Battery voltage booster

[Kappes Buur]

I see a few offerings for boosting an AA battery, but does anyone know
of a similar item for boosting a low reading 9V battery to 5V?

[Rbastler]

Do you want a stepdwon converter (or low drop reg) from 9V(or whatever the battery voltage above 5V is) to 5V ?
Or a stepup to 5V ? But wy then ? Id say at about 7V a standart Duracell 9V Alkaline battery has barely any capacity left(so its unusable basically). Its DEAD at 5V.

[kalel]

I also doubt that at 5v, there's much energy left in a 9v battery.

Taking a quick look at an energizer battery chart:
http://data.energizer.com/pdfs/522.pdf

They seem to stop at 5.0v (and one at 7v).

It still doesn't say how much energy is left at what voltage, but I assume it will really not be useful.

One way I tried to use a "used up" 9v battery is to make a simple LED tester. An LED with a resistor (I used 2 - about 400 ohms total) doesn't draw much current, so for the short time needed to test it, an old battery works fine. It's been working well for months (it's not used very frequently or for long durations).

[mariush]

Find a switching regulator chip with really low quiescent current (very low current while idle) and then figure out the maximum current the circuit you try to power will need (how much current would normally be pulled from a battery, for example multimeters will take up to 5-6 mA if you don't have backlight on)
The max current is important to know when you're picking inductors (no point to use big 1A peak current inductors when you could use a 100mA smd inductor in your circuit)

Then you probably want to a regulator that runs at a high switching frequency, because while not as efficient as lower switching frequency regulators, they'll allow the use of ceramic capacitors and surface mount inductors and diodes.

Then you're gonna have to pick an output voltage and then decide what battery you will use because that will decide if you're gonna make a simpler BUCK (step down only) or BOOST (step up only) or if you're gonna make a circuit which can do both (but will be more complex, usually requiring two inductors and more diodes etc)

For example, if you want your fake battery to always have 6v, then you could use a small lithium battery which has voltage between 3.7v and 4.2v and pick a boost regulator only which will always have to boost to 6v, there's no way the battery will be higher than 6v at any point
Or if the current is very low, you could use two non rechargeable 12v batteries in parallel or series to step down to 6v

Some time ago I made a boost only circuit using LT1307, boosting the energy from a 25 Farads 2.7v supercapacitor ( 0.6v to 2.65v)  up to 5.6v ~ 4-6 mA. Very low quiescent current but kinda bad efficiency .. 80% for 1v to 3.3v 75mA... would be even worse for 5v+ output.
Even with not so precisely chosen inductors and with electrolytics on prototyping board, i got 30 minutes to 45 minutes of life from the circuit multimeter.  Batteries will hold way more charge and the voltage won't drop so fast (as is the case with supercapacitors)
It worked for me... capacitor was charged from plain usb using a linear regulator set to 2.65v and which had an internal limit of 0.8A (a 1117 ldo)  ,,, capacitor was charged within 2-4 minutes and meter worked for 30m+