What rates should I set on my new charger?

I seems this question is being asked frequently enough to justify an answer. To that end the Battery Clinic offers the following.

FAQ – What should I set my charger to?

1) There is really no difference in the Battery Size vs. charge/discharge characteristics - radio batteries vs. power batteries (electric flight). There are many using Cs cells for radio batteries, particularly in larger planes as well as many using N or AAA for radio batteries as opposed to park flyers.

2) Max charge voltage – Does not apply for Ni-Cd or Ni-MH - these are constant current systems and the voltage will follow the charge current. Giving the maximum charge voltage inevitably ends up with some people setting up constant voltage supplies for Ni-Cd and Ni-MH with the disastrous results.

3)The same for Lithium and Lead Acid but using voltages as the primary criteria. Set max charge voltages: 4.2 volts/cell for lithium (there may be separate settings depending on mfg.) and 2.45-2.50 v/cell for Lead Acid and current limits C/10 (for best life) - Lithium again will depend on manufacturer and type, lithium ion or polymer at this point.

4) A conditioning charge should be used for new packs or packs that have been in storage for some time. This would be C/10 (or as close as you can get to it) for 16 hours. Same for packs that may have been left with the switch on and completely discharged, with some cells reversed - 24 hours @ C/10 is suggested.

5) Discharge rates - a brief statement as to why one would want to discharge in the first place. 1) To check the pack capacity against the manufacturers rating and 2) to check how much capacity was taken out for a flying session in order to know the systems capacity usage rate. In both cases the discharge should be at C/5 as this is what the manufacturer uses. Discharging both Ni-Cd and Ni-MH at rates as high as 10C is not detrimental to the cells should one want to, for some strange reason, discharge the pack quickly. I don't know what the Lithium manufacturers rate there cells at but C/5 should be a good range for these also.

6) Depth of Discharge (voltage cut off)
Minimum discharge voltage per cell is the same for Ni-Cd and Ni-MH and that is the industry standard of 0.9 volts per cell, or if you want to get technical it is V= (N-1) X 1.2 volts, where N= number of cells. You can see from this where the 1.1 volt cut-off voltage came from as it covers you safely up to 12 cells while 1.05 volts takes us down to 8 cell packs. The 0.9 volts on the other hand covers our applications quite well and does give a truer representation of the capacity as measured by the manufacturer without the "heavy" math.

Lead Acid should not be discharged below 1.8 volts per cell (open circuit voltage) and should NEVER be allowed to set at this level or sulfation will take over and kill the battery for all practical purposes.

Minimum discharge rates and levels for Lithium systems are a bit more critical with the limits established by the manufacturers - although 3 volts/cell seems to be the set standard for now to allow for some safety factor. While deep discharge has minimal effect on Ni-Cd or Ni-MH it can be both cell destroying and dangerous with Lithium systems.

Note: In battery discussions the term C refers to the batteries rated capacity. So if we say to charge at 0.1C (which is the same as C/10) we mean to set the charge rate to 1/10th of the capacity. For a 700 mAh pack this would be 70 mA. Similarly if we were to charge this pack at 2C it would mean 1400 mA (or 1.4 amps).

So now we can lay down some rules of thumb.

Ni-Cd
Slow Charge or Conditioning rate 0.1C,
Maximum Fast Charge rate 4C (Reduced fast charge rates enhance battery service life if you don’t need it don’t use it, your battery and pocket book will be happier in the long run). Peak sensitivity 8 to 10 mv/cell. - Excessive heating indicates that sensitivity should be increased (number reduced).

Discharge (to establish capacity per mfg. rates) C/5 (Higher discharge rates may be used with10C is a safe limit – much higher rates are possible but few cyclers will accommodate them.)

Ni-MH
Slow Charge or Conditioning rate 0.1C, Fast Charge rate 2C (Reduced fast charge rates enhance battery service life – same as stated above for Ni-Cd), Peak sensitivity 3 to 5 mv/cell - excessive heating indicates that sensitivity should be increased (number reduced).

Discharge (to establish capacity per mfg. rates) C/5 (Higher discharge rates may be used but 10C is a safe limit)

Lead Acid
Charge - Voltage 2.45-2.50 volts/cell with current limit of C/10. When the charge voltage is attained the current must be reduced to C/50.

Discharge (to establish capacity per mfg. rates) C/20 (Higher discharge rates may be used but 1C is a safe limit)

Lithium Ion/Polymer*

Lithium Polymer
Charge - Voltage 4.2 volts/cell with current limit 0.7C - When the charge voltage is attained the current must be reduced to zero. Lithium systems will be damaged with any overcharge.

Discharge (to establish capacity per mfg. rates) C/5. Max discharge 2C

Lithium Ion
Charge - Voltage 4.2 volts/cell with current limit 0.7C - When the charge voltage is attained the current must be reduced to zero. Lithium systems will be damaged with any overcharge.

Discharge (to establish capacity per mfg. rates) C/5. Max discharge 2C

*Offerings of Lithium Ion or Lithium Polymer technology are constantly being introduced. It is best to consult the manufacturers recommendations when setting charge and discharge rates.

Sanyo, LG Chemical, and Kokam have charge rates exceeding the above, with 1C or even higher being accepted. Panasonic and Gold Peak are a little more conservative setting their max charge (at this time) in the 0.75C range. As this technology matures look for continued changes in the acceptable and recommended rates.

I hope this helps make your New Year Happier.