NIMH or Li-Po Battery for My RC Car

Many new RC car beginners get confused about what kind of batteries they should choose for their RC car NIMH or Ni-Po battery? What are the differences between them? As you may hear some questions like “ What kind of RC car batteries should I buy? “ “Can I use LIPO battery for my RC car?”“What are the differences between 6s 7.2v NIMH battery and 2s 7.4v Lipo battery?” So let’s take these batteries as an example and make a comparison between them in several ways.


NIMH, Nickel Metal Hydride, serves as a replacement for NiCd as it has only mild toxic metals and provides higher specific energy.

LIPO, lithium polymer battery, is a rechargeable battery of lithium-ion technology in a pouch format.

Let’s compare these batteries in the following aspects:

Price:

NiMH:It is cheaper than the Lipo battery. You may find the price of an 7.4V Lipo battery could afford two 7.2V NiMH batteries for your rc car.

Lipo: It is more expensive, but as the production scale of Lipo battery gets bigger, the production cost will get lower and lower and so as the price.

Voltage

NiMH: It’s nominal voltage is 1.2v per cell. As we can see from the picture, the common voltages for NiMH battery are 6v, 7.2v, 12v, 14.4v.
Lipo: It’s nominal voltage is 3.7v per cell. So we usually find Lipo batteries in 7.4V, 11.1V, 14.8V, 18.5V, 22.2V.

Weight/size

NiMH: 7.2v NiMH batteries are heavier than 7.4V Lipo batteries for the same capacity. And it’s bigger than the Lipo one.
Lipo: Lighter and smaller than the NiMH one, this makes a great difference for rc cars . That’s why Lipo batteries are getting more and more popular in the RC cars.

Memory effect

NIMH: It has the memory effect. It is commonly believed that when rechargeable batteries are not fully discharged between charge cycles that they remember the shortened cycle and are thus reduced in capacity. Which is also called “voltage depression”.

Lipo: Do not have the memory effect. So the full capacity of the Lipo will always larger than the NIMH one after being used for weeks.

Charge and discharge:

NIMH: It is not that dangerous if it is over discharged or over charged. But over charging or over-discharging will shorten the life span of the NiMH batteries.

Lipo: Can never be over discharged or over charged. Or it will damage the battery, explode, or even get your rc car on fire.

Battery charger

NIMH&Lipo: You have to choose the correct battery chargers for them or use charger that can both charge NiMH and Lipo batteries.

Power&Capacity

NIMH:A nimh battery has a consistently less power as you run it down.

Lipo: Lipo batteries will provide longer and quality run time for its larger capacity and C rates.
To make a conclusion, lipo batteries are better choice for RC cars.

3S or 4S Battery Question

The KV rating indicates an unloaded ( no prop) RPM that the motor will spin at. The actual RPM you get will depend on the prop you use. The ESC will work with a 4S battery, but for any given set up ( load and throttle setting) the 4s will result in a higher average current delivery than the 3S.

The 11.1 v assumption you are making on the the 3S battery is the low end of the charge/ discharge voltage curve, I would use a fully charged battery voltage to think about current draw. If the top end the motor will deal with is 660 watts, at 12.5 v that will be a current draw of 52 to 53 amps, which exceeds the capacity of your ESC ( 45 amp continuous, 50 burst). The ESC seems to be the limiting factor in your power system, so to use the Gens ace 4s battery you may need to consider reducing your prop size and or pitch to maintain the same overall current draw on the system.

Because the ESC deliver current in a time gated fashion ( full current on then off with the interval determined by your thottle setting) you cannot depend on the just assuming you can fly at less than full throttle to keep your current delivery in a safe range for your ESC.

There is the benefit of getting more watts out of your setup. The Afro 20A slim ESC (2S-4S) and 18AWG wire are rated for 20A. Amps is what melts your wires and ESCs. Watts is what makes your motors powerful generally speaking.

For each motor:
A 3S setup will have a theoretical watt limit of 12v x 20A = 240 watts
A 4S setup will have a theoretical watt limit of 15v x 20A = 300 watts

Also the LiPos “contain” more watts per mAh in a 4S lipo vs a 3S lipo:

Lets compare 2200mAh 40C lipos

2200mAh 3S 40C lipo: 2200mAh x 12v = 264 watts – continuous draw: 88A
2200mAh 4S 40C lipo: 2200mAh x 15v = 330 watts – continuous draw: 88A

4S tends to be a bit heavier than Gens ace 3s lipo with the same mAh and C rating though. What makes most sense is to compare the watt to weight ratio.

Another vital factor to be aware of with the Silver series ESC is that they use a % of initial voltage to determine the LVC, as opposed to the fixed voltage that most other manufactures use. As stated in their instructions, never use a partially discharged battery on thier ESC as it may set the LVC at a point that will result in battery damage if flown to the LVC.

The motor will WANT to increase speed by 1000 rpm (per volt), but will lose some to heat and what have you. I wouldn't get too bogged down on that part right now.

Watts = volts X amps

A higher VOLTAGE setup is often used to REDUCE the AMPS required. If you look at the formula above, if you increase the voltage, you reduce the amps.

The esc amp rating has NOTHING to do with the amount of power your setup will draw. It is a MAXIMUM rating for the esc. Think of an electrical fuse. You can flow anything from zero up to the fuse limit with no problems. Exceed the maximum rating and bad things happen. Smaller esc's are usually used to save weight, room, and money.Similar with the C rating of the battery pack. C rating x the battery capacity (in amp hours) = the number of amps the battery CAN provide at its maximum level. You can certainly use a higher C rating battery, but not a LOWER C rating.

Why Lipo Battery Is Different from Ohter Battery Types

Modern Lithium Polymer batteries (LiPo, Li-Poly) are able to store and deliver large amounts of energy from light-weight packs. Think of and treat LiPo batteries as fuel. Lithium Polymer cells, as with any high energy source (petrol, electricity, gunpowder etc) must be handled with appropriate precautions and care. Lithium Polymer batteries have been proven world-wide to be a practical and enjoyable power source for model aircraft.

LiPo batteries may take a wide variety of shapes due to the gelling agent separating the cells. Machines and tools with strange sizes and shapes often use LiPo batteries for this reason. However, because they are stronger and potentially more dangerous than other rechargeable batteries like NiCd or NiMH batteries, special care must be taken.

What is the composition of a Lipo Battery?

LiPo battery consists of one cell or 2 or more cells connected to provide a specific voltage and/ or current capacity.  The battery is interfaced into an AV using a basic connector, such as a Deans. The connector consists of a red (+) and black (-) lead as well as a Cell Balancer Connection.
 

Each cell in a multi-cell LiPo pack is rated for 3.7 volts and requires a charging voltage of 4.22 volts. When operating with a  LiPo battery it is EXTREMELY important that you do not allow any cell to drop below 3 volts. Discharging a LiPo cell to less than 3 volts can cause irreparable damage to the internal chemistry causing dramatic reduction in battery life, charge capacity, and discharge time.

It is not uncommon for individual cells in a battery pack to discharge at different rates over the duration of a flight.  Cells may vary in voltage by a couple tenths of a Volt during discharge.  It is important to program your transmitter or Autopilot to alert you before a cell drops below 3 volts. A LiPo cell has a nominal voltage of 3.7V. For the 2s lipo 5000mah above, that means that there are two cells in series (which means the voltage gets added together). This is sometimes why you will hear people talk about a "2S" battery pack - it means that there are 2 cells in Series. So a two-cell (2S) pack is 7.4V, a three-cell (3S) pack is 11.1V, and so on.

Special Considerations when Using LiPo Batteries

The main concern that people using LiPo batteries have is that they have a tendency to explode. This is an obvious problem and means that extra care must be taken to prevent this from occurring.

General usage tips:

1.Lithium batteries don't work well in cold air. If you are flying in the winter keep the batteries in your car for best performance.
2.Don't let the batteries overheat. Try and keep them under 140-160 degrees F. This will prolong your battery life.
3.Don't push the batteries past their rated maximum C rating. This will damage the battery and the apparent capacity of the batteries will drop. If when you recharge you are only putting ½ to ¾ of the rated capacity back into the batteries you are probably pushing them too hard.
4.If your building your own cells then put spacing between each cell in the pack to help cooling of the pack. This is most important when building packs larger than 2 cells.
5.Some LiPoly cells use aluminum tabs that you must solder to. Normal soldering procedures will not work on aluminum. You'll need to purchase aluminum soldering paste. The vendor where you purchased your aluminum tab cells should stock this paste.

Lipo battery Voltage and Capacity

When talking about a LiPo, the primary characteristics to understand are the battery’s voltage and capacity. This is typically noted in a shorthand such as “4S-2200”. In this example, “4S” denotes that the battery has four cells in series. The nominal voltage of each cell is 3.7 volts (4.2v fully-charged), so the total pack voltage is:

4 cells x 3.7v = 14.8v.

The second number denotes the capacity of the battery in milliamp-hours (mAh). A fully charged 2200mah is rated to provide a current of 2200 milliamps (2.2 amps) for one hour before it is fully discharged. This capacity value is completely independent of how many cells are in series. In simple terms, the capacity value allows you to estimate how long a battery will provide useful power in a given application. In practical terms for RC use, the capacity rating is typically only helpful for rough comparisons of different batteries. i.e. a lipo 3s 5000mah battery will provide about double the run time of a 2S-2500 lipo in the same RC car.

A 4S2P-2200 battery would consist of two 4S-1100 batteries wired in parallel to provide a total 2200mAh capacity. All other things being equal, you would care for and use this battery the same as you would the previous 4S-2200 example (which is really a 4S1P-2200, but we ignore the 1P). There may be a difference in physical size, but a 4S-2200 and a 4S2P-2200 are functionally equivalent. The differences will really only matter to the guy at the factory who has to assemble the battery.

Check for Lithium Cell Balance

Before charging a new pack, check the voltage of each cell in the pack individually by using the taps. Keep a record and check every tenth use cycle. An unbalanced pack, or a pack with voltage levels not within 0.1 volts of each other, may catch fire. If the pack becomes unbalanced after every discharge, it should be replaced.

Do Not Charge Batteries Unattended

LiPo batteries don't take too long to charge, but there should always be someone near the battery and charger to make sure nothing bad happens. This is very different from other rechargeable batteries, which are more set-it-and-forget-it products. Charge in an open, ventilated space on a safe, fire-retardant surface such as a fire-safe Pyrex dish with sand or a fireplace. Keep a bucket of sand nearby in case a fire starts. If it does, dump the sand on the fire to put it out.

Life of Lithium Polymer Batteries

A Lithium Polymer battery needs to be replaced when it holds 80% or less of its capacity. If used at the maximum continuous discharge rate on every cycle and every charge is at 1C or greater and it’s used down to 3.00V per cell under load (when your LiPo compatible ESC cuts power to the motor) then don’t expect to get  more then 40 or 50 cycles from your LiPo Pack. Please see my guide to prolonging the life of your LiPo’s.

10C 15C 20C Lipos - What does the C mean

How quickly a battery can discharge is it's maximum current capacity. Current is generally rated in C's for the battery. C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2C discharges the battery in ? or half an hour. All RC batteries are rated in milli Amp hours. If a battery is rated at 2000 mAh and you discharge it at 2000mA (or 2 amps, 1 amp = 1000mA) it will be completely discharged in one hour. The C rating of the battery is thus based on its capacity. A 2000mAh cell discharged a 2 amps is being discharged at 1C (2000mA x 1), a 2000mAh cell discharged at 6 amps is being discharged at 3C( 2000mA x 3). I think gens ace lipo is best choice, they are the proffessional manufacturer of Lipo battery.


Currently LiPo technology does not allow currents as high as NiCad or NiMH batteries do. For this reason, this many LiPo batteries are put in parallel to increase the present capacity of the battery pack. When 2 batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have 2 2000mAh cells and you wire them in parallel then the result is the same as 14000mAh cell. This 4000mAh cell has the same C rating as the original 2000mAh cells did. Thus if the 2000mAh cells could discharge at a maximum of 5C, or 10 amps then the new 4000mAh cell can also discharge at 5C or (4000mA x 5) 20 amps. This method of battery pack building allows us to use LiPo batteries at higher currents than single cells could produce.

The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the XSXP method. The number in front of the S represents the number of series cells in the pack so 3S means it's a 3 cell pack. The number in front of P means the number of cells in parallel. So a 3S4P pack of 2100mAh cells has a total of 12 cells inside. It will have the voltage of any other 3s lipo 5000mah since the number of cells in series determines the voltage. It will have the current handling of 4 times the maximum C rating of the 12 individual cells. So say our 3S4P lipo pack had a maximum discharge of 6C. That means that it has a nominal voltage of 10.8 volts (3x3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).

The maximum continuous current delivering capability of a battery can be determined by multiplying the C-Rate X (mah of battery).

A 2000mah, 20C battery would be capable of delivering 40000ma (40A).
A 2000mah, 25C battery would be capable of delivering 50000ma (50A).

That's in a perfect world - however, there are no standards for how a manufacturer/vendor decides how to C-Rate their batteries. The big boys like Enerland probably do a good job of accurately descrbing the C-rate. Other pack makers may be another matter. They may call a real 15C pack a 25C pack in order to sell more. You just have to go by the RCG reports on the packs to get some idea on how accurate the C-rate is.

The other side of the coin is flight time. To get a 10 min flight, the average C-Rate draw needs to be 6C. A 20C pack operated at 20C will last a max of 3 min flight time and you'll maybe get 50 cycles out of it at the best.

Here is a good rule of thumb to go by.

3min flight to 80% drain = 16c avg and it can peak 32c = use 30c+
4min flight to 80% drain = 12c avg and it can peak 24c = use 25c+
5min flight to 80% drain = 10c avg and it can peak 20c = use 20c+
6min flight to 80% drain = 8c avg and it can peak 16c = use 15-20c+
7min flight to 80% drain = 7c avg and it can peak 14c = use 15c+

For me, high C-rates (>15) have no value as I want at least 8-10 min flight time. However, I'll take them if the price differential is small as it generally means a battery with a lower IR.