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USB Powerbank Efficiency Testing

These days USB powerbanks are very common – ranging in capacity from about 1Ah, to about 20Ah. Internally, they’ve all got much the same format:

  • Lithium Ion cylindrical or Lithium polymer pouch cells for energy storage
  • DC-DC boost converter
  • Microcontroller & LED Battery Gauge Display
  • Lithium cell protection & charge control

As the maximum voltage of a lithium cell for common chemistries is 4.2v, there needs to be a DC-DC converter to boost the voltage up to 5v for the USB ports – There are dedicated chipsets designed for powerbank use available everywhere for this part, and this section is going to be the most energy-wasteful part of the system.

To get a handle on the discharge efficiency of these units, I ran some tests with a constant current load, on different powerbanks from different manufacturers. All were in the range from 5Ah-20Ah, and all had ports rated for 2.1A Max output current.
The load was set for a nominal 2A current, and the powerbanks were fully charged before a discharge cycle. All powerbanks were in new condition to ensure that age-related degradation of the cells wasn’t going to be much of a factor.

Without further ado, here’s some test results:

Nameplate Capacity (Wh @3.7v Cell Nominal)Nameplate Capacity (Ah)Measured Capacity (Wh, Calculated @5V Output)Measured Capacity (Ah)Ah Efficiency %
44.4Wh12Ah31.656.33Ah52.75
37Wh10Ah33.86.76Ah67.6
74Wh20Ah56.511.3Ah56.5
22.2Wh6Ah17.93.58Ah59.67
18.5Wh5Ah15.043.01Ah60.2
18.5Wh5Ah12.452.49Ah49.8
22.2Wh6Ah193.80Ah63.33
37Wh10Ah30.856.17Ah61.7
55.5Wh15Ah48.69.72Ah64.8
18.5Wh5Ah15.253.05Ah61

Overall, these efficiency numbers are pretty poor with an average of 59.735% across these 9 samples. I expected at least high 80’s for efficiency on powerbank DC-DC converters, which must be pretty well specialised for the input voltage range by now. I suspect this is mostly to do with keeping costs down in mass production.

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IPL (Intense Pulsed Light) Hair Removal Device Internals

IPL hair removal is rather similar to laser hair removal – high energy photons are directed into the hair follicle to heat the cells up until death occurs, stopping the hair from growing. These units use high-energy Xenon flash tubes to do the job, and operate in exactly the same fashion as a camera flash. Here are the internals of such a device.

Mainboard
Mainboard

The mainboard in the top of the unit deals with all the functions of the device. There’s a main microcontroller, which in this case is an unmarked IC. The UI is a simple LED display, showing the number of shots on the tube remaining. These units come with 500,000 shots programmed in. The limit is to prevent the Xenon tube from exploding when it reaches end of life. In the case of these devices, after the counter reaches zero, the unit is disposed of. The trigger transformer is visible at the right of the board, along with it’s capacitor. This is triggered by a small Thyristor at the bottom edge of the board just to the right of the LED display. There are some power handling components on the left side, along with the main switching FET & gate drive IC for the high voltage supply.

There are several power settings, and the power itself is varied by the voltage on the main capacitor, from around 270v to 400v. There’s no dedicated switching IC here – The microcontroller generates a 65kHz square wave, from 0-50% duty cycle to drive the main switching FET. There’s a resistive feedback network to regulate the boost converter’s output voltage.

Mainboard Bottom
Mainboard Bottom

There’s less on the bottom of the PCB, apart from the connections to the PSU, cooling fan, capacitor & trigger button, there is the transformer for the HV supply. At the lower centre, is the driver IC for the LED display.

Flashtube
Flashtube

The flash tube is mostly hidden inside a plastic shroud with an amber filter glass on the front to provide the correct light spectrum. The metal ring around the outside edge is part of a capacitive detection mechanism that prevents the tube firing unless it’s placed against skin. I have no doubt that the intensity of light from these devices could quite easily cause eye damage. Even absorbing the energy into the skin is slightly painful – hitting black tattoo ink with it feels like a needle prick!

Cooling Blower
Cooling Blower

Since the flash tube passes such a large pulse of energy, it is force cooled by this small blower which sits under the mainboard, and directs air into a slot in one end of the tube housing. The other end allows the air to exhaust back out, taking the heat with it. This fan runs continually while the device is powered on. This fan shifts an impressive amount of air for it’s size.

Flash Capacitor
Flash Capacitor

Finally, there is the main storage capacitor for the flash tube. This sits down in the handle, and is massive at 450v 680µF, providing 69 Joules of energy at full charge. In the case of this unit, the energy is variable from 25-55J.

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HLD-100+ Refrigerant Leak Detector Lithium Battery Mod

My refrigerant leak detector has one flaw – from the factory it uses C-size alkaline cells. Considering I don’t normally stock these (nothing else I have uses them!), and they’re big & heavy, I decided to do a modification to run the unit from lithium cells instead. This would allow charging via USB.

Factory Battery Compartment
Factory Battery Compartment

Here’s the factory moulding, with extensive strengthening ribs around the battery compartment. Some of this will have to be removed to get the new cells into place. Cue the scalpel! The battery contacts will also have to be removed from the casing.

Battery Fitted
Battery Fitted

After removing some of the plastic, the new cells are a snug fit inside the battery space, with plastic ribs still in place to stop them falling out of the end of the case.

Lithium Charge IC
Lithium Charge IC

A small hole drilled in the back of the unit allows access to the USB port for charging, through this TP4056 3.7v charge module.

Board Refitted
Board Refitted

A bit of rewiring, and the original battery connector is refitted to the board.