BATTERY INFORMATION

THE CHARGING OF NiCad AND NiMh BATTERIES by Jerry Houser

• BATTERY CAPACITY
• CHARGE RATES

A link to "Fuel Gauge” for lithium battery packs by Rex Geivett

THE CHARGING OF NiCad AND NiMh BATTERIES

by Jerry Houser

This discussion is intended to present some practical information with regard to the charging of NiCad (Nickel Cadmium) and NiMh (Nickel Metal Hydride) batteries for use in electric powered aircraft. While some of the principals are similar for Li-Poly batteries, many of the principles are not; therefore, this is not intended to apply to Li-Poly or Li-Ion batteries. Li-Poly batteries require specific considerations that will NOT be addressed in this discussion.

The term “cell” applies to an individual cell and not a group of cells. Connecting two or more cells in series creates a battery”.

One milliamp (ma) is 1 /1000^th of an amp of current.  In other words 1000 ma = 1 amp. 

One millivolt (mv) is 1/1000^th of a volt.  In other words 1000 mv = 1 volt.

BATTERY CAPACITY.  The manufacturer specifies a battery capacity for a particular cell.  This capacity is usually stated in “mahr” (milliamp hours).  For example, a 2000 mahr cell (or battery) should be able to deliver 200 ma (milliamps) of current for approximately 10 hours (200 ma X

10 hours = 2000 mahr).   It would also follow that this cell could deliver 100 ma for 20 hours (100 ma X 20 hours = 2000 mahr).  However, our electric aircraft tend to draw a lot more current.  If an aircraft were to draw 20 amps (20,000ma), the battery would only be good for about 1/10^th (6 minutes) of an hour of flight time (20,000 ma X 0.1 hours = 2000 mahr).  While this is nice in theory, batteries are usually rated at the 10 hour (200 ma) or 20 hour (100 ma) current draw rates.  When one draws more than this amount of current out of a battery, the capacity drops.  In other words, the 2000 mahr battery will not actually deliver that much capacity when we are drawing 20 amps from it.  At a current draw of 20 amps, the 2000 mahr battery will probably only last about 5 ½ minutes instead of 6 minutes.   Its capacity will be more like 1800 mahr instead of the rated 2000 mahr.

CHARGE RATES.  There are 3 common terms used when charging NiCad and NiMh batteries.  They are slow, fast, and quick. 

-A slow charge is commonly made at 1/10^th C (C is the capacity of the battery).

In this example we are using a 2000 mahr battery so we would want to charge it at 200 ma for a slow charge.  Charging is not 100% efficient, so we would need to charge our 2000 mahr battery for about 14 hours (40% longer than expected) to bring a fully discharged battery back up to a full charge.  It is recommended that the first charge for a new battery be done at this rate.  It is also recommended that this be repeated after a battery has received 25-50 quick charges – this slow charge process will tend to “rebalance” the cells.  

-A fast charge is commonly made at about 1/3^rd C.  In this example we are using a 2000 mahr battery so we would want to charge it at 667 ma for about 4 hours.

-A quick charge is commonly made at somewhere between 1C and 3C.  To charge our 2000 mahr battery at 2C, we would use a charge current of 4000 ma for a little over ½ hour.

As the rate of charge increases from 1/10^th C to 1C, the charging efficiency gets better.  When charging at 1C, a fully discharged battery will be charged in slightly over 1 hour.

Batteries are designed for a particular maximum charge rate.  While all batteries may be charged at the slow rate, some batteries are not intended to be charged at the fast or quick rates.  Attempting to charge “slow rate” batteries at a faster rate could result in very (dangerously) hot batteries and damage to the cells in the battery. 

Some manufacturers state that NiCad batteries may be charged at rates of up to 3C while NiMh batteries should not be charged at a rate of over 2C.  At the field, there is a tendency to charge at the maximum possible rate so that another flight can be made without having to wait so long for the battery to be recharged.  However, the faster one charges a battery, the shorter will be the life of the battery in terms of the number of cycles before the battery won’t hold much of a charge.  A cycle is one charge and one discharge.  For this reason, many pilots charge NiCad batteries at no more than 2C and NiMh batteries at no more than 1 1/3C (4/3C).   A good solution would be to have a second battery pack so that one will not be tempted to charge at quite so high of a rate/current at the field. 

It is common for a battery that is being quick charged to get slightly warm to the feel - especially near the end of the charge as the battery is reaching its fully charged state.  However, a battery that is very warm early on during the charge or one which gets very hot near the end of the charge indicates trouble.  Either the charge rate is too great for that battery or some of the cells in the battery are going bad.  Very warm temperatures during charge should be avoided.

As a NiCad or NiMh battery reaches its fully charged state, its voltage will peak (reach a maximum) after which the voltage will fall slightly.  This is the characteristic that a “peak” detection charger looks for to signal that the battery is fully charged and that the charger should shut off (or move into trickle mode).  As soon as the voltage starts to decrease slightly from its peak (maximum voltage) the charger knows that the battery is fully charged.  Many chargers allow the user to set the value for the voltage decrease after the peak in order to signal the charger to quit charging.   The chargers usually ask for this value in mv (millivolts, 1/1000^th of a volt) per cell.  Most users enter 5 mv/cell for NiCad batteries and 3 mv/cell for NiMh batteries.  NiCad batteries tend to have a slightly greater voltage decrease as they reach their fully charged state than do NiMh batteries.  Setting this value too high results in the charger not shutting off quite as soon after the cell reaches it fully charged state – the extra energy being put into the cell turns into heat which is not good for the battery and will likely reduce the number of cycles available from the battery before it becomes unusable.  Setting this value too low may cause the charger to think that the battery is fully charged when it isn’t and the charger will “false peak” and shut off too soon.  One may need to experiment with this to get the setting correct for a specific battery pack. 

It is an advantage to have a charger that will not only allow one to program various parameters into “battery memories” but also one that has a digital display so that one can monitor the progress of the charge.  Two items are of particular interest during the charging of a battery – voltage and the amount of charge put back into the battery. 

As a battery becomes fully charged, it will read its maximum voltage which is followed by a very slight decrease in voltage that signals the charger to shut off (or go into trickle mode).  NiCad cells peak at about 1.55 volts per cell (a 10 cell battery would peak at about 15.50 volts) while NiMh cells peak at about 1.60 volts per cell (a 10 cell battery would peak at about 16.00 volts).  Both of these voltages assume that one is charging at 1C.  If the charge rate is lower, the peak voltage will be slightly lower.  Monitoring the voltage not only gives one an indication of  approximately where the battery is in terms of reaching its fully charged state, but also gives an indication of a problem with one or more cells in the battery pack.  An unusually high voltage reading may mean that there is a defective cell(s) in the battery pack while a low reading may mean that there is a shorted cell(s) in the battery pack.   

The amount of charge put back into the battery indicates (approximately) how much energy was used on the previous flight.  Remember that charging is not 100% efficient, so if one puts 1500 mahr (as indicated on the chargers readout) back into a battery, it means that probably 1300 mahr to 1400 mahr were actually used during the flight and put back into the battery.  Charging is about 90% efficient when charging at 1C and only about 70-75% efficient when charging at 1/10^th C.  Also remember that a 2000 mahr battery is only going to provide about 1800 mahr of capacity when drawing high currents like we do with our aircraft.

As an example, one makes a flight with a 2000 mahr battery.  After landing, this battery is charged at 1C (2000 ma).  When the charger shuts off, its display indicates that 1400 mahr were put back into the battery.  What this really means is that the 2000 mahr battery should be considered only a 1800 mahr battery at the high currents that we are drawing and that about 1300 mahr of energy was used during the flight and replaced during the charge cycle (remember charging efficiency).   Considering this and monitoring the amount of energy put back into the battery during charge, one can determine approximately how much energy was left in the battery upon landing and one can also make sure that the charger didn’t “false peak” and go out of the charge mode before it should have.  Let’s assume that a “normal” flight requires that we put about 1400 mahr of energy back into the battery during charge.  We have just landed after making a “normal” flight and put the battery on the charger.  About 20 minutes later, the charger indicates that the battery is fully charged although we notice that the display on the charger indicates that the battery accepted only 850 mahr of energy (550 mahr less than usual after a normal flight).  Something is wrong – the most likely thing would be that the charger “false peaked” and terminated the charge cycle too soon).  If this happens, feel the battery for warmth.  If the battery is quite warm, then it may be fully charged – but if it is not warm, press the charge button again to re-start another charge cycle.  The two charge cycles together should equal about 1400 mahr of energy being put back into the battery.   If this “false peaking” occurs often, the voltage decrease after reaching peak may need to be re-programmed and  increased from 5mv to 6-7mv for a NiCad battery or from 3mv to 4-5mv for a NiMh battery.  Repeated false peaking may also be caused by a cell in a battery going bad or from a badly imbalanced battery pack – try slow charging it to restore the balance. 

Some pilots program multiple charge routines into a charger for one particular battery. 

Routine 1:  Set to charge the NiMh battery at 1.5 C.  This would be used to recharge the battery at the field after a flight and prior to another flight.  This rate gives a fast field charge to the battery.

Routine 2:  Set to charge the NiMh battery at 1C with a programmed cutoff at 1200 mahr.  This could be used after the last flight of the day.  Note that the charge rate is a bit slower so as to prolong the life of the battery and that it is set to cut off at 1200 mahr (200 mahr before where the charger normally cuts off when charging the battery after a normal flight). This means that the battery is not quite fully charged.  This battery is now ready for a very brief peak charge just prior to flying on the next trip to the field.  Using this method, the next day one needs only a few minutes to peak charge the battery (using routine 1) for the first flight of the day.  Charging the battery just before use warms the battery up a little which causes it to put out a little higher voltage at the beginning of the flight.  During the flight, the battery will remain warm due to the high rate of discharge.  It is normal for a battery to be warm (but not hot) after a flight.   Fully charging the battery after the last flight of the day and then attempting to charge it again the next day just before a flight seems to shorten the life of the battery.

Routine 3:  Set to charge the NiMh at 1/10^th C.  This is the slow charge that should be performed on a brand new battery as well as after 25-50 fast charges (to rebalance the battery pack).

Please remember that there are many variables in batteries including the manner in which they were manufactured.  Therefore, slight differences may be noted between brands of batteries.