Battery Charging

These set-up instructions give basic advice to beginners, individual set-up preferences may improve the cars performance further

Batteries. 

Constant Current Chargers

The two types of batteries used in RC cars are Nickel Metal Hydride (Ni-MH) and Lithium Polymer (LiPo). Ni-MH are individual Sub C cells of 1.2V made up into a pack of six cells (7.2V). LiPo packs must be of the hard case  type for cars. Not the soft case used in aircraft. The packs are made up of 3.7V
cells, these are available as single cells
1s or 2s packs, 2s 7.4V is the common type used in touring cars.

Before I describe the different types of charger, it is necessary to understand what is known as the "C" rate. "C" is the battery capacity divided by hours, so for example, "C" for a 3000 mAh battery would be 3000 ma, (or 3 A since 1000mA = 1A) For maximum life, manufacturers have determined that Ni-MH batteries should be charged at 0.1C rate. In the case of the 3000mah battery this would be 300ma.

Trickle Chargers

Chargers that work near the 0.1C rate are called trickle chargers, and are generally small and inexpensive. Unfortunately it takes 12 hours to charge a battery at this rate, and it is impossible to tell when your battery is fully charged.
Although charging at this rate does not produce excessive heat, batteries charged in this way do not perform as well as those charged by other means.
Trickle chargers are not a practical proposition for model car racers.
Delta Peak Voltage Chargers

Ni-MH Battery / cell manufacturers generally recommend "Fast Charging" at a rate no greater than 1C, (or 1 x the capacity of the battery 3000 mA = 3Amps).

For the Delta Peak Voltage setting, its probably easier if I give you a basic explanation of what it is. When a Ni-MH battery reaches full charge, the voltage hits a maximum (PEAK) value, and then "changes" (DELTA meaning change) and the voltage starts to drop or reduce as the battery starts to go into overcharge. This setting refers to the point at which the Batteries Voltage reaches its PEAK, and then Changes (DELTA) and starts to drop.

What a "Delta Peak" charger does is monitor the batteries voltage, and when it senses the Change in Peak voltage, the charger turns off with the battery being fully charged.. With your charger, you can adjust the point at which the charger registers the change, so as to remove the possibility of having a pack false peak (not fully charge), ensuring that the pack is fully charged.. The greater value you set the Delta Peak setting at, the more your charger will overcharge your battery.

 Ni-Mh packs don’t like to be overcharged at all, so I generally use a setting of between 3 - 6mV per pack.

Never use a Ni-MH charger to charge a LiPo battery

Charging  LiPo Batteries correctly with a charger specifically designed for lithium chemistry batteries is critical to both the life span of the battery pack, and your safety. Always charge a LiPo battery in a LiPo safety bag, It is a BRCA and club rule.

Maximum Charge Voltage and Current

A 3.7 volt LiPo battery cell is 100% charged when it reaches 4.2 volts. Charging it past that will ruin the battery cell and possibly cause it to catch fire.

It is critical that you use a charger specified for LiPo batteries and select the correct voltage or cell count when charging your  LiPo batteries if you are using a computerized charger. If you have a 2 cell (2S) pack you must select 7.4 volts or 2 cells on your charger. If you selected 11.1V (a 3S pack) by mistake and tried to charge your 2S pack, the pack will be destroyed and most likely catch fire. Luckily, all the better computerized chargers out there these days will warn you if you selected the wrong cell count.

All LiPo battery chargers will use the constant current / constant voltage charging method (cc/cv). This means is that a constant current is applied to the battery during the first part of the charge cycle. As the battery voltage closes in on the 100% charge voltage, the charger will automatically start reducing the charge current and then apply a constant voltage for the remaining phase of the charge cycle. The charger will stop charging when the 100% charge voltage of the battery pack equalizes with chargers constant voltage setting (4.2 volts per cell) at this time, the charge cycle is completed. Going past that, even to 4.21 volts will shorten battery life.

RC LiPo Battery Charging Current

Selecting the correct charge current is also critical when charging RC LiPo battery packs. The golden rule here use to be "never charge a LiPo or LiIon pack greater than 1 times its capacity (1C)."

Most LiPo experts now feel you can safely charge at a 2C rate on quality packs that have a discharge rating of at least 20C or more, and low internal resistances, with little effect on the overall life expectancy of the pack, as long as you have a good charger with a good balancing system.

Once again, the four main things that shorten LiPo battery life are: HEAT, OVER-DISCHARGING (voltage & current), OVER CHARGING (voltage & current) & INADEQUATE BALANCING

 

Ni-MH Battery Maintenance.
The battery  is one of the key elements in a cars performance, and is not cheap, so it pays to look after it.

Each Ni-MH cell in the pack should be covered in insulation, that usually carries the manufacturers logo. Check that it is not damaged. If it is it should be repaired. This is particularly important in modern cars made from carbon fibre, which could cause a short circuit across a cell or cells, causing damage to the battery pack or car.

The individual cells in a pack are soldered or spot welded together. These joints should be in good condition. A bad joint reduces the amount of power available to the motor, and will also affect charging.

The connectors on the end of the battery need to be well secured to the battery pack. For the novice, one of the first mistakes made is that of connecting the battery the wrong way round and damaging the speed controller. Polarity sensitive connectors will help prevent this.

Overcharging will quickly and permanently ruin a battery pack by boiling off the electrolytes within the cells. Once this has happened a battery pack will never perform as well.

Memory is built up gradually within a Ni-MH battery pack through its charge and discharge cycles, and a defective memory will cause poor performance and a lack of duration. Every time you use your battery, you need to use 100% of its capacity, if you don't, the battery remembers this and will not perform as well next time.

Copyright Central Radio Car Club 2003© 02/03/2009