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Topic: Powering a 5v circuit with car and backup lipo (Read 2230 times) previous topic - next topic


I am going to need to power an arduino and several of these sparkfun led strips.  2 strips would almost be 4 amps.

I was thinking about using a Traxxis 7.4v 12800mAh R/C car battery.  Those can handle the large loads.  I am not an expert in power supplies.  I've been researching what I can do and I have hit a wall.  I would like the car power system to charge the battery while it is running and the project use the power from the car.  Then when the car isn't running to use the battery's power. 

i am not sure how to accomplish this.  I see some obstacles.  One, need to regulate the car voltage as it can spike.  Two, how to charge the battery while using the car power system.  I would obviously used a switched power line in the car, however, to protect form accidentally draining the car battery if it isn't running I could use a voltage switch that disconnects the car battery form my project.

My backup plan is to just have a single switch that when off the car power is connected to a charger to charge the lipo battery an the lipo is no powering the project. When on the switch will cut off the car's power system completely and connect the lipo to the project.  i would rather have an automatic way instead if possible.

I found this but I am not sure if that can be used for what I want.


i would just have a lipocharger in the car(could be turned on/ by relay from arduino so its not running all the time) so when you put your stuff in the car it runs by the 12V through the charger, and when u remove it your lipo starts to lose charge and everthing runs on that.
now this is probably not the best way as it can lower life of ur batt
oh wait ur not removeing it ur turning the car off.
have a acc line from the car run to the arduino split it to a signal, and a line.
(one goes through resistors etc to your input pin the other to a relay. on NO)
use a lm317 voltage reg or something else to reg pwr if u want (or caps and resistors) etc.

your lipo is also connected to the relay. on a NC

Code: [Select]

carRunning = digitalRead(inputPin);
if( carRunning == HIGH){
  dighitalWrite(relayPin, LOW);  //relays i like activate on a low others may be high.
   digitalWrite(relayPin, HIGH); // when the acc is off it switches to battery

the problem i see is that their will be a short iturupt in power between the 2 power sources.
so add a 9v battery in parral with the relay so it provides power durring the inturupt.

hope it helps let me know if it dont or u need clarification


So you are saying have the arduino be powered by which ever source and control which source is powering it?  That sounds like it could fail at some point and the arduino would need powered at all times.

Yes, I will have a master switch to shutoff the project.  I am hoping to keep the project component separate from the power, the power supplies (car and lipo) only supply power.

How about a little more detail.  The project will run off the lipo most of the time.  Most of the time I will just shut it off while the car is running (not legal to have this on while driving on public roads).  So while I am driving why not have the car charge the battery is what I am thinking.  However, if parked at a show and the car is running I figure why waste the lipo battery when the alternator is running also.


why a lipo then? if you have room just add a second battery like you do for stereo systems/rv's
your entire project always runs off the second battery parrellel to the cars 1st battery. alternator directly
and a ssr connects them when the car is running.
u can't really do this with a lipo cause you might destroy it.

car alternator charges >battery>goes through diode or relay to prevent start batt from discargeing
> into second battery(or lipo with safeguards (charger))>> into your project box/disconnect switch.
that would be best i think but i have room in my car and the 1st battery is in the trunk. you dont need any code or arduino control and u can power anything 12v for a while off a second battery, and everthing charges up when u start the car.
make sure its a deep cycle battery

Give me more info about your restrictions, and preferences. i love to help


Mar 11, 2012, 10:50 pm Last Edit: Mar 11, 2012, 10:55 pm by SirPoonga Reason: 1
Space is a concern.  But you just gave me an idea I can't believe I didn't think of.  A pocket bike battery is also lead acid.  This should be enough.


Thanks, your idea will work with that battery.


glad to be of service. just match that battery cause its smaller u can damage it.
i dont know any safeguards.
check it periodicaly post some pics of the finnished project they are always fun to see.


Mar 13, 2012, 03:58 pm Last Edit: Mar 13, 2012, 04:09 pm by SirPoonga Reason: 1
So then this begs the question, what diode should I be using?  Actually, I am trying to figure out how to hook it up.  Don't I need a relay in there instead of a diode?


you need to prevent back flow, period
a diode will do it
a relay will do it
others search battery desconect/ second battery etc
id use a simple auto motive relay
it could stick over long use, and it uses power constanly when on (or off if u change types)
but is reliable could use 2 in series to provide more security

but i dont know what kind of diode you would need just knowit is possible. i use a relay to a switch on my dash to manualy turn chargeing on auto off with the car.
you can turn auto on  auto off with the coil to the acc
to be short:
yes use a relay.

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