Lithium polymer batteries or LiPo batteries are currently the most efficient power sources available with the highest possible specific energy. This means that you get more power and capacity from a smaller and lighter battery. To accomplish this, a different technology is used compared to NiCd, NiMH, and Li-lon batteries; subsequently, slightly different handling and charging applies. Please read the following safety guidelines to familiarize yourself with the specifics of these batteries.
Some technical terms explained: Voltage and capacity
Because 1 Lithium element or cell has a (low) fixed voltage of 3.7 V. most batteries or packs are built up from a number of Lithium cells in series. The amount of Lithium cells in series is the indicated by the letter ‘s’ e.g. 3s battery. You can determine the total nominal voltage by multiplying this number with 3.7V. In our example 3s battery gives 3 x 3.7V = 11.1 V. this is an important value when It comes to choosing charger parameters, the charger settings must match the LiPo. (See chapter Lithium polymer batteries)
3.7V is the average voltage; LiPo can be charged up to 4.20V and discharged down to 3.0V per cell, any lower or higher in voltage and the cell will almost certainly be damaged and will become unstable which can be very dangerous (fire risk).
The capacity of the battery is indicated with ‘mAH’ e.g. 1500 mAH. This tells you how much energy it can contain. MAH is the abbreviation of milli Ampere Hour or 1/1000th Ampere Hour. (1000 mAH = 1Ah)
The capacity tells you how long it takes to discharge a battery of 1500 mAH can provide 15000 mA (milli-Ampere) or 1.5A during 1 hour, or twice as much (3000 mA or 3A) during half an hour, ect. Because Lithium batteries come in variety of capacities, the ‘C’ rating is used to determine the maximum allowable current x C value e.g. 1500 mAh x 30C = 45000 mAh or 45 A.
Maximum allowable discharge current is usually between 10C and 50C, maximum charge current is always 1C. (Whilst some LiPos can be charged at higher C ratings up to 5C, doing so will shorten the life of the battery, so it is good practice never to exceed 1C)
In case of a 3s 1500 mAh battery with a 30C rating, the maximum (continuous) discharge is 45A and the current is 45A and the charge current is 1.5A (15000mAh x 1C = 15000 mA or 1.5A)
CHARGING LITHIUM POLYMER BATTERIES
As mentioned before: if the voltage of a single Lithium cell exceeds 4.20V, it will become unstable. When this happens the cell will over heat, swell up and rupture whereby the electrolyte can spill and catch fire! Because the technology of the LiPo batteries is different from other batteries, you must always use specialized LiPo charger. Under no circumstances attempt to charge a LiPo with a NiCd/NiMH charger.
The charger parameters must match the LiPo and must be verified each time before you start the charging process:
- The programmed number of cells must be the same as the LiPo or the nominal voltage must be equivalent. A 3s must be charged with 3s or 11.1V setting etc.
- The charge current must never exceed 1C (for a 1500 mAh battery: 1.5A)
- A balancer must be used to balance the voltage of individual cells during charge. (See chapter: Balancing)
To further avoid fire risk always respect the following precautions:
- Never charge the battery unattended
- If you have a multifunction charger, thoroughly read the instructions and make sure it is in LiPo mode.
- Verify that the total voltage in the pack never exceeds the amount of cells the amount of cells in the pack may never exceed (5 x 4.20V =) 21.00V
- Put the charger and battery on a non-flammable surface, like a ceramic tile, during charge and make sure no flammable objects are within a radius of 1m.
- If possible, put the battery inside a special LiPo bag or in a fireproof container during charge.
- Never charge inside a car, clubhouse or garage and if possible, keep a dry extinguisher or fire blanket ready. Never charge the battery while it is in the model.
- Stop charging immediately if the battery gets warm or swells, disconnect the pack and put it in a safe place for observation for a few hours.
- Never charge or recover a battery pack that damaged/bad/swollen cells, or cells that are below 3.0V
- Use properly insulated connectors and charge leads at all time.
Because the charger only measures the total voltage of the pack during the charge, the individual cells must all have the same voltage at all time. Otherwise the total pack voltage might be correct, but some individual cells might already exceeded 4.20V. E.g. a 2s pack that has reached 8.4V is fully charged, but if one cell has a voltage of 4.10V, the other cell will be at 4.30V and will overheat. To avoid this situation, specialized balancing circuits or balancers are available or are built into the charger. Make sure that the connector type of your balancer match the battery and always check if all cells are recognized. (The indicated number of cells on the balancer must match the number of cells in the pack.)
Always use a balancer during charging and preferably use a charger with integrated balancer. If a separate unit is used, always use an active balancer that has the ability to interrupt the charging process if necessary.
USING LITHIUM POLYMER BATTERIES
Lithium Polymer batteries can store a tremendous amount of energy and therefore must be handled with care. A fire risk exists and precautions should be taken to minimize this risk. There are two main factors that can possibly cause a fire during use:
- Physical damage: one or more cells are punctured, deformed or exposed to extreme conditions
- Internal damage: one or more cells are punctured, deformed or exposed to extreme conditions.
- Internal damage: one or more cells are discharged too fast or too deep.
To avoid this please respect following guidelines at all time:
- Never expose LiPo batteries to extreme heat (heater, fire, flame, exhaust, sun); never leave them inside a car for a long time.
- Never puncture a LiPo cell, as it will immediately catch fire. When installed in the model, make sure it cannot touch protruding screws, or sharp metal parts. Make sure the battery is well protected in the case the model should crash.
- Never put a LiPo in your pocket, as key chains or other objects may puncture it.
- Do not deform the battery. If it should have deformed during a crash or transport, never try to force it back into its original shape but dispose of it safely. (See chapter: disposal)
- Always transport LiPo batteries in a fireproof case where it cannot come in contact with other objects.
- After a crash, put the battery in a safe (fireproof) place for observation for 24 hours before using it again. It might have suffered internal damage, which might cause a delayed reaction.
- When one or more cells have swollen, dispose of the battery pack immediately, do not try to recover it or recuperate separate cells. Reassembling packs from different cells is dangerous if not done by professionals. Furthermore all factory assembled packs are composed of matched cells, when random cells are used to compose a battery packs, the different characteristics of the cells will affect performance adversely and this can be dangerous.
- Don’t let the LiPo cells come in contact with water, if this should happen; dry them quickly with a towel.
- Never exceed the maximum stated discharge rate. (e.g. 30C)
- Never allow the battery to short circuit. Use properly insulated connectors and isolate them additionally during transport if necessary.
- Never allow a battery to exceed 60°C (140°F) at any time.
- Never allow the pack to be discharged below 3.0V per cell; preferably use an ESC with LiPo low voltage safety cut-off. Over discharging causes permanent damage to the cell.
- When one or more cells are below 3.0V after use, don’t try to recharge them.
- Stop driving as soon as you notice a drop in power. However tempting it may be to continue, you will almost certainly discharge one or more cells in the pack.
- After use, let the battery pack cool down completely before recharging. Failure to do this will result in the rapid deterioration of your battery pack and can be dangerous!
- When the battery consistently gets very hot after use, check the gearing and make adjustments if possible. Discharging batteries at maximum discharge rate for prolonged times will shorten the life of the battery considerably.
- Disconnect the battery from the ESC or load after use avoid slow over discharge.
- Make sure the battery is well ventilated in the model to avoid heat build-up or thermal runaway.
- Don’t let children handle these batteries without adult supervision.
STORAGE OF LITHIUM BATTERIES
For long time storage it is recommended to discharge/charge the batteries to 50-40% of their capacity (or 3.8V per cell). Never store LiPo batteries fully charged, not even for a week, the performance will degrade noticeably. Keep the LiPo batteries stored in a fireproof location at room temperature.
DAMAGED LITHIUM BATTERIES AND DISPOSAL
Please take great care when a battery has been involved in a crash: there might be visible and/or invisible damage to the cells, which may cause a violent (delayed) chemical reaction. If possible, remove the battery from the model as soon as possible and put it in a fireproof location for at least 1 hour for observation. Be careful as the cells may be very hot and they might be leaking electolyte; wash your hands with soap immediately. Should you get the electrolyte in your eyes, flush generously with water during 10 minutes and seek medical attention. Wait at least 24hours and inspect thoroughly before reusing the battery.
If the battery should catch fire immediately after the crash, don’t attempt to remove it from the model, use a fire extinguisher!
When a Lithium battery pack is damaged or needs to be disposed of, proceed as follows:
- Submerse the battery into a container filled with about 10L of salt water. (Add a cup of salt per 10L)
- Leave the battery submerged for 2 weeks; this will slowly and safely discharge the battery until the voltage has dropped to 0V, which eliminates the risk of any chemical reaction.
- After 2 weeks, place the battery in the trash.
Terry Crew – Reality Racing