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I am currently looking at my stack of old batteries at home, did some measurements and found that most of them contain at least some energy left. I was then thinking. What is I could make a device with several mountpoints (holders for different types of batteries such as LR6, LR03, 6LR61, LR20, CRC032, LR44, and so on). All have an expected operating voltage but since they are removed as being "empty" they usually are below the expected voltage.

Is there some kind of low voltage (like under 1 volt) transformer that could take in a variable voltage and convert it into another so that I can unload all old batteries into a rechargeable battery pack? Are there already existing diy projects that attempt this? Or out-of-the-box solutions I've missed?

But, I'd like to build this as a hobby project maybe. A simple device where I plugin all old batteries, and they are drained into a battery pack. So I suppose "energy harvester" would be the proper terminology.

I saw this video on Joule Thief: https://www.dailymotion.com/video/x6jytol. It's a good explanation but I need to get a broader understanding of this subject in order to control the output voltage and to charge a rechargeble battery with it.

Bob Ortiz
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    What research have you done on this question? What did you think of the "Joule thief"? – Harper - Reinstate Monica Apr 27 '21 at 20:11
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    Possible - sure. Practical/worth doing - no. – Ecnerwal Apr 27 '21 at 20:12
  • Transformers the wire wound type won’t work with DC but there are DC to DC converters. Building different charging systems and then converting the voltages to a single voltage is doable not only in theory but reality. The cost to harvest will be about 10x what new batteries will cost and quite bulky. – Ed Beal Apr 27 '21 at 20:23
  • I've added my opinion about the Joule Thief. @Ecnerwal why not worth it? Aren't most diy projects? It's a nice learning practice. – Bob Ortiz Apr 27 '21 at 20:24
  • @EdBeal can you explain why the cost to harvest would be so high? I mean old batteries are basically free and available. – Bob Ortiz Apr 27 '21 at 20:26
  • Off topic as too broad and not about home improvement. VTC. – isherwood Apr 27 '21 at 21:22
  • Bob you want to use the batteries this means different voltage setups or different draw rates as they discharge the cost of a single cheap DC to DC converter for a cr33 or lr44 would take hundreds of these spent batteries plus holders wiring and time converting to a usable voltage then putting into another storage medium that is never 100% efficient plus the losses in the conversion all adds up to costs and we are not counting the time in setting up. What’s the current online price for new lr44’s I think I got 20 for under 3$ fresh not 2 years old how many to charge to pay ? 10x may be low. – Ed Beal Apr 27 '21 at 21:25
  • Whoopsie doodle! You probably should have asked this question on Electrical Engineering Stack Exchange. It has different rules, so read up first if you post there and check if your question has been asked. One popular project to suck old batteries dry is the [joule thief circuit](https://en.wikipedia.org/wiki/Joule_thief) which can power a small LED for a bit from what you would consider a dead battery. Low voltage energy harvesting is challenging because of arbitrary voltage drops over many common components like diodes, but it has been fairly well researched. – K H Apr 28 '21 at 00:03
  • The [LTC3107 for example](https://www.analog.com/media/en/technical-documentation/data-sheets/3107f.pdf) is an IC designed to build low voltage energy harvesters with sources like thermoelectric generators. It can step up a voltage as low as 0.02V. It even has a crude form of Maximum Power Point Tracking (MPPT). Disadvantage is it references output voltage with a reference battery and supplements the output from the battery so it might be tricky to use that particular chip, but it was first datasheet on the list. – K H Apr 28 '21 at 00:19
  • You can google "Low voltage energy harvester" and search datasheets until you find something you can work with. You can also consider building a regulated joule thief (Normally it's kind of a self regulating runaway converter) or flyback converter to charge a capacitor instead of powering an LED. You can set up another circuit to dump the capacitor into a battery charger periodically to produce short bursts of current suitable for charging. The engineers would have better solutions, but these are what I could pull off reasonably easily as a hobbyist. – K H Apr 28 '21 at 00:27
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    Odd that this was closed and not migrated to [electronics.se]. Seems that it would be appropriate there... – FreeMan Apr 28 '21 at 12:28

1 Answers1

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Is this possible? Of course it is. But is it practical? Not at all.

The comparison is not to the cost of batteries. Batteries are expensive. The comparison has to be to the cost of electricity. Using some very rough numbers:

Cost of electricity per kWh: US average is around 13 cents. Let's round up a bit to make the math simple and call it 20 cents per kWh.

How much does power does a battery hold? That varies a lot, depending on size. A few examples:

  • LR20 = "D" = 2.5 Wh (that's Wh, not kWh) - 27 Wh
  • A typical 12V car battery = 48 Amp-hours = 576 Wh
  • A large 12V car (or marine) battery = 100 Amp-hours = 1,200 Wh
  • A large Tesla battery pack = 100 kWh

Let's assume that each battery starts out filled to 50% capacity (unlikely), and that the magic energy harvester can get every last drop of electricity out of the battery (pretty much impossible) and can do that with 100% perfect efficiency (also impossible). The $ value is:

  • LR20 = "D" = 0.0135 x 0.2 = $ 0.003 (less than a penny)
  • A typical 12V car battery = 0.288 x 0.2 = $ 0.058 (less than a dime)
  • A large 12V car battery = 0.6 x 0.2 = $ 0.12 (less than a quarter)
  • A large Tesla battery pack = 50 x 0.2 = $ 10.00

In the case of the small batteries, toss 'em (in an environmentally friendly way - there are plenty of commercial and government locations that accept used batteries for recycling).

In the case of the large batteries - the safety precautions needed to properly process the batteries will likely cost far more than the potential value of the stored electricity.

In general, subject to local laws, you can usually toss small batteries, especially non-rechargeable batteries (e.g., alkaline). However, if you do that, be extra careful with 9V batteries. 9V batteries have a potentially dangerous design flaw - the + and - terminals are right next to each other. If you take a 9V battery, even mostly dead, and short circuit the terminals, you can start a fire. With most other battery types the danger is incredibly small. But a 9V battery tossed in a trash bag that landed on a piece of aluminum foil or other conductive material could actually start a fire. A piece of electrical tape across the terminals before tossing the battery eliminates the danger. Video about 9-volt batteries.

manassehkatz-Moving 2 Codidact
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    ...and if you want "sort-of-free" or lower cost (or just non-grid - sometimes it's higher cost, depending on battery lifetimes, etc.) energy, a solar charger for rechargeable batteries is a far more practical project than a "battery drainer" (other than the straight-up JT to drain them by making light, and for practical uses a rechargeable flashlight is more practical there, too.) – Ecnerwal Apr 27 '21 at 21:07
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    Well thanks for putting $ to it, I think finding a new way to recycle all the battery materials might be worth the cost because if we don’t we will have dead battery stockpiles next to the nuclear waste + – Ed Beal Apr 27 '21 at 21:31
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    @EdBeal I think the big problem right now is all the "new & improved" (and they really are "improved") battery technologies. There is well-established infrastructure for recycling 12V lead-acid car batteries. But all the Lithium stuff (multiple chemical technologies) - and all the different packaging, integrated electronics, etc. gets super complex to recycle efficiently. – manassehkatz-Moving 2 Codidact Apr 27 '21 at 21:47