Batteries

Batteries are available in different sizes, weights and capacities C, which refer to their stored energy expressed either in amps-hour Ah or milliamps-hour mAh. For example, a battery with a capacity of 500mAh should deliver 500mA during one hour before it gets totaly discharged (flat).

The radio control sytems are usually powered by rechargeable batteries. There are two main rechargeable battery types available on the market today: NiCads (nickel - cadmium) and NiMH (nickel - metal hydride). Lead-Acid batteries are also used as ground power source.

Normally the NiCads stand more "abuse" which means that they may be charged at higher rate (normally 2 - 4C) and have the ability to deliver higher current, i.e. discharge rates up to 2C continuous or 8 to 10C during 4 - 5 minutes and even up to 100C during very short time. They have some designations such as the Sanyo AE for high capacity and AR or SCR for quick charge/discharge.

A NiCad cell consists basically in a positive plate foil of nickel metal with nickel oxide/hydroxide, a negative plate foil of cadmium metal with cadmium hydroxide and an isolating porous separator film moistened with an electrolyte of potassium hydroxide (caustic potash). The two plates are sandwiched between the isolating porous separator films, rolled up and enclosed in a nickel-plated steel can. A spring-loaded vent is fitted at the positive terminal end in order to release the electrolyte and/or gasses, in case overpressure occurs due to overcharge. See picture below.

The NiMH have higher capacity/weight compared with the NiCads but are more sensitive to high charge rates (max recommended 1C) and normally it is not recommended to discharge the NiMH batteries at higher rates than 3 - 5C.

The NiMH self-discharge rate is also about 50% higher than the NiCads. However, the NiMH are more environment-friendly.

A new type of NiMH battery known as HeCell has recently been developed, which is claimed to allow higher discharge rates than the conventional ones (about 12 - 16C).

Both battery types lose their stored charge due to internal chemical action, even when not in use. Normally the NiCads lose around 10% of its charge in the first 24 hours after been charged and keep losing it by 10% per month. The rate of self-discharge doubles for a rise in temperature of 10 degrees C. Some NiCads can discharge themselves completely in a period of six months.

The best way to keep batteries which are not in use for a long time, is by having them stored in the refrigerator (not in the freezer). Just allow the battery to reach the ambient temperature before using/recharging.

Some manufacturers claim that these battery types are able to stand at least 1000 charges/discharges during their lifetime, assuming they have been subject to the ideal charging and handling methods. In practice however, we may expect about 600 - 800 charges/discharges.

A safe method to charge both the NiCads and the NiMHs is by using a constant charge current (CC) at 1/10 of their capacity (0.1C) during 14 hours. For other charge current values one may use the following formula:
Charge Time (Hours) = 1.4 x Battery Capacity / Charge Current (assuming a constant charge current is used).

However, low cost CC chargers provide no way of detecting when the battery is fully charged. The user is then expected to estimate the charging time based on the constant charging current value and the battery capacity, according to the formula above. And providing the NiCads' are discharged to about 1.1V p/cell each time before recharging, this charging method can be used to achieve a reasonably long battery life. Since repeatedly recharging an already fully charged NiCad or one with a large part of its charge remaining will degrade its performance.

Some chargers provide the option to discharge the batteries down to about 1.1V per cell before starting the charging process. There are also fast battery chargers on the market charging from 1C up to 4C. But due to the high charging current level, it is required a reliable method of stopping the charge once the battery is fully charged, otherwise overheating and battery damage may occur.

Since the NiMHs' and NiCads' voltage actually starts dropping after they have reached the fully charged state, the fast chargers use the so-called Delta Peak detecting method. There are "negative delta V (-DV)" and "zero delta V (0D)" detectors. Also "change of temperature (dT/dt)" detectors are commonly used. Some manufacturers use negative or zero delta V together with change of temp. detection, in case of one method fails to detect. Since NiMHs' voltage drop (delta V) after the fully charged state is lower than the NiCads, a more sensitive delta V charger is required for the NiMH batteries. Some chargers allow the user to set the value of the delta peak detection, which may be between 10 - 20mV per cell for NiCads and 5 - 10mV for NiMHs. A too low value may cause false peak detection due to electric noise, preventing the batteries from getting fully charged, whereas a too large value may result in overcharge, which reduces the batteries' life.

Some fast chargers offer the possibility to automatically change over to slow charge (trickle-charge, for ex. at 0.05C) when the fully charge status is detected.

The graph on the right shows the voltage and temperature variation of a four cell NiCad during charging at 1C constant charge current.

Notice how the voltage drops after it has reached a top value, whereas the temperature keeps rising.

The battery is considered fully charged when the temp. rises about 10°C above the ambient temp. ( 24 + 10 = 34°C )

The NiMH batteries tend to dissipate heat during all the charging process, while the NiCads get warm only when they reach the full charge point. The nominal voltage is 1.2V per cell for both battery types and a charged cell may have about 1.45 - 1.50V.

It's not possible to know exactly the NiCad's or NiMH's cell charge status by only measuring it's terminal voltage, as the cell's charge status is not a linear function of the cell's voltage. A reliable method to know how much charge is left or whether a cell still has its nominal capacity, is by discharging it with a known constant current and measure the time until the cell voltage reaches about 1.1V. For example, it should take about two hours to discharge a fully charged 500mAh cell by using a constant discharging current of 250mAh.

Battery researchers have in the recent years come to conclusion that NiCads respond better to a pulsed charging waveform than to a steady DC current. By applying the charge current in one-second pulses with brief "rest" periods between them, ions are able to diffuse over the plate area and the cells are better able to absorb the charge.

This is particularly true at the higher charge rates used by fast chargers. These chargers have a microprocessor that samples the "rest" periods between the charging pulses to read the battery terminal voltage. Another interesting discovery is that the charging process actually improves even further if during the "rest period" between charging pulses, the cells are subject to very brief discharging pulses with an amplitude of about 2.5 times the charging current, but lasting only about 5mS.

It is claimed that these short discharge pulses actually dislodge oxygen bubbles from the plates and help them diffuse during the "rest period". The use of these brief discharge pulses is known as "burp charging". Tests done by both US military and NASA have shown that NiCads charged by using fast chargers employing the burped pulse system tend to last up to Twice as long as those charged by traditional CC chargers. Many of the high-end fast pulse chargers for NiCads use a charging method according to those findings.

A battery pack consists of several cells connected in series, which inevitably age at different rates and gradually develop individual different charge status, and since the battery pack as a whole is charged and discharged repeatedly, these differences may become accentuated. The result is that some weaker cells can eventually be discharged well below 1.0 V and even driven into reverse polarity before the others reach the fully discharged state. During the recharging process, the weaker cells will be improperly recharged and tend to suffer increased crystal growth, while the others will absorb most of the charge and overheat, which dramatically degrades the whole battery pack performance. It's therefore advisable checking if the battery cells get different temperatures during the charging process, specially when high charge current rates are used.

It's claimed that individual cell differences may level out by slow charging the battery pack from time to time at 0.1C during 14h or so.

Some few examples of many battery chargers available on the market:

GWS-MC-2002 Charger
Input Voltage range: 9-15V DC
4-12 cells of 50mAh - 3000mAh
NiCad or NiMH pack can be charged.

TRITON Charger, Discharger
Handles 1-24 NiCd or NiMH cells,
1-4 Li-Ion cells or 6,12,
and 24V Lead Acid batteries.

For those who like to tinker with electronics and can't afford an expensive and sophisticated charger, there's a cheap alternative based on the National Semiconductor® LM317 low cost regulator. The circuit diagram below shows a constant current charger using the LM317.

The constant current may be set anywhere between 10mA and 1.5A by choosing the appropriate resistor R.
R = 1.25 / I
Where R is the resistor value in ohms, 1.25 is a reference drop voltage in Volts and I is the constant current in Amps.

For example, to charge a 500mAH battery at 0.1C, (50mA) the R value will be:
1.25 / 0.05 = 25ohm.
The dissipated power on the resistor R in this example is:
P = V x I = 1.25 x 0.05 = 0.0625W or 62.5mW.

The dissipated power on the LM317 IC is:
(Vin - Vout) x Charging Current.
It's advisable to use a heatsink to prevent the IC from getting too hot. Notice that the IC's metal package or tab also carries the Vout, so it's necessary to use isolating washers in case you attach the heatsink to a metal case.

NiCads and NiMHs may be on charge during relatively long time without the risk of overcharging damage when using a constant current equal or less than 0.1C. However, it is not advisable to have the batteries continuously on charge longer than 24h, so one may connect the charger to a timer in order to cut the charging after about 14 -18h.

For those who prefer a more sophisticated D.I.Y. NiCad charger based on delta peak method, as well as other interesting circuits, check here or here

New rechargeable battery types, which are often used with slow-flyer and indoor models are the Li-ion (liquid electrolyte) and the flat Lithium-polymer (gel electrolyte). The cell shown on right (Kokam®) has 3.7V as nominal voltage, 4.2V max and 3.0V minimum. Other brands may have different nominal voltage. For example PowerfLite® has 3.6V and Duralite® includes a built in charge safety circuitry. These battery types have much higher energy density than both the NiCads and the NiMHs.

The max charge rate recommended is 1C while the discharge rate should not be higher than 3 - 4C continuous or 5 - 6C during short time. The self-discharge rate is claimed to be very low, typically 5% per year. These batteries cannot be charged with the same chargers that are designed for NiCads or NiMH.

In order to correctly charge the Li-ion/Lithium-polymer batteries, it must be taken into account the number of cells in the actual battery pack, since both the max charging current and voltage have to be set according to the cells' specifications. Charging these batteries with a wrong charger may cause them to explode. Also a short circuited pack may easily catch fire. According to Kokam®, the Lithium-polymer batteries should not be discharged below 2.5V per cell, otherwise a rapid deterioration will occur.

The basic charging procedure is by limiting the current (from 0.2 C to max 1C depending on manufacturer) until the battery reaches 4.2 V/cell and keeping this voltage until the charge current has dropped to 10% of the initial charge rate. Since the batteries only have 40 to 70% of full capacity when 4.2V/cell is reached, it's necessary to continue charging them until the current drops as described above. A charge timer should be used to terminate the charge in case the top voltage and/or termination current never reach their values within a certain time, which depends on the initial charging current, (e.g. 2 hours at 1C or 10 hours at 0.2C). Trickle charging is not good for Lithium batteries, as the chemistry cannot accept an overcharge without causing damage to the cells.

The circuit diagram below shows a simple Li-ion/Lithium-polymer charger based on National Semiconductor® LM317 low cost regulator.

Before connecting the cells to the charger the max charging voltage has to be set by adjusting P1 (2k potentiometer). The max charging voltage must not exceed 4.2V per cell (Kokam), e.g. 8.4V for two serial connected cells. It is recommended using a digital voltmeter. The max charging current is set by choosing the value of Rx.
Rx = 0.6 / max charging current

For example, for a max charging current of 600mA, Rx should be 0.6 / 0.6 = 1ohm, while for a max charging current of 1.2A it should be 0.6 / 1.2 = 0.5ohm. The dissipated power on Rx at a charging current of 1.2A is:
P = V x I = 0.6 x 1.2 = 0.72W

The dissipated power on the LM317 IC is:
(Vin - Vout) x Charging Current.
It's advisable to use a heatsink to prevent the IC from getting too hot. Notice that the IC's metal package or tab also carries the Vout, so it's necessary to use isolating washers in case you attach the heatsink to a metal case.

The LM317's max output current is 1.5A. For higher charging currents one may use the LM350 rated at 3A or the LM1084 rated at 5A.

Note: if a Li-ion battery gets discharged below 2.9V/cell, it needs to be slow charged at 0.1C until 3.0V/cell is reached before a higher charging current rate may be used. Also discharging below 2.3V/cell will damage the battery.

According to the manufacturers the Li-ion batteries should be stored charged to about 30 - 50% of capacity at room temperature. For prolonged storage periods, store discharged (i.e. 2.5 to 3.0V/cell) at -20° to 25° C.

Important!
Make sure to set your charger to the correct voltage according to the number of cells. Failure to do this may result in battery fire!

Before you charge a new Lithium pack, check the voltage of each cell individually. This is absolutely critical as an unbalanced pack may explode while charging even if the correct cell count was chosen. If the voltage difference between cells is greater than 0.1V, charge each cell individually to 4.2V so that they are all equal. If after discharge, the pack still is unbalanced you have a faulty cell that must be replaced.

Do not charge at more than 1C.
NEVER charge the batteries unattended.

Caution:
If you crash with Lithium cells there is a risk that they get a latent internal short-circuit. The cells may still look just fine but, if you crash in any way remove the battery pack carefully from the model and place it on a non-flammable place, as these cells may catch fire later on. (A box with sand is a cheap fire extinguisher).

The lead - acid batteries have much lower energy/weight ratio than all those previously mentioned. Which means that the lead - acid batteries are heavier for the same capacity.

They are not suitable to be used airborne, but since they are rather cheap, they are often used on the flying fields as ground power supply for engine starters and/or to charge the smaller ones.

There are various versions of lead acid batteries:
The Gel-Cell, the Absorbed Glass Mat (AGM) and the Wet Cell.
The Gel-Cell and the AGM batteries cost about twice as much as the Wet Cell. However, they store very well and do not tend to sulfate or degrade as easily as the Wet Cell.

Lead - acid batteries get "sulfated" when the soft lead sulfate normally formed on the positive and negative plates' surfaces re-crystallises into hard lead sulfate when the batteries are left uncharged during long time. This reduces the battery's capacity and ability to be recharged.

Both the Gel-Cell and AGM are the safest lead acid batteries one can use. However, Gel-Cell and some AGM batteries may require a special charging rate.

There are sealed (maintenance free) and serviceable non-sealed Wet Cell batteries. Non-sealed batteries are recommended in hot climates since distilled water can be added through the filler caps when the electrolyte evaporates due to the high environment temperature.

The lead acid batteries have a self -discharge rate of about 1% to 25% a month. They will discharge faster at higher temperature. For example, a battery stored at 35°C (95°F) will self-discharge twice as fast than one stored at 24°C (75°F).

Lead acid batteries left uncharged during long time will become fully discharged and sulfated. The best way to prevent sulfation is by periodically recharging the battery when it drops below 80% of its charge. It is possible to determine a non-sealed battery's charge status by measuring the concentration of the sulfuric acid of the battery electrolyte ("battery acid") with a hydrometer.

The lead- acid batteries have normally 3 or 6 cells connected in series. Each cell has a nominal voltage of 2V resulting in a nominal pack voltage of 6V and 12V respectively. They are usually charged with a constant voltage of 2.4 - 2.5V per cell having the charging current limited to 1/10C. It is not recommended charging these batteries with a charging current exceeding 1/3C. A lead -acid battery pack is considered fully charged when the charging current falls below 10mA and/or the cell voltage reaches 2.4 - 2.5V.

Should a lead - acid battery be continuously left on charge (when used as power backup); the charging voltage should not exceed 2.25 - 2.30V per cell. It is also advisable to charge these batteries in a well-ventilated area/room, since it produces hydrogen-oxygen gases that can be explosive and also the electrolyte contains sulfuric acid that can cause severe burns. Lead - acid batteries' life span is about 4 - 6 years depending on the treatment.