measure individual battery voltage when they are in parallel ?


I build a system based on arduino, the thing is powered by two 9v batteries in parallel, for longer operation and battery hot swap

I wanted to know if it was possible to measure individual battery voltage (to display on an lcd) ?

since both + poles are connected together I can only measure both batteries

would adding a diode do the trick ?

here is a quick sketch I made


That looks good and should work well.

so you confirm no current from the first battery would interfeer with the "real" voltage of the other battery beeing measured and vice versa ?

I thought the current would be indeed kept from passing, just to be sure

what would you recommend as diodes ? shotky that go up to 10V-20V or something ?

You will get a .5-0.7 volt drop across those diodes, look for good schottky diodes with the lowest voltage drop.

How much current draw does the unit take ?

How long do you need to last ?

What's in the circuit ?

If you do use and LCD then this could draw 12-47Ma depending how bright you have the back light or what of type of LCD you use. 9V batteries only have 500Mah and will not last very long.

I've used these in one of my projects where I've used 2 X AA batteries in series then paralleled with another 2 batteries the same then you get 4800Mah which will last a lot longer, You could even do the same but use D type cells and then you will get 26000Mah which would even last longer. By doing it this you can just measure the battery pack as it is with them in series and the parallel

You can even get cheaper ones depending on how much current draw you need the one in the link gives 500-600Ma output current.

Suppose one battory is completely empty and one is full. Then there will be a very, very small reverse current flow backwards into the empty cell. That would hardly give any significant voltage reading from the emty cell.
Schottky has a lover voltage drop than silicon diodes, that is correct. However such Schottky diodes are mostly used for signals but check their data, that they allow the current You need, and the voltage drop at that current. I don't know if You loose much if You use silicon diodes.

I made a quick test and it appears to be working

indeed 9v batteries have such small mAh , I did'nt know
it's for a guitar, and I use a 2x16 LCD +RGB leds

damn I alread ordered a dual 9v battery rack to put in the guitar

there are 9v batteries here with bigger capacity

Yes, there are 9 volt alkaline battorys with quite different capacities. Using rechargable cells is most likely cheeper in the long run.
Check the data regarding current consumption of the parts in Your build, consider the time of ooeration You want and there You will see what mAh capacity You need.

I'll check Your link later. USB, that is 5 volt? Step up technic used?

I use a buck boost(9v-35v) then a 5v regulator

it goes straight into raw pin of an arduino pro mini

I totaled the current I need, should be a bit less than 400mA

so 2x800mAh batteries should give me 4 hours of play...more than enough :slight_smile:

since they come from UK, I guess I can count on the quality

these ship to my country :

I think You've done Your homework very well. I'm mobile now, will look at Your links later.
I use UNOs so I don't know Minis but You know I think.