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A Small Rechargeable USB Power Supply from an E-Cig LiPo

USB Power Supply from an E-Cig LiPo

We’re still sadly finding E-Cig litter on a near daily basis and despite having incorporated the small LiPo’s they contain into drones, building MP3 players using the LiPo and other parts, and building a UV torch, we still need to find uses for the growing single cell LiPo pile.

A common project we’ve seen over the years is using a single cell in combination with a boost converter to make a small USB/5V power supply. Across the maker communities, you can find these circuits crammed into all manner of mint tins, matchboxes and other containers to make useful little devices. We’d ideally considered making such a project inside one of the E-Cig body tubes that we have in abundance, but when looking for budget PCB modules to create this project we realised they would be too big for the tubes. As such once we’d decided on our circuit and modules we did a quick and easy 3D-printed enclosure.

In the first article about recovering E-Cig LiPo’s we looked briefly at the most common and possibly cheapest way people are approaching charging these cells, the common tp4056 module. TP4056 actually refers to the IC chip on the module and as such there are numerous different small PCB modules being made around this common charge chip. The simplest ones simply charge a cell. As a reminder, you can change a reference resistor which limits the current depending on the resistance. Most TP4056 modules arrive set up to charge at a 1 amp rate which is a little too much for our cells. Most of the LiPo we find are around 550mAh and so looking at the datasheet we can work out that around a 2.5K ohm resistor limits the charge current to match. This means that our 550mAh battery should be fully charged in one hour, but often will take less if recharging from a partially charged state. There’s potentially another problem that can effect LiPo cells, over-discharge. Over-discharging a LiPo cell can damage the cell and or decrease its capacity so when making up some kind of LiPo-based power supply it’s nice to have a feature that prevents this. Rather conveniently there is a different TP4056-based battery charging module that incorporates an over-discharge protection feature. This module is often called the “03962a”. In terms of the charge current limiting, this works in exactly the same way as the other module, you simply swap out the reference resistor and replace it with one to get the desired charge rate. Again, we swapped ours for a 2.5K ohm resistor.

TP4056 module

You can see that as opposed to the more common TP4056 module the 03962a module has some extra components and most tellingly, some extra pads to connect to. There is a set of pads to connect to the battery, similar to the standard TP4056 module but there’s also a set of output pads which you can connect to a load. The 03962a board then allows the battery to power the attached device but cuts the power when it detects the battery has reached the over-discharge threshold of 2.4V.

03962a module

For the boost side of our project, we picked a really cheap DC-DC step-up power supply module that accepts any DC input between 0.9V and 5V and outputs 5V at a maximum continuous rating of 500mA. The only additional component is a small switch so that we can switch on and off the DC-DC step-up module. One slight issue to avoid is that the 03962a board also protects from overcharging by monitoring the voltage and as such you can’t run a load, ie run the DC-DC step-up module connected to something, whilst the 03962a board is plugged in to charge the LiPo. We’re happy to just accept that as a limitation and wired the modules, switch and a decent salvaged LiPo, known to be holding a good charge, from our salvage pile.

Circuit Diagram

When wiring to the LiPo we made sure to minimise the amount of soldering time to introduce as little heat into the LiPo as possible, the other consideration is to make sure that the LiPo and the wire connections are well insulated and cannot short against either the LiPo foil casing or any other parts of the build. It’s fair to say that the “disposable” E-cig manufacturers sail close to the wind in this aspect of their designs, so we added lots of little sections of Kapton tape to insulate as well as to reinforce and protect the cell and cell wiring. With everything hooked up and checked we then designed a very simple case for 3D printing. As a quick project, we did nothing too fancy and simply made a base enclosure with a mount hole for the switch and everything else glued to the sides. Using a few dabs of hot glue we got everything mounted securely before printing a simple flat lid, and a small end closure piece which we again glued into position. It’s not going to win any design awards, but it’s functional and fit for purpose.

CAD drawing of enclosure

Whilst it’s not got the capacity to give modern smart phones much of a charge, it is a really handy device. For testing, we found an old USB LED light which makes quite an excellent torch/lantern but we’ve also used it on our workbench to connect up to prototyping boards for 5V power supply duties and for quickly testing small USB devices' functionality. It really is a pretty useful little gizmo!

General tinkerer! Freelancing writing about making things, rocketry, boats, electronics and a mahoosive pile of unfinished and unstarted! Author of "FreeCAD for Makers" book on Raspberry Pi Press and writes for Hackspace Magazine, Tindie, Kids Code Computer Science, Toms Hardware and more!
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