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Battery Basics

01/19/2006 by

Copyright:© 2006 Horizon Hobby, Inc.

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Whether you are involved in electric or glow-powered flight, rechargeable batteries have a dramatic impact on the performance of your particular model. Combine the varying range of experience that someone might have before they even pick up a radio with the numbers of different battery types, chemistries, and capacity and it can be rather easy for someone to do the wrong thing when it comes to battery selection or maintenance. Quite often people may damage or otherwise reduce the life of their rechargeable cells before they even use them for the first time. While it may seem like there are too many different types of cells and it might seem confusing, knowledge is power.

These are three of the most common park-flyer battery packs. Each one has a slightly different capacity, size, connector, and chemical composition. You can see here how much smaller a comparable LiPo is versus a standard battery pack.

Choosing the Right Pack for You:

Regardless of what type of model you will be using your particular battery in, there will undoubtedly be a number of different chemical compositions to choose from.

Nickel Cadmium (NiCd), Nickel Metal Hydride (NiMH), and Lithium Polymer (LiPo) cells are currently the most commonly used, but each needs to be charged, discharged, and stored differently. On top of that, each model may require a different cell count or battery configuration as well. To determine what pack configuration you will need, check the owner’s manual of your particular model for more info. The battery you will need should be listed in the “Items Needed to Complete” section of your manual.

Battery Basics:

One of the most common misconceptions about batteries and battery packs is that a battery pack is made up of one very large battery. Truth be known, a battery pack is actually constructed from a number of individual batteries, called cells, that have been connected together to work as a single pack. There are two ways that the cells can be connected together. The first is called “Series”, where the positive terminal of one cell is wired to the negative terminal of another cell. This method is used when you want to increase the output voltage of the total battery pack, as the individual cell voltages are actually combined to create one large voltage output. For example, a 6-cell NiCd or NiMH pack is made up of cells rated at a nominal 1.2 volts each. When wired in series, you take the individual voltage (1.2V in this case) and multiply that by the number of cells in the pack (6) to get the total pack nominal voltage. If you do the math, you’ll see that a 6-cell pack has a total nominal voltage of 7.2 volts. This is the most common cell connection method found in the RC hobby.

While a battery may be a battery in many instances, the connectors can vary quite a bit. This battery from a HobbyZone Firebird Commander 2 is similar in dimensions to the standard ParkZone J3 Cub’s pack, but the connector is significantly different.

Here we have a winner. The right battery with the right connector makes all the difference in the world.

The second way to build a pack is called “Parallel.” In this method, you connect the positive terminal of one cell to the positive terminal of another, and do the same with the negative terminals. Unlike a Series connection that increases the voltage output of a battery pack, wiring cells in Parallel increases the total capacity of the pack. Much like the voltage calculation, but to figure out what the actual end result capacity will be, simply add the mAh rating ( milli-amp hour) of the cells being paralleled together to figure out what the capacity of the pack is. If you are using 2100mAh cells in a 2-cell parallel pack (commonly referred to as “2P”), multiply 2100 by 2, and you will get a total capacity of 4200mAh. Just remember though, a Parallel connection does not change voltage, so while you can get 4200mAh out of a 2P pack, , the nominal voltage will remain the same.

Chemistry Class:

Rechargeable receiver packs save you money in the long run and are generally lighter than equivalent alkaline packs.

As I mentioned before, there are three major chemistry types used in constructing a rechargeable battery. The first one is called Nickel Cadmium, or NiCd (pronounced Ni-cad) for short. While not as commonly used as they once were, there are still a number of NiCd packs sold and used each year. NiCd batteries are relatively inexpensive, but they have a number of negatives. NiCd batteries need to be fully discharged after each and every use. If they aren’t, they will not discharge to their full potential (capacity) on subsequent discharge cycles, causing the cell to develop what’s commonly referred to as a memory. Additionally, the capacity per weight (also known as “energy density”) of NiCd cells is generally less than NiMH or LiPo cell types as well. Finally, the Cadmium that is used in the cell is quite harmful to the environment, making disposal of NiCd cells an issue. In fact, several countries in Europe have banned NiCd batteries for just this reason. This ban is what sped up the demand for alternative cell types, and the first to really answer the call was Nickel Metal Hydride (NiMH).

NiMH cells have many advantages over their NiCd counterparts. With the removal of Cadmium from the cell, the NiMH cells were able to fill the need for industrial and hobby-grade batteries all over the world. NiMH cell manufacturers were also able to offer significantly higher capacities in cells approximately the same size and weight of comparable NiCd cells. NiMH cells have an advantage when it comes to cell memory too, as they do not develop the same performance issues as a result of improper discharge care.

Lithium Polymer batteries have really taken a foothold in the air market due to their high capacities, high voltage outputs, and light weight. A LiPo pack can weigh as much as 50% less than a conventional can-style battery pack.

Lithium Polymer (LiPo) cells are the newest and most revolutionary cells to come to market. LiPo cells typically maintain a more consistent average voltage over the discharge curve when compared to NiCd or NiMH cells. Add to that the higher nominal voltage of a single LiPo cell (3.7V versus 1.2V for a typically NiCd or NiMH cell), making it possible to have an equivelant or even higher total nominal voltage in a much smaller package. LiPo cells also typically offer very high capacity for their weight, delivering upwards of twice the capacity for sometime ½ the weight of comparable performance NiMH cells and packs. That’s right, with LiPos you can often achieve higher voltage and power output, with more capacity, in a lighter weight package.

With all of these benefits, why aren’t LiPo packs more widely used? With so much energy packed into such a small space, there are some important safety measures to take when dealing with LiPo cells. A LiPo cell needs to be carefully monitored during charging as overcharging a LiPo cell (to beyond 4.2v), or the charging of a physically damaged or overdischarged cell (discharged to below 3.0v under load) can be a potential fire hazard.

Many LiPo batteries come with safe charging circuitry integrated into it. This circuitry prevents over charging, over discharging, and in some instances helps to balance the pack out. If your pack has a “Charge” lead on it, always charge through that connector.

While some battery chargers can charge either NiCd/NiMH or LiPo cells, chargers that do all three major chemical types are starting to surface. The Dynamite Vision Peak Ultra can charge your LiPo packs along with NiMH and NiCd batteries as well.

If you are going to go the LiPo route, use a charger that can correctly charge them (using a constant current, constant voltage method of charging as LiPo cells can not be “Peak Charged”), such as the Vision Peak Ultra (DYN4053) or the E-flite Celectra 1-3 Cell charger (EFLC3005). Not only must care be taken when charging LiPo cells, but when discharging them as well. You should never over-discharge a LiPo pack to below 3.0v per cell under load, and you must use an ESC programmed to provide the proper low voltage cutoff for your pack (for example, a 9v cut off for a 3 series LiPo pack). Also, you should never dead short a LiPo pack, even if only for an instant, as the large amount of energy stored in the small package can catch fire quite quickly as a result. While these seem like major deterrents to using a LiPo battery, these usage guidelines are quickly becoming well known as they are typically well outlined in the instruction manuals included with most LiPo packs, ESCs and LiPo chargers. However with all of their performance benefits, there is little doubt that lithium polymer battery packs are currently the future of battery technology for electric powered models.

To Build or Not To Build:

Some people prefer to assemble their own battery packs rather than buy preassembled packs. Before you solder on the cells, scuff up the terminals to provide a better connection and more secure solder joint.

Many people damage their battery packs before they use them for the first time with poor soldering techniques. Remember to use an iron with a large enough tip to transfer heat, apply solder to the items being joined and not the iron itself, and don’t hold the iron onto the battery for too long.

There are two different ways to purchase your batteries, either as loose cells or as pre-assembled packs. With individual batteries (cells), you’ll need to solder the cells together yourself to create your own battery pack. The second option is to purchase a pre-assembled battery pack that comes with the pack pre-assembled and shrink-wrapped, often times with the connectors pre-wired. If you do not have much soldering experience and/or do not have a high-quality soldering iron, it will generally be best to purchase a pre-assembled pack.

As a safety note, you should NEVER attempt to solder LiPo cells together into a pack configuration yourself. LiPo cells are very susceptible to heat damage, and excessive heat can cause them to possibly leak or even explode. For this and other reasons, most LiPo manufacturers willl offer pre-assembled packs only.

Discharging and Storage:

Discharging and storage really go hand in hand. For NiCd packs, you should completely discharge them, to 0.9v per cell, before you store them NiMH packs should be stored with roughly a 50% charge in them for best performance. And before you charge a NiMH pack for the first time in the day, simply drain the pack completely on a discharger or in the model and you are then ready to charge the pack for use throughout the day.

LiPo batteries are completely different when it comes to discharging and storage. Depending on the output voltage of your pack, you should only discharge your pack so far. For example, during use, a 7.4V LiPo battery (also known as a “2 series” or “2S” pack) should never be discharged below 6.0 volts under load (3.0v per cell). For storage of 2 weeks or more, LiPo packs should be stored at approximately 3.8v per cell to prevent overdischarge or imbalance from developing among series cells in the packs due to differing levels of each cell’s self-discharge rate. In the case of a 2S 7.4v pack, the pack should always be stored at 7.6v. You should also store your LiPo batteries in a fireproof container or cabinet and never store your LiPo, or any other battery for that matter, in the model itself.

Pehaps no other item has quite as much effect and influence on our hobby as rechargeable batteries do. There are almost as many different theories and misconceptions out there about rechargeable batteries as there are individual battery sizes and types. Just remember that knowledge is key when it comes to batteries, as is consistent charging, discharging, and storage. Whether you are in the hobby shop or at the flying field, feel free to ask questions of those around you who may be more experienced. That is one of the best ways to learn and grow in the hobby.