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Frequently Asked Questions (FAQ) — The Basics




This FAQ answers the following questions:


Why should I discharge my NiCd or NiMH battery packs?
Proper discharging is as important as proper charging to ensure that your battery packs last as long as possible and to maximize their capacity and performance. For more information, see our Why Discharge? and Tech Tips pages.

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What does "C" mean? I see it all the time when reading about charging or discharging.
"C" is short for "capacity". It is the rated (by the manufacturer) amount of charge a cell can hold and is often used to specify the charging and discharging current a cell can safely handle.

For example, to prevent damage due to overheating most NiMH cells shouldn't be charged at a rate higher than "2C". If you're using 1100mAH NiMH cells, that is, the cell's rated capacity is 1100mAH, then those cells shouldn't be charged at a rate higher than (2 x 1100) = 2200mA, or 2.2A, to prevent overheating of those cells and possible damage. This is just a general recommendation and certain cells should be charged at a lower rate and others can be charged at a higher rate without overheating. Follow the recommendations of the cell's manufacturer or the dealer you purchased the cells from.

"C" is used the same way for discharging. Most NiCd cells can be discharged at a "3C-5C" rate without worrying about overheating. If you have 3000mAH NiCd cells, this means you can safely discharge them at a rate of (3 x 3000) = 9,000mA, up to (5 x 3000) = 15,000mA, or 9A-15A without overheating. Again, this is just an example. Please follow the recommendations of your cell's manufacturer or the dealer you purchased the cells from.

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What's the difference between CamLight Systems' Pack Dischargers and Auto-Cutoff Modules?
Our Pack Dischargers combine a built-in load to discharge your pack with an auto-cutoff switch to disconnect the load from the battery pack when the voltage has dropped to the proper level. Our Auto-Cutoff Modules do not have a built-in load, just the auto-cutoff switch, and use a load you supply to discharge the battery pack.

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Which should I use, a Pack Discharger or an Auto-Cutoff Module?
Our Pack Dischargers provide an all-in-one solution for discharging your battery packs—both a built-in load and an auto-cutoff switch to end the discharge at the right time. Our Auto-Cutoff Modules provide more flexibility and greater discharge current ratings by allowing you to pick the type and rating of the load you wish to use.

Our Auto-Cutoff Modules are perfect for testing the efficiency and run time of your packs with motors and other loads where you are measuring the performance of your packs. Our Pack Dischargers are perfect for general conditioning and tracking of your pack's capacity over time due to their built-in load and easy hookup.

If you have additional questions regarding our products, please contact us. We'll be happy to discuss your application and help you pick the product that's best for you.

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What settings can I adjust on your Pack Dischargers and Auto-Cutoff Modules?
You can adjust the following settings on our Pack Dischargers:
  • Pack Voltage - Tells the discharger how many cells are in your pack, i.e., the pack's nominal voltage. Depending on the model, our Pack Dischargers have 5 or 6 Pack Voltage settings, for 4-10 cell, 10-20 cell, or 15-30 cell packs.
  • Cutoff - This is the per-cell voltage that the discharger uses, along with the Pack Voltage setting described above, to determine the "cutoff voltage" at which the discharge stops. The discharger multiplies the Pack Voltage (# of cells) by the Cutoff (per-cell cutoff voltage) to arrive at the final cutoff voltage. The discharger monitors the pack's voltage during a discharge and disconnects the load when the cutoff voltage is reached. For example, a 10-cell pack discharged at the 1.0V/cell Cutoff setting would have the discharge stopped when the pack's voltage dropped down to 10.0V. Depending on the model, our Pack Dischargers have either a fixed 0.9V/cell per-cell Cutoff setting or 4 selectable settings, 0.8V, 0.9V, 1.0V, or 1.1V/cell.
  • Load - All of our Pack Dischargers allow you to adjust the Stage-1 discharge current by selecting the number of power resistors used for the load. Each discharger has 4 Load settings. See the Pack Dischargers page for the discharge current specifications for each of our dischargers.
You can adjust the following settings on our Auto-Cutoff Modules:
  • Pack Voltage - Tells the Auto-Cutoff Module how many cells are in your pack, i.e., the pack's nominal voltage. Our Auto-Cutoff Modules have eight Pack Voltage settings, for 5-20 cell packs. The Cutoff is not adjustable and is set to 0.9V/cell. The Load is determined by the type and rating of the load you connect to the Auto-Cutoff Module.
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How do I hook up my battery pack to a Pack Discharger?
The battery pack is connected to the BATT wire leads. Since each Pack Discharger has a built-in load, no other connections are needed.
Battery Pack Discharger hookup diagram
The current flows from the battery pack, through the auto-cutoff switch, and through the built-in load. When the auto-cutoff switch detects that the battery pack's voltage has dropped down to the selected cutoff voltage, the load is disconnected from the battery pack and the discharge stops.

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How do I hook up my battery pack to an Auto-Cutoff Module?
The battery pack is connected to the BATT wire leads. Since an Auto-Cutoff Module doesn't have a built-in load, you also need to connect a load (bulbs, motors, power resistors, etc.) to the LOAD wire leads.
Auto-Cutoff Module hookup diagram
The current flows from the battery pack, through the auto-cutoff switch, and through the user-supplied load. When the auto-cutoff switch detects that the battery pack's voltage has dropped down to the selected cutoff voltage, the load is disconnected from the battery pack and the discharge stops.

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You recommend discharging to approximately 0.9 volts/cell. Does that mean 0.9 volts while the battery pack still has a load on it (still discharging)? Or is it 0.9 volts after the pack is off the discharger and has cooled down?
They should be discharged to approx. 0.9V/cell while under load, that is, while being discharged.

Once the load is removed, the cell's voltage rises back up. This makes it very hard to determine the cell's charge (or discharge) level since even a fully discharged cell's open-circuit voltage (i.e., the voltage without a load attached) can be a lot higher than 0.9V.

Also, the 0.9V/cell cutoff level is only the approximate cutoff voltage to aim for. If you are discharging at a very high rate (let's say, >3C-5C), the internal resistance of the cells will cause the pack's voltage to drop. This will make it seem like the pack has reached the 0.9V/cell cutoff before the cells are truly discharged. This is why we use a 2-stage discharge in our Pack Dischargers. The first stage quickly brings the pack down to 0.9V/cell (or whatever the cutoff is set to) at a high discharge rate. The second stage again brings the pack down to 0.9V/cell, but at a much lower discharge current level. This minimizes the effect of the cell's internal resistance on the pack's voltage and ensures a complete discharge since the discharger (or us) won't be "fooled" into thinking that the pack is discharged before it really is.

If you're only doing a single-stage discharge (manually or with your own cutoff switch) at a high discharge current level, you can bring the pack down to 0.8V/cell. This will help prevent the voltage drop due to the cell's internal resistance from making the pack appear discharged before it really is.

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How do I hook up my Whattmeter (or other AH-meter) to my battery pack and one of your Pack Dischargers or Auto-Cutoff Modules?
The battery pack is connected to the SOURCE leads of the meter since it's the source of the current flowing into the Pack Discharger or Auto-Cutoff Module. The meter's LOAD leads connect to the BATT leads of the Pack Discharger or Auto-Cutoff Module since the load that the discharge current is flowing into is contained within the Pack Discharger or connected to the Auto-Cutoff Module. Hookup diagram for AH-meter and Pack Dischargers
The current flows from the battery pack through the meter (where the voltage, current, and other parameters are displayed) and on to the Pack Discharger or Auto-Cutoff-Module.

Note: Be sure not to exceed the maximum voltage rating of the AH-meter or go below its minimum. Also be aware that not all AH-meters, voltmeters and ammeters are the same. Some are more accurate than others and some have specific requirements in order to get the greatest accuracy. Carefully read the instructions/specifications that came with your meter.

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How do I use an AH-meter to maximize the performance of my battery packs and help to extend their life?
The AH-meter will tell you the voltage of your battery pack and the charge or discharge current level along with the total charge (in milliampere-hours or ampere-hours) taken from, or put into, the battery pack.If you're serious about taking care of your battery packs and getting the best performance from them that you can, you need to use an AH-meter while discharging. Here's why:
  • While you can use the ampere-hour readout of a battery charger to see how much the charger has put back into your cells, this won't tell you how much you can get out the cells during a discharge. A battery pack is a lot less efficient at the higher discharge current levels we typically run at. The amount of charge you can get out of a battery pack at these higher discharge current levels is less than you can get out at lower discharge current levels. An AH-meter can help you choose the highest discharge current rate that still gives you a decent run time by measuring the capacity of the cells at various discharge current levels.
  • Using a fixed load and a fixed discharge cutoff voltage when discharging with your AH-meter allows you to measure the capacity of the cells over the years and detect when their performance has dropped to an unacceptable level. It also allows you to compare the useful capacity of the cells with different loads (different discharge current levels) to see what the maximum discharge current level of the cells really is.
  • If you don't have a voltmeter or ammeter, an AH-meter can perform many of the same functions.
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Which AH-meters do you recommend?
Three companies (in no particular order) selling meters you should check out are: Go to top


For more information on discharging NiCd/NiMH packs, see our Why Discharge? and Tech Tips pages.