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Lithium Properties

Most lithium batteries purchased in the RC hobby industry are not directly from the factory. At times you may be getting the wrong information. The critical specifications for packs that you need to know are:

 

Capacity  mAH - stored energy measured in amp and time

Voltage  V  - multiples of 3.7V nominal lithium cell: 4.2V max, 3.0v min

C rate  - measure of power.  Just a multiplier of capacity, not units.

 

How are packs specified?

Capacity is a measure of energy stored in a battery with units of mAH (mili-amp hour)

Basically how many mA of current can you draw from the cell/pack in an hour.

 

eg 5000mAH 6s pack can give you

5000mA of current for 1 hours

2500mA of current for 2 hours

 

What is capacity?

C rate of lithium batteries is defined as a multiple of the capacity in AMPS of the full discharge rated current.

Eg 2200mahr 3s pack 40C can deliver 2.2amps * 40= 88amps continuous current.

 

note; how many cycles a battery pack can perform at its full C rate is determined by the quality, some can do more and others less

What is C rate?

Watch our video to your right

How do I charge properly?

Yes you can....IF the cell has been designed by the factory to do so and if your charger is safe enough to handle the safety issues like our PowerLab and CellPro chargers.

 

Fast charging causes heat and heat can either cause fires or at a minimum decrease the cycle performance of your pack.  If you don't need to fast charge over a 1C rate then don't.   It will save you money too.

 

Now some RC cells have been designed for fast charging but....with limitations.

1. Typically the stated fast charge rate is at a higher temperature of the battery say ~35 degC. Above or below that and the manageable C rate is lower.

2. Also the temperature is of the pack at steady state, not the ambient temperature.  

eg: having a warm pack that is stable at 35 degC in a cold room you can fast charge having a cold pack that you have taken into a warmer environment you cannot fast charge until the pack has reached the same warm temperature. This will usually take a few hours.

 

Be careful with the fast charging capabilities of lithium, it does come with limitations.

Can I fast charge?

In all lithium battery packs the cell with higher resulting SOC (capacity) is exposed to higher voltages. For example, what happens if one cell has less capacity than the other three serially connected in the pack, if they all start in the same state of charge?

CC/CV (constant current/constant voltage) charging will bring the pack to 4.2 x 4 = 16.8 V. However, individual cell voltages will not be equal. the “low capacity” cell will have a much higher voltage than the remaining cells, while the normal capacity cells will have a lower voltage than achieved in normal charging. When the lower cell has a total capacity deficiency above 10%, its cell voltage begins to rise into dangerous area above 4.3 V which will result in additional degradation of this cell or even become a safety concern.

 

To make the matters worse, the effects of cell degradation caused by imbalance is auto accelerating. Once a cell has a lower capacity, it is exposed to increasingly higher voltage during charge which makes it degrade faster so its capacity becomes even less.

Note that not all battery chemistry is equally affected by cell imbalance. While Li-ion chemistry is specially vulnerable because of its ability to store almost 100% of all energy delivered, Lead-acid, NiMH and NiCd-s are relatively tolerant to overcharge because they can respond to increased voltage by internal shuttle reactions that are equivalent to a chemical short-circuit inside the cell.

 

Imbalance of cell is inevitable in a battery pack with multiple cells due to

- capacity variation in cells (1 - 2%)

- state of charge difference

- impedance variations ( up to15%)

- localized heat degradation of cells more than other (eg middle cell in a pack)

 

What is imbalance?

Standard lithium IR is measures using a 1kHz frequency voltage applied to a cell or pack. Battery manufacturers use this alternating current method to measure IR.

 

RC hobby chargers on the other hand can only use a DC method of checking IR usually during the charging or discharging process. Both of these will give you varying results.

 

1. Passive chargers

These type of chargers will basically calculate the voltage across a cell in a pack with V = I R the main calculation. the DC resistance is a measurement of all the IR values including cables and connectors and typically readings are higher than factory IR values

2. Active chargers

These types of chargers use pulse charging to charge the battery and have individual control of each cell level via the balancing ports. The calculation used is DELTA V = I R where DELTA V is the change in voltage between on/off switching of the charging circuit (PWM modulation). This is more accurate than passive charging due to the removal of the effect of wires and leads but still not as accurate as a true AC measurement of IR.

 

We find that many RC hobbyists use their charger to measure and compare IR values; note

1. You can do this for your own packs and compare packs of the same cell count only.

2. Changing cell counts will show different levels of IR as passive chargers calculate the whole pack DC resistance.

3. You cannot compare your IR values with someone else as the variance from one charger to another is too great for such small measurements. Component tolerance in chargers can be as wide as 10% and this milli-IR measurements become almost impossible.

4. Half of your pack IR is usually in the silicon cables.

 

 

 

What is IR?

No.  There is no need to run-in or pre-cycle lithium polymer batteries before playing.

In the formation process of lithium batteries a SEI layer is formed on the exteneral later of the anode plate.  This is very common and the layer is removed by the factory formation process during one of the quality control processes, thus pre-cycling of lithium polymer batteries is not required.

 

On the other hand lithium phosphate batteries do get stronger in power and capacity within the first few cycles due to the electrolyte seeping through the outer coating of nano phosphate material. Once the electrolyte is fully soaked into the anode material electron transfer is faster and easier thus a better performing battery.

 

Do I need to run in the packs?

Firstly, the lithium Ion polymer battery has no risk of explosion due to the characteristic of its own package, since the battery explosion occurs only in the abuse or battery internal short circuit.

 

The abuse mainly refers to overcharge. We have evaluated a 20AH lithium battery; it will not explode even it is overcharged to 10V.

Because the outer layer of plastic film of battery can withstand the pressure about 2Mpa. There is not large enough pressure to incur an explosion. Generally, the explosions occur in steel and aluminium shell batteries, because such batteries may produce gasses which are likely to form a high-pressure, high-temperature.

 

 

We conducted overcharge test up to 5V for 2.5hours for RC cell and there was no explosion.

The explosion occurs when the positive active material and electrolyte cannot stand the high voltage such as over 5V.

 

 

To stay safe we suggest 4.3V (Blend427) and 4.35V per cell (Bloend427) per cell should be safe for any well manufactured cell

 

Now here is a tip or two.

 

1. if your charger can control the voltage per cell down to 4.1V then you will see an extended life cycle of your pack.  So its not full you say. Correct.  But you never fully empty your packs anyway.

 

2. if your charger can control the voltage abover 4.2.  Say 4.22 - 4.3, it is still safe especially if you have active charge control and you'll get extra capacity out of your pack on race day.  Up to 25percent more...but, life cycle will be sacrificed but that is ok for those special race day competitions and the little extra boost you need on the day.

 

 

What is maximum cell Voltage?

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HV difference?

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You may be wonder what the benefits of our "HV" series of lipos are for RC Hobby?

 

Much like iPhone and Galaxy who are already using HV lithium one gains the following:

 

1. With higher full charge voltage you get more power

2. With higher capacity you get longer play time and more power at the end of your run

3. With Lower IR you get longer pack life