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Suppose I'm replacing batteries in some powerful device like a flashlight - the flashlight will not even run on those batteries but some low power device like a TV remote or a wall clock will continue running on them for months. So there's a lot of usable energy in those batteries, just the flashlight can't make use of it anymore.

Besides using those batteries in wall clock (the problem is you run out of wall clock faster than you run out of half-drained batteries) what else can be done to give them a second life?

I've seen some marketing projects like this one from Duracell which I personally find incredibly stupid because they wasted a ton of wire and a gazillion of LEDs just to build a stupid sculpture but not a working battery management system that would allow to actually prevent half-drained batteries from being disposed (recycled) prematurely.

Is there any known practically working solution?

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    The bunny in the movie certainly is nonsense, but (small) LEDs do require little energy, so that's one option for which you can use your half-drained batteries. – THelper Oct 11 '13 at 10:45
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    @THelper: Great, except producing those LEDs is not at all clean, so if I don't need them in the first place I wouldn't use them just to make use of batteries. – sharptooth Oct 11 '13 at 10:56
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    @sharptooth good point but lets use a headlamp for instance that runs on AA battteries and has a halogen bulb. The batteries may not have the life to power it. Instead of disposing of them you could use them to power your LED headlamp. I think this was THelper's point. – Charlie Brown Oct 12 '13 at 22:29
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    you can also get rechargers that work on single-use batteries. They are limited but if you have the batteries it's worth a look. I found I only got 4-5 cycles out of the batteries (still better than one, though), but since the mostly-discharged batteries were free buying the charger paid off. You might find the same. – Móż Oct 13 '13 at 23:56
  • @CharlieBrown: I'd then use an LED headlamp at all times - it would have much better runtime. – sharptooth Oct 14 '13 at 9:12
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Technical terms

To partially understand the process of draining batteries it is important to know some technical parameters:

  • voltage - Battery operated electric devices need certain minimal voltage level and current to work correctly. As a battery is being drained the voltage drops down. In some batteries the voltage goes down very slowly during most of the discharge cycle but rapidly at the end (NiMH accumulators) in some the voltage goes down more evenly (Alkaline batteries). For illustration below are discharge curves (showing the voltage going down) from this page: Batteries and Battery packs enter image description here
  • current - When a device needs more power it takes higher current. (or needs higher voltage -> batteries connected in series) As the current rises the voltage drops down. See the internal resistance which contributes to the voltage drop below.
  • internal resistance - Higher internal resistance causes larger voltage drop and also higher energy loss. (The energy transforms to heat inside the battery.) The internal resistance is higher in older and lower-quality batteries.

Conditions to reuse partially discharged battery

From this information you can see that if some of the following conditions are fulfilled you will be able to use the battery in a second device after it is unusable in the first:

  • 1st device drains relatively high current
  • 1st device needs relatively high voltage from a single cell to operate
  • battery has an even discharge curve
  • battery has high internal resistance
  • 2nd device drains lower current
  • 2nd device needs lower voltage from a single cell to operate

So lower the current and voltage the second device needs - more power it can utilize from a partially discharged battery.

Examples of suitable devices which can be powered by partially discharged batteries

I think that the following device types can satisfy the conditions stated above:

  • clocks (both digital and electro-mechanical)
  • digital thermometers, pocket digital games with simple graphics
  • digital thermostats
  • remote commands, wireless keyboards, mouses
  • pocket radios and digital audio players
  • bicycle led lights, led head lamps, cyclocomputers

Other solutions

Other possibility is to build a power source which will need relatively low voltage per single battery cell.

In fact some modern devices use more and more efficient power management circuits. The devices need low voltage inside which also helps low power consumption. The low voltage is produced by a power efficient switching voltage stabilizer/regulator which can possibly operate with wide range of input voltage so that batteries can be efficiently discharged to a low voltage and more energy can be retrieved from them.

It is certainly possible to construct a power source which will generate stabilized electrical power from a possibly large number of half-discharged batteries by using a switching regulator. I have never seen a mention of such a device. The procedure of replacing the batteries would be probably annoying. Because of dependability of such a power source (varying run-time) I would imagine its use for example for charging of other batteries e.g. in cell phones.

Because partially discharged batteries have various properties they cannot be effectively used connected in series. On fully discharged cells connected into the circuit there would be unnecessary losses and they can even leak the electrolyte. For this reason there should be a separate regulator for each cell which makes the device more expensive.

In my opinion it is practical to use half-discharged batteries in a low power demanding devices mentioned above.

More for sustainability

Regarding sustainability it is best to avoid using batteries as much as possible. If you use them, prefer low self-discharge and rechargeable batteries (accumulators) with minimum of dangerous substances. Also prefer low power devices.

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    The problem with your "other solutions" is that you either need to feed the DC-DC converter independently out of each cell, or put a pile of them in series. With cells in series current will continue to get pushed through them after they are completely discharged. That's inefficient and can cause cells to leak. So the reason you don't see such devices is that they're hard to build, expensive because you need a lot of DC-DC converters and so are not worthwhile. – Móż Oct 18 '13 at 0:44
  • Interestingly, DC-DC converters will run off very low voltages - digikey.com/product-detail/en/LTC3108EGN-1%23PBF/… goes to 20mV (but is limited to 50mV or spikes of 500mV). I suspect you could find one that will accept anything up to 2V, which would work nicely. You'd still have very low power available, but for LEDs it should work. And only $3.50 each in quantities of 100 (external components also required). – Móż Oct 18 '13 at 0:47
  • @Ӎσᶎ: Thank you for the good point! I have added the information about problems of discharging batteries connected in series into the reply. The link to the DC-DC converter is useful too. Maybe I should include it into the reply too :) – pabouk Oct 22 '13 at 3:08
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I've rebuilt two single-battery operated appliances in our home to a "battery juicer". I'll describe one of them here to explain the idea, the other one is remade in a similar way to this one. Both of them operate happily with half-drained batteries and I've reached the balance of incoming half-drained batteries and draining them almost totally in those appliances.

The appliance (thermometer) was originally run by one AAA battery (1.5V) and would operate to about 1 Volt. I replaced the battery holder with two AAA batteries holder and attached it through a simple voltage regulator. A blue double switch is visible in the picture (rear side of the appliance):

Rear side of the appliance

There are two switches and three diodes added to the circuit. Those are germanium diodes taken from an old HDD (reused :), each with 0.3V potential drop in forward bias.

Circuit diagram

  • When the batteries have enough power I turn off both of the switches, which lowers the voltage of the bank by 0.9V.
  • With less powerful batteries I switch on switches 1 or 2 to shortcut the diodes. The decrease is 0.3V or 0.6V.
  • In the last stadium I switch both switches on and 1V is reached by two 0.5V batteries.
  • When I want to deplete the batteries to 0V, I replace one of the almost empty (0.5V) batteries with a half-depleted one (1V) and run them until the death of the weaker one. This is in fact the most common situation because once you have not equally drained batteries, one of them is going to die earlier and will be replaced.

At first I started with resistors, not diodes, but their voltage drop depends on the current - when the current fluctuates, the voltage drop changes too. This is not wanted, diodes provide a constant voltage drop to the bank.

Tweaks: more batteries can be grouped to reach a better use of used batteries. But then more switches are needed to regulate the wider scale of voltages. My other appliance is using a bank of 3 batteries, but they are external, because they wouldn't fit inside.

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There is a small DIY circuit called a "joule thief" that will take a AAA or AA battery down to about a half a volt or less. Most of my AAAs are from my computer keyboard which starts acting up when the battery reaches about 1 volt. I can take that battery and put it in a joule thief circuit and it will run an LED for maybe 24 to 48 hours. The joule thieves I make only take the battery down to about .7 volts, but there are better ones that will take it as low as .35 volts. Here's a link to a Youtube video that shows how to make the circuit. It can be made from components salvaged from old electronics too. The guy in the video is an expert, so I'm pretty sure if you followed his instructions exactly it might take the battery down to maybe a third of a volt. The LED would probably run for several days on a dead battery as opposed to my 1 or 2 days.

How to make an authentic joule thief

  • Will the battery leak if discharged that low and left for a month in the device? – sharptooth Jul 21 '17 at 15:25

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