How much wireless power is enough to replace AA batteries?

10 October 2018

One exciting opportunity for using wireless power is power delivery instead of disposable batteries. Smart devices – such as smart locks – require frequent battery changes. Replacing batteries is inconvenient – whether because one needs to always buy batteries, or because the process of replacing the batteries takes time and effort.

Some have suggested that the adoption of smart battery-operated devices could be accelerated if batteries were not needed. Customers are sometimes worried that if they forget to change batteries on time for their smart lock, they might be locked out of their own home.

If disposable batteries were no longer needed, there would be environmental benefits. Whether because of cost or capacity, most batteries purchased are disposable, not rechargeable, and these end up in landfills as toxic waste.

Additionally, device manufacturers report that some exciting features for these smart devices are left out of the product because of battery constraints.

In short, the ability to use wireless power instead of batteries is valuable to everyone, but how much wireless power is needed to replace a battery?

Many smart home devices use multiple AA batteries. AA alkaline batteries typically have a capacity rating of about 2500 mAh (milliampere-hour) and AA NiMH batteries have capacities of only 1200 to 1900 mAh. Let’s assume 2500 mAh. Given a nominal voltage of 1.5V, this translates into 2500 x 1.5 = 3750 MWh (milliwatt hour). A high-end device might have 8 such batteries for a total capacity of 30000 mWh and might need battery replacements every 3 months.

But what if we provided this via wireless power? Let’s assume a small rechargeable battery is built into the lock. That rechargeable battery can provide power if the wireless power is not available (for instance, during a power outage) and also to provide occasional peak power when a high-current activity such as turning the mechanism of the lock is required. The rechargeable battery is then charged through the wireless power.

Let’s examine this using a wireless power system that can deliver 150 mW (this is the low-end model of the current Wi-Charge line). Every hour, such a system can deliver 150 mWh. In a day, it can deliver 3600 mWh. Thus, the 30000 mWh – what 8 AA alkaline batteries can provide over 3 months, can be delivered in just over 8 days with a low-end Wi-Charge system.

What are the implications?

A wireless power system can easily replace batteries for high-end smart home devices
A single wireless power transmitter can charge multiple devices. For instance, a smart lock, a security camera, a smart thermostat, etc.
Manufacturers using wireless power can unlock new and exciting features on their devices, now that the energy constraints are removed.

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