# 12 VDC Battery charging circuit from 14VDC

#### sflyscan

Joined Sep 16, 2021
23
At all times? What is the flight time? And what else is on the main busbar?
Right now on the main bus bar there is aviation equipment that drain on their own about 24 A. Generator is rated 60A with a safe factor of 80% so 48A. There is 24A left for the secondary system and battery recharge. But I talk about this with my boss just now and he said that it's find that the battery is slowly recharging itself and we will keep the secondary bus connected to the alternator. Flight time will vary but probably around 3 to 5 hours. So battery is continuously recharge during the flight.

So after thinking about it, I will choose a resistor so the battery is charge at about 4 -5A. Probably around 0.3Ohm. Am I right thinking like that or I am missing something.

#### Irving

Joined Jan 30, 2016
3,185
So we need to put the diodes back in? Is the 30A breaker between alternator and primary bus or between battery BT1 and bus? There should be a breaker between battery and bus (and for BT2 as well).

#### sflyscan

Joined Sep 16, 2021
23
So we need to put the diodes back in? Is the 30A breaker between alternator and primary bus or between battery BT1 and bus? There should be a breaker between battery and bus (and for BT2 as well).
Yes there is already breakers for Battery 1 and alternator. This is the original circuit of the aircraft wasn't modified for this. A 30A circuit breaker was added between the main bus and the secondary bus. Schotty diode will be back in with a resistor of probably 0.4Ohm is what I am thinking right now for the recharge circuit of the battery.

#### sflyscan

Joined Sep 16, 2021
23
Original Cessna172 Electrical Schematic for the one interested.

Main battery seem to be recharge with resistor.

#### crutschow

Joined Mar 14, 2008
31,109
I don't know how long it will take to charge the battery in this case.
Likely the only way to know that accurately is to do a test.

Note that to measure the charging current, just measure the value across the 1Ω resistor and calculate the current from that (1V/A).
Inserting an ammeter will add some additional resistance that will reduce the current.

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#### Irving

Joined Jan 30, 2016
3,185
Main battery seem to be recharge with resistor
Yes, so it seems. But only when fed from ground connector or MASTER_BAT is off!

Are you taking feed off ALT or PRIMARY?

Not sure resistor will cut it...

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#### sflyscan

Joined Sep 16, 2021
23
Are you taking feed off ALT or PRIMARY?

Take the primary feed. I would prefer to have a charge control like you showed in your diagram but I really don't know how to make one.
I think with 0.5 ohms your simulation showed that the battery would fully charge in 3.33 hours which would be perfect in our case.

#### Irving

Joined Jan 30, 2016
3,185
Take the primary feed. I would prefer to have a charge control like you showed in your diagram but I really don't know how to make one.
I think with 0.5 ohms your simulation showed that the battery would fully charge in 3.33 hours which would be perfect in our case.
I can help with the charge controller.

With the diode in there too the charge rate is much lower, because you lose 0.4v in the diode and the charge rate slows down faster the closer to 100% SoC you get, its more like 4 - 5 hours, and remember that's just the bulk charge. For a 100% charge you need 4h or more absorption time at constant float volts after that. Otherwise the battery will deteriorate faster. A proper constant current charger is a much better solution. But remember it will only fully recharge if its not being used, so the Avionics bus must be powered from the Primary bus and the secondary battery only used for the short switchover between alternator and ground support.

I found this site very helpful (esp Fig 5) in understanding the poor Cessna schematic. Basically if the MASTER_BAT switch is off there is no power to the Primary bus from the main battery, though it appears that there will be from the external feed (which also trickle charges the main battery through a resistor, this is a float (maintenance) charge not a restorative charge)

#### sflyscan

Joined Sep 16, 2021
23
I found this site very helpful (esp Fig 5) in understanding the poor Cessna schematic. Basically if the MASTER_BAT switch is off there is no power to the Primary bus from the main battery, though it appears that there will be from the external feed (which also trickle charges the main battery through a resistor, this is a float (maintenance) charge not a restorative charge)
Oh ok that make sense. Thank you for this website indeed it help me to have a better understanding of that poor schematic.

I can help with the charge controller.
It would be really appreciated

#### Irving

Joined Jan 30, 2016
3,185

I've shown charging to BT2 as diode/resistor - this isn't my recommended approach.
Surprisingly there is no circuit breaker on the output side of the Master battery.
Its also worth noting that the internal Electronics (Avionics) bus (not shown in this diagram) is disconnected from the Primary bus both when starting or when EXT power is applied to protect it from surges/transients.

#### sflyscan

Joined Sep 16, 2021
23
I've shown charging to BT2 as diode/resistor - this isn't my recommended approach.
Surprisingly there is no circuit breaker on the output side of the Master battery.
Its also worth noting that the internal Electronics (Avionics) bus (not shown in this diagram) is disconnected from the Primary bus both when starting or when EXT power is applied to protect it from surges/transients.
Thank you very much, it is much appreciated. I'm going to continue from there and use a resistor to recharge the battery. Guess if there's a problem with this I'll have to rethink it, but for now it's good enough to give it a try.

Thanks again,
Sam

#### Irving

Joined Jan 30, 2016
3,185
Updated diagram, I just realised the 25A circuit breaker was in wrong place!

Thank you very much, it is much appreciated. I'm going to continue from there and use a resistor to recharge the battery. Guess if there's a problem with this I'll have to rethink it, but for now it's good enough to give it a try.
OK, your resistor needs to be rated at around 20W, eg something like this one

#### sflyscan

Joined Sep 16, 2021
23
Updated diagram, I just realised the 25A circuit breaker was in wrong place!

OK, your resistor needs to be rated at around 20W, eg something like this one
Yes I found one rated at 50W like this one.