Lithium Polymer Cells

The Pros and Cons.

By Peter Barrow

Introduction.

Lithium technology, used as I have since they came out, can be very rewarding. I have tried to make this user sheet to point out both the good and bad points. They can be used as successfully as Nimhs and NiCads.

In the late 1970s I can remember people getting their hands damaged by exploding NiCads. But we learned to use them reasonably safely. Lipos are becoming safer, because we are becoming to understand what we can and cannot do with them. Some of the makers of lipos are on there 6^th or 7^th generation cells now. These latest cells are subjected to some very stringent tests to make them even safer.

The following is my own personal research from the web and experiences over the last 6 years of using lipos.

History

Lithium ion cells were first thought of in the early 1960s and the first commercial cell came from Sony in 1991. This revolutionised the portable electronics industry. Their have been numerous improvements over the years and lot of legislation to make them safer before they were allowed to be sold to the public.

The latest requirements are the following: verification of compliance now required by UL1642, UL2054 and SBA G1101. These requirements included crush and penetration tests. For those that want to see all the data follow this link:

http://www.kokam.com/english/product/kokam_safety_02.html

Also now Robbe have this statement on there cells (this is a simplified version of the kokam pages):

Extra safety:

Specially treated Lithium material: batteries do not burn and do not explode, even if mishandled in any of the following ways:

Drilling a 5 mm Ø hole through the pack
Bending the pack along its length with a force of 13 kN
Slitting the centre of the cell, approx. 15 mm
Heating to 150°C for ten minutes
Overcharging at 5 Volts and 0.4 C for about 2.5 hours
Reversed-polarity charging at 5 Volts and 0.4 C for about 2.5 hours
Short-circuiting with 100 mOhm for 2.5 hours

http://at.robbe-online.net/rims_at.storefront/en

So even the main retailers are showing how safe there cells are. These cells are the lastest generation. I would advise everyone who buys Lipos to only buy cells that are rated 20c or greater. These cells have past the compliance regulations.

Basics

Lithium polymer and Lithium Ion cells are similar technology. The main differences are:

Lithium Ion cells tend to be in cans like Nimhs.

Lithium polymer cells comes in plastics sacks with a special polymer insulation material between the plates in the sack. They are also lighter than lithium ion.

The nominal voltage for a lipo cell is 3.7 volts which is part charged. The nominal voltage on lithium Ion is 3.3v. Again, this is part-charged

These are the cell equivalents we tend to use to work out what number of cells to use.

NB: this tends to be a industry standard.

2 cells = 6 cells 7.4v
3 cells = 10 cells 11.1v
4 cells = 12 cells 14.8v
5 cells = 15 cells 18.5v
6 cells = 18 cells 22.2v

Disadvantages

A drawback of the lipo battery is that its life span is dependent upon aging from time of manufacture (shelf life) regardless of whether it was charged, and not just on the number of charge/discharge cycles. However, this is far better for the modeler than the latest Nimh cells that degenerate almost in front of our eyes.

At a 100% charge level, a typical lipo battery that is full most of the time, kept at 25 degrees C or 77 degrees F will lose roughly 20% capacity per year. The capacity loss begins from the time the battery was manufactured, and occurs even when the battery is unused.

Different storage temperatures produce different loss results with a fully charged cell:

6% loss at 0 °C (32 °F)
20% at 25 °C (77 °F)
35% at 40 °C (104 °F)

When stored at 40% - 60% charge level, these figures are reduced to 2%, 4%, and 15% at 0, 25 and 40 degrees Celsius respectively.

This can be reduced by storing your cells in a cold room or fridge. Cells must be stored around 40% charged in a cold area 0c to 10c. This reduces loss in capacity to around only 2% a year. So a 5000mah pack will only lose 100mah in capacity.

As batteries age, their internal resistance rises. This causes the voltage at the terminals to drop under load, reducing the maximum current that can be drawn from them. Eventually they reach a point at which the battery can no longer operate the equipment it is installed in for an adequate period. This varies from maker to maker. Some will only give 50 or 60 charges. Where as some will give over 700 charges.

Also there are problems charging unbalanced cells. This can be very dangerous. Also high and low temperatures are not the friend of lipo cells. A cell going above 60c will lose performance and will balloon. A cell going above 70c will break the bag. Newer types of cells will not explode or burst in to flames, but give out a gas and smoke (see the link in the history section). Older cells will ignite and you will not be able to put the fire out. If you have several cells together, the 1^st cell will burn and when you think it has gone out, the next cell will ignite. Also sub zero temperatures can cause damage to your cells. Any prolonged sub zero temps can cause cell to go out of balance or to balloon within a couple of charges.

Any cell going in to deep discharge (below 2.8v) can be irreversibly lost. Some times by using a good charging system they can be brought back to life. But the charge time can increase to over 40 hours. So be patient. Also going above 4.23v reduces the life of the cell and if the voltage continues to climb can cause the cell to balloon and degrade.

There are folded and flat plate cells. The folded cells have a greater internal resistance and are heavier. These cells get hot very quick in our applications. The Graupner cells I tried recently went over 60c in a mono 1 and only lasted about 8 runs before my Schulze charger would not charge them. They were shown on there site as 100 amp discharge and having a 4800mah capacity.

The flat plate types are very thin in comparison and lighter. They have a lower internal resistance. These are the best types to use. They have C ratings up to 30C now, but they are larger. Some of the newer cells are over 5000mah but are very long

Advantages

Lipo batteries do not suffer from the memory effect. They also have a low self-discharge rate of less than 5% per month, compared with anything with a Nimh Cell. We know some Nimhs that will lose over 30% per day. Some are less, but the average is over 30% per month in Nimh batteries and 10% per month in NiCad batteries. The loss is voltage rather than capacity. I have recently done some tests to check this out. lipo’s take around 5 or 6 charges before they are at there best.

Maverick 5000mah 20C cell.

Charged at 5 amps from flat: 5647mah in
Discharged at 2.5amps on auto on a schulze 5324mah out.
Charged at 5 amps from flat again 5653mah in
Left for 48 hours and discharged 5298 mah out

If we left Nimh cells for that time we could have lost anywhere from 10% to 50% or some I have had this year could be flat. The cells were left in a cold shed during the test.

The main advantage is there weight. A 4s 5000 pack will be around 600 grams (including cables), against 840 grams for the 12 Nimh cells (plus solder, connecting bars and wire).

Some of the new types of lipo are getting lighter. 4800 kokams weigh less than 250 grams. They are also similar in size to a 6 cell Nimh pack and thinner.

Another advantage is voltage and capacity:

The Voltage between charged and flat is 4.2v to 3. This is only a 29% drop.

But a Nimh drops from 1.5 volts to 1 volt; a 33% drop. The speed we would run at the end of the race would be faster than with the Nimh’s.

The capacity advantage come from the higher discharge rates allowed. For example:

All the GP cells we have been using since are only rated at 30amps, where as the lipo’s are rated in C rates. IE if a 5amp capacity lipo is rated at 15c then the maximum discharge is 15x5 = 75amps. 5amps at 20 c = 100amps and 5 x 30c =150amps.

Therefore we gain extra capacity from the lower voltage drop you get. Most good lipo’s rated at 5000mah will give around 4000mah out at there maximum constant discharge rates.

Here are some test figures:

All the cells were discharged at 48amps.
Maverick 5000ma 4637 ma out
Fullymax 5300ma 4492 ma out
Kokam 4800 ma 4327 ma out
Graupner 4800ma 3521 ma out
The test on the Graupner cells was cut short due to temperature going above 60c. How ever the voltage was almost at the switch off point, so they would have only run for another couple of seconds.

An example of the differences in performance.

My hydro 2 does 4minutes 20sec with the GP4600 cells. With the 5000mah Lipo I get nearly 6minutes. Doing the calculations gives us:

GP 4600 (4.4capacity @ 30amps) amps 4.4 x 60 = 264amp minutes ÷ 4.33 = 61amps average discharge rate.

A maverick 5000 (4.8capacity @ 50amps) 4.8 x 60 = 288amp minutes ÷ 6 = 48amps average discharge rate.

Or is it? Maybe it is because the NiMHs get very hot and lose so much energy in heat. The heat has to come from some where and the only available energy is the cells. However the lipos come off a lot cooler. Therefore being more efficient.

We all know that if we discharge at higher rates then the run time comes down, but if the cells were designed to be discharged at higher rates then the voltage drop would not be as bad. That’s where the lipos have a major advantage. With the high discharge rates we have even in mono 1 the capacity is dropping in the Nimh cells every run. Also because we discharge the Nimh’s at high rates there life span is limited. We have had quite a few Nimh cells blow up over the last season. Even the Germans are having problems with limited life span and exploding Nimhs.

There were some tests done recently, where some lipo’s were charged and discharged until they reached 80% capacity. They varied from 75 charges with a well advertised lithium retailer to 750 charges for kokams. They were charges at 1 C and discharged at 50amps.

At the moment the cells I am using have been used over 35 times and no reduction in performance. That’s over a season of meetings for less than £60. Some of my Kokams that I use for flying are over 5 years old and still working. On average if you buy good cells you should get a lot higher number of charges than Nimh cells.

Selecting Equipment

An area not covered yet is chargers and controllers. Most of us that have bought controllers and chargers in the last couple of years, will have noticed they are lipo compatible.

All the latest controllers for brushless motors are lipo friendly. Some can be programmed for cut off voltage manually as well, but most are automatically set. If you set lipo mode, they detect the connection voltage and this in turn sets the cut off voltage. This is one of the disadvantages of the new A123 cells. They have a lower voltage per cell and at the moment the controllers are very few. Some of the controllers can be set to NiCAD mode for A123 Cells. They then work on a percentage of voltage change. The NiCAD setting is similar to the A123 voltage differences from full to empty.

This is the same for charging the A123 type cells. A special charger that charges Li-fe or Li-Fe4po4 is required for A123 cells. There are a few chargers about for charging the Li-Fe cells, like the cheap Schulze lipo card2. Of course the more expensive schulze chargers will charge Li-fe cells as well as nearly every thing else. So if you are investing in a new charger.

Check for the following:

Charges as many different types of Lithium cells as possible.
Check the makers web site for up dates for the charger. If you can do it your self, send off for up dates or last of all send it away.
If your buy a cheap charger be prepared to change it as new technology comes along.
Built in balancers can be cheaper.
Look for one that charges at least 5S. This will help when charging the lower cell counts
Only buy an expensive charger if you need to charge faster than 5amps or large cell counts. Or unless it charges more than one pack at a time.
At the lower end on price. The charger must have leds for error warning or connectors for connection to a PC for checking every now and then.
Chargers for Lipos with balancers start from around £40. I have tried a cheap one with balancers built in and it worked great. It even gave me individual voltages for the cells.
The lower cost chargers can take over 2 hours to charge a pack because of a number of reasons. First and most common reason is the voltage multiplier are smaller and second reason is the balancer limits. The balancer discharge rates can be only a few milliamps. Even the expensive Schulze system has only a 1 amp discharge system on the balancer. Charging can take a long time if your cells are out of balance. The higher the discharge rate or balancing rate the quicker the balancer will work. If you are only charging low cell pack numbers. Ie., up to say 4s then you are better off using several low cost chargers with balancers included, than one expensive charger.

How to get started

When first selecting which lipo’s to buy I follow these general rules:

Always read any instructions that comes with any item used with Lithium cells
Always take precautions when charging lipos
use a lipo sack or some thing similar
try and charge out side if you can not be with the cells most of the time
Continually check the cells
Never buy Thick short cells. These cells long cells folded to reduce the length. These tend to be older technology and have a lower C rating for the discharge. They will also be heavier.
Thinner and longer cells are flat plate type and have better discharge rates. Some now up to 30C and by the end of next year 40C
Select a sell with a minimum of 20C and a capacity of 4000mah or greater. This will give you a minimum discharge rate of 80amps. Almost double what we use in mono 1 and 2. For hydro 1, hydro 2 and mono 3 go to 5000mah for the extra discharge rates.
Always check on different web sites the C rating on the cells to compare the details.
If you know anyone using lipos drill them for info on what equipment they are using and which cells.
Buy a reasonable charger. There are good balancing chargers from around £40.
If you already have a good charger that does lipos. Then get it up dated to the latest software if possible and buy a good balancer. A good balancer will protect you and your investments. Most balancers are voltage balancers. When the charge finishes the balancer makes all the cells the same voltage. There is a different type of balancer but these are more expensive. The schulze system balances the cells during charge. If one cell increases in capacity before the other it distributes the extra between the cells. This helps at the end of the charge by not taking as long to balance the pack. The Schulze system starts at around £65 for a 4 cell charger.
Always charge at 1c. If your pack is 5 amps charge at 5 amps or less. This will help with the life of the pack. The lower the charge rate the more life you will get out of your pack. Charging at a higher rate will not give you more power like Nimh’s
Always balance yours cells at least every 3 times you charge them. If your discharge rates are high, then balance them every time
If you fasten them in your model with sticky Velcro then put a band of heat shrink around your pack where the Velcro is going, before sticking the Velcro to the heat shrink.
When racing you can either charge on the day if you want or part charge the day before.
Always allow plenty of time for the balancing if you use a voltage balancer on the day.
If your model slows down unexpectedly bring it in. If a cell has ballooned for what ever reason you may be able to save the other cell. You can buy individual cells for repairing packs.

What you must not do!

Problems usually come from the following reasons:

Charging cells that have fallen below 2.8v
Charging cells too fast
Charging cells that are hot
Discharging cells when the outside temp is too low (cells need to be warm if used around freezing point).
Discharging cells over their recommended discharge rates.
Using cells of the same capacity together from different manufacturers
Using damaged cells
Not balancing cells
Not using a lipo charger
Discharging your cells on a balancer and leaving them connected for long periods. Some balancers use power from the cells. This can take them in to low voltage. So check your balancers instructions. I always do a discharge and charge when checking capacity on Lipos.

Summary

Some times we have to make decisions we don’t want to, unfortunately Nimh cells are too fragile for competitive use. They are costing between 4 and £10 per cell. This is killing our sport. The use of the new technology will bring our sport to new levels. Gone will be the expensive matched cells. The Lipo system has had hundreds of millions of dollars thrown at it and with the latest safety features making them easier for every one to use them safely. Batteries with the same power will be available to every one. No one will be faster just because they have better cells. If you look after your cells you will get hundreds of charges from the makes I have informed you of. The transformation that fast electric racing will take over the next few years should see our sport grow to be even more exciting.

Links for info:

Document last modified 16th November 2007 at 17:49