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Make a Precision Battery Discharger

This is a precision battery discharger for 8.4V 7-cell Ni-Cad or Ni-MH batteries. I made a cheap, reliable discharger circuit with an easy visual indication of the battery’s state (discharging or discharged) with less than 10$ of parts. Most parts can be found around the house or taken from broken electronic equipment – or at worst, purchased from Radio Shack.



The circuit as shown only works with 7-cell (8.4V) NiCd or NiMH battery packs, but with a couple additional parts it can also work with 8 cell 9.6V packs.

Mine discharges at the rate of .38A (or 380mA). This is VERY close to the “textbook” discharge rate of C/5 for a 1700mAh battery.

A 1700mAh battery (for example) cannot supply 1.7A for one hour, although that is implied by the rating. No battery can. The standard used by manufacturers to gauge capacity is C/5 – meaning the battery can supply current at the rate of C/5 (in our case 1700/5, or 340mA) for 5 hours. An optimal battery, when loaded at one-fifth of the mAh rating, should take about five hours to discharge.

Of course, not all batteries are created equal. Batteries get old, they get tired, their capacities change. But it does mean that C/5 is a “safe” ballpark discharge rate to use my battery collection.

Nitpick: The term “battery” refers to a pack of at least two cells. So there is no such thing as, say, a “C battery”. It is more properly called a “C cell”.

WHY USE A DISCHARGER?

Battery life of NiCd and NiMH packs will improve if you exercise your battery.

“Exercising” your battery has two stages. First, you discharge the battery to (ideally) 1 volt per cell. (NiMH and NiCd cells are nominally 1.2V per cell.) Second, your battery should be slow-charged (usually a 16 or so hour cycle) until fully charged – which is to a level of approximately 1.35V per cell for NiCd, and 1.39V per cell for NiMH. (Or 9.45V for an 8.4V NiCd pack and 9.73V for an 8.4V NiMH pack).

If you don’t have a slow charger, you can still charge your battery normally but I wouldn’t recommend making a habit of it.

By the bye, modern batteries should only need exercising once every few months. The exception is brand-spanking new (as in, fresh from the factory and not pre-conditioned by a manufacturer) packs, which should be put through 3 full discharge-charge cycles before being put into service.

HOW IT WORKS

This image above shows the completed discharger, complete with hideous epoxy putty as a shell to cover and insulate the exposed bits.

When the battery is plugged into the discharger, the lamp (a 12V auto signal lamp) lights up. When the battery has reached 7V (1V per cell) the light quickly dims and turns off.

The core of the discharger is the LM7805 linear regulator; a very common +5V regulator. Feed the 7805 some DC power (like, from a battery) and it will output 5V and burn the rest off as heat (it’s actually a little more complicated than that, but that explanation will do.)

The 7805 has a handy feature (limitation, actually) that we will take advantage of: it only functions when the input voltage is higher than 7V. That happens to be the amount we want our battery to discharge to to get 1V per cell for a 7-cell 8.4V battery!

So, all we do is plug the battery into a 7805 regulator, use the regulator’s +5V output to light something visual (like a lamp) and wait. Power will be used up from the battery, resulting in a heated 7805 and lit lamp, until the battery voltage reaches 7V – at which point the regulator stops producing a 5V output and the light goes out.

SCHEMATIC AND PARTS

Here is the schematic for the discharger – click for a larger version. It has two versions of the design; one in the usual schematic format for those of you who can read electronic schematics (top), and a more user-friendly version (bottom).

Note: the diode shown inline between the battery and the input of the 7805 (circled in red in the bottom diagram) is OPTIONAL and is NOT NEEDED for the basic version of the discharger. I’ll get into what the diode is for later. If you are using the discharger with a regular 8.4V 7-cell battery, you do NOT need any diodes at all.

Here are the parts I used:

  • 12V auto signal lamp (two light elements in one lamp – only used one).
  • PC power supply connector (took female plugs to connect to battery)
  • LM7805 5V regulator (with metal tab heatsink)
  • 1uF (one microfarad) capacitor (should be marked with a – for one end; the other end is +)
  • 0.1uF (point one microfarad) capacitor (non-polarized)

All electronic parts should be easily available in a well-stocked parts bin, or at (shudder) Radio Shack (which, by the way, often charges you a few dollars for parts that should cost a few cents).

ASSEMBLY

Follow the schematic diagram of your choice and solder together the various parts. If you do it quick and dirty, you may end up with something like this.

TESTING

When you’ve completed soldering everything together, test the discharger by plugging in a charged battery. The lamp should light up. After a short time, the 7805′s metal tab should also get warm. That’s normal.

If the 7805′s metal tab gets HOT, or (more likely) the discharger runs for a short time, then turns off (the light goes out) and the 7805′s metal tab is HOT, then your lamp is discharging too much current – the 7805 is overheating and going into thermal shutdown. If this is happening, you need something that sucks less juice than the bulb you’re using. See the OTHER BULBS AND LOADS section below.

If something else happens, disconnect the battery and check your wiring. If you connected things as sloppily as I did then you should make extra sure there are no shorts (places where exposed wires are touching where they should not) and that your connections are solid.

If the discharger works, let the battery discharge in it and disconnect the battery soon after the lamp dims and goes out. Although the regulator is no longer powering the lamp, it is still sucking a small amount of current – so it’s best not to leave it connected any longer than you need to even though the lamp is out.

Allow the battery to “settle” for a few moments, then measure the output voltage with a voltmeter. It should read almost exactly 7V, or 1V per cell for an 8.4V 7-cell battery!

If all is well, seal and insulate the discharger’s parts in epoxy or something so they don’t bend and break or touch each other. Don’t use hot glue since the regulator gets warm and will melt the glue.

ADVANCED OPTIONS

OVER-DISCHARGING

There is a chance that the discharger – for whatever reason (manufacturer variance and/or loose adherence to standards, for example) doesn’t discharge to 7V. This can be addressed.

If your battery is being discharged too much (ie discharged to lower than 7V) you can add one or more diodes between the battery’s + terminal and the input pin of the 7805. Refer to the schematic diagrams for diode placement.

As for diode selection, you have two options. To make the discharger think the battery is 0.7V lower than it really is (that’s 0.1V per cell), add ONE silicon recifier diode in the spot indicated in the schematic diagram. A suitable diode is the 1N5800, but you can use pretty much any silicon rectifier diode that is rated for at least 3A (or at least 5mm in diameter). When in doubt, get the fattest one. 2A will do if you’re in a pinch.

If 0.7V doesn’t suit your needs, you can use a schottkey barrier/rectifier diode (or germanium diode, rather than silicon) which has a voltage drop of 0.3V (and is also more expensive). A suitable diode would be the 90SQ030. Again, if using a different diode make sure it is rated for at least 3A (or at least about 5mm in diameter). Again, 2A will do if you’re in a pinch.

MORE THAN ONE DIODE

You can place diodes in series (end to end, one after the other) like this:

        --->|--->|---

So one of each would together result in a 1.0V volt drop (0.7 + 0.3), meaning the discharger would discharge the battery to a level 1.0V higher than whatever it was discharging to before.

By using either of or a combination of the two diodes you should be able to alter the shutoff point to match approximately what you need.

8-CELL (9.6V) BATTERY PACKS

A couple of diodes will allow this discharger to also work with 9.6V 8-cell batteries.

By adding one diode of each type as mentioned above, the discharger will think the battery is a full 1.0V LOWER than it really is, meaning it would shut off when the battery is at 8V instead of 7V. That happens to be just right for an 8-cell battery. Shutting off at 8V will be 1V per cell.

OTHER BULBS AND LOADS (Updated May 2003)

You should also be able to use a flashlight bulb or a small motor in the place of the 12V auto lamp. Anything that draws around 300mA or so or less should do the trick. Most small toy motors will draw from 100-200mA and should be fine, for example.

PARTS SOURCES

If you’re really stuck for parts, you can order online from Digi-key.