Testing USB power banks

March 20, 2016

USB charging is very common in portable electronics today and USB ‘power bank’ batteries are popular for topping up devices on the go. Beware that some USB power banks have low capacity and charge slowly. This post explains how USB power banks work, how to test them using simple equipment and power bank sizing.

How USB power banks work

A common USB power bank contains Li-ion cells, a boost converter, a charge controller and, hopefully, a protection circuit (Li-ion cells usually do not have built-in protection).

Nominal cell voltage for Lithium nickel manganese cobalt cells is 3.7 V and maximum charging voltage is 4.2 V. Nominal USB voltage is 5 V. The boost converter steps up voltage from 3.7 V to 5 V output. The charge controller stops USB charging at 4.2 V.

The capacity rating for USB power banks usually refers to the Li-ion cells and is substantially greater than the output capacity at USB 5 V.

Block diagram of USB Power Bank. Measure points indicated with *

Block diagram of USB Power Bank. Measure points indicated with *

The output USB voltage is higher than the Li-ion cell voltage. The cell current is greater than the output USB current:

(Power out) = (Power in) × (Boost converter efficiency)
(Volts out) × (Current out) =  (Volts in) × (Current in) × (Efficiency)

Substituting nominal voltages:

5 × (Current out) =  3.7 × (Current in) × (Efficiency)
(Current out) =  3.7 × (Current in) × (Efficiency) ÷ 5
(Capacity out) = 3.7 × (Capacity in) × (Efficiency) ÷ 5

where capacity is measured in mAh. Substituting the nominal 3.7 V voltage is slightly inaccurate. The Li-ion voltage actually declines during discharge to about 3 V when empty (see example below).

Substituting DC-to-DC switching converter efficiency of between 75% and 98%:

(Capacity out) = 3.7 × (Capacity in) × (0.75 to 0.98) ÷ 5
(Capacity out) = (0.56 to 0.73) × (Capacity in)

This equation is important because the mAh at USB 5 V (see block diagram above for measuring points) is substantially smaller than the mAh drawn from the Li-ion cell.

Known-capacity device testing method

Charging a known-capacity device is the cheapest method to test discharge capacity of a USB power bank. My smart phone is new, the battery is new and the battery meter seems to be reasonably accurate:

Discharge capacity = (End % − Start %) × (Device capacity) ÷ 100
Start % = Device battery meter when a full-charged power bank is plugged in to the device.
End % = Device battery meter when the power bank is empty.
Device capacity is for 3.7 V nominal Li-ion cell voltage, the same voltage as the power bank cell.
The factor 100 is used to convert percentages to fractions.

Average discharge rate = (Discharge capacity) ÷ (Discharge duration)

To discharge in one step, the device capacity should be greater than the USB power bank capacity at 5 V (see previous section). The device should be switched off, so that it drawing current for charging and little else. This method will be unreliable for devices with old batteries having reduced and unknown capacity. This method does not measure voltage.

USB meter testing methods

USB meters can be used to measure both discharging and charging. I purchased a simple USB voltage and current meter on ebay for AUD 2.

Simple USB voltage and current meter.

Simple USB voltage and current meter.

To measure capacity, I record time, voltage and current at regular intervals, enter the data in a spreadsheet and integrate current over time. Longer sampling intervals can be used for high capacity batteries. Accuracy is limited by the meter accuracy (±2 % current accuracy for my meter) and sampling frequency.

I later purchased a USB voltage, current and capacity meter for 5 AUD, which integrates current over time and reports capacity.

Integrating USB voltage, current and capacity meter.

Integrating USB voltage, current and capacity meter.

Example: Laser 2200 mAh Powerbank

The Laser 2200 mAh USB power bank is popular in Australia, where it retails for between AUD 8 and AUD 15.

The Laser 2200 mAh USB power bank contains one 18650 Li-ion cell. Maximum current ratings are 1 A output and 0.5 A input at 5 V. I removed the lid to measure cell voltage with a multimeter.

The Laser 2200 mAh USB power bank contains one 18650 Li-ion cell. Maximum current ratings are 1 A output and 0.5 A input at 5 V. I removed the lid to measure cell voltage with a multimeter.

I discharged the Laser power bank into my phone. Average output voltage of 4.96 V was very close to 5 V nominal USB voltage. Average current of 684 mA was typical for my phone. Maximum current of 870 mA was close to rated 1 A output for this power bank. Integrated discharge capacity was 1254 mAh. My phone’s battery meter indicated 1383 mAh.  Another discharge test with an integrating meter gave 1230 mAh. Converting to 3.7 V nominal Li-ion cell voltage and assuming 75% to 98% boost converter efficiency (see introduction above), the estimated cell capacity was 1718 to 2240 mAh. This range includes the 2200 mAh rated capacity.

Discharge curve for Laser 2200 mAh USB power bank. Integrated discharge capacity was 1254 mAh at an average output voltage of 4.96 and current 684 mA.

Discharge curve for Laser 2200 mAh USB power bank. Integrated discharge capacity was 1254 mAh. USB voltage should be between 4.75 and 5.25 V.

I charged the Laser power bank with a mains charger (maximum rated output current 1 A at 5 V). The average charging current of 503 mA agreed with 500 mA rated charging current. Integrated charging capacity was 2305 mAh (no voltage conversion is applied to charge capacity because current out = current in). Assuming 80% to 90% Li-ion charge/discharge efficiency, the estimated cell capacity was 1844 to 2075 mAh, which agrees with the discharge capacity.

This Laser power bank performed as rated but the low capacity charges my phone only 50%. Nearly identical USB power banks can be purchase on ebay for AUD 1, excluding the battery. High capacity 2900 to 3600 mAh Panasonic 18650 batteries cost about AUD 10. We can buy these componenets and assemble a higher capacity power bank for about the same price as the 2200 mAh Laser power bank.

Sizing USB power banks

From the introduction:

(Capacity out) = (0.56 to 0.73) × (Capacity in)

This capacity equation can be used for sizing USB power banks (i.e. capacity of the Li-ion cells). Li-ion charge/discharge efficiency is between 80 and 90%:

(Device capacity) = (0.80 to 0.90) × (Capacity out)
(Device capacity) = (0.80 to 0.90) × (0.56 to 0.73) × (Rated capacity)
(Device capacity) = (0.80 × 0.56  to 0.73 × 0.90) × (Rated capacity)
(Rated capacity) = (1.53 to 2.25) × (Device capacity)

An easy to remember rule of thumb is (USB power bank rated capacity)= 2 × (Device capacity). Now you understand why USB power banks often seem to be inadequate!

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Could Activ Energy rechargeable batteries be Eneloop killers?

February 6, 2016

All of my rechargeable nickel-metal hydride AA cells are Eneloops. I was surprised to find that Activ Energy cells from ALDI give similar performance at half the price.

Active Energy NiMH rechargeable AA cell.

Active Energy NiMH rechargeable AA cell.

Product description

The Activ Energy AA cells tested were 2400 mAh capacity, low self-discharge and “extra long life”. Here is a comparison with Eneloop specifications:

Cell Origin Date Min. cap. (mAh) Max. cycles Diam. (mm) Length (mm) Mass (g)
Activ Energy MSN 5004 9143 China Jan. 2015 2400 14.4 50.4 27
Sanyo Eneloop HR-3UTGA (2nd Gen.) Japan 1900 1500 14.3 50.4 27
Panasonic Eneloop BK-3MCCE (4th Gen.) China 1900 2100 14.3 50.2 26
Panasonic Eneloop Pro BK-3HCC Japan Feb. 2014 2450 500 14.4 50.3 30
Activ Energy AA cells compared to Eneloops. Minimum rated capacity reported. The standard eneloops I own and measured. The Panasonic Eneloop Pro dimensions and mass are from lygte-info.dk

The mass of the Activ Energy AA cell is about the same as for 1900 mAh Eneloops and lower than 2450 mAh Eneloop Pros. The capacity of the Activ Energy cells appears over rated in this comparison.

Maximum cycles was not specified for the Activ Energy AA cells. I assume maximum cycles is about 500, like Chinese Eneloops

Self-discharge performance was not specified for the Activ Energy AA cells. I assume 85% capacity retention in one year, like the Eneloop Pros and 2nd generation Eneloops.

The Activ Energy packaging included an LGA tested quality mark. It was not explained what product features were tested.

Capacity

I don’t have a battery tester. I discharged the cells using a 2.4 ohm resistor and recorded voltage and current at regular intervals. Discharge current was about 400 mA and discharge duration was about 5 hours. I stopped the discharge when voltage decreased below 0.9 V under load. Estimates of capacity are imprecise because of low sampling frequency and variable end-points.

I charged the cells with an Olympus Ni-MH Battery Charger BU-100 (around 2002 or 2003 vintage) at 490 mA per cell.

The Activ Energy cells were tested in the following sequence:

  1. Initial charge (top-up).
  2. Discharge 1.
  3. Recharge.
  4. Discharge 2.
  5. Recharge and one week rest.
  6. Discharge 3.

For comparison, I also tested one new Panasonic Eneloop and one well-used Sanyo Eneloop:

Capacity (mAh)
Cell Test Mean Low High
Activ Energy 1 1830 1743 1899
Activ Energ 2 1950 1880 2120
Activ Energ 3 1897 1849 1944
Panasonic Eneloop (new) 1 1755
Sanyo Eneloop (used) 1 1708
Discharge capacities. Four Active Energy cells were discharged three times. One Sanyo Eneloop and one Panasonic Eneloop were tested one time.

Average discharge capacity for Activ Energy cells was similar to Eneloops. Other tests of Activ Energy cells have reported mean capacities of 2045 mAh after a few break-in cycles (previous generation, 2300 mAh cells) and 2220 mA. Altogether, these tests show 200 to 500 mA less than rated capacity.

Voltage

The Activ Energy cells had flat discharge curves and mid-discharge voltage was close to nominal 1.20 V for NiMH cells.

Discharge curves for Active Energy Cell 1. Discharge test 3 had the highest voltage.

Discharge curves for Active Energy Cell 1. Discharge test 3 had the highest voltage.

Discharge curves for Active Energy Cell 1. Discharge test 3 had the highest voltage.

Discharge curves for Active Energy Cell 2. Discharge test 3 had the highest voltage.

Discharge curves for Active Energy Cell 3. Discharge test 3 had the highest voltage.

Discharge curves for Active Energy Cell 3. Discharge test 3 had the highest voltage.

Discharge curves for Active Energy Cell 4. Discharge test 3 had the highest voltage.

Discharge curves for Active Energy Cell 4. Discharge test 3 had the highest voltage.

Self discharge

Activ Energy cell voltages initially were 1.29 V at 12 months after manufacture. This was not far below freshly charged 1.42 V and well above nominal 1.20 V. Although I did not measure initial capacities, these voltage measurements strongly indicate that the Activ Energy cells are low self-discharge cells.

Referring to the previous section, discharge capacity after one week of rest was 3% lower than the preceding test. Another test of Activ Energy cells reported 3% capacity loss in 5 days and 8% capacity loss in one month. The self-discharge rate declines over time and maybe these Activ Energy cells can retain 80% of their capacity after one year.

Cost comparison

In four-cell packs, Activ Energy AA cells (AUD 1.75 per cell) cost less than the Australian retail price of Chinese Eneloop AA cells (about AUD 5.00 per cell). However, I am not confident about cycle life for either of these products. I would choose Activ Energy cells if I urgently needed to buy NiMH cells. I would choose Activ Energy cells over Chinese Eneloops if cycle life was similar. The best choice remains quality Japanese Eneloops.