Fly Electric!

Lithium Cells

A123 cells
Charger circuits
Balancing issues and circuits
In-flight voltage monitoring circuits

THE BASICS

This page is mainly about rechargable Lithium-Polymer (LiPo) cells (as used in RC aircraft). Their main advantages over Nicads and NiMHs are that they are very light, you don't need to 'top them up' if not used for some time and they have no 'memory' effect. Lipos are the most common lithium chemistry available. Not many people would consider buying Nicads or Nimhs these days.

The downside of lipos are that they are a bit fragile. Straying outside strict voltage limits usually damages them. They can also easily be damaged physically. Oxygen is released if overheated which makes them swell up. If allowed to reach 150'C thermal runaway can result in a fire.

Lithiums are great but require a healthy dose of respect.

To overcome some of these problems, various new lithium cells are being developed so expect to see changes over the coming years. eMoli is a brand of lithium cell that has a built-in single-use fuse and is a more stable Lithium-Manganese chemistry. It has similar voltage characteristics as lipos but are still only low C ratings (10-15C continuous). They are packaged in steel for safety and are sold as 'Milwaukee V28' power tool batteries.

A123 is another new cell in 2006 and are sold as 'DeWalt DC9360 36v' batteries. Technically they are Lithium Iron Phosphate. Their main claims to fame are low internal resistance, high 30C rating, high charge rates, wide voltage range and inherent temperature stability. They seem to have most of the good features of lithiums with the robustness of nicads. Read more on my A123 page.

This page is intended for Lipo cells but most of the facts also apply to conventional Lithium-Ion (LiIon) cells. In fact I am told that most cells labelled as 'polymer' are probably really 'ion' (true poly's have a dry electroyte and ion's are wet). The information is not meant to apply directly to other lithiums including those mentioned above, the older Lithium-Metal (eg: Tadirans) and some of the other types although much of it may. Very little is meant to apply to 'primary' or non-recharable Lithium cells (eg: camera or watch batteries). You HAVE to know what cells you are using! In the RC modelling space this is now mostly Lipos. Here are some photos of various rechargable cells.

800 and 1600 Li-Ion cells
800 & 1600mAh
laptop cells
Kokam type cells
Bare Kokam
LiPos

Various small
3 cell packs

Various larger
3 cell packs

I have been using lithiums for some years now. However, please note that I am not a qualified engineer or battery expert so you use the information I share from my experiences at your own risk.

VOLTAGE

The voltage of a Lipo cell rises as it is charged and falls as load is applied and the capacity is used up. They are designed to be charged up to 4.2v per cell and not discharged below 3.0v (more on this later). They have a nominal rating of 3.6 or 3.7v which means very little to modellers except to indicate that each lithium cell is equivalent to about 3 Nicads or NiHMs in voltage terms. They have a limited number of cycles (sometimes quoted to be 300-500). We tend to work them quite hard so you may not get even this life out of them. The cells to buy these days are the 'high discharge' types. 10 or 12C 'continuous' are the minimim you should consider these days (Sep'06) with 20 and 30C becoming more common. This is improving every 6 months or so.

A lithium charger needs to take the cells voltage up to 4.2v per cell at a charge current of about 1C (ie: 1 x the cell's capacity). Once the voltage reaches this maximum the current needs to keep declining until the cell is fully charged. These characteristics of the charger are called 'constant current constant voltage' (CC CV). Voltage peak is not an indicater of 'fullness' like with Nicads and NiMHs. This means they have to have a special charger. However, a charger that meets these requirements is extremely easy and cheap to make. Some simple circuits are provided below which in my experience can be as effective as the expensive commercial ones.

The voltage is not meant to drop below about 3.0v per cell. However, this is generally considered to be the 'no load' minimum. I set my controllers to cut off at 2.75v/cell and many controllers are programmed for 2.5v/cell. Since the voltage drops gradually as it is used, it can be difficult to detect when to stop flying. A speed control with adjustable cutoff voltages is therefore usually a necessity. Some require you to program the number of cells and some assume you are connecting fully charged cells and make the decision for you.

The 'penalty' for straying outside the defined ranges is death (of the cell!). You may be lucky and only lose capacity and inability to deliver full current. However in reality this can mean that the cell is no longer of great value to you. So, it is in your interest to charge and discharge the cells in a controlled manner as I have described.

SAFETY

Two safety warnings are important:
1. The cells contain HF (Hydrogen Flouride) which is a very bad substance. However the percentage is said by one manufacturer to be extremely small. Sensible precautions would be to avoid contact with the internal gel should the casing split. Standard HF treatment is to flush exposed areas for 15 minutes in running water and to consult a doctor.

Encouragingly, some safety sheets now recommend piercing the plastic casings and soaking them in water before disposing them. I would not do this but it does imply that the risk is very low. Also, you may be able to see from the following photo that this Kokam 3200mAh cell has three layers of insulation (the silver outer, a latex-type inner and each cells is actually made of many very thin individually wrapped cells):
Burst pack

2. The cells do ignite from time to time, usually when over-heated. It has been said that they suffer from thermal runaway once over 150'C. Charging is the highest risk because it is easy to set a charger to 3 cells to charge just 2. At best the cells may just puff up; at worst burst into flames. Drawing too much current can also generate too much heat, as can leaving them in a hot car without ventilation. Be careful! Have a foam extinguisher to hand or a bucket of sand. Don't charge unattended. Don't charge on top of flamable surfaces. Double-check settings.

TEMPERATURE & OTHER BITS

Safety aside, you should understand that Lithiums are very sensitve to temperature in use. Even on expensive cells rated at 20C continuous they need forced air cooling even at moderate currents. Overheat them and as mentioned above they usually puff up and don't work as well.

In winter they need the opposite. Their voltage tends to drop when cold so you may need to blank off cooling air to allow them to generate some temperature to keep their voltage up (to avoid premature ESC cut-off). In winter I bring my lithiums into the house to warm them up before flying and keep them in a polystyrene 'cooler' box to reduce temperature loss before use.

It's probably also worth noting that laptop computer battery packs usually comprise 3 cells in series to achieve their required operating voltage (nominally 10.8v) with 2 to 4 cells in parallel to increase capacity (eg: 3 in series and 2 in parallel =6 cells in a pack which in modelling terms is often abbreviated '2S3P'). They always have built-in charging mechanisms and these monitor voltage of each set of parallel cells. There are also usually one or two temperature probes and current sensitive switches. These are the manufacturers normal requirements for safe operation. Most modellers don't have these controls so we are taking greater risks.

If you ever have to solder a lithium cell you may find that one lead is aluminium which does not take ordinary solder. An 'Aluminium Solder Paste' does the trick and I use one manufactured by Solder-It.


CHARGERS

It is easy to charge Lithiums so DIY chargers are entirely practical. Many are safer to use in practice than commercial ones due to their simplicity. Some circuits can be found here:

Lithium charger circuits
BALANCER/CHARGE GUARD

Lithiums need to be kept in balance and details of how to achieve this can be found here:

Balancing issues and Circuits

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