Wednesday, October 07, 2009

HB-1A charger mod

This afternoon I wired up the battery holders and installed them, with a set of 2400mAH NiMH batteries, into the HB-1A. Before doing that, I added a modification to allow the batteries to be charged in situ, as is shown in the not very sharp photograph (sorry, photography just isn't my thing.)

The modification is very simple. It consists of a 5 ohm half watt resistor, made up of two 10 ohm quarter watt resistors in parallel, in series with a general purpose silicon diode with a current capacity of at least 500mA.

Locate the power switch contacts on the circuit board. The power switch is a two-pole two-way switch, with both poles connected in parallel. Only one side of the switch is used. The power input from the power socket goes to the centre pair of contacts. So the charger circuit is soldered to the unused pair of contacts, so that charging takes place only whilst the transceiver is switched off.

The other end of the charger circuit is connected to the positive line coming from the battery connector. A convenient connection point is the connector side of reverse polarity protection diode, as can just about be made out from the photo.

When the battery pack is installed, it more or less touches the components on the circuit board. (There really isn't much wasted space inside an HB-1A!) For peace of mind, I made an insulating layer out of thin card, cut to fit so it rests on the circuit board between the board and the batteries. That will help to prevent any accidental short circuits.

With the component values I used, I measured a charging current of 200mA when 13.8V DC is applied to batteries that had recently been charged. This current is suitable for charging a pack of 8 cells with a capacity of 1800mAH to 2400mAH in 14 - 18 hours. [Charging time = (capacity / charging current) x 1.5.] You should not exceed this time by more than 25% as overcharging can harm the cells. If the battery is not fully discharged then judging the charging time can be difficult. The cells will get warmer once they are charged, but they won't get hot like they do in a rapid charger and you probably won't notice this through the HB-1A steel case.

This is not a constant current circuit, and the charging current might be higher than 200mA if the batteries are heavily discharged. In that case, it might be advisable to start charging at a lower voltage to avoid exceeding the power rating of the resistors. I would recommend monitoring the charge current with a meter the first couple of times. I have not had a chance to check the current drawn when the batteries are discharged for myself yet.

A 500mA 12V DC unregulated wall-wart makes a good power source for this charger. Off-load it will measure something like 16-17V but when 200mA is drawn the voltage will drop to something in the range 13.5-13.8V. The nice thing about this is that it provides some current-limiting built in: If the cells try to take a higher current, the voltage from the wall-wart will fall below 13.5V, reducing the current.

Don't try to use a wall-wart rated for less than 500mA as the current rating is not for continuous use and it will get too hot. Beware of using a larger one (e.g. 1A rating) too, as the voltage may be too high at 200mA. Don't ask me where to buy one: the one I used came from my junk box.

The characteristics of these unregulated wall-warts vary, so assume nothing and measure the voltage and current. Finally, don't switch the HB-1A on whilst the wall-wart is connected. The voltage delivered when the receiver is on and drawing only about 50mA can easily exceed 15V DC, which could damage the electronics.
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