Affordable battery pack for occasional Wi-Fi “AP on a stick” site surveys

Since I don’t survey every day, I could not justify the purchase of a full-blown battery pack. My goal is to get a universal battery pack, which would allow me to survey for 5 hours and provide power to my laptop or USB device (like the WLAN Pi) at the same time.

A quick research made me to test the RAVPower AC 27000 mAh Power Bank. Here is what I’ve learned after using it for a few days:

  • The 2 built-in fans kick in when AC device starts drawing more than 20 Watts. Below this threshold, the fan is off. With the fan on, it actually becomes annoyingly loud (watch this video).
  • The AC inverter seems to operate with 82% efficiency
  • MacBook Pro 61 Watt power brick charges the battery using USB type C port
  • It powers USB devices and 230V AC devices (power injector in my case)
  • Capacity of 99.9 Wh

Things I like about this battery pack:

  • It is universal and power USB and AC devices
  • Its size and capacity are great

Things I don’t like:

  • When AC load exceeds 20 Watts, the 2 fans become generate significant noise
  • The adapter from its AC socket to UK socket is really poor, does disconnect very easily and cuts power. This is a huge downside.
  • AC power automatically switches off when the connected device draws less than 8 Watts or so. If you need to power a very low power device, use the USB port or plug one more device in to increase total load.

Battery life tests

I tested a few access points powered by a PoE+ 802.3at injector:

Cisco Catalyst 9115 in site survey Embedded Wireless Controller mode stays powered for 6 hours and 24 minutes and draws around 13 Watts.

Cisco Aironet 1560I (in 2SS only mode) in Mobility Express site survey mode stays powered for 5 hours and 28 minutes and draws around 15 Watts.

Formula to estimate battery life

Cisco Aironet 1540I draws around 8 Watts (measured by a smart plug) and estimated battery life is:

Run time = Battery capacity in Wh * Battery inverter efficiency / Power drawn by device in Watts = 99.9 * 0.82 / 8 = 10.2 hours

Tested devices powered by AC power

MacBook Pro (13-inch, 2019, Four Thunderbolt 3 ports) powered using MacBook 61W USB-C power adapter connected to the battery pack charged the laptop with no problem. The only annoyance is the battery fan noice. I can’t imagine using this in an open plan office as it would disturb others. Charging while on site or in a car is not a problem.

I was able to power Cisco WLC 2504 and 3504 with no problem at all and they drew around 25 Watts. You can use the above formula to calculate estimated battery life.

Cisco Catalyst compact switch WS-C3560CX-8XPD-S is not able to be powered by this battery, the battery goes into overload mode and cuts power. I suspect the AC wave output of the inverter is far from “perfect sine” and it prevents some devices to be powered. A different battery pack with better filters would be my suggestion if you need to power a device like this.

Another Cisco Catalyst compact switch WS-C3560CG-8PC-S works perfectly fine and draws about 17 Watts with no Ethernet ports connected and no PoE provided to its downstream devices.

Maximum AC load test

It tried connecting as many devices to the battery pack and power them using the inverter. These devices can be powered concurrently just fine:

  • Cisco Meraki MS220, Cisco Small Business SF100-08P switch, Aironet 3800 AP, Catalyst 9105AXI AP, Aironet 1800S Wi-Fi active sensor, Aironet 1560I, MR32 AP, MR20 AP, Aironet 1815W, a Bluetooth speaker and Raspberry Pi 4

How I fixed the supplied AC adapter issue

As I mentioned, the provided power adapter is a joke and not fit for purpose if you want to connect a device using a UK power plug. Just the weight of the power cable itself pulls the adapter from the battery pack socket and stops power supply to the connected device.

I decided to keep the battery pack as everything else works quite well and I replaced the provided adapter by a power cable with European plug and IEC C14 to UK socket “UPS” power adapter. This on its own stays connected in the battery inverter’s socket quite nicely and I added a couple of velcro straps to keep it securely in place at all times.

Fan noise

Published by

Jiri Brejcha

Jiri is passionate about mobility ranging from Wi-Fi to folding bikes;-) He is a Wi-Fi Technical Architect at Cisco UK and proud member of the Cisco Live Network Operations Team and WLAN Pi development team. If not working, he is most likely riding his Brompton bike.

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