# 12 VDC Battery charging circuit from 14VDC

Thread Starter

#### sflyscan

Joined Sep 16, 2021
23
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.

So I want to make a recharging circuit for this battery with the 14 VDC coming out of the alternator. Anyone know a circuit for recharging a 12VDC battery with 14 VDC?

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.

#### MrChips

Joined Oct 2, 2009
24,618
Welcome to AAC!

Here is a circuit for charging a 12V lead acid battery.

Thread Starter

#### sflyscan

Joined Sep 16, 2021
23
Thank you very much MrChips
I am going to look at the LM317 datasheet, with a quick glance I think this is exactly what I need.

#### MrChips

Joined Oct 2, 2009
24,618
One problem is that the drop-out voltage is 3V. In other words you need a headroom of >3V over the charging voltage.
If the charging voltage is 13.75V you would need an input voltage of at least 16.75V.

#### MrChips

Joined Oct 2, 2009
24,618
A simple solution would be to use a zener diode as a voltage regulator. We can show you how to select the zener voltage and the value of R.

The simplest solution is to use a diode such as a 1N4001 between the alternator and the battery.

Thread Starter

#### sflyscan

Joined Sep 16, 2021
23
Yes indeed the battery needs a constant voltage of 14V for recharging. As you said with the LM317 the voltage will drop to 11V which is too low.

Your solution with the Zener diode and a resistor seems to be a good solution.

So if I understood correctly, I will need a 14v zener diode, but for the resistor value I'm not sure how to determine it.

Is it related to the recharging current of the battery?

Thanks again!

#### MrChips

Joined Oct 2, 2009
24,618
Is it related to the recharging current of the battery?
Indirectly, yes. The purpose of the resistor is to protect the zener diode.
The closest suitable zener I can find is 1N5351 which is a 14V 5W zener. This should be able to handle about 0.35A @ 14V.

If the alternator voltage should ever reach 16V, then the calculated value for the series resistor R would be
(16V - 14V) / 0.35A = approx. 6Ω @ 1W.

Of course, you still have to tell us the specifications of the battery.

Thread Starter

#### sflyscan

Joined Sep 16, 2021
23
Indirectly, yes. The purpose of the resistor is to protect the zener diode.
The closest suitable zener I can find is 1N5351 which is a 14V 5W zener. This should be able to handle about 0.35A @ 14V.

If the alternator voltage should ever reach 16V, then the calculated value for the series resistor R would be
(16V - 14V) / 0.35A = approx. 6Ω @ 1W.

Of course, you still have to tell us the specifications of the battery.
The battery is a 12VDC 11Ah. It is the RG12-LSA from concorde. On the datasheet what I found is that the minimum charging current is 0.2X11 = 2.2 A.

Maybe I am not interpreting the datasheet in a good way.

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#### Ian0

Joined Aug 7, 2020
3,757
I thought you meant Concorde, and wondered how you'd got hold of it!
What is the capacity of the other battery that is charged from the same alternator?

Thread Starter

#### sflyscan

Joined Sep 16, 2021
23
I thought you meant Concorde, and wondered how you'd got hold of it!
Only in my dream haha.

The other battery is 24Ah.

#### Ian0

Joined Aug 7, 2020
3,757
The other battery is 24Ah.
My opinion is that it would work just fine with a split-charge relay. The alternator will be set up to charge a fairly small battery, so it won't put in too much current, and the voltages will be the same.

Thread Starter

#### sflyscan

Joined Sep 16, 2021
23
My opinion is that it would work just fine with a split-charge relay. The alternator will be set up to charge a fairly small battery, so it won't put in too much current, and the voltages will be the same.

The thing is that in my installation I do not have access to the direct output of the alternator. I only have access to a 14VDC output on the avionics bus. I made an electrical distribution unit, which is basically an aluminum box with a circuit breaker and copper bus for all the payload equipment. It is this box which is connected to 14VDC. From there, I recharge the battery. Do you think I can add this split-charge relay in the power distribution unit or do I have to connect it to the output of the alternator?

#### crutschow

Joined Mar 14, 2008
28,194
Do you need to block the new battery from discharging back into the bus?

If that's the case, and I think it might be, you would need to add a diode or a relay to prevent that.
The relay could be activated by the main power (ignition?) switch.

#### Ian0

Joined Aug 7, 2020
3,757
I know a thing or two about charging batteries, and about road vehicles and alternators, but I don't know much about the 14V Avionics Bus. When you mentioned "14V from the alternator" I was in familiar territory.
Is "14V Avionics Bus" anything special, or is it just a fancy name for the output from the alternator?

#### Juhahoo

Joined Jun 3, 2019
250
You need a buck boost battery charger, a bit complicated device but can easily handle situations like above. Too bad you're looking for ready made solutions and not trying to build one. This device can accept lower or higher voltages than output, current is adjustable and can handle different battery chemistries, cut off automatically and is battery reverse protected.
Just havea look at LTC4020. You can even buy a ready made board from Analog. But this is only if you are enthusiastic enough to dig into something like this.

#### Irving

Joined Jan 30, 2016
2,311
There's no need to complicate things but no one's asked all the right questions. Is the secondary battery there to power the avionics independently of the primary battery, or to increase overall battery capacity? How much does your avionics consume and are they left on for long periods when the main battery is not charging/connected?

If the former, you need:
• something to stop the secondary battery feeding back to primary - diode or split-charge relay. This is only strictly necessary if the secondary battery is not connected directly to the primary or if you wish to ensure the primary isn't discharged by the equipment connected to the secondary.
• something to limit charge current and voltage when secondary battery << primary/alternator. This should be the max charge current when battery completely discharged (battery will automatically take less if fully charged, ie batt volts ~13.8v) and limited to float volts when battery current << max charge current. However, if your battery is connected to the charger most of the time when in use and is rarely discharged below say 80% you can connect it directly without any additional current limiting (subject to point 1 above)
• something to disconnect secondary battery from avionics when overall system is 'off' (assuming you have the equivalent of a car ignition switch that disconnects all accessories).

The 0.2C charging rate of 2.2A is the minimum recommended charging rate for a static charge lasting at least 6 hours (10h inc 4h absorption time), You can charge slower or faster (up to 88A according to spec), but faster generates more heat and can reduce battery life in terms of recharge cycles, however this depends a lot on how far you discharge the battery. If your secondary battery is never fully discharged and is used primarily in parallel with the primary battery and normally charged at the same time then you don't need anything fancy., just connect directly (subject to point 1 above).

Thread Starter

#### sflyscan

Joined Sep 16, 2021
23
There's no need to complicate things but no one's asked all the right questions. Is the secondary battery there to power the avionics independently of the primary battery, or to increase overall battery capacity? How much does your avionics consume and are they left on for long periods when the main battery is not charging/connected?

If the former, you need:
• something to stop the secondary battery feeding back to primary - diode or split-charge relay. This is only strictly necessary if the secondary battery is not connected directly to the primary or if you wish to ensure the primary isn't discharged by the equipment connected to the secondary.
• something to limit charge current and voltage when secondary battery << primary/alternator. This should be the max charge current when battery completely discharged (battery will automatically take less if fully charged, ie batt volts ~13.8v) and limited to float volts when battery current << max charge current. However, if your battery is connected to the charger most of the time when in use and is rarely discharged below say 80% you can connect it directly without any additional current limiting (subject to point 1 above)
• something to disconnect secondary battery from avionics when overall system is 'off' (assuming you have the equivalent of a car ignition switch that disconnects all accessories).

View attachment 248291

The 0.2C charging rate of 2.2A is the minimum recommended charging rate for a static charge lasting at least 6 hours (10h inc 4h absorption time), You can charge slower or faster (up to 88A according to spec), but faster generates more heat and can reduce battery life in terms of recharge cycles, however this depends a lot on how far you discharge the battery. If your secondary battery is never fully discharged and is used primarily in parallel with the primary battery and normally charged at the same time then you don't need anything fancy., just connect directly (subject to point 1 above).
The secondary battery is there to power the new equipment that we have added to the aircraft. It is a system independent of the avionics system. The secondary battery was mainly used to power the equipment before the flight when the aircraft is off and after the flight. During flight, the system will be powered by the output of the alternator. It must be charged during the flight as it is needed after the flight. The system consumes almost 24A, which is why it must be recharged because the battery can only deliver 11Ah, which is not long for 24A. The system is controlled by a circuit breaker switch in the cockpit.

The secondary battery will probably never really be depleted, so as I understand it, I don't need to limit the recharge current. I just need to put a diode. Because at the moment in my last system I had put a resistor to limit the current but the battery does not seem to be recharging at all.

I think I don't really understand how the charging current will change over time. Is there any internal resistance in the battery playing a role in this? As when the voltage difference between the charging voltage and the battery is smaller, the charging current will be smaller? I.E. if the charging voltage is 14V and the battery is 13.8V, the charging current is I = (14-13.8) / Rbatt

Thank you again for your help everyone it is really appreciated

Thread Starter

#### sflyscan

Joined Sep 16, 2021
23
Ah yeah and I was forgetting that there is also a solenoid with a switch for the secondary battery to allow it to power the system in addition to the circuit breaker switch that is used to power the system from the alternator. A diode is there after the solenoid.

The only thing now is the battery recharge circuit, at the moment there is a diode and a resistor, but if I understood, I don't need the resistor.

Here I have made a schematic

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#### Ian0

Joined Aug 7, 2020
3,757
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.