Charging Batteries
For the longest battery life and best reliability, choose a good charger. AstroFlight and Hitec both make high quality chargers that can charge up to 36 and 24 cells respectively using a 12 V DC power source such as an automobile battery.
These chargers are the peak detection type, necessary for the proper charging of Ni Cad batteries. Peak detection chargers are necessary because during the first 70% of the charge cycle a Ni Cad battery absorbs almost all of the energy. After this point the battery cells start to generate gases and the temperatures rise.
Excess heat, due to overcharging, is the greatest enemy to battery longevity.
A peak detection charger will sense when the battery is at maximum charge and then stop charging before excess temperatures are reached. For long life do not recharge a battery while it is still hot from a recent discharge. Most peak detection chargers available at hobby suppliers only detect peak voltage, not heat.
A test carried out by GTE Government Systems on Ni Cad batteries used in radios by the Navy showed that when these batteries were simply charged and used, 45% had to be replaced within one year. When the NiCads were exercised by bringing their voltage down to one volt per cell once a month,the annual replacement rate dropped to 15%.
The typical sports flying we do several times a month is usually sufficient exercise for our NiCads even if the voltage doesn't drop down to one volt per cell. Running down NiCads until the prop is barely turning after each flight is not a good idea. It can cause battery overheating, cell reversal, and armature warping of the motor.
When cells were reconditioned by using a slow, deep discharge, bringing each cells voltage down to 0.6 volts, with careful control to prevent cell reversal, the annual replacement rate dropped to just 5%. Reconditioning is typically done with cells that have not been exercised for several months.
Matched cells, especially when used in high current applications, are a good investment. Mismatched cells are sometimes found in new sport battery packs, as well as those that have aged.
The reason for unevenly matched cells is poor quality control from the manufacturer and inadequate matching when assembling the batteries. If not too far off, cells in a new pack can adapt to each other with some initial slow charging at about 1 amp current.
In an unmatched pack, weak cells hold less capacity and are discharged quicker than stronger cells. This imbalance can cause cell reversal in weak cells. Cell reversal is defined as when the stronger cells of the battery impose a voltage of reverse polarity across a weaker cell during deep discharge. In other words the plus side of the cell will now show a negative voltage and the negative side a plus voltage making the cell unusable. Also, the weak cells will reach a full charge first and go into heat generating overcharge while the stronger cells still accept a charge and remain cool. An easy way to detect a weak cell in a pack is to feel if one cell is warmer than the rest after charging. The weak cells are always at a disadvantage, making them weaker and contributing still further to their mismatched condition. There is a strong relationship between matched cells and the longevity of a battery, especially at high-load currents.
When weak cells are detected it is best to replace them with cells that have a closer voltage to the other cells in the pack. Remove the weak cells and re-wire the pack temporarily Charge the remaining cells to capacity. Then charge the replacement cells to full capacity before soldering them into the pack. That way you can assure that all cells will have about the same state of charge before putting the pack into use.
Anyone considering purchasing Nickel-Metal Hydride (NiMH) batteries should take into account that proper chargers for these type batteries are more complex than those for NiCads. Also NiMH cells tend to have more resistance than NiCad cells.
They are recommended for use in radios and with motors that do not have a high current draw such as speed 400 or smaller motors. The 3000-mAh Sanyo NiMH cells seem to work o.k. up to about a 20 Amp discharge rate, but only have an actual capacity of around 2700 - 2800 mAh. At higher discharge rates they really heat up and the current flow decreases, severely effecting motor performance and eventually battery life.
Manufacturers recommend that to achieve maximum capacity and service life, NiMH batteries be rapid charged rather than slow charged. Typically this should take around 2 hours for a fully discharged cell. Also, the amount of trickle charge needed to maintain full charge is critical. It MUST BE SET LOWER than for NiCds. A trickle charge that is acceptable for a NiCd will overcharge the NiMh battery and cause irreversible damage.
The typical NiCad charger is not designed to provide such a fully saturated charge to the NiMh battery. Full-charge detection usually occurs with these chargers immediately after a given voltage peak is reached. Because of the absence of a topping charge, NiMH cells will not reach their maximum capacity. Another area to watch for carefully with NiMH cells is overcharging, even when they feel cool to the touch.
If charging a battery for 14 to 16 hours is good, why not charge them for even longer periods of time at lower than the 10% rate? Battery manufacturers are strongly against doing this, warning that it can lead to serious battery failure. A NiCd battery loses about 10% of its capacity in the first 24 hours after being removed from the charger. Thereafter, the self-discharge rate is about 10% per month. Charging a battery at less than the 10% rate is like trying to fill your basin with the drain open.
If we charge a NiCad battery at a rate of 10% of its capacity, shouldn't the charge time be 10 and not 14 - 16 hours? During the first 70% of the charge cycle a NiCad battery absorbs almost all of the energy, thereafter the energy absorption goes way down. The extra 4 to 6 hours used to slow charge receiver batteries gives them the time to absorb the remaining energy for a fully saturated charge. This long charge time and low charge rate are particularly important with radio and receiver batteries, where the cells may not be as well matched as in a motor battery.
You can do a rough calculation of how long your receiver battery will fly your model in combat. Figure on a current rate of about 100 mA for each servo and about 25 mA for your receiver. If your wing uses a 225 mAh receiver battery you will have approximately 1 hour of flying time (100 + 100 + 25 = 225). These are numbers for full size servos in heavy-duty use.
Less strenuous flying will draw less current resulting in longer flying times.
If slow charging at a rate of 10% of their capacity is good for transmitter and receiver batteries, why not slow charge motor batteries? Motor batteries usually have higher capacities and higher charge rates than transmitter and receiver batteries.
In fact manufacturers agree that generally it is better to fast charge these cells.
This is because slow charging of these batteries can cause a build up of large crystals in the cells causing lower life and performance. This is also why these batteries can not be left on trickle charge indefinitely. Many battery manufacturers include instructions in new battery packs. You can your cells by using the minimum charging voltage recomme
These chargers are the peak detection type, necessary for the proper charging of Ni Cad batteries. Peak detection chargers are necessary because during the first 70% of the charge cycle a Ni Cad battery absorbs almost all of the energy. After this point the battery cells start to generate gases and the temperatures rise.
Excess heat, due to overcharging, is the greatest enemy to battery longevity.
A peak detection charger will sense when the battery is at maximum charge and then stop charging before excess temperatures are reached. For long life do not recharge a battery while it is still hot from a recent discharge. Most peak detection chargers available at hobby suppliers only detect peak voltage, not heat.
A test carried out by GTE Government Systems on Ni Cad batteries used in radios by the Navy showed that when these batteries were simply charged and used, 45% had to be replaced within one year. When the NiCads were exercised by bringing their voltage down to one volt per cell once a month,the annual replacement rate dropped to 15%.
The typical sports flying we do several times a month is usually sufficient exercise for our NiCads even if the voltage doesn't drop down to one volt per cell. Running down NiCads until the prop is barely turning after each flight is not a good idea. It can cause battery overheating, cell reversal, and armature warping of the motor.
When cells were reconditioned by using a slow, deep discharge, bringing each cells voltage down to 0.6 volts, with careful control to prevent cell reversal, the annual replacement rate dropped to just 5%. Reconditioning is typically done with cells that have not been exercised for several months.
Matched cells, especially when used in high current applications, are a good investment. Mismatched cells are sometimes found in new sport battery packs, as well as those that have aged.
The reason for unevenly matched cells is poor quality control from the manufacturer and inadequate matching when assembling the batteries. If not too far off, cells in a new pack can adapt to each other with some initial slow charging at about 1 amp current.
In an unmatched pack, weak cells hold less capacity and are discharged quicker than stronger cells. This imbalance can cause cell reversal in weak cells. Cell reversal is defined as when the stronger cells of the battery impose a voltage of reverse polarity across a weaker cell during deep discharge. In other words the plus side of the cell will now show a negative voltage and the negative side a plus voltage making the cell unusable. Also, the weak cells will reach a full charge first and go into heat generating overcharge while the stronger cells still accept a charge and remain cool. An easy way to detect a weak cell in a pack is to feel if one cell is warmer than the rest after charging. The weak cells are always at a disadvantage, making them weaker and contributing still further to their mismatched condition. There is a strong relationship between matched cells and the longevity of a battery, especially at high-load currents.
When weak cells are detected it is best to replace them with cells that have a closer voltage to the other cells in the pack. Remove the weak cells and re-wire the pack temporarily Charge the remaining cells to capacity. Then charge the replacement cells to full capacity before soldering them into the pack. That way you can assure that all cells will have about the same state of charge before putting the pack into use.
Anyone considering purchasing Nickel-Metal Hydride (NiMH) batteries should take into account that proper chargers for these type batteries are more complex than those for NiCads. Also NiMH cells tend to have more resistance than NiCad cells.
They are recommended for use in radios and with motors that do not have a high current draw such as speed 400 or smaller motors. The 3000-mAh Sanyo NiMH cells seem to work o.k. up to about a 20 Amp discharge rate, but only have an actual capacity of around 2700 - 2800 mAh. At higher discharge rates they really heat up and the current flow decreases, severely effecting motor performance and eventually battery life.
Manufacturers recommend that to achieve maximum capacity and service life, NiMH batteries be rapid charged rather than slow charged. Typically this should take around 2 hours for a fully discharged cell. Also, the amount of trickle charge needed to maintain full charge is critical. It MUST BE SET LOWER than for NiCds. A trickle charge that is acceptable for a NiCd will overcharge the NiMh battery and cause irreversible damage.
The typical NiCad charger is not designed to provide such a fully saturated charge to the NiMh battery. Full-charge detection usually occurs with these chargers immediately after a given voltage peak is reached. Because of the absence of a topping charge, NiMH cells will not reach their maximum capacity. Another area to watch for carefully with NiMH cells is overcharging, even when they feel cool to the touch.
If charging a battery for 14 to 16 hours is good, why not charge them for even longer periods of time at lower than the 10% rate? Battery manufacturers are strongly against doing this, warning that it can lead to serious battery failure. A NiCd battery loses about 10% of its capacity in the first 24 hours after being removed from the charger. Thereafter, the self-discharge rate is about 10% per month. Charging a battery at less than the 10% rate is like trying to fill your basin with the drain open.
If we charge a NiCad battery at a rate of 10% of its capacity, shouldn't the charge time be 10 and not 14 - 16 hours? During the first 70% of the charge cycle a NiCad battery absorbs almost all of the energy, thereafter the energy absorption goes way down. The extra 4 to 6 hours used to slow charge receiver batteries gives them the time to absorb the remaining energy for a fully saturated charge. This long charge time and low charge rate are particularly important with radio and receiver batteries, where the cells may not be as well matched as in a motor battery.
You can do a rough calculation of how long your receiver battery will fly your model in combat. Figure on a current rate of about 100 mA for each servo and about 25 mA for your receiver. If your wing uses a 225 mAh receiver battery you will have approximately 1 hour of flying time (100 + 100 + 25 = 225). These are numbers for full size servos in heavy-duty use.
Less strenuous flying will draw less current resulting in longer flying times.
If slow charging at a rate of 10% of their capacity is good for transmitter and receiver batteries, why not slow charge motor batteries? Motor batteries usually have higher capacities and higher charge rates than transmitter and receiver batteries.
In fact manufacturers agree that generally it is better to fast charge these cells.
This is because slow charging of these batteries can cause a build up of large crystals in the cells causing lower life and performance. This is also why these batteries can not be left on trickle charge indefinitely. Many battery manufacturers include instructions in new battery packs. You can your cells by using the minimum charging voltage recomme