I have built a USB charger with a 9v battery and a voltage regulator however i want to build a new one. i have found when the 9v battery drops below 5v the output of the regulator is nearly nothing. So i want to put a load indicator led on the 5v output that comes on when something is plugged in and goes off when its unplugged or the voltage of the 9v battery is at 5v or less.

Thank you all for taking your time to read this.

 

A 9V Battery is horrible at capacity. They are generally made for long term use with very little current draw.

USB powered devices, on the other hand, want 2.5W (5V @500mA) continuosly, very power hungry.

A 7.2V Lithium cell with a switching 5V Regulator may be closer to what you are trying to attain, or even 4AA batteries with a switching regulator. Linear regulators waste a good deal of power when dissipating half their voltage.

 

I agree with your concern however i am using rechargeable 9v batteries. I also use 4AA battery pack with a 9v battery clip on it so i can use them interchangeably. lithium's are a extremely bad idea unless i redesign my circuit to have a battery level monitor in it and I'm trying to keep it supper small and simple. My initial design was so i could keep it under my seat of my motorbike in case i broke down and had a flat phone it would give me enough life to make a phone call. I have just been using it more often than i first intended.

Thank you for your fast reply.

 

Here's the output of a script I keep on my computer; I run it whenever I want to remember something about a common battery.

Battery capacity in mA*hr
        Alk.
       Mass,g   Alkaline    C-Zn    NiCd        NiMH        Lithium  Dia Length
       ------   --------    ----    --------    ---------   -------  ----------
AA       20      2890       1100    600-1000    1700-2900     3000   14.5  50.5
AAA      11.5    1250        540                800-1000             10.5  44.5
AAAA      6.5     625                                                 8.3  42.5
C        66      8350       3800                4500-6000            26.2  50
D       140     20500       8000                900-11500            34.2  61.5
9 volt   46       565        400    120         175           1200    18x27x49
CR123    dia=0.67,  length=1.36  3 volts                      1500   17    34.5
  3 volt (nomimal) coin cells (open circuit about 3.6 V):
CR2025   dia=0.787, height=0.098                               160
CR2032   dia=0.787, height=0.126                               225
CR2450   dia=0.965, height=0.197                               610

Alkaline data from wikipedia; in turn from Energizer.  Masses were measured.

Silver Oxide   Volts mA*hr     Alkaline   Volts  mA*hr   Dia, in   Height, in
------------   ----- -----     --------   -----  -----   -------   ----------
    SR41        1.55   42        LR41       1.5    32     0.311      0.142
    SR44        1.55  200        LR44[2.0g] 1.5   150     0.457      0.213