So, i needed a 9v battery for my multimeter, as it happens i had none, so i looked at a bunch of power bank circuits on my table and decided i would convert them into 9v and 12v batteries for various purposes, really any purpose that needs a small 9v or 12v rechargeable portable power supply, that can include fiber optic modems in case of emergency for instance so that you can have internet when the power goes down, a portable router, radio controllers for modelism, guitar pedals, audio instruments, amplifiers, alarm clocks, whatever rocks your boat, you can replace any 9v battery system with a smart lithium system with charging trough usb port that gets full in one hour and can supply more current than any tradicional 9v battery ever could for basically the knowledge, that mod can save a lot of time to makers that don't have time or money to buy the same thing divided into modules on the internet, or people like me that have enough spare power banks and just don't want to waste them.
In the next tutorial i'll show you how you can enclose this power bank into a 9v battery shell so that you can make your own 9v rechargeable lithium battery if you want, i just want to mod some electronic stuff that uses 9v batteries to use lithium and i'll mod the enclosures of the devices to fit the charging port instead of using a shell battery that i have to remove to charge, but each case is one case, and it's good to have a spare shell battery because they're compatible without further mod of the device, but the mod of a device allows for a not removable battery approach, that i would like to have with my multimeter because it allows bigger batteries or more batteries in parallel.
Power bank circuit
scrap electronic boards
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Step 1: The Basic Mod Diagram
So, this mod consists of basically creating a secondary coil around the inductor with a higher number of turns than the inductor has, so that this new secondary will discharge more voltage than the original has, as the original can only increase voltage at a maximum of pi*Vin but with no impedance (no load) only, wich results in poor performance at higher output voltages, while our modded secondary bypasses this need changing the coupling factor of impedance trough having more turns and thus higher voltages by default, it's literally turning a power bank into a switching mode power supply.
The coil mod alone isn't enough to increase the voltage, as the chip has a pin called feedback that measures the output voltage trough two resistors in a resistor voltage divider configuration, the input voltage on this pin of the chip is always a proportion of the output voltage, and it should reach 0,6v when the output voltage is in the desired maximum voltage, at wich point the regulator on the chip stops the coil activity and hibernate for long periods of time, just waking up to mantain the charge on the output capacitors as they leak and to compensate for the drop in current on the resistor divider.
For instance, my model of powerbank has a TD8208 step up chip.
But the great 99.9% majority of power banks use a similar 6 pin or 5 pin converters, you can generally google them or you can use your intuition and find the 0,6v or sometimes 0.7v pin of feedback and do the rest of the mod, sometimes you just have to see wich ci has a circuit with two resistors pointing at the output and gnd and it will almost certainly be the feedback pin, and the coil mod is always the same case.
Step 2: Doing the Mod
Well, that's what you have to do hardware-wise, first do the coil mod as the power bank should still somewhat work normally after the mod, but with less current capability, it'll be the same current limit you'll have once working on 9 or 12v output but less power/hour because of lower voltages, after you do this part of the mod you'll plug a battery and measure voltage on the output, if it's 5v that's ok and you did the first part right because the regulator on the chip is holding the voltage at it's programmed voltage level.
Next part is changing the output capacitor for a 16V+ tantalum capacitor, then changing the resistors from the resistor divider to new values so that it will allow for 9v or 12v operation, but before you test the circuit be sure to remove all of the solder flux from around these new resistors as the solder flux is conductive and will drain your battery for nothing and will mess the output voltage regulation.
If you can't find R1 with exactly that many ohms, you can use 2 MegaOhm for 12v and 1.5 MegaOhm for 9v then take a conductive graphite pencil and brush the resistor a little bit to make it more conductive, so that you can reduce the resistance and therefore the voltage to get the required voltages. This is known as pencil mod, and it's common in overclocking video boards.
If you need 9v and 12v operation, you can put 2 resistors in series in place of R1, a R1A 1.5MOhm and a R1B 500k, then put a switch to short the 500k resistor, with the open swith the system will regulate to 12v and if you close the switch the short will limit it to 9v.
Step 3: And the Result Is....
Well, i got 8.92v, and that means a perfect success as it's at the same voltage level as a charged 9v battery, i used 1,47MegaOhm resistance as R1 and i'm pretty satisfied with the result. If i really wanted to get nominal 9v i'd change r2 to a 90k resistor, but that's hard to get and wouldn't improve the performance much, as a greater voltage generally means more power spent for nothing.
Take care of the polarity on the tantalum capacitor, as it has odd markings that signal the positive terminal unlike electrolitic capacitors that mark the negative terminal.
Step 4: And Then the Next Project...
This converter primary use will be in my multimeter, as you can check in the previous picture i had a low battery while taking the voltage read from the converter, and on this new photo i'm showing the converter installed and the low battery icon gone, i'm happy with the converter and the way it turned out.
The second back photo from the multimeter have the led lighting on while i turn the multimeter on.
Step 5: Advanced Mod, How to Increase Efficiency of the Converter
If you have a little bit of space available or your inductor is broken you may consider changing the inductor for a toroidal transformer, you can find a small one inside every compact fluorescent lamp circuit, due to increased efficiency the new transformer should allow for more output current or alternatively better battery life and less ripple and EMI from the converter, wich might be important when using the converter to power measuring devices.