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As Raspberry Pi 5 accessories are only available for pre-ordering, I was curious if attaching a real time clock battery also preserves Raspberry Pi 5's last power state. The latter is definitely not preserved without a battery. Though there are a number of compatible rechargeable batteries from different makers available for pre-ordering, I haven't been keen to wait. Instead, I've used some leftover electronics that I already had at home.
C R 2032 battery is non-rechargeable, but it is still a lithium battery with capacity of about 235 milli-ampere hours. A rechargeable M L 2020 battery has only around 45 milli-ampere hours, but it can be recharged about a 1000 times before it loses 90 percent of its original capacity. Many P C mother boards use C R 2023 to support real-time clock operation and preserve bios settings for five or more years. C R 2032 battery should therefore keep Raspberry Pi 5's real-time clock running for about the same time.
But C R 2032 unlike M L 2020 should never be recharged. A diode must be wired in series with C R 2032 battery to prevent any recharging electric current. The diode causes a battery assembly voltage to drop to around 2.8 volts, which is well within the operating range of M L 2020 battery while discharging.
How it works? The hardest thing to make is a battery plug to fit in a very small two-pin J S T battery connector between USB-C connector and HDMI zero connector. I tried to make a plug myself from a 1.28 millimeters female header, but it was still way too big. Luckily, one of the battery connector pins is ground, which can also be drawn from 40-pin extension header. I therefore shortened my plug to a just 1 pin and I somehow managed to insert it to connect the battery voltage pin. However, I must admit that buying a proper male connector for the battery is a much better option.
The rest has been easy! Again my Raspberry Pi case has been too small for in-building the battery. Therefore, I've just connected a battery cable and drawn it out of the Raspberry Pi 5's case to a small battery module with 2 N 4001 rectifier diode and a C R 2032 battery slot. The module is small enough to fit into a discarded blue dental thread case.
I've also tested, if the battery had changed Raspberry Pi 5's power on behavior after a power outage to have sadly discovered, that it has remained the same. I will discuss the solutions to this problem in one of the forthcoming videos, but until then you may use the relay solution mentioned in Adding Raspberry Pi 5 External power button video.
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