Voltage, Cell Count(S) And C Rating:
The S count and voltage:
A Lipo battery is built of one to a few cells. You know the number of cells included in your pack by its voltage as each cell is 3.7v but also by the S number written on the pack: 1S = 3.7v, 2S=7.4V (so 3.7v x no of cells = V of pack). So if you need 11.1v you will need a pack with 3S or 3 cells (3.7v x 3 = 11.1v). In our boats the RMG can take up 9v so we will be using a 2S.
The C rating:
The C rating is a multiplier that indicates the maximum power a pack can provide for a short time. This figure is important for those racing RC cars, RC offshore boats, anything that requires speed or excessive power. In our situation we are looking for time on the water so the actual capacity of the pack is more meaningful then its C rating.
A pack with a rating of 1C continuous would mean that a 2000mAh battery should not be discharged any faster than 2000mA or 2A, which would take one hour to discharge. A 2000mAh pack rated at 12C continuous would be able to discharge at 12 times its capacity (12 x 2000mA = 24000mA or 24A) at which rate it would discharge in 1/12th of an hour but with a max capacity of 24A.
Charging a Lipo:
The nominal voltage of each cell is 3.7V and it can be discharged to 3.3V per cell, 3V minimum without damaging the cell, and up to 4.2V when fully charged. A battery pack is usually composed of two or more cells put together in series for increased voltage, or in parallel for increased capacity.
IT IS IMPORTANT TO CHARGE EACH CELL IN A PACK TOGETHER.
This is why you can’t use a regular charger to charge Lithium batteries but one programmed for this purpose. This function is called a balanced charge as it will ensure that each cells in the pack are charged at the same rate and together and to do that Lipo batteries include a second cable which communicate with the charger/balancer to ensure all cells are charged at the same rate together.
The other advantage of the Lipo is that they don’t need to be cycled to ensure best capacity and they can be charged at much higher rate hence much quicker than other batteries. Each pack will say the maximum charge capacity. It will say for example that the max charge can be at 5C which means that the battery pack of 2000mah with a 5C max charge means it can be charged at 10A which will take just minutes to recharge the pack. Typically you’d want to charge the pack at 1C or at the nominal capacity of the pack.
How to choose my Lipo battery for our IOM?
First you need to know the voltage required to power your electrics. In our case 7.4v (2S)
Then you need to know how much power you will need. This is important for RC vehicles requiring a lot of power or speed. We don’t, the power need of a RMG is quite low. You can go for a 1000mah 2S battery which will last you for a few hours of sailing. Lipo batteries goes up to 6000mah or more now with a C rating of 50 or more (giving 300A with up to 6S). These are not for us.
So what C rating we need for our RC boat ? well since our power requirement is low a 1 or 2C is sufficient if you find more it will be ok but then watch the size of the pack so it fits in your pot or boat.
Example of choosing a right pack for your need (not needed for our need in RC boat)
If you know how much continuous current you will be drawing and the capacity of the pack you want to use, you can easily determine what C rating you require. If you are drawing 5A from a 1320mAh pack, just take the current and divide by the capacity: 5A = 5000mA, 5000mA / 1320mAh = 3.8C. Using a pack with a higher C rating than you require will leave some headroom, and extend the life of your battery. Batteries are also given a C rating in terms of burst, which is how quickly the battery is able to discharge for a short period. A burst rating of 20C would mean a 2000mAh battery could supply 20 x 2000mA = 40000mA or 40A for a few seconds at a time.
Lithium Batteries are also know as LiPo, Li-
Which Battery Is Best For Me?:
To select a battery, you first need to know what voltage you require and how much current you will be drawing continuously. If you have a motor that works with 11.1V, you would need a 3 cell battery. If you need to draw 20A, and you would like to have a 10 minute (1/6th of an hour = 6C) flight, you would need a battery with 20A / 6 = 3.3A = 3300mAh. This means you would need a 3s 3300mAh battery with a C rating of 6 or higher.
Never charge a lithium battery if it is below 3.0V per cell, puffed up, or damaged.
Always place on a fire-
Only use chargers designed to work with Lithium Polymer batteries.
Never leave your battery unattended while charging.
Lipo batteries don’t like sea water so make sure you keep your battery pack dry – in a pot but avoid Lipo altogether if you don’t have a pot and put your battery at the bottom of your boat, particularly if you boat tends to take in a bit of water.
How to Prepare Your Lithium Polymer Battery for Storage -
If your lithium polymer (lipo) batteries will be unused this winter or you’re planning on leaving them alone for a month or two, please read this helpful article.
If you need to store your lipoly batteries, we would like to help you make sure they
are fresh the next time you need to use them. Li-
To insure your lipo batteries do not loose capacity while left unused, please follow these basic steps:
Place your lipoly battery in a LipoSack
Charge the lithium polymer battery pack for storage to between 3.8V and 3.9V per
cell (if your charger does not allow you to set the termination voltage, you can
also fully charge the battery, and then discharge the battery to between 40-
Check the voltage of your lithium polymer batteries with a Digital Multi Meter once a month while they are in storage
Keep lithium polymer batteries inside the LipoSack and stored in a cool, dry place with a stable temperature of between 4 to 21 degrees C.
When you need to use your lithium polymer batteries again, fully charge the lipo pack and it will be ready to go. Following the 5 easy steps above will help extend the life of your valued lipo batteries.