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I would like to build a little water mill to place under the sink of the kitchen to produce electricity. Considering that the mill will work when the user opens the tap to wash a dish, using a dynamo and a little electric circuit (bridge rectifier and a low pass filter), I would be able to generate a continuous electric signal of about 6 V, 0.1 A for a couple of minutes. The idea is to charge a battery, typically an external one used to charge a smartphone, i.e. a battery with a capacity of 2500 mAh.

I am wondering if the repeated short sessions of charges will affect the battery in any way. Obviously, the battery will need a couple of dish-washing sessions to be fully charged, but will these short sessions affect its efficiency, or even alter the charging time ?

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  • $\begingroup$ I'm not sure it's an answer, but solar is also intermittent, so you might want to look at what's inside a charge controller. $\endgroup$ – DLS3141 Aug 18 '15 at 14:42
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    $\begingroup$ How much energy do you think you can extract from a sinkfull (50 kg?) of water with at most about 18" (0.5m) of "head"? Maybe 200-250 Joules, or Watt-seconds, tops. A 2500 mAh, 3.7V battery holds about 33 kJ of energy, so you'd need at least 100 loads of dishes to give it any significant charge, and this is ignoring all of the power conversion efficiency issues! $\endgroup$ – Dave Tweed Aug 19 '15 at 0:45
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The slower you charge a Lithium battery the better. They are not keen on very fast charge/discharge fluctuations.

  • One reason is that faster charging also results in higher temperatures which stresses a battery and therefore reduces battery life.

  • Another reason is that lithium plating might occur, where the transport rate of the Li-ions exceed the rate at which they can be 'stored' in the anode in a process called intercalation. Instead, the lithium deposits as Li metal onto the anode, which can cause short circuit and degrades capacity, since there are less lithium ions to store electrons. The following image might visualize the storage process better: Li-ion intercalation intercalation in cathode. The opposite happens in the anode(electrons are stored during charge and released during discharge) Source

EDIT: To answer your question directly:no, short charge cycles do not influence battery life as long as you charge slowly and try not to stress it by making full charge swings from 0 to 100%. In fact, ideally, but maybe not practically, you should try to keep it near 50% at all times(at least, this is the rule of thumb used in EVs).

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  • $\begingroup$ Thank you for your answer. Could you expound on the subject? $\endgroup$ – Wasabi♦ Aug 21 '15 at 19:59
  • $\begingroup$ I disagree. Many Li-Ion manufacturers recommend charging at 1C or at minimum 800mAh. As for a practical test, if you charge a smartphone via USB it will actually last less in the same consumption conditions as compared to it being charged at high current (even 2A). $\endgroup$ – Overmind Sep 16 '16 at 9:26
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I don't believe periodic charging would damage a smartphone, however it likely wouldn't charge it much either. A typical phone charger today outputs 1 Ampere of current at 5 Volts, and takes perhaps 2-4 hours to charge a phone. At 0.1 Ampere or a tenth of this current, it would take ten times as long to charge the phone. It would actually be longer since the phone is self-discharging during this time, and may not ever charge appreciably.

As long as the dynamo's output was rectified, filtered, and regulated to 5.0v, it should work for demonstration purposes. It must be regulated however, a phone will never expect 6V and could be damaged. It should also be "smooth" DC or direct current, meaning there is little noise or variations in it's output.

If the dynamo can be expected to produce less than 5V, then a "buck-boost", "boost-buck", SEPIC, Zeta, etc. converter/regulator can all output a fixed voltage (5v) for an input which goes below and above the target. I don't recommend inexpensive eBay items, but something like this "Step Up Step Down DC-DC Boost Buck Converter" can convert a varying input voltage to a relatively stable output voltage. Even so, I'd recommend having it output 6V, and use a second "buck" regulator (with better "line" and "load" regulation and less noise) to produce a clean 5.00v for the phone. Of course, the more pieces that are added, the more efficiency is lost. The best solution would be to buy (or build!) a custom converter designed around the particular dynamo being used. Questions about such would be best directed to electronics.stackexchange.com.

If you wanted to recharge the phone from a kitchen sink tap or garden hose quickly, consider a plumbing fitting which replaces the existing nozzle with an impeller under pressure from the water supply. The water pressure can do much more work than the free-flowing water mass and gravity, allowing the dynamo to spin faster and produce more current. Such topics as fluid pressure, work, fluid head etc, all fall under the category of Fluid Dynamics in physics.

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