Hacking a Power Tool Battery Charger

I bought a cordless power tool only to find out later that the charger was faulty. So, whilst searching for a replacement charger I set about converting the faulty charger into an intelligent charger. Peter Hayles in Australia sells the circuit board and processor chip for the effective fast charger. (See http://www.shapely.asia/charge1.html for the details). In summary this design detects the small drop in cell voltage when the battery is fully charged.

You can order a pre-assembled board from Peter, but I ordered the bare board and the processor chip. (http://www.shapely.asia/downloads/PH-000002-BOM.xlsx). This saves money and gives more personal satisfaction. If you choose this route the components can be ordered from Maplin Electronics in UK, RadioShack in USA, and/or eBay. You need to add an 18 pin IC socket to Peter's bill of materials. You may wish to order several values for R1 to allow you to experiment with a range of maximum charge currents. (see Peter's notes on this at http://www.shapely.asia/charge1.html)

Although there should not be any LV connections exposed, you need to unplug the charger from the AC mains supply.
NOTE you open this unit at your own risk. The author is not responsible for anything which results from the opening of this charger unit

open the charger case (after making sure the unit is not plugged in to the mains). This requires a tamperproof T20 torx bit. There are five screws. Then lift out the circuit board (PCB) by removing the three small screws with a pozidrive screwdriver

Cut the PCB free from the black low voltage leads from the transformer. Make the cut as close to the PCB as possible, or unsolder the leads

Remove the heat sink from the PCB and drill holes to accept the TO-3 regulator. To provide extra capacity a small TO-3 heat sink should fit on the existing heat sink and still fit in the case. I used the basic one from Maplin. Fix the regulator with its insulation kit to the heat sinks, solder some leads to the regulator including a solder or crimp tag to one of the screws fixing the TO-3 regulator because the body of the regulator is the output connection then put to one side. Check with a DVM that the TO-3 case is insulated from the heatsink. Later you will need to trim the Heatsink to fit around the LEDs

You need to keep the battery connections for the conversion so with a fine hacksaw cut out a piece of the old PCB large enough to keep the hole for the fixing screw and the other two supporting lugs. Solder leads (up to 5A capacity) to the positive and negative connections, ignoring the side connector which is now unused. Make sure you identify which lead is the Positive and which the negative.

In the old design the light from the LEDs was sent along three clear plastic rods. Push these rods out from inside the case, cut off the top 10mm then glue these three short sections back into the holes in the case. When you are ready to fix the new PCB to the case glue the two new LEDs in place behind the clear rods, then connect them to the PCB with suitable wires making sure the polarity is correct. The flat on the side of the LED is the cathode. The other lead is the anode and should be connected to the square pad on the new PCB.

To make space for the heat sink you modified earlier you need to cut away the post nearest the transformer which supported the old PCB (not the post which supports the base of the case). I cut most of it away with a hacksaw then drilled out the remainder. Take care not to drill through the case.
Position the heatsink between the new PCB and the transformer, mark where the screws for the Heatsink will go and drill the two 3.5mm holes. Then fit the Heatsink to the case. You will need new M3 screws, about 10mm long for this.

You need to extend the leads from the transformer to reach the terminals on the PCB. I re-used the cores from a lead from an old domestic appliance, again making sure the lead is not connected to the mains supply.
Connect the leads from the regulator to the terminals on the PCB (input, output and adjust). Double check you have the connections correct before powering it up.
Connect the battery +ve and -ve leads to the Bat + and Bat - terminals on the PCB. Note the the Bat+ terminals are doubled up. If you're planning on charging with a high current you may wish to double up the leads.
If you haven't done so connect the LEDs to the pads on the PCB.

Before inserting the 18 pin processor chip check the following:-
Test the AC input leads for the correct voltage - approx 21vAC
Test across the big capacitor for the correct DC voltage - approx 27vDC
Test the output of the 5v regulator - at pins 5 (ground) and 14 (+5v) of the IC socket

When you are confident that all is as it should be, insert the IC, checking for the correct way round - look for the cutaway shape at the end of the socket and the chip

Having built the new charger PCB you can prepare to fix it in the case. (Don't solder the two LEDs to the PCB, but solder the LEDs to lengths of wire about 15cm long and solder these wires to the pads for the LEDs on the PCB). The new PCB fits very nicely on edge between the two posts that were used to hold the old PCB. It stays in place with blue tac

Once you are confident that all is ok prepare to check the operation.
Before you plug in the power tool battery, plug the unit into the AC mains and check that the two LEDs flash briefly.
You will see Peter Hayles' charging algorithm showing the charging stages