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Electronics -> Single cell low current 3.7V Lithium Ion smart charger

Last modified on Tue, 5th Aug 2008 at 13:13 BST by zipplet

What is it?


This is a simple but smart low current 3.7V charger for Lithium Ion batteries that I am currently designing. It has been designed especially for small cells such as the 10180 cell used in the Orb Wee series of lights. It was born out of the desire to have a cheap smart charger that would be safe for smaller cells. Lummi supplies the Nano charger which is not safe if you want to charge unattended and leave it on after charge is complete for hours as it overcharges cells because the maximum charge voltage for that charger varies (mine is 4.26V) but it is fine if you charge then take the battery off soon after charging. The nano charger also charges at a rate that is too high for 10180 cells, shortening the life of the cells.

I am currently maintaining a thread on CandlePowerForums about this charger.

The charger will be designed to only charge 3.7V standard lithium ion cells, not 3.0V protected cells or LiFePO4 cells. That will be in a later revision as needed.

(parts copied from my posts at CPF)

A PIC microcontroller will supervise the charging and force power to be cut once current drops to a low level (say, 10ma). The microcontroller would also watch for bad battery polarity and voltage, refusing to apply any power if the cell is below say 2.5V, and applying small pulses until the battery reaches 3V before trying to bulk charge.

Simple status LEDs would be used (an LCD display is overkill) and the charger would have a button that needs to be pushed to start charging, so a power failure interrupting the cycle will simply cause it to stop. For added protection there will be a thermal fuse + normal fuse inline with the battery.

This charger will be especially suited for small cells such as those used in the Lummi Raw / Wee lights. A higher current version would be useful for RCR2/RCR123A cells.

The thing will be designed so that even when the worst happens (crashed microcontroller?) the worst possible case is the charge current stays on with the voltage at 4.2V. As it will be tweaked to be exactly 4.2V, theroetically the current should approach zero and the cell will be unharmed. However this should not happen with careful attention to detail and use of the watchdog timer. I plan on trying a PIC with a 10-bit A/D converter because 8-bit will give a poor resolution (I worked it out at ~19mv).

How does it work?


The charging part of the circuit is a linear regulator set to supply 4.18-4.20V (probably at the lower end for extra safety) after the voltage drop from the diode and switching transistor (so, the regulator will actually be set nearer 4.8V!) with a current limiting circuit that will be set to 60ma in the initial model for charging small cells. Charging current is switched using a transistor by the PIC microcontroller.

When the cell is inserted and the user pushes the start key, the microcontroller will sample the voltage across the battery terminals. If the voltage is out of range (<2.5V, >4.15V (fully charged)) it will refuse to charge and flash an error condition. Assuming the voltage is in range, charging will begin.

For 2.5V-3.0V, the charger will pulse the charging current to slowly bring the cell up to the bulk charge rate. At >3.0V, the charger will fully switch on applying a current of 60ma (constant current charge). When the battery hits 4.2V, the voltage will stop rising due to the linear regulator, and effectively enters a constant voltage charge. The PIC microcontroller will note this change and inform the user that charging is nearing completion. Once charge current drops below 10ma, the charger will terminate completely and indicate a full battery condition.

Safety features:

Current status


I haven't spent too long on this but I'm prototyping bits right now.

Later model


Once this small model works I will build a larger model that can charge at higher variable currents with a small LCD display!