I haven't yet done this, but I have a device (Fluke 881AB voltmeter) with dead NiCd batteries, and when I get around to replacing them that's what I was thinking of. NiCd batteries are still available. That has vastly less capacity, and a very different discharge curve (exponential vs. nearly flat). Depending on the circuit using it, that might work slightly (at 10% capacity, maybe) or not really at all. It hasn't happened to me, but have heard of one case of a leaking Li primary cell. Not a coin cell, but a C-sized one: I remember a report of one leaking in an automatic parachute opener (AAD), wrecking the parachute. In fairness, that was one left in quite a number of years beyond its expiration date. (Before someone asks the obvious: the problem was found during a required inspection on the ground.) paul

On 2016-Feb-06, at 1:21 PM, Mark J. Blair wrote: My favorite solution, for the right circumstances, is to mount a 2, 3 or 4 cell AA or AAA battery holder on the outside/rear and use common alkaline cells. Right circumstances comes down mostly to current draw. If the function is purely CMOS memory retention the current draw is equivalent to static/idle leakage and you can expect shelf life from alkaline batteries. An RTC is going to draw some current due to the active circuitry. I'm not sure what the current draw for an RTC of that era is, the tech should be CMOS and the draw small but not as small as simple memory retention. The advantages of this solution: - AA/AAA alkaline cells are commonly available and relatively inexpensive. - depending on current draw, you can often use otherwise-worn-out alkaline cells. For example, cells that are no longer usable in your remote control or flashlight can still have enough potential to maintain CMOS memory for years. - alkaline cells tend not to leak and generally can be ignored for years. - if mounted on the exterior, damage in the event of leakage will be minimal. - if mounted on the exterior, they're accessible for replacement. The more cells you use, the more headroom/longevity there is as the cells wear down, but too high a voltage with newer cells also has to be accounted for. If the original design was for rechargeable batteries one will have to disable the charging function and may have to install isolation diodes.

This is what I normally do, preferably mounted away from the board. Generally you can get away with simply using a diode to prevent the circuit trying to charge the battery. Chris