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Currently, I have a USB powerbank powering my arduino Uno which powers my micro servomotor. I would like to have the motor running for long periods of time (~24 hrs). Sometimes, I am able to achieve this run-time but in most cases the power bank shuts off after about 3-4 hrs of running. I am not quite sure about the variability. It seems that my powerbank has a current threshold (don't know exactly but probably 50-60 mA) and my arduino/servo motor are probably right at that cut-off.

What is the easiest way to handle this problem? Could I program my arduino to draw more current from the power bank (have it calculate pi to many decimal points?)? Or, would just buying a proper grade resistor do the job.

Thanks

kingrumak
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2 Answers2

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A direct resistor connection will work, but will be very wasteful. Trying to calculate PI won't actually increase the load much. Instead, a simple intermittent pulsed load will work. A resistor and suitable N-channel transistor or mosfet, pulsed every n seconds would trick the power supply into thinking there is a large load, but also reduce the average load, hence longer battery life.

schematic

simulate this circuit – Schematic created using CircuitLab

A 1 second pulse of 75mA every 10 seconds is only a 7.5mA draw over 10 seconds, compared to a constant 75mA load.

Adjust the period and load as needed.

Passerby
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  • If you are so belligerent, your solution is also "patently" false. The idea is good, and it is true that powerbank circuitry [might] have some delay after the current drops below threshold. However, you have no idea how often the internal firmware scans the sensor ports, and how long you need to apply the load, or what should the period be. My powerbank drops the power after 7-8 seconds. So your algorithm already is not working. More, I tried 1s on, 4 s off with 100 mA, and it worked for a while (1 minute or two), then dropped off. Without knowing the algorithm, the pulsed method fails. – Ale..chenski Feb 15 '17 at 02:02
  • @alichen you think all power banks cut off on low loads. Not even half do. You think me calling that a lie is somehow belligerent. You also thought no power bank could take and provide power at the same time, only to be proven wrong by your own device. You may want to rethink your approach. – Passerby Feb 15 '17 at 18:15
  • Passerby, wrong again. Adjusting period without EXACT SYNCHRONIZATION with inner workings of powerbank controller will fail again, since your external frequency will be never the same as internal, and at some point your schema will fail catasrophically. If you know a power bank that does not shut down by itself, simply post brand/model/link, so people here will get it and not suffer from abrupt disconnect of power, myself included. So far this site has a half-dozen of questions about powerbanks, with no clear answer. – Ale..chenski Feb 15 '17 at 18:36
  • Passerby: regarding the powerbank working as UPS (charging and providing power at the same time), I have retracted my statement yesterday, after additional testing - the charge progress was disproportionally slow while external power was well available, so the bank can't be really qualified as UPS, and re-design seems necessary. – Ale..chenski Feb 15 '17 at 18:43
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Most "powerbanks" are designed to cut internal operations off when the current falls below certain threshold (50 mA - 100 mA). Having additional load to keep it running will waste the battery runtime.

The best way is to modify the powerbank circuit to remove this protection. Some cheap powerbanks have a single control circuit that apparently has the current shunt circuit built-in, and it is not possible to disable. Some powerbanks will have low-Ohm sensing resistors on-board, so increasing their values will lower the cut-off threshold, but it may have adverse effect on the overall current delivery. It might be possible to somehow fool the cut-off circuit applying some internal bias, or else, it depends on powerbank control circuitry.

FOR ILLUSTRATION: My Powerbank (Model CJ-6000) has a pretty sophisticated circuitry, I might start playing with it: looks like a uP and three sensing resistors

See also https://electronics.stackexchange.com/a/270342/117785

ADDENDUM: after some reflection (based on powerbank circuit shown above) it becomes clear that a simple change in current sensing resistor will not work well - while the cut-off threshold gets lower, the overcurrent will also go lower, which might be unacceptable. The workaroud would be to use a non-linear element like a Schottky diode, with something like 10 Ohms in parallel (to define new cut-off threshold). This circuit has something like 3.5A overcurrent threshold (which amounts to 350 mV threshold over 100 mOhm shunt), so the diode must have a pretty low forward voltage like this one to avoid the overcurrent flip.

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Ale..chenski
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  • Thanks Ali. If there is no way for me to tweak the power bank circuitry, would you say that my best bet is to use an appropriate resistor. The powerbank is rated at 50,000 mAh (not sure if I believe this) but a 100 ohm resistor rated at .25 W should suffice. – kingrumak Feb 14 '17 at 19:23
  • @kingrumak, wow, 50Ah, this is a sizable jump-start quality pack! At 50mA load it should last about 1000 hrs, not counting for your arduino. If it is good enough, the extra load is probably your easiest solution. – Ale..chenski Feb 14 '17 at 19:44
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    `All "powerbanks" are designed to cut internal operations` patently false! – Passerby Feb 15 '17 at 01:33
  • @Passerby , my statement about "all powerbanks" is based on simple logistics. The powerbank industry is dominated by China, and all devices use same cheap Chinese ICs for control. Please provide evidence by pointing to any available powerbank that doesn't turn off on low loads, and I will happily replace "all" to "most". – Ale..chenski Feb 15 '17 at 18:57
  • Multiple generations. The cheapest banks, and thus the ones with the highest quantity out there, do not. Look at any youtube review of power banks. Every single one in this form factor. http://wildernessmastery.com/wp-content/uploads/2016/09/Power-Bank.jpg all the single cylinder ones. All the slimmest credit card shaped ones. Sold retail in any CVS, Walgreens Walmart, even dollar stores now. All the eBay multi cell ones that are sold with inflated stats and fake cells. Saying "most" have these auto off features is ignorant, as is thinking there is only one type of IC or board layout. – Passerby Feb 15 '17 at 19:29
  • @Passerby, your statement that "all the single cylinder ones" have no auto off is proven false. I went around (Wallmart/HEB) and purchased 3 cheapest power banks, two single-cell, one two-cell. There were a dozen more power banks with start-up buttons. The "Vibrant" one-cell has the auto-off, period. Only the "Pocket Juice 2000 mAh Pink" keeps voltage. However, I am not sure if this is not a broken sample, because the package says "Pre-Charged", which is obviously impossible for long shelf life. Mine came discharged. So please stop your groundless nit-picking. I am changing "all" to "most". – Ale..chenski Feb 16 '17 at 03:05
  • This one-cell power bank shuts off after 10-12 seconds, https://www.heb.com/product-detail/vibrant-rechargeable-usb-backup-2000mah-power-pack-colors-may-vary/2027651 – Ale..chenski Feb 16 '17 at 03:14
  • This "Pocket Juice 2000 mAh" one-cell seems to keep VBUS voltage on, https://www.walmart.com/ip/Pocket-Juice-2-000mAh-Solo-Power-Bank-Pink/46444851 – Ale..chenski Feb 16 '17 at 03:15
  • "It doesn't work like I wrongly think so that means it's broken". – Passerby Feb 16 '17 at 03:17
  • And Lithium cells should be stored at 60 to 80 percent charge for long shelf life... – Passerby Feb 16 '17 at 03:19
  • @Passerby, what makes you think that "came discharged" means 60% discharged? Mine came fully discharged, to the extent that it didn't output ANYTHING out of package until charged. Comprende vu? – Ale..chenski Feb 16 '17 at 03:24
  • That's a trick. Some power banks won't turn in their output after a battery is inserted until you connect it to power first. Look at some of Bigclivedotcom year down videos – Passerby Feb 16 '17 at 03:26
  • @Passerby, not turning on with partially charged battery sounds like utter nonsense. Power banks have SOLDERED batteries. The product must be tested after that. Think how. – Ale..chenski Feb 16 '17 at 03:39
  • Did you look at the Bigclivedotcom videos? – Passerby Feb 16 '17 at 04:24
  • @Passerby No. Which one I should be wasting my time on? – Ale..chenski Feb 16 '17 at 04:34