six cell Battery Charger

Article Date June 2006.
I have a Pure Elan DAB radio, and it's great sound and great quality. Unfortunately it EATS BATTERIES. In fact it costs around 50 pence per hour to listen to it. It takes me back to the '60s and Perdio Trannies that used to devour PP9's in a similar fashion.
The obvious answer is rechargeables, but charging them is very troublesome because all the commercial chargers I found in the shops seem to be four-cell, running at about 50ma. That means it took me two days to charge my six C-cell balleries! Calls for a Jarviser make-not-buy decision.

This charger is part woodwork project and part electronic. The case holds six C-Cells three on top and three on the bottom in series. The cells are connected by spring contacts from a cheap battery holder. The power comes from the guts of an IBM laptop power unit. The current control comes via a home built circuit containing an L200CV integrated circuit controller. Circuit is shown below.
NB. UK mains electricity will kill. If you are not comfortable with mains electricity or don't ave the necessary skills or training you should build the L200 controller circuit only, and put it in the box, and feed power to it with a purchased universal power supply of around 12 volts DC and 0.5 amps. If you do build this project but with a metal case instead of wood it must be properly earthed.

Circuit and Veroboard Layout

This circuit is from an old Maplin Electronics web page. Unfortunately they no longer sell the L200. I got all the other components from Maplins, including some 0.1 inch pitch stripboard (also known as Veroboard). About 2.5 inches are needed of 7 strips wide. Just two track cuts are needed as shown.

Obtain pdfs of these diagrams, including circuit description, by clicking
here and here
Note that R2 and R3 are each 3.9 ohm to give a parallel equivalent of just under 2 ohm which sets the unit at around 200ma fixed current.

Construction details

Essential hardware includes the L200CV which is a 5 pin design especially for printed circuit boards. The spring battery contacts are taken from a cheap Maplins battery holder by heating with a soldering iron and removing when hot. Strip board is 0.1 inch pitch to fit the L200 and the miniature variable ("pot") resistor

I found it easier to mount the connector springs to a small wooden sub-plate. You need to allow for the two sub-plates, the springs at half-compress and the length of 3 cells to work out the battery compartment length. Obviously at the feed-in end the spring has its spur cut off and is mounted with its own screw to take the -ve wire, and a simple screw takes the +ve wire (pictured) At the other end which is mid-point of the six cells the spring is kept intact with its side wire spur.

Depending on the size of the components purchased you should be able to get them all on a piece of board 2.5 inches long. If you keep the input voltage just a couple of volts higher than the working voltage, which in turn is a couple of volts higher than the combined battery voltage, there should not be a huge amount of waste heat to dissipate. A small chunk of metal as s simple heat sink will take away the heat. Buy a smart finned one if you have the money. I used the guts from a 12 volt IBM laptop power unit and set the regulated voltage to 10 volts.

Check that none of the copper strips are linked to neighbours by careless soldering. A small Antex type 15 watt soldering iron is best size if you have one. Scrape a sharp screwdriver blade between the strips after you are finished soldering to ensure you can see the brown insulation board between all the strips.
Make sure you interrupt the two copper strips as in the layout, either with the proper Vero tool. or drill the strip with a 5mm drill til it just cuts through. Visually check all connections before applying power.

The guts from the IBM power unit are stuck into the box with silicone sealant. If you use a metal case do a proper job and EARTH the case with a 3-wire power cable. (we have 240 volts in the UK - nasty!).
If you are not happy about playing with 240 Volt mains, keep the power unit intact and connect the 12 volt cable through ointo the box, which can be made smaller. As I was housing six C-cells I decided to build the whole thing into a piece of furniture, being a woodwork fan.

The completed spaghetti. A nice lid hides a great deal.

After testing for 12 hours, I felt it got a little warm. These few holes were perfectly adequate to keep the insides comfortable.

Commissioning

Follow the set up instructions on the data sheet. Basically set the voltage pot to half way point. Connect a good voltmeter to the output and move the pot until 10 volts (or what you will) is achieved. It should be at least 1 volt above the combined voltage of the six cells. As the current was set with R2+R3 in parallel = 1.95 ohm, all that is necessary is to check the current with six cells in the charger. You can either interrupt the outputwires inside the unit and check with a milliameter, or what I did was to fix aluminium foil to both sides of a piece of thin card with double-sided tape, then set this between the last cell and its spring contact to interrupt the circuit. Connecting a milliameter to the two foil faces (in this case using sellotape!) will remake the circuit and measure the current.