Buck converter to charge second battery.

[Pietpower]

Can you use a buck boost controller to charge a second battery in a vehicle?

You can set the input battery voltage to draw a little power from the main battery or to just operate when alternator is on and voltage high.

Output voltage set to second battery 100% value (13.0V for AGM)

Output current can be set to say 10A drawing less load from the alternator and on the wiring (smaller wiring)

Not to replace a proper dual battery system but an secondary charging system while driving and while camping the main system will be a solar system.

 

[Gidsie]

Maybe in conjunction with a cutt off type controller as below

Adding a Diode will also be a good idea to stop backflow

It is uncertain what the Max Amps is even though relay handles 20/30A

 

Maybe an all in one Buck/Voltage Switch is available ?

I would also be interested 

https://www.robotics.org.za/XH-M603

https://www.robotics.org.za/XM-M604

 

 

 

 

[BritishRacingGreen]

Hi @Pietpower , I was scratching my head why you would employ such a converter , and not charge directly from the 12V DC bus of the vehicle , but then I realize that you want to limit the maximum charge current to the additional battery in order not to violate the charging specification of the alternator. Well this is quite a novel idea of yours. So the converter acts basically as a current limiter and plus minus nothing else. 

So its a good idea , you will just need to take extreme care when adding this converter to your vehicle , as the alternator transients in vehicle can become nasty, very nasty . There I would recommend you an 18V Transzorb and 100nF polyester capacitor in parallel with the input . Then  make up an inductor  with thick wire on a toriodal core with a number of turns . The inductor must handle the 10-20A DC max charge current , and it must be able to attack the rate of change in current of input transients . this inductor must be in series with the input. Add a 35A  automotive fuse also inline with the inductor. 

You have not indicated the input DC range , but it should be able to handle a max of 18V to be safe.

Again , quite a novel idea of yours.

[Pietpower]

Thanks

Here is one scenario:
Set the input battery protection to 13V and output to say 14.2V and 10A max.
When the car is off and the main battery is at 12.7V then the circuit is off.
When the car starts the alternator kicks in charging the main battery at say 50A then the converter charge the second AGM battery at 14.2V.

I presume when the battery gets closer to 100% the current will drop but how do you prevent overcharging?  Can you just select the correct voltage?
 

[Scorp007]

On 2022/04/22 at 8:41 AM, Gidsie said:

Maybe in conjunction with a cutt off type controller as below

Adding a Diode will also be a good idea to stop backflow

It is uncertain what the Max Amps is even though relay handles 20/30A

 

Maybe an all in one Buck/Voltage Switch is available ?

I would also be interested 

https://www.robotics.org.za/XH-M603

https://www.robotics.org.za/XM-M604

 

 

 

 

Just something I found with the XH-M604. As soon as the charger starts charging the battery the voltage rises and as the output to the battery is also the battery sense input to the controller it tends to switch charging off. At this point the battery voltage drops to below the set point and it charges again for a few seconds. This keeps on repeating itself. 

I found by using one of the other models where the 220V input to the charger is switched which has a 3rd input/output for the battery voltage sense works out a lot better. The 220V input that switches the charger also powers the controller aux voltage for operation. It thus functions better but is thus only suited to using 220V AC as the source for creating an auto on/off charger.

[Pietpower]

On 2022/04/22 at 8:41 AM, Gidsie said:

Maybe in conjunction with a cutt off type controller as below

Been thinking about that for a bit.  When you charge the battery the voltage will be slowly rise but when you reach the set voltage you want to keep it there for a while so that the battery can reach full capacity while the current drop.

[Pietpower]

I have investigated the idea a bit further.

The input voltage and the output voltage is too close together.  Thus a buck/step down converter or a boost/step up converter will not work.
You need an all in one buck/boost converter.  I can't seem to find one that is rated at say 20A to be able to give a continuous 10A charge.

For lead acid batteries a higher voltage for saturation charge is preferred and then a lower voltage for float charge.
For LifePo4 it seems you need to disconnect the charge current to prevent damage.