How To Power NEST Protect Fire Alarm Using Rechargeable Batteries And Charge From Wall Adapter
DISCLAIMER: The following info are my notes about modifying the power input on the NEST fire alarm. Do not attempt this at home. It could cause a malfunction of the fire alarm and injury or death if there is a fire.
NEST Protect Fire Alarm uses expensive AA Lithium batteries that need to be changed every 1-2 years. It will not accept regular AA batteries, or rechargeable batteries. This is very annoying, not solely due to the cost of the batteries, but more so the maintenance frequency required especially if you have many of these fire alarms and not to mention any other wireless home automation or alarm systems that require occasional battery replacement. I end up feeling like I'm spending all my time replacing batteries. The marketing tells you that the battery is in these devices last two to five years, which sounds fine. The reality is that when you have a lot of these devices that consume power at different rates and have different battery change intervals you start to have to replace batteries all the time.
There is a model of the NEST protect fire alarm that plugs into the AC system and still has a battery backup. But from what I can tell the battery backup still requires non rechargeable lithium batteries, which means that they will be need to be replaced at some point.
I would like the nest protect fire alarm to plug into the AC power and have a rechargeable battery backup.
The nest protect fire alarm requires lithium batteries because it wants a very stable 4.5 volts. It uses six lithium batteries but they are wired as two sets in parallel of three batteries and series for an output of 4.5 volts. Four nickel metal hydride rechargeable batteries have a nominal voltage of 4.8 volts.
Float charging NiMH for extended periods of time is generally not recommended, but the expectation is always that the flow charge is at the max charge voltage. If you float charge the nickel metal hydride battery below its max charge voltage it can be done indefinitely with no negative effect on the battery. The downside is that the battery will never reach full capacity. However this is acceptable in the application of a battery backup system for a device that has very low power consumption. Charging a NiMH battery to 1.2V will only reach 40-50% of it's capacity.
A constant voltage buck converter can be used to charge the NiMH battery to 1.2V per cell (4.8V for the 4 pack). Most buck converters require a 1.5V minimum difference between the input and output. That would mean that a voltage supply would need to be a minimum of 6.3V, which is more inconvenient than being able to use a ubiquitous 5V USB power supply.
Buck Converters
Since 4.8 volts is so close to 5 volts, a USB power adapter that outputs a constant 5 volts could be used to charge the batteries to 1.25 volts per cell, which is still far below the max charge voltage.
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