iamdarkyoshi's Highly Dangerous (but extremely simple) Powerbank Capacity Tester - Page 1
Forum section: electronics / general

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: May 28, 2016, 06:57:58 am
content: I need something for testing the capacity of powerbanks. Can't be bothered to buy a cheap tester.So I made this device:It has a relay that self latches when the two brown wires are touched, and once the voltage dips below about 4v, the relay will turn off and disconnect the load. This will detect when the battery bank decides that the battery is too low to output anything, and due to the fact that for it has to be manually reset with the two brown wires, it is especially good for powerbanks that tend to turn back on once their battery voltage floats up a little.Here is the load:Five 1 ohm resisistors in series with a fan on them. I have hand picked (or rather lucked out) 5 resistors that in series, pull exactly 1A at 5v.So what makes this device so dangerous? What is the second relay used for? Why are there green and yellow wirew coming off the breadboard?Glad you asked.The second relay's coil is connected where the load is, so when the load turns off, so does this relay. This relay's contacts go off to......A mains powered lamp timer. The second relay is connected to a timeswitch. I figure that you could set the dial for midnight, and once the powerbank switches off, whatever the timer reads in hours ahead of the original midnight position is the battery bank's true capacity in amp hours (as we drew 1A, and the timer gives us the hours it ran for)Now, this results in what I would consider the "true" amp-hour rating, as a 10Ah battery bank contains 10Ahs worth of lithium batteries, which on their own, do not put out 5v. They need to be stepped up in voltage, which means that a constant 1A 5v load, I will draw more than 1A from the batteries.I don't expect particularly great acuracy with this thing, but its a hell of a lot better than just trusting the ebay listing or my secondhand lithium cells.Anyway, I hot snotted the wires in place so they don't easily fall out, and I ain't running this deathtrap tonight. Anyway, hope you guys found this idea at least somewhat entertaining, tomorrow I will test it!

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 28, 2016, 07:21:25 am
content: Why not use one of those battery operated analog clocks?

author: SeanB
author_rank: Super Contributor
timestamp: May 28, 2016, 07:22:27 am
content: Safer to use a simple wall clock, and connect the relay to the power leads off the single AA cell in it. Just make a shim connector out of either some blank double sided PCB or 2 thin pieces of brass with some paper between them for insulation, and you can do the same.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: May 28, 2016, 07:26:16 am
content: Quote from: SeanB on May 28, 2016, 07:22:27 amI thought of that, but I looked around the house and couldnt find ANY wall clocks (aside from our 40in one in the living room, which is off limits.)I do have a HDD clock, but it is mainly for show.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: May 28, 2016, 09:25:58 am
content: Or get a constant current sink, and a comparator that detects when the voltage is too low, that will turn on a buzzer, that will tell you to stop the timer and disconnect the load.And then you can calculate the capacity.Or you can even connect it to a servo that will stop the timer automatically from your phone so you will not have to manually stop it.(that's at least how I will do it).

author: JoeO
author_rank: Frequent Contributor
timestamp: May 28, 2016, 10:50:38 am
content: OR you could spend $20 and get one of these.http://www.ebay.com/itm/181846099882?rmvSB=truereview:

author: Ian.M
author_rank: Super Contributor
timestamp: May 28, 2016, 12:19:05 pm
content: The cheapest easy way of automating stuff like that and log the result is to use an Arduino. It can also be used to sense voltage directly and current through a shunt + PWM a power MOSFET chopping the load resistance to maintain either constant power, or constant current as desired. Either log direct to PC via the UART or add a datalogger shield for SD card and RTC.Google:Arduino battery tester sketchP.S. *NEVER* use high power resistors directly inserted in your solderless breadboard. The combination of heat and relatively thick leads usually damages the breadboard contacts. I'd have mounted the resistors to solder tags screwed to standoff pillars screwed to the fan mounting holes then run flyleads to the breadboard.

author: dannyf
author_rank: Super Contributor
timestamp: May 28, 2016, 01:58:21 pm
content: I liked its simplicity and elegance.I would have used an active element of the heater - maybe a power mosfet (or a LM317-like CCS load) on a cpu cooler -> entirely self-contained.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: May 28, 2016, 04:06:17 pm
content: Quote from: Ian.M on May 28, 2016, 12:19:05 pmYeah, I know that it is not the greatest idea, but I have a whole bin of these things, school was throwing them outAlong with probably $1500 of other stuff I grabbed

author: ZeTeX
author_rank: Frequent Contributor
timestamp: May 28, 2016, 06:08:45 pm
content: Quote from: iamdarkyoshi on May 28, 2016, 04:06:17 pmSchools just throw expensive electronics free? Wish they would do the same in my country

author: Ian.M
author_rank: Super Contributor
timestamp: May 28, 2016, 06:14:42 pm
content: If they close a course and there's a turf war for storage space, its not unusual.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: May 29, 2016, 03:14:49 am
content: Quote from: ZeTeX on May 28, 2016, 06:08:45 pmOne of them is a "temperature calibrator" which seems to be worth around 500 bucks on ebay. I got two other bits of related gear, two 10A variacs, two simpson 360-2 multimeters, a 5KV 35uf capacitor, four genuine hakko digital temp soldering stations, several hundred 74xx series logic gates, box of unopened USB logic analyzers (perhaps an EEVBlog giveaway?) and that aint even half of it.

author: Rick Law
author_rank: Super Contributor
timestamp: May 29, 2016, 08:43:55 pm
content: Quote from: Ian.M on May 28, 2016, 12:19:05 pmI did something similar using a Nano - I use 3x5W 1 Ohm at various serial/parallel or bypass configuration to obtain 0.33ohm to 3ohm behind a fan.A rubber band mounted LM35 on the battery for temperature sense.The Nano does the V/I/Temperature monitoring and any other adhoc (program change) control/interrupt. Where situation permits and requires, I can use the INA219 (with zero, one or more of the 5W1ohm in parallel to the current sense) for better accuracy with the monitoring.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: May 30, 2016, 06:12:52 pm
content: I kinda forgot that my bench supply counts mAh, so I can just plug the powerbank into it, set it for 5v, and let it charge a fully discharged powerbank until its full.