Batteries

Batteries (rechargeable)

There is a wide array of rechargeable batteries to choose from but it will pay dividends to consider the pro’s and con’s of each battery type before making a choice to suit your application. You will need to take account of the physical space available, the weight of the battery and its impact on the balance of the model, the voltage necessary to propel the model and the amp rating (power) to ensure the battery doesn’t go flat while the model is in the middle of the lake!

Lead acid. (Pb) These batteries have been around for a long time, first used in 1859. They are robust but the main advantage to the lead acid battery is that they are cheap. They are also heavy but for some applications this can be a useful feature as they can also be used as ballast in a model boat.

Most are now supplied as sealed units from the factory so require no maintenance but they will last longer if they are stored fully charged, disconnected from their application, and they will self-discharge so should be recharged every 6 months or so. They are not designed to be deeply discharged and this can result in permanent damage.

Each cell has a nominal voltage of 2.1 volts so a 3 cell battery will be rated at 6 volts.

Nickel Cadmium. (NiCad). These are similar to NiMh (below) but have now fallen out of fashion and are no longer widely available as NiMh is considered superior as they are cheaper, produce more power for weight and less harmful to the environment

Nickel Metal Hydride. (NiMH) This type of battery is very versatile. They have 2 to 3 times the power of the NiCad by weight and can be fast charged but will give longer life if trickle charged. If used in the same way each time, they can develop a “memory” but this can be removed by a number of charge and discharge cycles.

They are available in a wide variety of shapes and sizes but will be in multiples of cells with each cell having a nominal voltage of 1.2 volts. A pack of 4 or 5 AA size NiMH batteries will power the radio and servo’s on a model yacht all day long. They are safe to use and will take some abuse but will self-discharge over time so should be recharged every 6 months or so.

These batteries are also available in different physical sizes, AAA being the smallest and lightest and C and D being the largest. The size is not necessarily the determining factor for power as this is determined by the mAh rate.  

Lithium Iron (LiFe). Also a versatile battery which has a wide variety of uses similar to the NiMH. It is capable of producing more power than a NiMH but are more expensive so where more power is required most users will use LiPo. Compared to the LiPo, the LiFE is safer, non-flammable in normal use, cannot be overloaded and will last longer with more charge cycles.

Each cell has a nominal voltage of 3.2 volts and the battery should be stored fully charged. They can be fast charged at a high rate (up to 10C, see later), they are more robust than a LiPo and safer for the environment.

Lithium Polymer (LiPo). This is the “go to” battery for power but at a price as they are expensive. They are hazardous, flammable and can be overloaded and if so will often explode with disastrous results for your model.

They have a highest discharge rate than other batteries so are suited to models where large amounts of power are required over short periods but must be used with an appropriate ESC as if a cell discharges to less than 3 volts the battery will become deeply discharged and irreversible loss of performance will result.

Each cell has a nominal voltage of 3.7 volts and will fully charge to 4.2 volts. After use the battery should be recharged to its storage rate, about 3.7 volts. You must use a battery charger with a LiPo facility.

Fast speed boats will tend to use LiPO batteries but they are less appropriate for other applications.

Battery capacities.  The power of a battery is normally expressed in mAh, milliamp hour. A 2200 mAh battery will discharge at 2.2 amps for 1 hour. If your motor draws 1 amp then the battery will be full discharged after 2 hours and 12 minutes 

LiFE and LiPO batteries will also have a C rating. This rating is important for both charging and discharging the battery.

A 5000mAh battery with a C rating of 30 can deliver up to 150 amps if demanded by the motor (5 x 30) but the battery will be discharged very quickly, in this case in about 2 minutes but such a burst of power is likely to lead to very exciting performance from the model! (Note that in this case, if the motor is rated at 150 amps then the the ESC should be rated at the same or more)

Sometimes the C rating is expressed as two figures, eg 30-60C. In this case the battery can take "bursts" of up to 300 amps, for very short periods. 

Recharging the battery is best at 1C, in the case above, a 5000mAh battery should not be recharged at more than 5 amps and if time permits a rate lower than this will give a longer battery life and better performance.  

Battery chargers. Each type of battery requires to be charged in a specific way or damage will result. These days “smart” chargers are available with various programs for the different batteries and which will deliver the charge in the way that suits the particular battery. They are not expensive and as its likely you will use different types of battery for different applications they represent a good investment.

Battery power. Each battery of whatever type will have a mAh rating (milliamp) which represents the power the battery can produce. A single cell AA NiMh will always be 1.2 volts but will have a different amp hour rating. The higher the number, the longer the battery can deliver its 1.2 volts.

A 2200 mAh (milliamp hour) will deliver 2.2 amps for 1 hour. If the motor is drawing 5 amps in operation, the battery will last for something less than ½ hour (26 minutes). Most of us are probably bored with the maths so will use a battery with a higher mAh rating than necessary which isn’t a problem in most applications and probably a good idea but a battery with a higher mAh rating will be heavier than one with a lower mAh rating.

“C” rating. The C rating refers to the safe rate at which the battery can be charged, and also discharged. For a 2200 mAh battery, 1C will be 2.2 amps and unless specified the battery should normally be charged at no more than 2.2 amps. Fast charging will go above this figure but repeated fast charging will reduce the performance of the battery in time. Where possible a battery will give longer life if it is charged at below its C rating.

LiFe and LiPo batteries will also have a C rating much higher than this for discharge. I’m looking at a LiPo which has a 2200 mAh rate (1C for that battery) but also has a 40C discharge rate and a burst rate of 80C. This means that in normal use the battery can be discharged at 88 amps but with bursts of up to 176 amps. 

This typical LiPo battery comprises 6 cells so will deliver a nominal voltage of 22.2v.

It is rated at 5000mAh (5 amps) so will deliver 5 amps for 1 hour. 

It has a discharge rate of 65-130c so if demanded by the motor can deliver 325 amps (65x5) and in bursts up to 650 amps (130x5) 

These high discharge rates cannot be maintained for long and if your motor is demanding 300 amps will discharge in 1 minute