Real Time Clock (RTC) battery charger


The RTC developed for use with PIC-a-STAR, using Trxavrb, has proven to be a worthwhile addition for operator convenience (and bling).

However, some RTC chips seem to use more standby power than anticipated and the CR2032 battery (3.6v) goes flat in a few months.

To get around this issue, the later Combo C2a boards, now have built in charger circuitry. The extra parts are optional and sit in the area where the CR2032 battery was fitted. Because of that, it uses all SMD parts.

In the meantime, I made a small PCB that can be retrofitted to the C2 version Combo boards. I have a few spare for those interested.

Stiff wires are soldered into the C2 board then the adapter board is fitted over the top. The rechargable battery is soldered to the same wires. The right hand hole is positive lead.

Details are in this PDF file. Charge rate as shown is about 18mA. This should suit many rechargeable batteries, but can be altered if you wish. It is set by the 68R value which sets the current for constant current operation of the LM317.

Current = 1.25v / Res value

12 volts into the PCB is via a pad at bottom left. 12v is available on Combo C2, adjacent to IC303.

You MUST NOT use the CR2032 battery in this mod. It is only for rechargeable batteries. I used a 3.6V 3 cell pack from a portable phone. Found on eBay.

Thanks to John, G8MM, for this suggestion.


Page created 17th May 2012 by vk3pe