Today we are going to talk a little about chargers and some of the frequently asked questions pertaining to chargers and batteries used in UAV industry.
Batteries are one of the key components when building a UAV, and a good charger is required to keep things in the air.
There are many different chargers on the market and it is important to know some of these facts when making your purchase.
AC vs DC
When picking a charger, it is necessary to know how the charger is powered. Chargers can plug into the wall (AC) or can be powered directly from a power source like a 12 volt car battery (DC). Some chargers can do both, but typically require additional components to make then work with both power supply types. DC Chargers are great for pilots who only fly away from a source of mains power. They can be plugged into a car cigarette lighter or directly to a car battery and is great for pilots who need the additional portability. DC chargers are typically less expensive than a comparable AC charger, but are limited in where they can be used.
AC Chargers plug into a wall outlet and can be used indoors. Most AC chargers can also be be adapted to use DC voltage for its supply current, and give the pilot more flexibility where they can charge. AC chargers typically are more expensive because they need to add additional circuitry to convert mains voltage (110V in the US, 220V worldwide) to a lower DC Voltage. This circuitry is expensive and heavy, so it is no surprise the cost is higher.
Most chargers are rated in both amps and watts.This corresponds to how much power can be added to the batteries at a time. Amps are usually front and center with watts secondary. This is great for marketing, but not a great way for a customer to choose which charger. For an example, lets assume the charger is rated at 8 amps and 100 watts. If you recall, watts define a unit of power, and is volts x amps. The higher the wattage and amperage, the faster a battery can be charged (up to a certain point… see below for more info)
If we took a real world example of charging a 3000mAh 3S battery with a 100 watt, 8 amp charger, we would know we can charge up to a rate of 9 amps with a 100 watt charger. (3S x 3.7V/cell = 11.1 volts. Divide 100 watts by 11.1 and we get approximately 9 amps.) The circuitry is limited to only 8 amps, so even though it is rated as 100 watts, we are capped at charging a battery at 8 amps. In this case, the charge rate is capped by the rated amps..A 3000mAh 6S battery would charge at only a max of 4.5 amps (6S x 3.7 = 22.2 volts. Divide 100 watts by 22.2 and we get 4.5 amps.) So we can already see the 6S battery will charge at a rate half as much as the 3S battery, even though the overall battery capacity is the same. So even though we demand charging a 6S battery at 8 amps, we could only get a 4.5 amp charge rate because we are capped by the chargers wattage rating. If we took the same example and double the wattage rating, we could charge the 3S battery at 8 amps (the wattage increased, but the amperage did not) which is the same rate, but the 6S battery could be charged at a rate of 8 amps also (this time we are within the wattage rating, so we can get the full amperage even with the higher voltage battery pack)
The battery itself also has a limit to how fast it can be charged. This is related to the capacity of the battery. As a rule of thumb, charging a LiPo battery should be done at 1C for best battery life. Some battery manufactures are allowing higher charger rates, but lets use the 1C example as it is the safest. A 3000mAh battery x 1C would give us a value of 3 amps (3000mAh * 1000 (covert millamps to amps) = 3 amp hours. multiply by 1C for a max charge rate of 3 amps. So even though we could charge the example 3S battery at 8 amps, it would be better to charge at 3 amps. Same calculation applies to the 6S battery. For this example, the 6S battery should also be charged at the same are of 3 amps!
As we get larger and larger capacity batteries, we can bump up the charge rate without worry. For example, if we have a 16,000mAh battery, we could safely charge at a rate of 16 amps using the same rule of thumb (16,000mAh x 1000 = 16Amp hours x 1C = 16 amps. So for the smaller packs, we should regulate the charge rate to the battery capacity and larger batteries we should choose a charger that can provide the most amps/wattage as demanded by the battery.
The key component to a LiPo charger is how well it can balance the individual cell voltages in a battery. As a battery is discharged, each cell might discharge at a slightly different rate. A good LiPo charger will balance the charge on an individual cell level and will ensure the battery will perform at its peak. A LiPo charger should have a balance plug that allows the charger to monitor the voltage on each cell and will modify the charge rate for each cell until they all reach a fully charged state.
Storage mode is another great feature on good battery chargers. If a LiPo battery is not used for an extended period of time (lets say 10 days) then it should be stored in a partially charged state. Storing a fully charged battery will cause the cell chemistry to breakdown and create a ‘puffy’ LiPo battery. Placing the battery on a charger and running a storage cycle will help drain the battery to a level best suited for storage.
Multiple pack charging is another great feature on high-end chargers. On these chargers, it is possible to hook more than battery at a time to the charger. Each of the batteries has its own charging circuit built and can be charged or discharged concurrently. This is great when you have a fleet of UAVs or need to charge multiple packs at the same time. It is like buying more than one charger, so these chargers do tend to be more expensive.