Lithium-Polymer Battery Guide

Care of Batteries | Discharging | Charging | Safety

Care of Batteries

Initial Use

Lipo batteries are supplied partly charged. It is advised that they are not charged before first use. Simply fly a model for about several minutes. As soon as power begins to fade, land the model and charge the battery. Check battery and wait until temperature is back to ambient before flying them again. It is not advised to carry extended ground runs of any power system. Check the current draw briefly if required and then go fly the model.

Next Use

Short Circuit : connecting or touching discharge leads together.

Short circuit may cause a fire! Short-circuiting, even momentarily, of the battery pack leads may cause damage to the pack or even a fire. Be careful to avoid short-circuit, but if one does accidentally occur, move the battery to a safe place for at least 30 minutes. Use extreme caution because delayed chemical reactions may occur. When fitting the connector to the leads, work with only one lead at a time until the connector is fitted and insulated. Do everything possible to avoid short-circuit.

Storing

The speed by which lithium based batteries age is governed by temperature and state-of-charge. The following table illustrates that we can get the most prolonged life from Lipos when they are stored in the cool place with 40% charged . The most harmful combination is full charge at high temperature. this is the case when placing a Lipo battery in a hot car.

A charge state of 50% reads about 3.8V and 40% is 3.75V. Store lithium batteries at open terminal voltage of 3.75-3.80V. Allow the battery to rest 90 minutes after charge before taking the voltage reading.

Some lithium batteries fail due to excessive low discharge. If discharged below 2.5 volts per cell, the internal safety circuit opens and the battery appears dead. A charge with the original charger is no longer possible. However, if the cell voltage has fallen below 1.5V/cell and has remained in that state for a few days, a recharge should be avoided because of safety concerns. To prevent failure, never store the battery fully discharged. Apply some charge before storage, and then charge fully before use.

Do not store lithium batteris fully depleted. If empty, charge for about 30 minutes before storage. Self-discharge on a depleted battery may cause the protection circuit to trip, preventing a recharge.

Precautions when storing Lipos

Battery life

Aging of lithium based batteries is an issue that is often ignored. Lithium based batteries have a lifetime of 2-3 years. The clock starts ticking as soon as the battery comes off the manufacturing line. The capacity loss manifests itself in increased internal resistance caused by oxidation. Eventually, the cell resistance will reach a point where the pack can no longer deliver the stored energy, although the battery may still contain ample charge.

A lithium battery provides around 500 discharge/charge cycles. The battery prefers a partial rather than a full discharge. Frequent full discharges should be avoided when possible. Instead, charge the battery more often or use a larger battery.

Disposal

Batteries that have lost more than 30% of their initial capacity should be removed from service and correctly disposed of.  Discharge all cells individually to recommended cut-off voltage of 3.0 volts per cell. Caution!! Cells may be warm or hot during this discharge process. After cooling, puncture small hole in cells and immerse in salt water for several hours. Apply tape over terminals, put in a bag and dispose of in trash.

Incinerating or disposing of Polymer in fire is strictly prohibited.

Lipo Discharging

For a couple of years after commercialization of Lipo, one of the major disadvantages was low discharge rate. The maximum discharge current for earlier versions of Lipos were typically below 5C.  this was low compared to 15C to 20C for Ni-Cd's. Now, with the advent of 20C products, the major disadvantage to Ni-Cd is about to completely disappear.

Internal resistance determines discharge rate

Measured in milliohms, the internal resistance is the gatekeeper that determines the runtime. The lower the resistance, the less restriction the battery encounters in delivering the needed power spikes. Voltage drop caused by high internal resistance causes battery to reach low-end voltage cut-off sooner and this causes bigger internal energy losses. Due to voltage drop, the available energy cannot be delivered in the required manner and remains in the battery.

As the discharge current increases, the voltage drop increases even more (Voltage drop = Current X Resistance). this is why all batteries can't generate high discharge rate.

What is C-rate?

The charge and discharge current of a battery is measured in C-rate. At 2C, a 1000mAh battery would deliver 2000mA for 30 minutes. 1C is often referred to as a one-hour discharge; a 0.5C would be a two-hour discharge, and a 0.1C a 10-hour discharge.

The capacity of a battery is commonly measured with a battery analyzer. If the analyzer's capacity readout is displayed in percentage of the nominal rating, 100% is shown if a 1000mAh battery can provide this current for one hour. If the battery only lasts for 30 minutes before cut-off, 50% is indicated. A new battery sometimes provides more than 100% capacity.

When discharging a battery with a battery analyzer that allows the setting of different discharge C-rates, a higher capacity reading is observed if the battery is discharged at a lower C-rate and vice versa. By discharging a 1000mAh battery at 2C, or 2000mA, the analyzer is scaled to derive the full capacity in 30 minutes. Theoretically, the capacity reading should be the same as with a slower discharge, since the identical amount of energy is dispensed, only over a shorter time.

Due to internal energy losses and a voltage drop that causes the battery to reach the low-end voltage cut-off sooner, the capacity reading may be lowered to 95%. Discharging the same battery at 0.5C, or 500mA over two hours may increase the capacity reading to about 105%. The discrepancy in capacity readings with different C-rates is related to the internal resistance of the battery.

The result of over discharge

Over discharging will result in chemical changes within the cell that cannot be reversed by the following charge and this reduces the capacity of the cells. Cells should not be discharged below an on-load voltage of about 2.5V to 3.0V although if the period is kept very short it may be acceptable.

If the voltage is allowed to remain low for some time as a result of self discharge, then metallic copper may be formed within the cell. this copper could result in a partial or total short circuit rendering the cell useless. If the cell does not permanently short, then it would be possible to recharge it but this should not be done as copper will still be within the cell and a short circuit could occur at any time. I assume this is the reason why lithium chargers will not allow a charge to commence if the voltage is below the minimum specified, the cell or battery being safe whilst it has a low state of charge. A cell in this condition is likely to have lost a lot of capacity so throw it away to ensure safety.

Manufacturers rate the lithium based battery at an 80% depth of discharge. Repeated full (100%) discharges would lower the specified cycle count. It is therefore recommended to charge lithium more often rather than letting it discharge down too low. Periodic full discharges are not needed because lithium is not affected by memory.

Discharge over-rated?

Manufacturers usually don't indicate internal resistance of the batteries in the specs nor announce discharge performance graph shown above. this makes it difficult for users to determine the quality of the batteries.

Discharge current can be overrated. Some users say most Lipos are overrated by 3-4Cs. As we can see from short circuiting a battery, for the short period of time, current draw can be very big like 100C or more.

Lipo Charging

Lipos are designed to operate safely within their normal operating voltage but become unstable if charged to higher voltages. When charging above 4.30V, the cell causes plating of metallic lithium on the anode; the cathode material becomes an oxidizing agent, loses stability, and releases oxygen. Overcharging causes the cell to heat up. If left unattended, the cell could vent with flame.

Charge process

Stage 1 : Max charge current is applied until the cell voltage limit is reached.

Stage 2 : When Max voltage is reached, charge current (Red line in the graph) starts to drop until full charge is reached. The usual charger stops charging when current reaches below 100mA. (0.1C for 1000mAH Lipo, 0.03C is considered to be full charge condition)

Stage 3 : Stage 3 is topping charge state. Topping charge usually happens every 500hours due to self discharge.

No trickle charge is applied because lithium is unable to absorb overcharge. A continuous trickle charge above 4.05V/cell would causes plating of metallic lithium that could lead to instabilities and compromise safety. Instead, a brief topping charge is provided to compensate for the small self-discharge the battery and its protective circuit consume. Depending on the battery, a topping charge may be repeated once every 20 days. Typically, the charge kicks in when the open terminal voltage drops to 4.05V/cell and turns off at a high 4.20V/cell.

Recommended charge rate

Most cells are charged to 4.20 volts with a tolerance of +/-0.05V/cell. Charging only to 4.10V reduces the capacity by 10% but provides a longer service life. Newer cells are capable of delivering a good cycle count with a charge to 4.20 volts per cell.

The initial charge current determines the overall charge time. At 0.5C, over 93% of the capacity is reached when the second phase starts, and at 1C about 96% capacity. this means that even though the second phase is terminated early in an attempt to avoid any possibility of over charging, very little capacity is lost. At the recommended 0.5C initial current, it takes about 2.5 hours to achieve a full charge and at 1C about 1 hour 20 minutes. Fully recharging currently available cells at the flying field is probably not an option unless you have plenty of time.

The lower charge current reduces the time in which the cell resides at 4.20V. It should be noted that a 0.5C charge only adds marginally to the charge time over 1C because the topping charge will be shorter. A high current charge tends to push the voltage up and forces it into the voltage limit prematurely.

Increasing the charge current does not shorten the charge time by much. Although the voltage peak is reached quicker with higher charge current, the topping charge will take longer.

Some chargers claim to fast-charge a lithium battery in one hour or less. Such a charger eliminates stage 2 and goes directly to 'ready' once the voltage threshold is reached at the end of stage 1. The charge level at this point is about 70%. The topping charge typically takes twice as long as the initial charge.

How Lipo chargers work

1. Most chargers can detect the number of cells using auto voltage sensing. Charge voltage is determined by number of cells.

Sometimes, a charger can MIS-calculate the number of cells when one or more cells are dead or in the range of abnormal voltage.

2. Constant voltage is applied and charging stops when current is below a certain level. 4.2 Volts are applied for each cell (12.8 Volts for a 3S pack) until the end of charge process. Current remains constant for about an hour, and then continues to drop until a full charge is reached. Most chargers stop charging when current drop below 100mAh.

Battery Safety

Charging Guidlines

1. Use Lithium-Ion Polymer charger only. Do not use a NiMH or NiCd charger - Failure to do so may a cause fire, which may result in personal injury and property damage. Some Lipo chargers on the market may have technical deficiencies that may cause it to charge the batteries incorrectly or at an improper rate.

2. Never charge batteries unattended. When charging Lipo batteries you should always remain in constant observation to monitor the charging process and react to potential problems that may occur. If you notice anything abnormal happening to the battery pack during charging, the charging process must be stopped immediately, disconnect the battery and put in a safe place away from all flammable materials for at least 30 minutes. Unusual signs during charging include excessive heating, odd smells, smoking, ballooning or swelling of the battery pack. A safe place would normally be outside, away from any vehicle or dwelling place.

3. Never Charge your lithium polymer cells or packs on flammable materials such as wood, foam or plastic. Use a fire proof container. Have sand or dry fire extinguisher handy for the event of fire.

4. Do not charge at a rate greater than the capacity of the pack (1C). Charging at higher rates may cause cell damage or a fire.

5. Cell count selection on your charger is vital when charging. Please double-check that you have the charger set for the correct number of series cells in your battery pack before you connect the battery.

6. Allow battery to cool to ambient temperature before commencing charging.

7. Do not charge battery packs in series. Charge each one separately. If you start charging a pack with the charger not set for the correct cell count, a fire could result.

8. You have to check pack voltage prior to charging. Do not attempt to charge any pack whose voltage per cell is lower than 3.0 Volts (i.e. a 3S pack should read at least 6.0 Volts).

9. Do not permit voltage during charge to exceed 4.2 Volts per Series cell under any circumstances (i.e. a 3S pack should not be allowed to exceed 12.6 Volts).

Handling Guidlines

1. Wire lead shorts can cause fire! If you accidentally short the wires, the battery must be placed in a safe area for observation for approximately 15 minutes. A battery can still ignite even after 10 minutes. Additionally, if a short occurs and contact is made with metal (such as rings on your hand), severe injuries may occur due to the conductibility of electric current.

2. If for any reason you need to cut the terminal wires, it is necessary to cut each wire separately, ensuring the wires to not touch each other or a short may occur, potentially causing a fire.

3. To solder a connector, remove insulating tape of Red wire and solder to positive terminal of a connector, then remove insulating tape of Black wire and solder to the negative terminal of connector. Be careful not to short the wire lead.

4. In the event of a crash, you must remove battery for observation and place in a safe open area away from any combustible material for approximately 15 minutes.

5. Never store or charge battery pack inside your car in extreme temperatures, since extreme temperature could ignite fire.