Battery Backup Circuit Design Help

Other Hardware General Electronics
1

Hey there! It is my first time posting to the forum so please let me know if I'm doing something wrong :confused:

So recently I've been interested in designing a circuit that switches from one battery pack to another using one digital pin of my arduino micro (yes, I understand that both power supplies can be added in series but its more of a proof of concept).

Anyways the specs of my batteries are as follows:
Normal battery: 6v @ 2800mah
Backup: 7.2v @2800mah
Peak current draw of the circuit to be powered: somewhere around 850ma to 1amp (w.i.p.)

Voltage regulator: Pololu Step-Down Voltage Regulator D15V70F5S3 (7amp version)

I have voltage sense circuitry that I read with my microcontroller to tell me when the normal battery drops below 5.5v (my regulator requires atleast 4.5v to output 5v) at which point the normal battery will switch off and the backup will turn on. Obviously this has to be done rather quickly (<50ms?) otherwise we loose power to arduino (bad).

Temporarily I've been using a 5vdc relay, but it requires like a 1000uf tank capacitor on the reg input otherwise the power to the arduino drops out. I suppose as the circuit gets more and more finished towards the full 850ma to 1amp peak current that I mentioned above this value will have to increase. This solution seems rather clunky.

So: I was thinking of using a couple of P-channel FETS and a logic inverter. Such that, when the arduino digital output is LOW to the circuit the FET for the normal battery is open. However, when the output is HIGH the FET for the backup opens and concurrently the FET for the normal battery closes.

I've attached a schematic with some possible part types and layout. I'm not sure if it will work, infact, all I am sure about is that the logic inverter will work.

The schematic can be viewed/edited here (I believe): EasyEDA(Standard) - A Simple and Powerful Electronic Circuit Design Tool

schematic.png
schematic.png1273×617 22.4 KB

2

Hi, I was dealing with a similar problem recently, and got very good sugestions in this post:
http://forum.arduino.cc/index.php?topic=434046.msg2991683#msg2991683
The solution of the diods is very simple but it will switch whenever the backup battery has higher voltage than the normal battery, so doesn't give you much control to fully use the normal battery before switching to backup.

The solution of using the LTC4415 is much nicer and gives you full control of the switching, I haven't tried that one yet as the part has not arrived.

Hope that helps.
Cheers

3

crada:
Hi, I was dealing with a similar problem recently, and got very good sugestions in this post:
Switching between two power sources - #4 by crada - General Electronics - Arduino Forum
The solution of the diods is very simple but it will switch whenever the backup battery has higher voltage than the normal battery, so doesn't give you much control to fully use the normal battery before switching to backup.

The solution of using the LTC4415 is much nicer and gives you full control of the switching, I haven't tried that one yet as the part has not arrived.

Hope that helps.
Cheers

That is an interesting solution that I have seen on stack exchange somewhere but I rather use non-specific power IC's and stick with FETs if possible. For no reason other than to learn. Anyways this specific IC wouldn't work as the input voltage would be higher than the rated voltages of the IC but it is an elegant solution that I will keep in mind.