longer term capacity and certainly woudn't last 30 minutes. I replaced it with a Duracell SLA, which directly checks at 12.5 volts and 11.9 volts through the diode, but has more guts and does provide 30 minutes of power, but it's 3 lbs. heavier.

Question: What is the best wiring setup to get the Aux Battery charged at the same 14.3 volt rate that the main battery sees?

Sounds like you need one of these:

https://www.tcwtech.com/product/integrated-battery-back-up-system-ibbs/?v=7516fd43adaa

I had the voltage sag problem during startup and this cured it. Also will give me more than an hour backup of all essential systems if the electrical system takes a hike.

It's a possibility, except that it's 8 times more costly than the Duracell SLA. Can I directly charge the Aux Battery without going through the diode without any issues? That's the question.

I have the same type of setup in my RV - not flying but I've ground tested this with a power supply. With a regular PN diode feeding the aux battery, the voltage it sees is just barely adequate to charge it - lead acid batteries really want to be float charged at 13.8V or higher.

I've basically decided that if the aux battery doesn't charge quickly enough in practice, I'll change out the existing diode for a Schottky diode. That will give me another few tenths of a volt at the aux battery, which should help.

I just wish I could find a Schottky bridge rectifier in this same type of convenient package:

mburch said:

I have the same type of setup in my RV - not flying but I've ground tested this with a power supply. With a regular PN diode feeding the aux battery, the voltage it sees is just barely adequate to charge it - lead acid batteries really want to be float charged at 13.8V or higher.

I've basically decided that if the aux battery doesn't charge quickly enough in practice, I'll change out the existing diode for a Schottky diode. That will give me another few tenths of a volt at the aux battery, which should help.

I just wish I could find a Schottky bridge rectifier in this same type of convenient package:

Click to expand...

Here you go B & C essential bus diode

While the TCW battery will definitely solve your issue, I have a dual battery system with a full size 16aH lithium battery (EarthX) and then use the diode above to power what I call my Technically Advanced Aircraft Bus, or Essential Bus, whichever works for you, to carry the G3X, COM1, G5, GMU25, and GMU11 through the engine start. But also to have as an immediately available power source (essential) for a loss of primary power. During normal operation, the diode simply keeps the aux battery charged.

They have a couple sizes....enjoy.

rongawer said:

Here you go B & C essential bus diode

Click to expand...

That's a regular silicon PN diode, not a Schottky diode.

mburch said:

That's a regular silicon PN diode, not a Schottky diode.

Click to expand...

Technically, it's a high current silicon bridge rectifier. The MB2505 B&C sells, already mounted on their nicely done sink, will provide >12.5VDC output with an input down to 7.5VDC with 25A capacity. It will carry output voltage for essential electronics through an engine start, providing a well regulated output that satisfies the need the OP stated. And with your alternator VR set to 14.4V (the recommended setting), you will receive 14.2VDC out of the rectified bridge to charge your aux battery.

If your only goal is to charge your aux battery, you could just screw a MBR1060G (a Schottky diode, which is also made of silicon) to the airframe near your aux battery, for a 10A output, available through Amazon or other vendors.

Practically speaking, the essential bus diode B & C sells is a nice, low cost ($5, $20 with the heat sink) package that will meet the need and likely outlive the aircraft.

I've used these Schottky Diodes many times:
DSS2X61-0045A

rongawer said:

Technically, it's a high current silicon bridge rectifier. The MB2505 B&C sells, already mounted on their nicely done sink, will provide >12.5VDC output with an input down to 7.5VDC with 25A capacity. It will carry output voltage for essential electronics through an engine start, providing a well regulated output that satisfies the need the OP stated. And with your alternator VR set to 14.4V (the recommended setting), you will receive 14.2VDC out of the rectified bridge to charge your aux battery.

If your only goal is to charge your aux battery, you could just use the same heat sink mount and screw a MBR1060G to it (a Schottky diode, which is also made of silicon), for a 10A output, available through Amazon or other vendors.

Practically speaking, the essential bus diode B & C sells is a nice, low cost ($5, $20 with the heat sink) package that will meet the need and likely outlive the aircraft.

Click to expand...

It seems we might be talking about different things. The bridge rectifier you posted is a fine choice for an essential bus, I have the same unit in my airplane. I (and the OP) are talking about charging an aid battery, an application for which the lower forward voltage drop of a Schottky diode would be helpful.

At present I'm using one of these same MB2505 rectifiers to charge my aux battery and have found the forward voltage drop to be higher than stated above, hence my desire to find a Schottky diode in a similarly convenient package.

Walt said:

I've used these Schottky Diodes many times:
DSS2X61-0045A

Click to expand...

That type of package is probably what I'll end up with, it's just a pity nobody seems to produce one with quarter-inch tabs!

- Matt (EE)

mburch said:

It seems we might be talking about different things. The bridge rectifier you posted is a fine choice for an essential bus, I have the same unit in my airplane. I (and the OP) are talking about charging an aid battery, an application for which the lower forward voltage drop of a Schottky diode would be helpful.

At present I'm using one of these same MB2505 rectifiers to charge my aux battery and have found the forward voltage drop to be higher than stated above, hence my desire to find a Schottky diode in a similarly convenient package.



That type of package is probably what I'll end up with, it's just a pity nobody seems to produce one with quarter-inch tabs!

- Matt (EE)

Click to expand...

I can't help you with pre-manufactured 1/4" tabs for a Schottky, although you can crimp spades on to the pins of the MBR1060G fairly easily. Or Walt's suggestion is a nice package as well that you can use ring terminals on.

I have both bench tested and have operating experience showing the MB2505 rectifier provides ~14.2VDC output @10A load and 14.4VDC input. I recommend checking connections and/or wire sizing to minimize voltage drop.

For what it's worth I use a sterling charger to keep my backup battery charged (PC310). Been working great for quite a few years and quite pleased with it.
Basically, a constant current charger for small batteries.



https://www.sterling-power-usa.com/sterling-power-usa12vto12vbatterymaintainer.aspx

"This Battery Maintainer enables the auxiliary battery
bank to remain topped up by using charge from the
primary battery (only when the primary battery is
connected to a battery charger, generator,
solar cell, etc)."

For ground charging, I have an SAE connector (baggage compartment wall) that is connected to both batteries via a couple 10A diodes for isolation, that way if I'm leaving my airplane idle for a couple weeks, I can use a maintainer like Walt suggested to charge both the main and aux batteries simultaneously.

alcladrv said:

Even with the direct voltage reading of the Aux Battery at 12.6 volts, the voltage reading through the diode on G3X is only 11.9 volts or so

Click to expand...

Since nobody else mentioned it - most diodes have a voltage drop of 0.7V across them (that's just how silicon works), so that sounds right.

My diode looks like this:



I may try a small charger connected to the Aux Battery like Walt uses. Thank you all for your input.

alcladrv said:

My diode looks like this:

View attachment 21888

I may try a small charger connected to the Aux Battery like Walt uses. Thank you all for your input.

Click to expand...

The diode you show looks like the Schottky diode Walt gave a part number for. It’s actually two separate diodes in a single package, so you connect them both for higher capacity, or just use one side (i.e. only connect posts 1 and 2) and reserve the other side for a built in spare. Or do what I do and connect the power input to both posts 1 & 3, and then 2 and 4 to the main and aux batteries respectively.