Lithium Polymer Batteries: How to Choose, Maintain and Store
This article gives an overview of Lithium Polymer (Li-Pol) batteries with their main characteristics, applications, cost, and manufacturers. It describes algorithms of charging Li-Pol batteries, their operation, nuances of application and precautions.
Lithium Polymer Batteries Types & Difference from NiMh and NiCd Batteries
Improvements are being made all the time, and the use of traditional NiMh (nickel-metal hydride) and NiCd (nickel cadmium) batteries has now been replaced by lithium batteries. With a relatively equal weight per cell compared to NiMh and NiCd, they have a relatively high capacity with three times the voltage per cell instead of 1.2V 3.6V per cell. Therefore, a battery of 2-3 cells is sufficient for most models.
Lithium batteries are divided into two basic types - lithium polymer (Li-Pol, Li-Po or LiPo) and lithium ion (Li-Ion). The difference between the two is the type of electrolyte used. LiIon uses a gel electrolyte, while LiPo uses a special polymer saturated with a solution containing lithium. Since LiPo batteries are most commonly used for power models, then we will talk about them. However, a strict section here is very conditional, because in both types the main difference is the electrolyte used, and everything said about LiPo batteries, almost entirely applies to the Li-Ion (safety technology, features of operation, charging, discharging). In terms of practice we are interested only in the fact that LiPo batteries at this time are able to provide high values of discharge currents. That is why they are widely used for RC cars.
Main Features of Lithium Polymer Batteries
At equal weight LiPo batteries surpass NiMh by 3-4 times, and NiCd by 4-5 times. With a discharge rate of 2C to 20% loss of capacity the number of operating cycles is 500 - 600 (NiMh - 500, NiCd - 1000). In general, there is still very little data on the number of operating cycles, so in this case, we should be critical about the given characteristics. Since their technological production is constantly improving, it is quite possible that today the values for this type of battery are already different. Lithium batteries, like all other batteries, are subject to aging. After about 2 years, the capacity of the battery is lost by about 20%.
Ordinary and High Discharge LiPo
Among the variety of LiPo power batteries available on the market, there are two main groups - ordinary and high discharge (Hi discharge). Their difference is the maximum discharge current, indicated either in amperes (A) or in units of battery capacity (C). For example, with a battery capacity of 1 Ah, the discharge current is 3C, then the current is 3A.
The maximum discharge current in simple rechargeable batteries usually does not exceed 3C, some manufacturers specify 5C. With fast-discharge batteries, a discharge current of up to 8-10C is allowed. These batteries, compared to their low-current counterparts, have a slightly higher weight (about 20%), and their name is supplemented with the letters HC or HD after the digits of the capacity.
What is a Lipo C rating?
If you have a Lipo battery, you may see some information on its cover, such as the C factor. The C factors are given to determine the charging and discharging capacity of each battery, usually the charging capacity of batteries is 1C, but in some cases it can be higher, and this information is obtained on the back of the batteries. The discharge capacity may vary depending on each manufacturer and product specification, e.g. 1C, 20C, 30C, 40C, 100C, 120C, etc. etc. This information is also displayed on the front of the battery and you do not need to contact the manufacturer. for this. Generally, if the lipo-C rating information is hard to find on the battery, it means that the product is of low quality and therefore unreliable.
Is a C-rated Lipo battery reliable?
C rating can be a useful tool to help us choose a LiPo battery. Unfortunately, many LiPos manufacturers today exaggerate C ratings mainly for marketing purposes. Consequently, as long as the C classification remains a sales volume indicator, we cannot fully trust it. The best thing you can do is find out the true overall performance of the battery by looking at third-party tests and user reviews. It's also good to note that cheaper batteries are usually of inferior quality, which means that internal resistance will increase rapidly and the voltage drop will become increasingly noticeable, which will negatively affect the overall battery life.
C Rating Application of the Lipo Battery
All Lipo batteries are rated C. Knowing the C rating and the capacity of the battery, we can theoretically calculate the maximum safe continuous discharge current of a LiPo battery and its application level.
This can be calculated as follows: Maximum Discharge Rate = C-Rating * Capacity.
For example, a 50C battery with a capacity of 1300 mAh has an estimated maximum continuous discharge current of 65A.
Some batteries have two C-ratings: "continuous" and "burst". The Burst value only applies for a short period of time (e.g., 10 seconds). Although the C rating can be a useful and reliable tool, it has now become mostly a marketing tool. If the C rating is too low, the battery will have difficulty delivering current to your motors. You can also damage the battery if the current draw exceeds the safety rating. When the C rating is higher than requested, you will not get much improvement in performance. Instead, the battery will be heavier and you will carry extra weight, which will reduce the overall efficiency and effectiveness of the application.
A Lipo battery C rating can also apply to cost, as this rating indicates the internal resistance of the battery components. A high C value indicates that the battery will be able to give off better energy internally and will be able to hold voltage well under load. Consequently, with the same performance between different 10C, 15C, 20C, 30C, 35C, etc. batteries. etc. It will be preferable to use a higher C index: 35C is better than 10C... but it will cost a lot more because the procedures to get it are more expensive.
Another factor that is usually relevant to the use of C-rated Lipo batteries is the fact that it indicates the maximum current at which the battery can be discharged without being damaged. It is a factor that is usually multiplied by the power to get the maximum current. Thus, safe charging and discharging can be effectively accomplished with sufficient knowledge of the Lipo battery's C rating.
Advantages of LiPo and Price Range
The use of LiPo solves two important problems - increases the working time of the motor and reduces the weight of the battery.
By replacing the 8.4V NiMh 650 mAh battery with two standard, non-fast-discharging 2 Ah lithium batteries you get three times the battery capacity, which is 11 grams lighter and has a slightly lower voltage of 7.2 V! Even large airplanes can fly when using fast-discharge batteries, and their power capacity will not be inferior to the internal combustion engines themselves. As confirmation of this, in the world championship model flying F3A 7th place was taken by an American electric plane. And it was not a small aircraft, but quite a normal two meters long plane, like other participants who had models on internal combustion engines!
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Manufacturers of LiPo Batteries
Currently, there are several companies that make LiPo batteries for RC cars.They are Spektrum, Venom, Traxxas, Hexfly, Zeee, HRB, Gens Ace, etc.
The capacity of batteries has a very wide range - from 50 mAh to 6000 mAh. Parallel connection of batteries is used to get the larger capacity.
All batteries are flat in shape. Their thickness is usually more than 3 times less than the shortest side, the leads are made in the form of flat plates on the short side.
LiPo Batteries Charging
The battery charging algorithm is quite simple - the charge requires a constant voltage source of 4.20V per cell and a current limit of 1C. Charge is considered complete when the current drops to 0.1-0.2C. After a 1C current drop, the capacity of the battery will be approximately 70-80%. It will take approximately 2 hours to fully charge the battery. The requirements for the charger are quite strict in terms of accurate voltage maintenance at the end of the charge - not less than 0.01 V/cell.
Main Types of Chargers:
- Conventional, non-computerized devices that are designed only to charge lithium batteries, cost appr. $10-$40;
- Versatile, which are designed for different types of batteries, including Li-Ion and LiPo, their cost is about $120-400.
The first, as a rule, has only a LED indication of the charge. The advantage of such chargers is the low cost. The main disadvantage is the inability of some of them to correctly show the end of the charge. They are only able to show the moment at the transition of modes from current stabilization to voltage stabilization, which is about 70-80% of the capacity. It is necessary to wait another 30-40 minutes for the charge to be fully complete.
The second group of chargers has more features. Typically, they are all capable of displaying the voltage, current and capacity values taken by the battery during the charge, so you can more accurately determine the battery's state of charge.
When using a battery charger, the most important thing is to correctly set the correct number of cells in the battery and the charging current, usually equal to 1C.
Safety Precautions and Usage Rules
LiPo batteries are the most "delicate" batteries existing, as they require certain simple but essential rules, which can otherwise lead to a fire or "death" of the battery.
What you should never do to a LiPo battery (listed in increasing danger order):
- Do not store the battery in a discharged state
- Do not store the battery unpressurized
- The battery should not be heated above 60°C.
- Do not discharge it below a voltage of 3.00 V/cell.
- Do not discharge with currents that exceed the load capacity or heat the battery more than 60°C.
- Avoid short-circuiting the battery.
- Avoid charging the battery to a voltage that exceeds 4.20 V/cell.
Note! Neglecting the last three points may cause a fire, all others may cause a total or partial loss of capacity.
What to Do to Avoid Fire
To avoid a fire, it is necessary to get an appropriate charger and correctly set the number of the charging cells on it. In addition, it is necessary to use connectors that do not short-circuit the battery (as it can cause a fire) and monitor the current consumed by the motor at "full throttle". Also, it is not advisable to cover the batteries on the RC car from all sides, thus blocking the air flow to them. If this cannot be observed, special cooling ducts must be provided for cooling.
If the engine consumes more than 2C current, and the battery is closed from all sides, you should stop the motor after 5-6 minutes of operation, take the battery out and check if the it is too hot. The bottom line is, once heated above the conditioned temperature (about 70°C), a "chain reaction" begins inside the battery and turns the energy it receives into heat, with the battery literally melting and igniting anything that can catch fire.
If you short-circuit a nearly discharged battery, then there will be no fire. It will quietly "die" due to over-discharge... Here we reveal another important rule: Check the voltage at the end of the battery discharge and do not forget to disconnect the battery at the end of work!
Of course, let's not forget the essential ability of the motor controller to handle lithium batteries, namely the adjustable motor cutoff voltage. We must remember to program the controller for the necessary number of cells. However, there are now a new generation of controllers that automatically detect the number of connected cells.
Depressurization is another reason for lithium batteries to fail, since air must not enter the cell. This can happen because the outer protective package (similar to a heat shrink tube) is damaged by impact or a sharp object, or because of severe overheating of the lead when soldering the battery. The lead must be soldered very carefully and care must be taken not to drop it from a great height.
Bearing in mind the manufacturers recommendations, batteries should be stored almost charged (50-70%), in a cool place with the temperature not exceeding 20°C. Discharged condition during storage, negatively affects the service life - LiPo batteries, like other batteries, have a slight self-discharge.
The most important points on the use of LiPo batteries should be emphasized again:
- Use a normal charger.
- Use connectors that do not short-circuit the battery.
- Do not exceed the discharge currents allowed.
- Monitor the temperature of the battery if there is no cooling.
- Do not discharge the battery less than 3 V/cell (be sure to disconnect the battery after you finish racing!).
- Do not subject the battery to shocks.
Some more useful examples should be given that follow from what was said before, but are not obvious at first glance.
- At high charging currents (2 A or more) using thin cables from the charger to the battery, or connecting with alligator cables instead of regular battery connectors to the charger can lead to a voltage drop in the cables and terminals, the charger then starts to go to voltage stabilization mode earlier, which increases the charging time. For example, using the standard crocodile cables on a Triton at 1.5 A will increase the charging time by 20 minutes compared to thicker (1 mm2) cables without crocodiles.
- When using brushed motors do not allow the motor to stall (e.g. when lying on the ground) at full throttle of the transmitter because the current is too high and there is a risk of battery explosion (if the regulator or motor does not burn out earlier). This problem has been discussed many times on RC cars forums. A large number of regulators in brushed motors, as a result of loss of signal from the transmitter, shut down the motor. If your regulator has this feature, it is advisable to turn off the transmitter when you drop your model in the grass, for example. This reduces the risk of hitting the throttle knob on a dangling transmitter strap without noticing it while you are searching for your model.
- If you use the battery for a long time its cells become unbalanced due to the initial small variation in capacity - some of the cells "age" earlier than others, losing their capacity faster. The greater the number of cells in the battery, the faster the process goes.
From this follows the following rule - sometimes it is necessary to monitor the capacity in each battery cell separately. To do this, measure its voltage after charging is complete. How often? So far it is difficult to establish this precisely. We suggest to do that after about 40-50 cycles of use, and every 10-20 cycles check the voltage of the battery cells during charging to identify "obsolete" ones.
- It is not recommended to drain the battery "to zero" by driving the motor until it stops rotating completely. For a new battery such treatment will not be harmful, but for a slightly unbalanced battery, there is a risk of discharging below 3 V, which will lead to even greater loss of capacity.
- If the difference in capacitance is more than 20% - you should not charge such a battery without special measures!
- Automatic balancing of the cells during charging is performed by means of so-called balancers which are small cards attached to each cell. They contain a control circuit, load resistors and an LED which indicates that the battery voltage reaches 4.17V to 4.19V. If the voltage of an individual cell exceeds 4.17V, a portion of the current is short-circuited by the balancer to prevent the voltage from exceeding the limit value. By lighting the LEDs you can see which cells have less capacity - the balancer LED will light up on them first. Balancers must meet one important requirement - the current they consume from the battery during the "standby" mode should be low and be about 5-10 µA.
Remember that a balancer is not capable of saving the banks from overdischarging in an unbalanced battery, as its function is only to protect the cells from damage during charging, and as a means of indicating "bad" battery cells. All this applies to batteries made up of 3 or more cells, for 2-cell batteries balancers are generally not used.
- There are opinions that the operation of LiPo batteries at minus temperatures is prohibited. And it is true, the technical specifications of batteries indicate the range of operation at 0-50 ° C (at 0 ° C retains 80% of the capacity). However, at temperatures around -10 ° C ... -15 ° C, you can still use them. To do this, you should put the battery in a warm pocket before the run, without freezing it. And the internal heat, emitted by the battery during the run turns out to be a useful property, because it does not let the battery freeze. Of course, in this case the battery output will be slightly lower, compared to normal temperature.
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