Battery backup power supply

Often it is desired to have more than one way to power a circuit in case one fails. It is pretty easy to have a battery suddenly take over when the main supply fails.

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I quickly made the video below for demonstration purposes. I use a lithium ion battery that in reality can only be charged to 8.4V even though it is labeled as being a 9V battery.

Close to 9V lithium ion battery backup power supply prototype circuit.

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Battery backup basic properties

Generally speaking, a battery backup means that there is almost always plenty of power available from a main power supply. The battery only takes over when the main source fails. Another situation can be where the main source of power is intermittent, such as solar. With intermittent supplies, the battery needs to recharge as fast as possible to take over a large number of times.
No matter the case, it is important that the battery has enough stored energy to cover the gaps where the main power supply loses power. The main power supply may also charge the battery. That is almost certainly the case when the main power supply is intermittent. Protection circuitry is likely needed for charging the battery, since the main power supply will likely not be ideal for direct charging.
Here is a list of some things to take into account when designing a battery backup
  • Power supply voltage
  • Battery voltage/capacity/protection/recharge-ability/replacement options/etc.
  • Instant switch over between supplies based on which has highest voltage.
  • Diode OR gate


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