Reverse battery protection
In a recent column I showed how one is unlikely to get the full capacity from a coin cell in any real embedded system. Let’s look at another issue: protecting the system from well-meaning users trying to change the battery.
A typical coin cell battery socket looks like this:
It’s cheap, it’s easy, it mounts well on a PCB, and it’s a disaster. A confused user, poor instructions, perhaps a martini-induced brain fart, and an upside down battery will fry all of the sensitive electronics.
I saw a system not long ago that used a small signal diode to ensure that the electrons flow in the right direction. But that sort of a design is a catastrophe for these systems. It takes 0.7 volts right off the top; as the battery discharges it’s not long before there’s not enough voltage to keep the MCU going, even though the battery is a very long way from being dead.
Another design used a Shottky diode. As we all know these devices have a very low forward voltage drop. Anecdotally people talk about 0.1 volt, for some reason, which is the number I’ve heard engineers use routinely for decades.
But that’s simply not true. Here’s the forward voltage drop for a typical Shottky diode:
Sure, you can expect 0.1 V or so… at 0.005 mA! At more realistic currents (10 to 30 mA or so) that jumps to 0.4 V at room temperature.
If the protection diode drops 0.4V, then once the battery drops below 2.4V the system gets 2.0V. It can no longer provide enough voltage to keep the MCU alive. Remember the graph that showed typical voltage from a CR2032 for various loads? This is the same plot with a red line at 2.4V.
The useful battery capacity with a diode is above the red line.
Once the battery voltage falls below the red line the MCU will be starved for electrons. In the case of a 30 mA load, pretty much the entire capacity of the battery is lost. Even with a 10 mA load over half the battery is unusable.
A much better solution is one that won’t properly engage the battery if the cell is inserted backwards. The following part from Memory Protection Devices is a good example:
The contacts are arranged such that the negative terminal simply will not be connected unless the battery is installed correctly.
It’s $0.25 more than the socket shown before. But if you care about the customer experience, use a socket that will simply leave the system unpowered when the battery is inserted backwards.
Jack G. Ganssle is a lecturer and consultant on embedded development issues. He conducts seminars on embedded systems and helps companies with their embedded challenges, and works as an expert witness on embedded issues. Contact him at firstname.lastname@example.org. His website is www.ganssle.com.