Your AC to DC Power-Supply/Charger should have a Bridge-Rectifier on it's Output,
so it doesn't need or want an additional Diode-Drop.
The other Circuit doesn't seem to make much sense,
there appears to be no Power-Source to supply the Battery.
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The power source is from the aircraft itself. -ACBUS28VDC is the primary aircraft Bus which is powered by the generator of the aircraft.

I don't understand why I should have a Bridge-Rectifier on the output of the AC/DC. It already output 28VDC I don't need to rectify it?

AC/DC will be use on the ground while in the air the aircraft will power the battery. Battery will power system

 

Diode conducting 100A @ 1V equals 100W.
Why not use a MOSFET switch instead?

 

Diode conducting 100A @ 1V equals 100W.
Why not use a MOSFET switch instead?

Honestly I don't know, the diode seems a simple choice. I need something reliable and simple to go in an aircraft. If you have other suggestion I'm open for sure.

 

"" I don't understand why I should have a Bridge-Rectifier on the output of the AC/DC.
It already output 28VDC I don't need to rectify it? ""

I think that You mis-understood what I was saying .........

The Alternator in the Plane,
AND
the Ground-connected Power-Supply,
both
already have Bridge-Rectifiers built-in to their respective Outputs by the manufacturer.

The Alternator, and the Ground connected Power-Supply
can be directly connected to each other.
There is no need for an additional pair of Diodes to keep them separated.

Both Power-Sources can Charge the Battery at the same time,
and run all accessories,
without any type of 'Switching" between them.

The Diodes are already there,
You don't need to add more Diodes.
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Hi LowQCab, I actually misunderstood what you meant by that, my apologies.

I remember with an AC/DC that I used when I didn't have a diode, there was a spark if I connected it directly to the battery. I don't know, maybe it was a cheap AC/DC, but since then I always add a diode as a precaution.

For the aircraft, the power comes from the aircraft bus. This bus powers pretty much all avionics instrumentation, radio, etc. If I don't add a diode, the aircraft instrument will still be powered by the battery.

 

"" For the aircraft, the power comes from the aircraft bus.
This bus powers pretty much all avionics instrumentation, radio, etc.
If I don't add a diode, the aircraft instrument will still be powered by the battery. ""

Everything is always going to be powered by the Battery regardless of extra-Diodes or not,
and is only affected by the Main Battery-Disconnect-Switch.

There is no need to "disconnect" anything when You
have a Ground-Based Power-Supply connected.
( As long as it's approved for Battery-Charging in your particular Aircraft )
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"" For the aircraft, the power comes from the aircraft bus.
This bus powers pretty much all avionics instrumentation, radio, etc.
If I don't add a diode, the aircraft instrument will still be powered by the battery. ""

Everything is always going to be powered by the Battery regardless of extra-Diodes or not,
and is only affected by the Main Battery-Disconnect-Switch.

There is no need to "disconnect" anything when You
have a Ground-Based Power-Supply connected.
( As long as it's approved for Battery-Charging in your particular Aircraft )
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Maybe I forgot to add that the battery we added in the aircraft is not the aircraft battery. So there is one battery for the aircraft itself and another battery to run our system. The aircraft battery will indeed be controlled by the Master Switch in the aircraft, but our system battery can be powered on the ground to only supply our system. We don't want the aircraft be powered by our battery on the ground. Maybe I don't understand correctly but this is how I am seeing things.

 

MOSFET power load switch

MOSFETs for Load Switch Applications - OnElectronTech

https://e2e.ti.com/blogs_/b/powerhouse/posts/when-to-use-load-switches-in-place-of-discrete-mosfets

 

I seem to remember something about your new added device pulling
large enough amounts of power to almost completely drain the additional, new Battery
once it has completed it's Duty-Cycle.
Is this correct, or am I thinking of a different post altogether ?

If this is correct,
the additional Battery should never be directly connected to the rest of the Aircraft,
unless,
it is definitely established that the onboard Engine-driven-Alternator can
comfortably handle the extreme-charging-Current-demands that You will be adding to it.

Generally speaking,
Weight is the enemy in an Aircraft.
This can result in a Charging-System, and a Battery sizing,
that are just barely adequate for the Factory-designed-Electrical-Load,
( this is also routinely the case in modern "economy" Cars ).
Adding a larger, and almost completely dead, Battery,
to a barely adequate Charging-System,
can possibly result in some very unhappy clients.

You can probably get away with a Circuit that will automatically Charge your
newly added, larger Battery from the Aircraft's Charging-System IF the Current
demanded by the almost-dead-Battery is Electronically limited to around maybe ~5-Amps.
I would recommend a heavy-duty "Linear-type" Voltage-Regulator/Controller
with an adjustable Current-Limiting-feature, such as ........
https://www.mouser.com/ProductDetail/Microchip-Technology/MIC5157YN?qs=qwh19ItNqyI6xi82zdg%2B9Q==

The required heavy-duty N-FET that this Regulator-Chip will control may have to
dissipate as much as ~20-Watts of Heat under extreme conditions
so it will require a Heat-Sink much like the one You have for your Diodes.
The difference being, that it is very unlikely to overload the Aircraft's Charging-System.
Which is a very real possibility, as a dead-Battery may attempt to
demand over ~100-Amps of Current from the Aircraft's Charging-System,
which I assure You, it can't handle without affecting other Systems,
including, but not limited to, the Aircraft's main-Battery

But,
5-Amps will take considerable time to completely charge the new-Battery.
The time required is easy to calculate when the
Load that the Battery is powering, and how long the Load will be applied,
is fully known and "characterized".
As an example .........

The Load draws ~20-Amps,
and will be controlled by a timer that disconnects the Load after ~20-Minutes of run-time.

This needs to be converted into "Ampere-Hours", or Amp-Hours, or "Ah".

20-Minutes is 1/3rd of an Hour, so multiply the Current by 33% or
0.33 X 20-Amps = 6.6-Amp/Hours of Power consumed.

If You then re-Charge the new Battery at ~5-Amps .........
6.6 / 5 = 1-Hour and 20 Minutes to re-Charge the Battery,
but in reality the re-Charging process is not loss-free, nor Linear in nature,
so add another ~50% to the Charging-time calculated,
1.33 Hours X 150% = approximately
~2-Hours to completely re-Charge the new Battery, at 5-Amps.

This is looking like an external-Charger will be a requirement,
because if the new Battery is not kept at Full-Charge, at all times,
it's life expectancy will be seriously compromised.
And the Aircraft's Alternator is most likely not going to
be able to do an adequate job of Charging it,
but the Alternator can keep the Battery Charge "topped-off" after external "Bulk"-Charging.

There is no good reason to have any high-Current-connections to the Aircraft's Electrical-System,
and doing so is bound to raise the ire of the Inspector, and rightfully so.
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Thanks for your complete answer.

For my first design I had done something similar to what you said. The other added battery was only there in backup, and current to recharge that battery was limited to ~5A. That battery was used only for 10-15 minutes before the aircraft engine was started.
Our system was powered directly from the aircraft and battery recharge with 5A max. Everything was calculated so there is no overload on the alternator. We have change the alternator because the one that came with the aircraft was not powerful enough.

The reason why I changed the electrical circuit is this. The problem here is something that I was only able to solve with connecting the system directly on the added battery and charge that added battery directly through the aircraft alternator. The problem is we have a computer which takes a lot of current not constantly, the variation of power demand created big voltage sag. As low as ~8VDC with 14VDC system. Our equipment were shutting down because of the low voltage. We solve that by connecting as I said the added battery directly to the system and charging that battery directly with the aircraft alternator. We keep a charger on the aircraft to charge the battery during the night. Also the system is not power on with only the added battery itself. The system will be power on if there is a source recharging the battery.

I really struggled with the voltage sag, I though maybe add a supercapacitor, but just wasn't sure if that would have helped.

 

I think that best solution would be a VERY stout Alternator,
( something like a "Mechman" Alternator ),
and a SINGLE, Large, LifePo-4 Battery, ( much lighter weight ).

Diodes, and secondary-Chargers, waste Power, which is in limited supply.
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