On our summer scouts camp, I need to power a tablet (it simulates the computer from The Lost, and the children must activate it every 108 minutes). It requires 4.3 - 5.3 V and about 650 mA. I need to power it for 6 days, but the problem is, there is no electric outlet or something nearby. So basically we must make the electricity or use batteries. And for it to be harder, the financial limit for this project is 18 EUR (20 USD), which isn't a lot obviously. I would also like to note that we have a 1000W electric generator, but it can run for only about 2-3 hours/day.

So I came up with two solutions how to power the tablet:

Solution 1: Use batteries

I can buy and use a bunch of batteries. Problems:

  1. Capacity - I need 15,6 Ah/day, which is a lot for batteries and this financial limit.
  2. Charging - I can use the electric generator, but can I charge a battery in 2 hours?

Solution 2: Generate electricity ad-hoc

I have a dynamo from old Czech car, which (according to the internet) can output 12V 55A (is it possible?) and maybe connect it to a stationary bike, and children will have to use the bike to charge 18650 lithium batteries, which will then power the tablet.

I like the second option more. What do you think? Do you have more solutions? Any advice appreciated. Or feel free to ask for more details. Is it even possible?


  • My level of knowledge of electronics is low-medium, but I don't have much practical experience.
  • I made a practical test. 4 AA NiMH 2300 mAh batteries charged the tablet for 8 hours. Those numbers don't make sense, theoretically it should have been 15 hours!
  • \$\begingroup\$ This is an excellently-asked question. (I'm editing out the comment about your English, because it's perfectly fine. :) ) \$\endgroup\$
    – Greg d'Eon
    Apr 11 '15 at 16:34
  • \$\begingroup\$ Does the tablet have to be on for the entire time? It should be fine keeping the timer running with the screen off, and that will dramatically reduce your power consumption. Also, have you actually measured the power consumption? If that number is from the adapter, it's possible it's the worst-case draw, and the idle-with-screen-on is much lower. \$\endgroup\$ Apr 11 '15 at 17:08
  • \$\begingroup\$ I can believe that the dynamo is capable of 55A, but not when pedal-powered. You should still be able to get a usable current from it though (2A would be a generous amount for charging). \$\endgroup\$ Apr 11 '15 at 17:11
  • \$\begingroup\$ The tablet must be on, because when I put it into sleep, it also pauses the timer. Yes, I have soldered a special USB "interuption", which is connected to multimeter and which measures voltage and current. When the battery is fully charged, it draws 650 mA @ 5V, when it is depleted, it draws about 950 mAh. (It was very cheap tablet, around 40 USD, and it is about 2 years old) \$\endgroup\$
    – user72028
    Apr 11 '15 at 17:39
  • \$\begingroup\$ The original adapter is certified for 2A @ 5V, but since I switched off WiFi and turned the brightness down, I get only 650 mA. \$\endgroup\$
    – user72028
    Apr 11 '15 at 17:40

First of all, how long must the computer be on? You mention that it must be activated every 108 minutes, and I assume this means it must be turned on every 108 minutes. Then for how long does it get turned on for? At a rate of 650 mA, if it is only on for say 1/2 hour every 108 minutes the you would only use 325 mA every 108 minutes.

Assuming you follow the 108 minutes 24 hours a day, then you would turn on the tablet 13 times for 1/2 hour in a 24 hour period. You are camping for 6 days so a total of 13 x 6 = 78 - 1/2 hour sessions (or however long you are turning it on for).

At 650 mA, 78 - 1/2 hour sessions (650 mA x 78 x 1/2) = 25.35 amps

Your Nimh are at 1.2 volts, so put 4 in series to get 4.8 volts (closer to 5.6 volts fully charged so be careful). When in series, the capacity is NOT added, so have only 2300 mAh capacity with the 4 in series.

25.35/2.300 = 11.02

So you would need 11 sets of batteries, or recharge them 11 times if you use the tablet for 1/2 hour every 108 minutes 24 hours a day for 6 days straight. I have a feeling that you won't be using the tablet this much, so you will have to adjust your calculations accordingly.

I know that you mentioned 18650 batteries, but given you budget and your options, I think that a good quality Nimh is your best choice. The main reason is because you can get a reasonably priced Nimh AA battery that can be charged in less than 2 hours! Many lithium ion batteries will take longer to safely charge. Plus the voltage of 4 Nimh batteries matches what you need. Get a fast charger that will charge 4 at once. You can get chargers that will charge Nimh in as little as 15 minutes, but that is hard on the batteries. I would go for a 1 hour charger with the generator.

Good luck!

  • \$\begingroup\$ Thanks a lot for your answer. However, the problem is, that when I developed the app, I didn´t know that it has to be energy saving, so I developed it in Game Maker Studio, which (I believe) doesn´t support background tasks on Android. And it is better for children to see the countdown. So ideally, the screen would be on for six days. Well about that 18650, I have found some in old laptop battery pack - the laptop was about 10 years old. Do you think they are useable? Thanks. \$\endgroup\$
    – user72028
    Apr 12 '15 at 6:00
  • \$\begingroup\$ @user72028 - I have salvaged hundreds of 18650 from old laptops and sometimes the batteries are in great shape, sometimes they are dead. I'm my experience, any laptop battery older than 5 or 6 years is likely to be dead. Lithium ion die from old age as often as they die from too many cycles. Re do your tests to see if you can get the tablet to use less than 650mA by turning off wifi etc. Try testing Nimh again and maybe you need 3 or 4 sets of them. The other problem with lithium ion besides higher cost for you is that their voltage doesn't work with what you need where 4 Nimh should. \$\endgroup\$
    – Filek
    Apr 13 '15 at 4:34

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