San Gabriel Valley Radio Control League

One of the outstanding characteristics of the A123 type cell is its ability to be fast charged. Very fast! A good street charger can typically charge at 10 ampere (A), 4 times its capacity or “4C” for 2300 mAh cells. Do-It-Yourself (DIY) chargers can often hit 20A or 8C.

At market, there are several chargers that are designed to charge A123 up to 12A. They are rated by the maximum voltage, in volts (V), maximum current, in amperes (A), and maximum power, in watts (W). Pay special attention to the maximum power, it is often the limitation for the charging speed!

How to charge A123 packs at 20A? One of alternatives is a laboratory type DC power supply. It supplies adjustable both current (A) and maximum voltage (3.6V per cell of A123, remember!). A popular good one is capable to supply 50V at 20A. That’s 1000W of charging power! It charges up to 13 A123 cells, in series (13s1p), in less than 7 min!

Looking for more affordable power sources? How about an old computer power supply (ATX)? Or even a 12V deep cycle battery? Just use them to charge A123s through 18 gauge 2-wire regular lamp cord. The wire functions as resistor in series. The right length of cord limits the current to 20A while brings down the voltage to the needed 10.8V (Ohm’s law).

A quick voltage check while charging with a $10 Digital Voltage Meter (DVM) will confirm the exact length of the cord to be used. One deep cycle battery, 20 feet of 18 gauge lamp cord and one DVM are all you need to fast charge 3-cell A123 packs. Really cheap!

Don’t have deep cycle battery? No problem. A vehicle starter battery is good for 1 or 2 charges. Just run the engine for a while between charges so the crank starter battery won’t get damaged by a deep discharge. Don’t run the vehicle engine while charging, unless more than 3 packs of 3-cell A123s, connected in parallel (3s3p), are charging though long 10 gauge wires. This setup will charge at almost 60A of peak current!

Before fast charge A123, remember to charge balancing A123 packs for the first 15 cycles. It is called break in process. Once every 15 cycles, it is a good practice to monitor the imbalance between cells in the pack. This way, A123 can last several hundred of service cycles without noticeable loss of capacity.

If there is DC power available greater than 3.6V, it can be used to charge A123s. Search, learn and be creative, you will find your preferred way to fast charge A123s cheaply and safely.

Tai Liao
tai.liao@gmail.com