12 VDC Battery charging circuit from 14VDC

Thread Starter

sflyscan

Joined Sep 16, 2021
23
Don’t overlook the Peukert effect (autocorrect wanted to call it “Peugeot effect”), because you won‘t get anywhere near half an hour from a 11AH battery at 24A. The 11Ah spec will be based on the “20 hour rate” and you are discharging it forty times as fast. Logic tells you that is should therefore last a fortieth of the time, but Herr Peukert says it won’t.
Oh I didn't know the Peukert effect until today. I learn everyday it's really cool haha.

But I guess to handle this we connect the system to an external power supply which is an AC / DC converter before the flight and after the flight as well. The battery will power the system for maybe 2 minutes maximum before the flight and the same after the flight. This is to give us time to disconnect from the AC / DC and power up the aircraft. At present, we have not had a problem with this as 24A is the maximum consumption value of all equipment, which will likely occur when the system is on AC / DC to allow time for the equipment. to return to their nominal state, the nominal is around 15A.
 

Irving

Joined Jan 30, 2016
3,190
@Ian0 is right, at 24A your 11Ah battery will last around 18min (v 27min 'straight line') to 10.5v, which is considered fully discharged.

You definitely don't need the resistor as that will completely screw up charging. Your charge current at 14v on a completely depleted battery could be around 70A typically (limited only by alternator output capability and then by output impedance + battery internal resistance + cabling resistance). While this will charge the battery rapidly (assuming wiring and breakers allow it) its not good for battery life or the system and a depleted battery should be charged externally and returned to service once fully charged. More info here.

If you can ensure the secondary battery is never significantly discharged then a diode may be all you need. If not, you need to limit the charge current to protect both battery and, potentially, the alternator/wiring as we don't know what that's sized for. However you will have issues if the secondary battery is never allowed to charge fully (ie engine must run long enough) as eventually it won't be able to sustain any usage before/after flight. After just 10min use pre-engine start the secondary battery will need 3+ hours at 2A charge rate to recover 100% charge; so a higher charge rate is advisable though must be careful not to stress the alternator. Below is an overview of what I'm thinking.

What's the hold-up time of your equipment (ie if power was switched by a relay from primary to secondary would it cause a problem?)

Edit: Just seen your earlier post - the external supply is a big+ - but charging time still needs to be considered as there's always the issue it might take longer than 2 min. Also those diodes will dissipate 12W or more between them and will need a decent heatsink - that's potentially a lot of heat to get rid of in a confined space - so some form of relay switching might be considered if the equipment can stand a short power glitch.

1632162396953.png
 
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Ian0

Joined Aug 7, 2020
6,704
Your charge current at 14V on a completely depleted battery could be around 70A typically (limited only by alternator output capability and then by output impedance + battery internal resistance + cabling resistance)
I'm just wondering if this situation will ever happen. Firstly, if the 11AH battery is only discharged to half its capacity (which shouldn't be exceeded in order to get a reasonably life), it's open circuit voltage will be a little under 12V ; and if starting battery is at a reasonable state of charge, say 12.5V open circuit, the engine will start and engage the charging circuit for the auxiliary battery. The difference between open circuit voltages will be only half a volt or so. The alternator will then bring both batteries to full charge together.
The same situation happens when connecting a car to a caravan, and I have never heard any reports of burned out wiring or blown fuses.
 

Irving

Joined Jan 30, 2016
3,190
I'm just wondering if this situation will ever happen. Firstly, if the 11AH battery is only discharged to half its capacity (which shouldn't be exceeded in order to get a reasonably life), it's open circuit voltage will be a little under 12V ; and if starting battery is at a reasonable state of charge, say 12.5V open circuit, the engine will start and engage the charging circuit for the auxiliary battery. The difference between open circuit voltages will be only half a volt or so. The alternator will then bring both batteries to full charge together.
The same situation happens when connecting a car to a caravan, and I have never heard any reports of burned out wiring or blown fuses.
Agreed if there is a small difference, and the current is likely to be limited by alternator output. However worst case the secondary could be at 80%DoD or around 11v and the alternator as drawn by the TS has, I believe, a 30A breaker so connecting the secondary to the charging circuit could trip the breaker. In a car/caravan setup the secondary feed to the caravan is direct from the primary battery via its own fuse/breaker, but here we're tapping into the 14v DC feed a long way from the battery/alternator with who knows what in between and a 24A load on what appears to be a 30A circuit. Given the avionics package is arguably critical I think best to be cautious and restrict unnecessary loading where possible.
 

Thread Starter

sflyscan

Joined Sep 16, 2021
23
Edit: Just seen your earlier post - the external supply is a big+ - but charging time still needs to be considered as there's always the issue it might take longer than 2 min. Also those diodes will dissipate 12W or more between them and will need a decent heatsink - that's potentially a lot of heat to get rid of in a confined space - so some form of relay switching might be considered if the equipment can stand a short power glitch.
Ok so if I understand your advice is that I should use a Charge Control, which I am not sure what is it, that will control a relay for the recharge instead of a diode.

I am not sure what's the hold-up time of the equipment since it is not given in their datasheet, but the power from the secondary battery to the equipment is already controlled by a solenoid which is controlled by an On/Off switch. There is a diode for each source, battery 1 and 2 that goes right on a copper bus. Each equipment are wired to this copper bus.

The recharge circuit for the battery is also wired to this copper bus. So it will be copper bus, wired to the control charge which will control a relay that recharge the battery 2. I tried to make a little drawing. Charge control is to protect from a very high current if the battery is almost fully discharge? And I am not sure what is a control charge.

1632243730981.png

Sorry I am pretty new in this kind of stuff. Thank you very much for your help it is really appreciated.
 

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Irving

Joined Jan 30, 2016
3,190
Ok so if I understand your advice is that I should use a Charge Control, which I am not sure what is it, that will control a relay for the recharge instead of a diode.
Yes, basically the charge controller performs 2 functions:
  • limits the charging current - to protect the battery and/or the charging source; in your case I think you have a 24A load on a 30A circuit; if the second battery was charging at 10A there might be some issues...
  • reduces the charge voltage to the second battery when its not being used but is fully charged

But it doesn't control a relay as such. I was just thinking on the heat/losses in those diodes and wondering if a relay to switch between battery 1 and battery 2 might be possible.

The recharge circuit for the battery is also wired to this copper bus. So it will be copper bus, wired to the control charge which will control a relay that recharge the battery 2
This is an odd arrangement. but maybe common in avionics. Battery 2 can't charge from the bus unless the bus is > battery volts, in which case your diode between battery 2 and the bus prevents it sending power to the bus. The only time there is power to the bus from battery 2 is when the generator is not running and either battery 1 is disconnected from the bus or its voltage is < battery 2 voltage, and, of course, CB2 is closed.

I've updated the drawing to reflect your sketch:

1632248733860.png
 
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Thread Starter

sflyscan

Joined Sep 16, 2021
23
Yes, basically the charge controller performs 2 functions:
  • limits the charging current - to protect the battery and/or the charging source; in your case I think you have a 24A load on a 30A circuit; if the second battery was charging at 10A there might be some issues...
  • reduces the charge voltage to the second battery when its not being used but is fully charged
Thank you very much, that clarify things for me. Do you happen to have one in mind? I can't find one on the Internet, I am not really sure what I am looking for actually.

This is an odd arrangement. but maybe common in avionics. Battery 2 can't charge from the bus unless the bus is > battery volts, in which case your diode between battery 2 and the bus prevents it sending power to the bus. The only time there is power to the bus from battery 2 is when the generator is not running and either battery 1 is disconnected from the bus or its voltage is < battery 2 voltage.
Yes this is exactly what we want here. It will serve when the alternator is off and battery 1 off for pre-flight and post-flight. I don't know if this is common in avionics but for what we want this is exactly that. And usually it serve for maybe 2 or 3 minutes max before we power the system with an AC/DC on the ground.

Thanks again!
 

Irving

Joined Jan 30, 2016
3,190
Thank you very much, that clarify things for me. Do you happen to have one in mind? I can't find one on the Internet, I am not really sure what I am looking for actually.
I've been looking. I've found examples of how it's done (quite simple really) but nothing ready-made that fits the bill. I could design/build one but I'm not sure about the need for "airworthiness" though it has no direct impact on the flight situation. What's your understanding of this aspect?
 

Reloadron

Joined Jan 15, 2015
7,079
I am building a system in an airplane and I need to install a 12VDC battery in addition to the one already there in the engine compartment. The battery will be used to power other equipment.
Forgive me if I am off base but can you just mess with and modify an aircraft electrical system even on a small private aircraft? Doesn't air worthy figure into things. A worst nightmare is the smell of smoke on an aircraft. Don't any and all parts used have to meet certain specifications? A typical small private aircraft electrical system should look close to this:

Aircraft Elect.png

The addition of a battery should also include another solenoid? Also aircraft alternators still use independent regulators I think but might be wrong. My advice is consult with your local aviation authority, for example in the US the FAA. This is not like working with a car or truck.

Ron
 

Thread Starter

sflyscan

Joined Sep 16, 2021
23
Forgive me if I am off base but can you just mess with and modify an aircraft electrical system even on a small private aircraft? Doesn't air worthy figure into things. A worst nightmare is the smell of smoke on an aircraft. Don't any and all parts used have to meet certain specifications? A typical small private aircraft electrical system should look close to this:
Yes, you are absolutely right, this is exactly the circuit of our Cessna172. Effectively all parts need to be approved, I am working with someone who can approve TSO. That's why I first build the circuit and then I introduce him to see what he thinks about it. Right now, the only thing I'm missing is our secondary battery recharge circuit which will power non-avionics related equipment. Everything is protected by a circuit breaker so there is no risk of fire in flight.

The addition of a battery should also include another solenoid? Also aircraft alternators still use independent regulators I think but might be wrong. My advice is consult with your local aviation authority, for example in the US the FAA. This is not like working with a car or truck.
The secondary battery is in fact controlled by a solenoid and a switch for the pilot. And yes, there is a regulator for the plane. I completely agree with you, we need the approval and consultation that we are doing at the moment. We are still in prototyping.

Thanks
 

Thread Starter

sflyscan

Joined Sep 16, 2021
23
I've been looking. I've found examples of how it's done (quite simple really) but nothing ready-made that fits the bill. I could design/build one but I'm not sure about the need for "airworthiness" though it has no direct impact on the flight situation. What's your understanding of this aspect?
At the moment, we are still in the prototyping phase, as you said, there is no direct impact on the flight situation. If there is a problem, the system will be shut down and the flight will be terminated so that I can figure out what is going on. If you can design something or show me the examples you found, that would be really grateful. It would give me an idea of how it's done and I will surely figure out what I can do next with it.
 

Reloadron

Joined Jan 15, 2015
7,079
Yes, you are absolutely right, this is exactly the circuit of our Cessna172. Effectively all parts need to be approved, I am working with someone who can approve TSO. That's why I first build the circuit and then I introduce him to see what he thinks about it. Right now, the only thing I'm missing is our secondary battery recharge circuit which will power non-avionics related equipment. Everything is protected by a circuit breaker so there is no risk of fire in flight.



The secondary battery is in fact controlled by a solenoid and a switch for the pilot. And yes, there is a regulator for the plane. I completely agree with you, we need the approval and consultation that we are doing at the moment. We are still in prototyping.

Thanks
Alrighty then, nice as you have a handle on all of it. :)

Ron
 

crutschow

Joined Mar 14, 2008
31,123
At the moment I was thinking of adding a resistor in series to reduce the recharge current, but I don't think this is the best solution.
But I think it may be.
If the battery is disconnected from the main circuit when the alternator is not charging, then all that I see you need is a small resistor in series to limit the charge current to the auxiliary battery.
For 2.2A its value would be about 1Ω since the voltage difference between the alternator charge voltage and the battery internal voltage is likely about 2V).
 

Irving

Joined Jan 30, 2016
3,190
At the moment, we are still in the prototyping phase, as you said, there is no direct impact on the flight situation. If there is a problem, the system will be shut down and the flight will be terminated so that I can figure out what is going on. If you can design something or show me the examples you found, that would be really grateful. It would give me an idea of how it's done and I will surely figure out what I can do next with it.
Sure, I can put something together for you.

Here is diagram updated to reflect new info...
1632336312605.png
 

Thread Starter

sflyscan

Joined Sep 16, 2021
23
Sure, I can put something together for you.

Here is diagram updated to reflect new info...
Thank you very much, that's what I had in mind. The only thing is the charge control which I don't know what to use. Is it possible to design one or are there already designed and available in the market?
 

Thread Starter

sflyscan

Joined Sep 16, 2021
23
But I think it may be.
If the battery is disconnected from the main circuit when the alternator is not charging, then all that I see you need is a small resistor in series to limit the charge current to the auxiliary battery.
For 2.2A its value would be about 1Ω since the voltage difference between the alternator charge voltage and the battery internal voltage is likely about 2V).
I also think about that. I think I'll do some testing on my own to see what happens if the battery is completely drained. In my head the current will probably go up to 3A with a resistance of 1ohm. The thing is, I don't know how long it will take to charge the battery in this case.
 

Irving

Joined Jan 30, 2016
3,190
I also think about that. I think I'll do some testing on my own to see what happens if the battery is completely drained. In my head the current will probably go up to 3A with a resistance of 1ohm. The thing is, I don't know how long it will take to charge the battery in this case.
I think this answers your question...
From 50% SoC... via 3Ohm, 1Ohm and 3A CC.

1632412769940.png
1632412926550.png
1632413109008.png
 

Thread Starter

sflyscan

Joined Sep 16, 2021
23
I think this answers your question...
From 50% SoC... via 3Ohm, 1Ohm and 3A CC.
Indeed, thank you very much for the clarification. So yeah, it takes a long time and after talking with my boss I might have to rethink the circuit. The load is supposed to be powered only by the secondary battery at all times. This secondary battery also needs to be recharged by the alternator all the time. I modified your diagram a little to adapt to the new configuration.1632413819856.png

I think now maybe I should switch to a battery that can deliver more Ah and have a recharge circuit that can recharge the battery quickly.
 
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