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Backup Battery wiring

N

N733JJ

Well Known Member
I have a TCW 3ah backup battery wired to my G3X touch. With the display dimmed the battery will last over an hour. I am willing to give up some of this time to power the GTR200 comm radio. Since the radio does not have a pin for a second power input (like the GTX45 transponder does)the only option is to tie into the line that supplies power from the bus. I have been told this is fine except I need to put a diode in both the line from the bus and the line from the e-batt to prevent a reverse flow.

Can anyone tell me the specs on the diode that I need? Now that Radio Shack is no longer around, where do I obtain them?

Thanks
Scott A Jordan
80331
 
Quite honestly I would re-think this. Diode isolation is good for things like an EFIS, where even a momentary power loss can cause a time-consuming re-boot. But re-booting a com is nearly instantaneous. The down side of the diodes is that they cost you about 0.6 volts, so your com thru a diode will quit a little earlier than without a diode, as your battery runs down. I’d suggest a simple main/eBuss switch for the power.
 
N733JJ said:
I have a TCW 3ah backup battery wired to my G3X touch. With the display dimmed the battery will last over an hour. I am willing to give up some of this time to power the GTR200 comm radio. Since the radio does not have a pin for a second power input (like the GTX45 transponder does)the only option is to tie into the line that supplies power from the bus. I have been told this is fine except I need to put a diode in both the line from the bus and the line from the e-batt to prevent a reverse flow.

Can anyone tell me the specs on the diode that I need? Now that Radio Shack is no longer around, where do I obtain them?

Thanks
Scott A Jordan
80331
Click to expand...

I have done BU bat's on com a few times but will only do it with a separate TCW 6AH unit. The 3A unit is not really able to handle the com current draw.
It would s**ck to actually be on the backup and have everything die when you keyed the mic.
 
Last edited:
Mouser.com has diodes. Part number 863-MBRF20L45CTG costs $0.83
Voltage drop while transmitting (5 amps) is about 0.4 volts.
The aircraft battery is a backup for the alternator. Is a backup needed for a backup?
 
Mich48041 said:
Mouser.com has diodes. Part number 863-MBRF20L45CTG costs $0.83
Voltage drop while transmitting (5 amps) is about 0.4 volts.
The aircraft battery is a backup for the alternator. Is a backup needed for a backup?
Click to expand...

Backup for a backup? Kinda. Long story short, recently while about 15 minutes from home, just as I started my descent, I had an alternator failure. I was getting advisories from center at the time and told them of the “issue” and asked them to relay to the (low use) tower that I MAY be NORDO and would be looking for a light. Well, the whole thing was blown out of proportion, center spent the next 10 minutes trying to get me to declare an emergency and land at a less suitable field. I told the controller that the more he made me talk, the higher the odds of my battery draining, he was causing an escalation of the situation. The tower closed the airspace till I was on the ground (still with radio contact) and a few days later I got a call from the FSDO asking about my engine failure!

After I explained it was an alternator failure they wanted to know who repaired it and copies of logbook entries returning the aircraft to service. When I told him that I repaired the broken wire on my home built he dropped all requests.

Well, at least now I know my main battery will last at least 15 minutes even with many transmissions!

Scott A Jordan
80331
 
Walt said:
I have done BU bat's on com a few times but will only do it with a separate TCW 6AH unit. The 3A unit is not really able to handle the com current draw.
It would s**ck to actually be on the backup and have everything die when you keyed the mic.
Click to expand...

I talked with the folks at TCW and they advised me I can wire it without the diodes, using the pass through function which basically does the same thing.

Thanks, Walt for the insight. I’ll give this a little more thought. My concern is not great enough for the flying I do to add another battery.

Scott A Jordan
80331
 
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