VeryLazyLogger (measure USB powerbank capacity) - Page 1
Forum section: electronics / projects

author: RoGeorge
author_rank: Super Contributor
timestamp: March 23, 2023, 08:44:41 am
content: Wanted to make a logger to measure the capacity of USB powerbanks.At first, the plan was to use a constant current load plus an MCU with a miniature 4 digits 7 segment to display the discharging time. The 7 segments display I happen to have is a vintage NOS from the 80s, priced $30on ebay. So now I'm keeping the little display for a more 'worthy' project than a timer. This is the little bubble display thingy:https://duckduckgo.com/?q=dl340m+led+display&iax=images&ia=imagesDisplay reserved for unknown future project, but still in need to measure the powerbank, so came up with a very lazy logger: a 100mA power LED as a load for the 5V USB powerbank, plus a 1.5V desk-clock to tick till the powerbank dies out.No constant current load, the USB powerbank outputs 5V stabilised, just a series 18ohm R with a 3.2Vf power LED. The desk quartz clock (with its 1.5V battery removed) is connected in parallel with the 18R.When it stops, the clock will "remember" the time.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 25, 2023, 11:39:55 am
content: Aaand... the clock stopped at 08:25 next day, after 23.25 hours. The non-volatile mechanical memory preserved the stop hour to be read later.The USB power bank in the pics is marked 4400mAh, max 1A in, max 1A out. It sit unused on a shelf, discharged out of curiosity, once or twice out since fabricated about 8 years ago. With a load of 100mA/5V the powerbank lasted 23.25 h which may seem as only 2325mAh instead of 4400mAh.However, the 4400mAh (and in general the mAh or the Wh specified on any USB powerbank) are not for the 5V output. Those numbers are for the Li-Ion cells inside the powerbank (usually one, or two Li-Ion cells in parallel). Marketing.This means the nominal 4400mAh have to be de-rated with the voltage ratio, 3.7V Li-Ion cell vs. 5V out, and de-rated with the efficiency of the 3.7 to 5V SMPS step up converter, about 80% or so. After applying these corrections to the 2325mAh measured with the 5V/100mA load:2325mAh * 5V/3.7V * 80% =3927mAh measuredcorresponding capacity of the Li-Ion cells, about 10% less than theadvertised 4400mAhwhen new. Not bad for an 8 years old NOS Li-Ion powerbank.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 26, 2023, 06:06:54 pm
content: Forgot to add a schematicDrawn on a 400x240 touchscreen.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 26, 2023, 07:13:18 pm
content: My very limited experience has been that the 'rated' capacity is basically what it takes to charge the bank to 100% while the discharge capacity is 60% - 70% of the 'rated' capacity. This is based on two 10AH Chinese units that use 18650 cells and one 10AH Philips unit that's about the size and shape of a phone. It must use pouch cells to get that form factor.Of course, I didn't have the benefit of your high-tech test equipment. I had to struggle on with a USB tester that measured and logged the mAH as the bank was charged or discharged.Ed

author: RoGeorge
author_rank: Super Contributor
timestamp: March 26, 2023, 09:42:17 pm
content: Quote from: edpalmer42 on March 26, 2023, 07:13:18 pmI didn't measure yet how much energy was put in. Made me curious, I might measure that, too.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 29, 2023, 07:05:48 pm
content: Found a second powerbank at e-waste, 6700mAh/24.8Wh, JUICEPEBBLEXL, in 5V/1A max, out 5V/2.2A max.By comparison with my 4400mAh, the 6700mAh one is bigger in size, more used, but also much newer. Found it discharged and with bad contacts at the micro USB charging connector. Fixed the micro USB and it started to work. Inside, there are two Li-Ion cells connected in parallel, both cells in very good state.I've kept measuring these days the discharging capacity for both powerbanks, with a load of 100mA/5V.What I don't understand is why from one test to the next, the capacity appears smaller.Test #, Nominal mAh, Time-on at 100mA, Calculated cells capacity mAh1 4400 23.25h 39272 4400 23.18h 39163 6700 41.63h 70324 6700 41.42h 6997Is this expected, is it normal to measure a slightly lower capacity at each discharge?

author: edpalmer42
author_rank: Super Contributor
timestamp: March 29, 2023, 07:12:21 pm
content: Two tests isn't enough to establish a pattern and you're only looking at ~0.5% difference. I'd chalk that up to normal measurement variation.