# Buck converter: how to estimate the discharging time of a battery from Vrated to Vgeneric?

Discussion in 'Power Electronics' started by casters, Oct 18, 2016.

1. ### casters Thread Starter New Member

Nov 28, 2014
4
0
Hello everybody, this is the first time I write on AAC's forum.
As the title says, i'm wondering how to estimate the dischaging time of a battery from , for example, 12V to 9.5V.
I'm projecting a Buck regulator for a guitar pedal board and I need 9V and, at least, 500mA output.
As source I thought for a 12V Li-Ion battery and, as a concequence of some preliminary steps, I obtained:

Duty=Vout/Vin=9/12=0.75
Iin=Iout*Duty=500*0.75=375 mA

If I think an average use of 4h (a very big gig ), I need a battery whose capacity is 375*4=1500 mAh

But here comes the problem:
the Vin mustn't fall below 9V (I have a buck regulator) and the 1500mAh are calculated in relation, IMHO, to a battery discharging from the rated voltage to the threshold voltage,which can be few volts.
How can I calcolate the minimum capacity of the battery that guarantees the operating condition of 500 mA*4 h knowing that I can't go below 9 V?
I'm sorry for the bad english, this isn't my mother tongue

2. ### crutschow Expert

Mar 14, 2008
13,477
3,362
Do you have the discharge curve for the battery you want to use?
Lithium ion batteries typically don't change voltage that much until they are completely discharged so your calculations are probably not accurate.

What is the voltage and current draw of your load connected to the buck regulator?

3. ### casters Thread Starter New Member

Nov 28, 2014
4
0
As I wrote before, output specs are: 9V @500mA
Since I have a battery as input, i must guarantee the operative condition for a certain period (4 hours) ,during which the Vin doesn't fall below 9V.
If i take a 12V 1500mAh Li-Ion battery that supplies 375mA, after how much time voltage drops to 9V?
I don't have the discharge curve and, for this reason, I'm asking for a method to evaluate the capacity of a battery that must ensure, on the output of a buck regulator, 9V and 500 mA*4 h
Why you say that my calulation aren't correct?

4. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
There is no way for us estimate the discharge characteristics of a particular battery because there is no generally accepted way to do it. You can make an empirical measurement, but as the battery ages the discharge curve may change. When manufacturers quote a battery life expectation, e.g. 1500 mAh, they don't specify what happens to the voltage in the process. Again, the only way you can be confident, is to make an empirical measurement. The folks at Samsung claim they have no idea why their batteries are getting hot and smoking. It has to do with what the batteries are being asked to do either intentionaaly or accidentally. It really doesn't matter now, does it?

5. ### casters Thread Starter New Member

Nov 28, 2014
4
0
What a cruel world

6. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
I don't see how you come to that conclusion. Some things are deterministic and some things are not. Nature always works according to mechanistic processes and so far as we know there is no such thing as magic.

7. ### casters Thread Starter New Member

Nov 28, 2014
4
0
Ok, then "What a partially non-deterministic world"

8. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
That's just like "mostly dead" means "slightly alive".

I'll have an MLT!

9. ### crutschow Expert

Mar 14, 2008
13,477
3,362
Because you calculations assume the battery linearly drops in voltage as it discharges and Li-ion batteries generally don't.
But that would give you more energy then you calculated so yours is a conservative estimate.
You need to look at Li-ion batteries with a capacity near to what you need and examine their discharge curves.