Maker Module 003: USB rechargeable power source

Originally posted 2015: information may be out of date, as this tech moves so fast! These 'Maker Modules' were a series of mini 'recipes' for extending components which can in turn be combined into something cool.

This Maker Module gives you a one-stop power source for your small project. By coupling a cheap and widely available charger module with a battery (of suitable chemistry) and switch, MM03 can be switched from ‘power source’ to ‘charging/off’ modes. ‘Power’ mode outputs nominal battery voltage, whilst ‘charging’ mode disconnects the load attached to the battery in favour of the monitored charging circuitry (and associated LED indicators).

*Disclaimer* – I cannot be held responsible for any combination of materials here described leading to personal damage or injury. Batteries are dangerous, particularly LiPo (Lithium Polymer) varieties, which can rapidly explode and cause fire if mistreated, charged too rapidly or else accidentally ‘shorted’. You have been warned!

Parts

• TP4056 USB battery charge module (available via eBay with either USB mini or micro connector)
• Suitable resistor (see below for appropriate rating)
• Battery (check the supported battery voltage and chemistries on your TP4056 based charge module)
• SPDT switch
  

Construction

There’s little to this make, beyond using the TP4056 module correctly. These typically come from the factory with a 1.2K resistor installed however, which equates to a 1A charge current. The important thing to note here is a basic rule that you should charge your battery with no more than 1C incoming current (i.e. a charge current of no more than the capacity of the battery). If you were planning to use a 1Ah (or greater capacity) battery, then that’s fine, but LiPo batteries are very common in much smaller capacities, and this default then represents TOO MUCH current; the resistor needs to be swapped for a higher value – failure to do so may result in a fire! The documentation details appropriate selection of a resistor ‘Rprog’ to manage the charging current supplied to the battery.

Consider my (depicted) example case: I have a battery rated at 190mAH. A safe (1C) charge current is therefore 190mA. The factory installed 1A charge current on average TP4056 board is over 5 times larger than this (5C)! I definitely need a different Rprog resistor to charge this particular battery. The table above gives appropriate values for my board – I can see that with a 5K resistor, I’d still be too high at a charge current of 250mA. The best value there is the topmost: 10K, giving a smaller than 1C charge current of 130mA.

Swapping out the resistor is fiddly, given that the default Rprog is a teeny tiny surface mount component. With a little care (and reference to the board to ensure that the targeted resistor is the right one), it can be cleanly removed. ‘R3’ is the Rprog resistor, on my module:

With as much care, a replacement resistor (in my case a normal sized component) can be inserted in place (here heat shrink wrapped too, to prevent conduction between random components)

The board is now ready to charge my 190mAH LiPo battery. Let’s add a switch to improve the board’s utility now. The following circuit diagram shows how this should work:

In one position, the switch creates a circuit from the battery across whatever load needs it. In the other position, the ‘load’ circuit is broken, and the normal ‘charge’ circuit completed instead. In this latter position, with a USB cable plugged in, the battery can be charged until the LED indicator on the module lights up to indicate that the battery is at capacity. Without a USB cable plugged in, this switch position is also a convenient ‘off’ switch for the load.