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Battery Relay

With Battery voltage less than 7 volt; is it possible for the battery to be recharged with external ac power?

Systematically… is the “Bat. Relay” can only be energized through a good charged battery..?

Or it doesn’t matter; as this relay can also be energized through external power for feeding the “Bat.Bus” as well for recharging the battery.  

Any feedback will be appreciated as we are having an issue of one of the C-130H.

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I don't remember the reference off the top of my head but I remeber there being a cutoff voltage for battery replacement.  It was something like if battery is found below 22 volts, replace battery. Can only recharge if it was above 22 volts if I remember correctly.  I would imagine that the battery is toast at 7 volts anyway and should be replaced, most electronics on the aircraft need a minimum of 18 volts to operate.

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Thanks brother for your response;

I agree with you about replacement of the battery, and I agree with you that most of the relays on the aircraft are required more than 14v to response;

But my Inquiry which may solve the puzzle is:…

Which power source energize the Battery Relay, is it the battery only? Or both battery and external power?

To make it more clear; we are having an aircraft (out of home station with no maintenance facility) crew reporting that the battery voltage is very low 7v, and Bat.switch failed to connect Bat.with isolated dc bus (which is normal due to low Bat.voltage); and as reported when trying to regain the voltage by recharging the battery with external power the battery failed to build up any voltage during the charge ….

So the question is; Is it a battery failure issue only ? or battery relay failure issue preventing the battery from been recharged and subsequently lead to battery failure ?

If systematically the Bat Relay is response only to battery voltage; then the possibility of failure is Battery only; but if Bat Rely should response to external power voltage, then Bat Relay is also suspected.    

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Battery relay is energised only when battery voltage on load > 15V once ,it is energised ,battery bus bar is tieup with isolated DC bus, battery will charge but it can damage to battery(Overheating) when selected long period of time ,when battery relay is once energised, the holding circuit of the  relay is completed and battery bus is remain hot  even invent of the battery to maintain battery services ready.


Munir Abbasi

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The BATbus is powered as soon as you connect the BAT in your case it would be powered at 7 VDC that value in NOT high enough to energize the BAT relay which will result in the BAT BUS remaining at 7 VDC even if you connect EXT power and get 28VDC on the Main, Ess and Iso DC BUS. The Volt meter will read 28 for the Main and Ess but if you select BAT it will read 7VDC and the BAT will not charge because it is NOT connected to the Iso DC through the BAT relay.

EXT DC will not charge the BAT in your case either as the ground that controls the BAT relay is open to apply EXT DC to the Main DC 

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Not sure what model you have or what avionics suite but.....does your aircraft have two batteries?

Most US models have a main utility battery and a second avionics battery (usually called SCNS battery).

If you have two, swap them to get a good battery in main location. 

Are you having issues with voltage on both batteries?

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There are a few get home tricks, such as jumpering ISO DC power to the battery relay to get it to close and flow current to the BATT Bus and battery, but it still leaves you with a possible bad battery.

Is it lead acid or NiCad? Lead acid you can mess with, Nicad are less forgiving.

Are there other batteries available at the location with the same connector, even if a smaller amp hour rating?

How far away from a replacement battery are you? Are you allowed a daylight VFR ferry flight with a suspect, but recharged battery?

Without a strong external DC power unit, you will not have enough current to start your GTC/APU.

Constant voltage charging (the aircraft) is the least desirable method of charging either Lead Acid or Nicad, but it can work. Another question, did this battery just up and fail, or did someone (like me once) leave the DC volt meter in BATT all night? A revived battery from an accidental discharge means it will be more reliable if recharged in the field off of the aircraft/ground power unit.

Do you have external power available? If not, unless you can charge the battery on a bench somewhere or get a local replacement, you are stuck until you can ship in a proper replacement.


Sorry about the rambling.

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Thanks a lot for all responses…

First of all, aircraft is on the air after replacement of both the Battery and the Bat. Relay.

Flying crew upon receiving the two items elect to change both items ….. discounting our advice… for to replace the battery at first and then to check the operation of Relay).  

 However I understand from “pjvr99” that a dead battery cannot be recharged regardless of the operation of the Relay ..

Thanks “Munirabbasi” & “NATOPS1”, for clearing the main point that I was asking for (And I understand from you that: Relay cannot be energized through power coming from external, it requires a good charged battery to connect BAT Bus with Isolated Bus).

“hehe” & “n1dp” our aircraft is equipped with only one battery (Lead Acid) and thanks for the ‘”get home tricks”… n1dp… the battery was newly charged …. and seems crew is like you … but they  “Blame it on Rio” … that’s way both items been replaced…;)!!!        

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Good job! Glad you got them back in the air...

One last condition I failed to add is with the BAT depleted to the point the BAT relay will NOT energize IF you use External AC or DC to "Start" the aircraft engines (which is possible) you will have NO FIRE PROTECTION as the power to discharge the agent comes from the BAT BUS.



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6 hours ago, tenten said:

Thanks NATOPS1 for the last added safety note ;


Actually for that reason, we asked them earlier not to fly or even start the engine without resolving battery issue.   


Good thing they listened to you... 

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