**By Mark McLean**

I apologise for writing yet another article of tedious electronics for the journal, but I think that there are several people who are interested in building their own battery chargers. This circuit is the backbone of several battery chargers that Julian Haines and I have built, including the yellow club charger and the inferno machine. Various twiddily bits may be added to make it switch off when the cells are charged, or to incorporate a timer, but I'm not going to go into these (I have yet to build a truly decent auto-switching off charger myself). This article describes a circuit which may be modified to perform either:

1) Constant current charging with a voltage limit. (This is the scheme required for Nicads. The voltage limit reduces the likelihood of overcharging the cells and damaging them.)

2) Constant voltage charging with a current limit (This is required for sealed lead acid batteries.)

Formulae are given for calculation of component values, plus a table for the common configurations. Do not look too hard at the formulae, they contain many fudge-factors! The circuit is based around a beast called the L200 which is an integrated circuit regulator that performs both current and voltage limiting. It is widely available and costs about £1.50. The whole circuit including box should not cost more than £20.

Cell Type | Max I_{out} | Max V_{out} | C | R1 | R2 | R3 | Transformer | Heat Sink |
---|---|---|---|---|---|---|---|---|

FX2 | 700mA | 3.2v | 4,700µF, 16v | 0.68, 3W | 2.2, 3W | 420, 430, 16k in parallel | 9v, 16VA | 12°C/W for 60°C |

FX5 | 700mA | 8v | 4,700µF, 25v | 0.68, 3W | 2.2, 3W | 5k1 | 12v, 9VA | 12°C/W for 60°C |

6v, 7Ah Lead Acid | 1.75A | 7.5v | 4,700µF, 25v | 0.27, 3W | 1, 7W | 4k6, 4k7, 240k in parallel | 12v, 20VA | 4°/W for 80°C |

- C = (I
_{out}*0.01)deltaV, deltaV = ripple voltage, 0.01 = period between ac voltage peaks - R1 = 0.45/I
_{out}, Defined by L200. - R2 = 1.5/I
_{out} - R3 = (V
_{out}-2.77)/0.00103, 0.00103 = current through R4. - V
_{smoothed}= (V_{rms}*1.414)-.5deltaV-2, 2 = drop across bridge rectifier, 1.414 = root 2. - Power into heatsink = I
_{out}(V_{smoothed}-V_{out}-3), 3 = drop across R1, R2 and D.

The circuit diagram shows a simple mains power supply that will produce the necessary smoothed dc for the regulator part of the circuit. If desired, this may be replaced by a general purpose power supply or car battery charger, but a smoothing capacitor may still be necessary and consideration should be given to fitting a bigger heatsink, particularly for the FX2 charger.

Transformers are rated by power handling (VA) and rms output voltage. The output voltage needs to be right, or else the regulator will be overloaded, but a transformer with more than the specified VA rating will work fine. The bridge rectifier must be capable of handling both the current through it and the voltage across it. However for FX2 and FX5 charging these are so small that anything will work fine. The W005 is a common rectifier which is perfectly suitable.

The capacitor must store enough charge to maintain the flow to the regulator in between the peaks of the mains waveform. Exactly how big it should be is related to the amount of ripple which is permissible and thus to the average power dissipation in the regulator. Around two volts of ripple is a typical target. The capacitor must also be rated for the peak voltage across it. Do not worry about the ripple current, it is almost never enough but it never seems to matter. The 0.22µF and 0.1µF capacitors provide high frequency filtering and improve the stability of the regulation.

The regulator limits both the output current and the output voltage. It operates by increasing the output voltage (and hence the output current) until either the current or voltage limit cuts in. The voltage limit works by limiting the voltage across R4 to be 2.77V or less. Thus the current through R3 is limited (to 1.03 mA) and so the size of R3 sets the output voltage limit. The current limit works by limiting the voltage across R1 to be 0.45V or less. Thus the maximum current (in Amps) is given by 0.45V divided by R1 (in Ohms).

The regulator dissipates a significant amount of power and must therefore be fitted to a suitable heatsink, which should ideally be mounted on the outside of the box. The power dissipated may be calculated from the current flowing times the average voltage drop across the regulator. This is equal to two volts (required for regulator operation) plus the average ripple voltage, plus a couple of volts spare. R2 is chosen to give a voltage drop of 1.5V, such that in series with R1 the total drop is 2V, which is what is needed to illuminate a LED. Thus the LED will only come on when current is flowing.

The rectifier diode D protects the regulator from being reverse biased in
the event of a battery being attached without power being applied on the
mains side. 1N4001 is suitable for I_{out} less than 1A, otherwise
use 1N5400.

Any form of mains power supply must be built into an earthed metal box and a fuse must be fitted. If Oldham style charging clips are to be used, then they should be mounted on a piece of plastic or bakelite which is in turn mounted on the box. Alternatively insulating washers may be used.

Index to Cambridge Underground

Table of Contents for Cambridge Underground 1994

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