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B & C 8 amp alternator with EarthX ETX104, Problem Help


Long title I know. I have a problem that I can not figure out. I only have 8 hours on the airplane now and the first 5 were without any issues in regards to the alternator and battery. Here is what has happened in the last 3 hours.
I was gone for a month and did not fly the airplane for around 8 weeks. This is a hand prop Lycoming 0-320, I am just using the battery to run GPS, com, transponder and a few EI gauges. The EarthX ETX104 is a light weight back up battery at 1.4 pounds that I was told would work for this application.

The problem started in the 5th flight hour after I returned from Alaska and the aircraft had sat. I was flying along and it trip the 2 amp breaker, this is the one that is related to the overvoltage crowbar protection. I tried to reset it with in flight but is would not. I landed and looked everything over and start the aircraft again and the charging system seemed to be working again so I took off. In about the same amount of time it took to trip the 2 am breaker it did it again so I went back and landed and called it a day. I hooked the battery up to an approved charger (the one EarthX recommends) and charged it overnight. I went a flew it the next day and it seemed to be working fine and then I heard a pop in my helmet/headset and looked at the EI volt meter and it had a red overvoltage light blink and it showed 8.5 volts and I thought this is weird. I turned the master off flew back to the field and landed. Once on the ground I checked to see if it was charging and nothing. No breaker popped so I figured the voltage regulator failed after talking to B&C tech support. I ordered a new voltage regulator and looked all my wiring over and can not find any problem. I installed the new regulator and went flying today thinking I have fixed the problem. I would say at about the same amount of time into the flight the voltage regulator shut down (low voltage light came on) and I noticed it had stopped charging. I did not have a circuit breaker pop so I turned off my alternator field switch and flew around for another 1/2 hour then I turned the switch back on and it showed I was charging normal again. I decided to turn the switch back off until I figure it out so I don't blow up another regulator. I did turn down the voltage regulator from 14.4 to 13.8 based on a discussion I had with B&C tech support when I installed the second voltage regulator.

Here is what I am thinking to try next, take the EarthX battery out and try a PC-680 battery instead.

Is anyone else running this set-up (little EarthX battery) with the over voltage protection that B&C sells with the 8mp alternator kit.

Thanks, Greg
So, double checking this. You had the B&C low voltage light on, but the 2 amp breaker did NOT pop, correct?

The first 2 times the 2 amp breaker did pop and the low voltage light came on. Today after replacing the voltage regulator the 2 amp breaker did not pop but the low voltage light came on.
Sd8 alternator?
i just put the same setup in a Pitts, (earthx, B&c with overvoltage) haven’t run it yet but will follow this closely.
i know they recently revised their overvoltage/regulator unit, assuming that’s what you’re running?



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I run the B & C 8 amp vacuum pad alternator with an EarthX (and handprop).

The EarthX will soak up a huge amount of amps when charging, but at least in my application, it will simply lower the voltage as it runs out of available amps to charge. I have yet to trip the 2Amp breaker on the overvoltage protection crowbar circuit, but can manually trip it and break the charging circuit. Are you sure the crowbar circuit is wired correctly? It took me a bit to fully understand what was going on when I initially installed it as the wiring was a bit goofy.

Somewhat related, I noticed the regulator gets warm-to-hot when at its upper range of amperage. I fab'd up a heat sink which helped in that area:


Edit: FWIW, I run an entire D180 glass dash, Trig radio and ADSB transponder, and charge my tablet (MFD) without any issues on this little B&C alternator without having to fallback to the EarthX. I set the charge rate at 14.3ish volts with no problems.


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Yes I followed that diagram and yes the set-up that motosix has is the one I have. Attached the regulator to .050 aluminum for heat sink, I felt the regulator as soon as it stopped charging today and it was barely warm.

Curious if you run this small battery or the larger earthX. I run the larger

Note in the diagram that the 2Amp breaker trips at 16volts. I am curious how you are getting to that voltage?

You have the diode installed correctly in the crowbar circuit, yes?
The first 2 times the 2 amp breaker did pop and the low voltage light came on. Today after replacing the voltage regulator the 2 amp breaker did not pop but the low voltage light came on.

The only way you can have a low voltage light (the alternator low voltage light, not the E.I. low voltage light) and the 2 amp breaker does not pop, is if the relay does not energize. This could be from a relay failure or from bad connections at the coil connections on the relay. To be honest, I don't use the relay sent with the SD-8 kit. I use a 'fog light' style relay as it's sealed and there are connector blocks available for them instead of just female spade terminals.

Well according to the tech guy at B & C if the EarthX battery is low it is like a sponge and will allow more than the 14.4 volts through. That is where the crowbar comes in and trips at 16, I never saw 16 volts on the voltmeter as I was flying or the overvoltage light I my EI gage so not sure about it other then it did trip the 2 amp breaker which the tech guy told me it was doing it's job. The tech guy also said I may have a problem with the relay so after wireweinie's last response I will switch it out since I also bought another one of them. The tech guy said if you have a problem with the diode it will split if you have it back ass wards which I know I don't (verified that already) he also said the capacitor is pretty bullet proof (will start to leak or buldge).

I think I am going to put a new relay in place and run the PC680 battery and see if the problems go away. If they do then I may look at why the EarthX is not working for me.

I run the ETX900 in Bushwacker with the 20 amp B&C accessory mount alternator and no problems, but it is a different animal so not apples to apples comparison.

Thanks for all the help.
Is the battery worth the weight savings and maybe grief? My B&C and Odyssey SBS J-16 whatever seem to work. 10# diff? I could never tell the diff. My winter gear weighs twice that.

Well according to the tech guy at B & C if the EarthX battery is low it is like a sponge and will allow more than the 14.4 volts through. That is where the crowbar comes in and trips at 16,

The voltage regulator will control voltage in spite of the current flow, that's it's job. EarthX batteries are sized according to the current output (amps) of the alternator. The battery needs to be large enough to handle the max output of the alternator in the system. This means you can pair a larger battery with a small alternator with no problems but not the other way around.

If the crowbar trips the 2 amp breaker in this SD-8 system, that means there has been, or it thinks there has been, 16 volts or more at the alternator output.

Yes. The diode will split in half if you connect it backwards. Every time. Sigh

If you are going to replace the the relay, consider going with a sealed style. If you stay with the ones from B&C, install new terminals on the wires during replacement.

Post #13, I kind of think it is not at this point:roll:

Web, It looks to me that it should work (max charge rate 10 amp) but at this point not sure. I have never seen the voltage of the battery below 13 volts when the system has shut down and I am simply running on battery power so not really sure what is going on. I read somewhere on EarthX site that the battery minimum voltage is 13.28 and maximum is 13.9. I have never seen this battery at 13.3 or higher so maybe I have a dud. When I charge it overnight with there recommended charger it only shows 13.1 when I flip master switch on, maybe my EI gage is off though.

I will go with your recommendations on terminal ends.

Which relay would you use if not the B&C, do you have a link.


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Two of my very favorite providers for tech support for my planes have been B&C and Reg @ EarthX. Both have been very accessible and very helpful.

Is the battery flashing a code?
Web, I have a friend who uses this alternator and this same battery, he eliminated the relay and crowbar and went direct with the red and black wire off the voltage regulator. He says the battery has all the fail safes built into the battery and that he can see if there is a problem with his gauge and shut system down if there is a problem. The battery has a indicator light so that is an additional warning light (I did not hook that light up but I could).

What do you think about running the system without the crowbar and relay, he told the owner of B&C what he was going to do and the guy just shrugged.

I like simple and if the battery has all these built in protections why even use the crowbar...
Is the battery flashing a code? Have not paid attention, I could look down and see it but actually had not thought of looking at the battery light as stupid as that sounds.

My ETX900 has an indicator fault light on it, I am actually not sure if this battery does so the answer is probably no because I did not hook up the pig tail off of this battery.

Just went and looked at the battery picture and it has the same built in indicator light but the answer is still no.

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Thanks Stewartb, sounds like maybe the battery is the problem. I will take my good voltage meter with me today and see what it checks on the bench when I pull it out. It has been charging all night so it is not going to get any more charged then it has been.81wxtwbB0mL._AC_SY450_.jpg
This is the charger I have been using and it is one that is recommended on EarthX web site. I do notice is says on the front of the charger 12.8V so not sure if that means that is the maximum it will charge the battery to...


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BEFORE you replace anything, troubleshoot.

You'll need to monitor the voltage closely to see what it does when the system goes off line. Does the voltage actually go above 16 volts? Does the system simply stop charging? And use a quality hand held meter to monitor bus voltage as they react much more quickly than panel mounted instruments.

I'm not sure about removing the crowbar from the control circuit. It's not only there to protect your battery, it's also there to protect your instruments and avionics. Can you really be sure that you won't miss a high voltage incident in time to protect you equipment?

If you leave a battery on a maintenance charger the recommended voltage is 13.3 to 13.9 volts. The charge voltage when on the aircraft is recommended to be 13.9 to 14.6 volts.

Troubleshoot first!!!

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An EarthX battery will have a tendency to expose flaws in your charging system where as the more tolerant PC-680 will cover these flaws up. Like said above nail down your voltages first before you start switching things out.

With regards to any initial EarthX battery, a reasonable practice is to partially discharge the battery down to say 13V. Then stick it on the Optimate overnight and check the top off charge of your battery the next morning. Then install in aircraft.

Good Luck - Jerry
If the battery says it's had enough charge there must be a transition from a previous state at the alternator or regulator. Does the alternator dump current through the regulator if not needed....the heat sink...to the firewall? Could there be a brief voltage peak if the battery declines a charge and before the regulator catches it? Harley Davidson dumps unneeded power through the regulator for example but maybe not the same apple.

Product specifications from there web site. It is interesting that the recommended charging amps is .8-5A
In flight at 2600 rpm I see more then that continuous so maybe that is the problem. I am running higher RPM then I normally would be while breaking engine in.

Voltage 13.2V
Capacity 4Ah @ 1C Rate
(See chart)
Capacity vs Temperature 25 °C = 100%
0°C = 92%
-30°C = 80%
Self-Discharge Rate < 3% / month @ 25 °
Absolute Max Discharge
10A (built-in auto
resetting fuse)
Continuous Discharge
Amps <20 °C
8A (built-in auto resetting fuse)
Continuous Discharge
Amps >20 °C
6A (built-in auto resetting fuse)
Standard Charge Voltage 13.9 - 14.6 V
Maximum Charge Voltage 15V
*****Recommended Charge Amps .8 - 5A*****
Max Charge Amps 10A (built-in current limiting)
Life (Charge cycles, 80%
depth of discharge)
2000 cycles @ 1C
discharge rate, 25°C
Life (Years) Up to 8 Years
Weight 1.3 lb. (.6Kg)
Dimensions 4.5 in (L) x 2.6 in (W)
x 3.7 in (H)
113mm(L) x 66mm
(W) x 95mm (H)
Environmental Rating
(resistance to water intrusion)
IP 66 (wash down
with a high pressure
Operating Temperature -30 °C to +60 °C
Storage Temperature -40 °C to +70 °C

I was curious about my other airplane with an EarthX EXT900 and what the voltage would read at the battery. I have not flown the airplane since I got home from Alaska on Sept. 26th and with my voltage meter it reads 13.96 volts at the battery.
I am using the B&C 20 amp alternator on that airplane so different I know.
Make sure your alternator voltage is between 13.9 and 14.6 volts. Then fly it and note the amp reading on the panel. If the ammeter is installed in the main bus wire, this reading is an exact measurement of the current being sent to the battery. If the ammeter is installed in the alternator output lead, this reading is a measurement of the current being sent to the battery PLUS the current sent to all the other electrical items on the aircraft, such as lights or radios. .8 to 5 amps recharge rate is ideal but damage will not occur until the amp flow exceeds 10 amps. And that should not happen with an 8 amp alternator.

Maybe a dumb question re the 8A alternator rating - is that the max the alternator is capable of producing? Or is that merely a thermal limit, beyond which the alternator would damage its self? Maybe that's not even pertinent to the situation, just tossing it onto the table - - -
It's a max output rating. If you try to overload the alternator, the voltage will drop off and limit the output.