Everything you need to know about RC Batteries

One of the most questions we get has to do with batteries, and a lot of the information out there is very questionable, and downright dangerous. Especially when it comes to LiPo batteries.

The info I am going to share with you obviously will be related to RC, but it will give you a better understanding of batteries in general.

The questions we get mostly is: “Which batteries are best for my car?” “How do I get the most power from my battery?” “How do I make them last?” etc.

Batteries mostly used in RC Hobby models is, NiMH and LiPo Batteries.

Then there are questions related to charging, C-Rating, capacity and so on.

Lets look at battery types.

1. NiMH (Nickel Metal Hydride)
2. LiPo (Lithium Polymer)

This refers to the chemistry-class of the battery and materials inside the battery that react to the storage and release of energy as electricity.

Each one has its Pro’s and Cons…

NiMH battery packs commonly named as stick packs, are constructed with cylindrical cells which looks like the common batteries we know. Stick packs are made up of 6-7 1.2 volt cells, to make up 7.2V and 8.4V.

Most Brushed Ready-to-Run (RTR) models that includes a battery comes with NiMH batteries.

They are durable, inexpensive and do not need too much in a way of special care. They’re heavier than LiPo batteries of similar specs., and their discharge rate decreases more steady while being discharged compared to LiPo batteries.

Even though LiPo batteries are more or less the same size, they are assembled with flat slab like cells. Each single cell has a voltage of 3.7 volts. So a 7.4 volt pack consists of two cells and a 11.1 volt pack is has 3x 3.7 volt cells and so on.

For more advanced cars like the Traxxas X-Maxx, XRT, Maxx, Sledge etc.  the MAXX as example runs on a 4S pack which gives you 14.8 volts. The Big Daddy X-MAXX 8S runs with 2x 4S (14.8 volt) packs connected in series to the ESC to give you 29.6 volts.

A LiPo battery is lighter than a NiMH of similar voltage and capacity which will in turn make your model feel more powerful. This “feeling of power” or commonly referred to as “punch” is the LiPo’s ability to maintain a higher voltage for a longer time of its run time.

Downside of LiPo’s compared to NiMH are cost and durability. NiMH is much cheaper and durable, where LiPo’s require special care to for longest life and safe usage.

The next topic is C-rating which many RC enthusiasts find confusing.

After capacity ( I will explain capacity a bit later in this article ), C-rating is often the most prominent number on a LiPo pack. That is the 35C or 50C that is displayed after the capacity on a pack. Ever wondered what that means?

A battery’s “C-rating” is a reference to the maximum amount of continuous current it can deliver.

The rating is expressed as the battery’s capacity ( the “C” ) multiplied by the rating number.

So if you have a 4000mAh, 35C battery, the math is 4000 x 35 = 140 000 milliamps. which is then divided by 1000 to get 140A. That means this battery can handle continuous current draw up to 140A.

The larger, heavier and more powerful your model, the higher the C-rating you’ll need and of course smaller, lighter and less powerful models can use lower C-rated packs.

Here come the confusion and discrepancy regarding C-rating. C-ratings are not consistent from brand to brand and Brand A’s 25C battery pack could actually be able to handle more current compared to Brand B’s 40C battery pack.

So comparison will only be accurate if you compare C-ratings of the same brand.

What does capacity and voltage mean and do?

The easy answer, is when you purchase a battery, whether it is NiMH or LiPo, the big number is capacity ( this determines how long your car will run on a charge ). Voltage on the other hand determines how much power and speed your model will deliver.

Lets cover the important thing first. SPEED and POWER! well, for me anyway.

As briefly explained earlier, the voltage of a pack is determined by how many cells it has.

With NiMH each cell delivers 1.2 volts per cell and for RC cars are most commonly made up of 6-7 cells. They are usually referred to as a 6 cell or 7 cell stick pack or on the other hand a 7.2V or 8.4V stick pack.

LiPo’s are different, even though the principle is the same, a single LiPo cell delivers 3.7V each, so LiPo packs have less cells for a given voltage. And similarly to NiMH are referred to as 7.4V or 11.1V packs, or 2 cell and 3 cell packs.

More volts is better because it delivers more power and speed, but only up to a point. Your RC Model can only handle enough voltage as to what the power system is designed handle. Exceeding that will at the very least, (if it has an over voltage protection installed) shut down, or at worst fry your electronics. Always check your model’s speed control manual for maximum voltage specs. before upgrading.

Run time!

This is the big number on battery packs, and indicates the capacity in milliamp hours ( mAh ). The bigger the number, the longer your car will run on a charge.

Think of it as your car’s fuel tank. The bigger it is, the longer you will drive, but also, it will take longer to fill up. Your battery will therefore also take longer to charge.

Which brings us to the next topic.

One of the best investments you can make, is get yourself a good Charger.

Choosing a Charger

Firstly, make certain that you get a NiMH charger for NiMH batteries, and a LiPo charger if you have LiPo batteries. This crucial to get that right.

If you use both types of batteries, there are chargers that can be set to charge both types of batteries, and other types too. Always make sure you have the right setting before you push the start button.

After making sure you have the right charger for your battery type, comes amperage. The higher the charger’s amp output, the faster it can charge your pack.

Do the math. Divide the battery capacity by the charger output. Example: your battery is a 5000mAh and your charger puts out 5 amps. 5000 milliamp-hours equals 5 amp-hours. Divide 5 by 5, and you get a charge time of 1 hour. The higher the amp rate, the quicker the charge time.

Charging Lipo’s works a little different than NiMH, and it is vitally important to always balance charge your LiPo. The little white connector you’ll find on most LiPo packs is called the balance plug. It is wired in such a manner, that the charger can read each individual cell’s voltage and charge it accordingly.

When a pack is balanced, it means that all the cells have the same voltage. Example: a 2-cell 7.4 volt pack is balanced if both cells have 3.7 volts. if you do not balance cells, their voltages will differ after a few cycles ( charging & discharging) Why is this important? LiPo cells do not tolerate being over charged either. While your ESC’s (Electronic Speed Control) low-voltage detection will prevent the pack’s total voltage from dropping past a certain point, the speed control only reads the total voltage of the pack, not the voltage of each cell.

Lets say your cut off is set at 3.3 volts per cell or 6.6 volts for a 2-cell pack. If the pack is unbalanced, that 6.6 volts might not represent 3.3 volts per cell. You might actually have one cell at 3.6 volts, and another at 3 volts meaning the 3V cell will be over discharged, and your pack is compromised or even ruined.

Note: there are very good contenders for the Darwin awards that give people very dangerous advise once a pack has been compromised, ( this includes hobby shops ), and when the charger, which has safety measures built in, and does not read a compromised cell anymore and gives an error. (This is there for a reason people!) I know LiPo packs are expensive, and everyone tries to save a buck here and there, but it is just not worth it. The end result will be way more costly than saving a quick buck and downright dangerous. This goes for buying second hand packs too ( Usually the pack puffs, but sometimes it does not. So what seems like a “healthy” pack, might have already been compromised.) Best is to discard it safely, and buy a new one. I have seen some spectacular and downright scary LiPo fires. DO NOT DO IT.

For extra safety always store and charge your LiPo batteries in a Lipo Safe Bag. This is a Flame retardant bag used to store LiPo batteries safely.

Charging safety tips

1. Always check the charging mode
2. Plug in safely ( never rig connections with exposed wires or clips, as these may short circuit and damage your pack, or start a fire)
3. Never, ever leave the charger unattended.
4. Keep your packs clean and always inspect them for damage after every driving session

How do I keep my LiPo’s in tip top shape?

• Always use low-voltage detection on your ESC’s. If you don’t, you run the risk of over discharging the battery. At a minimum, this will stuff up your batteries performance, and reduce the lifespan of your pack. At worst, the pack will swell or puff, and must be discarded ( ” it still works” famous last words ) Dispose of it safely.
• Packs can and will occasionally get bashed a little hard whether LiPo or NiMH. Repair damaged shrink wrap, busted connectors and frayed wires
• Always store Lipo’s on 50% charge. Do not store fully charged for longer than 10 days.  ( the battery’s voltage will degrade if stored full for longer periods )
• A discharged pack, if low voltage cut-off has been used, and assuming it has not been compromised before can be stored around three weeks without any damage. However, if you leave the pack too long, eventually it will self-discharge to the point where it will be over-discharged. There are chargers that has a storage charge function that will put the battery in the correct voltage to be stored safely.
• Store your batteries in a cool dry place. Once again there are people that say to store it in the fridge. Do not Store the battery in a cold refrigerator. Even though a cold battery has less chemical reaction taking place which can prolong its lifespan, taking a battery out from a cold fridge can cause condensation to occur on the inside of the battery, which can be very dangerous. The myth that a colder pack gives more performance is nonsense. Run your packs at normal ambient temperature. Avoid hot garages and leaving packs in the sun of course.

Hopefully I have cleared up a few dangerous misconceptions out there regarding LiPo batteries, and you can safely use your RC Packs for longer and safer.

Control Your Passion!!!