Basic Info

Every battery will have a few important specifications that you should look out for when buying/using/charging. I’ll try to summarize it as best as I can

Main Specifications

The voltage specified on most batteries is Nominal, which means it’s like the average voltage throughout the whole voltage range. You’ll most often see this listed as 3.7V for most batteries. There’s also a Maximum and Minimum safe voltage. For 3.7V nominal batteries, that is 4.2V and 3.5V in that order. You should not charge a cell over 4.2V or it could go up in smoke. You should not discharge it under 3.5V, any lower will start to damage the internal chemistry

The capacity, usually measured in mAh [milliamp hours] tells you how much current you can draw for an hour until the battery is “empty” and needs to be re-charged. 2000mAh is 2A for one hour, 1A for two hours, or 20A for 1/10th of an hour

And lastly, the C rating determines the rate at which you can input/output current. C × Capacity [in Ah] = Current in/out For charging, you should charge it at 1C, for a 2000mAh battery that would be 2A, 2C for 2000mAh would be 4A, and so on. Most batteries should list the output current rate as around 70-120C. Anything above ~75C is technically impossible, so the higher values are often just marketing and/or different ways of measuring the current rate. Make sure to pick a battery that has a current output higher than the maximum draw of all 4 of your motors

Cell Type

We mostly use 3 types of cells for our crafts and equipment. LiPo, LiHV, and Li-Ion

• LiPo (Lithium-Polymer) - The most commonly-used cell chemistry type. Can be optimized for both capacity and current output, we’re mostly using it for the latter. Usually flat rectangular cells. 4.2V max, 3.7 nominal, 3.5V min

• LiHV (Lithium-Polymer High-Voltage) - Basically LiPo cells that have been tested not to explode when charged to higher voltages. This higher voltage range can give longer flight times or more power output. 4.35V max, 3.85V nominal, 3.5V min

• Li-Ion (Lithium-Ion) - Optimized for capacity, they’re mostly used for stuff that doesn’t need as much current. Radios, goggles, or lightweight/current-efficient crafts. Usually cylindrical cells. Usually 4.2V max (with older cells it may be 4.1V, be careful), 3.7V nominal, 2.8-3V min

Cell Count

Tells you how many cells are in the battery, connected in series or parallel. A 1s battery is just a singular cell, a 6s battery is 6 cells connected in series, and a 6s2p battery is 12 cells, 6 in series, 2 of those series packs in parallel

You can see an example of different series connections below

Internal Resistance

Not usually specified, it’s still a very important measure. measured in mΩ [milliohms], it can be used as a rough estimate of battery health, the lower the better. Most modern chargers will measure it while charging, or have a separate mode for its measurement.

Choosing the Correct Battery

We’re lucky that for most sizes and types of crafts, there are tried-and-true capacities and cell counts that work best. Say for example, on a 5” freestyle/race quad, you will ideally be running a 1000-1500mAh 6s battery that is rated at least for 75C of current output. Or for a toothpick, you will have a range of different cell counts, usually around 300-600mAh

But standards aren’t everything. You should still know why and how those values are selected. The main thing you should watch out for is the maximum current draw. The manufacturer should provide datasheets with motor tests that will show a maximum current draw per motor. Obviously, if they’re in a quadcopter, you have to multiply that current rating by 4 for each motor to get the total. Let’s say a typical 2207 1850kv motor from FlyFish RC that I recently wrote about

We see that the motor draws about 38A at 100% throttle with a pretty generic 5.1” sized prop and a 6s battery. That gives us approximately 150A of total current draw. This may sound like a lot, but you’re not likely to be at 100% throttle all the time, and most batteries can take up to double their C rating for a short period of time.

Taking the maximum possible C rating (75C), multiplying by 2 to give us 150C, and thus we need at least a 1000mAh battery to be able to output that 150A of current that’s being drawn by the motors. You can of course pick a higher C-rated battery, but as said earlier, anything above ~75C is usually just for marketing. You can also go up in capacity, but there are diminishing returns with extra weight

Charging

Now that you know the basic battery information, let’s get into how to charge them without starting a fire. Battery misuse (especially during charging) can easily and very quickly result in a lot of damage. Knowing what you’re doing is very important

Hopefully, you have a decent charger by now. If you do not, you can find a list of some decent ones here (if I remember to put in the link). So, let’s get into the basics:

Basic Charging Principles

The charger is pretty much a more intelligent power supply. In goes a regulated 12-24V from a PSU, and out goes at a set voltage with a set current until the battery is at the set voltage. But you should know that there are a few charging modes that you will inevitably use:

• Balance Charge - What you should be using most of the time. It checks every cell using the balance plug of the battery and charges each one to the set voltage. A non-balanced battery is a dangerous battery

• Storage charge - Will either charge or discharge depending on the voltage of the cell to get it down to the set storage voltage. The storage voltage is often times equal to the nominal voltage (3.7V) and is used to preserve the life of the battery

• Discharge - Hopefully self-explanatory. Discharges the battery either to a set minimum voltage or until 0V to practically kill it. Useful for safely getting rid of old batteries

There are also different ways to charge depending on battery connection:

• Single-battery charging - Only one battery per charger port, usually the simplest and safest

• Series charging - Multiple batteries connected in series with a special series charging board. Usually for 1s batteries, most larger chargers make it somewhat hard to charge these small 1s packs. Plugging 4 1s 300mAh batteries into a series charging board will make it look like a single 4s 300mAh battery to the charger

• Parallel charging - Multiple batteries connected in parallel with a special parallel charging board. Usually for >2s batteries. Plugging in 4 1300mAh 6s batteries into a parallel charging board will make it look like a single 6s 5200mAh battery to the charger. You have to be very careful to make sure that all batteries are within 0.1V per cell of each other before plugging them in, otherwise they will violently equalize

Charger Parameters

There are basic values you should set before charging and watch out for during charging. Some of these may be repeated from the battery info section, but specified for charging:

• Chemistry - Refers to the battery you’re charging, likely LiPo or Li-Ion, or some other exotic battery you may want to charge

• Max cell voltage - For LiPo’s and most Li-Ion’s is 4.2V which should be the default on the charger. The voltage most often printed on batteries is the nominal voltage, but you don’t want to enter this as the max cell voltage. It wouldn’t cause any harm, but you’d end up with a half-full battery

• Cell count - The cell count on batteries is in series. A 1s battery has only one cell, 6s has 6 cells, and so on. This is what you enter as the Cell count. See the image below once again for a simple explanation of series connection and its effects on voltage

• Charge current - Depends on the capacity of the battery (usually in mAh) and the C rate. Also above for the battery info, but I’ll throw it here too, specifically for charging: \ To get the charge current, multiply the battery capacity in Ah with the C rate (ideally use 1C, but you can go higher if time constrained). A 1300mAh battery at 1C will be charged at 1.3A (1.3 × 1), at 2C it’ll be 2.6A (1.3 × 2), etc…
• Internal resistance - Usually measured while charging, some chargers may have a dedicated IR measuring mode. Used as a rough estimate of battery health - Under 10mΩ is great, under 20mΩ is fine, above 20 isn’t great, and you should retire the battery

Charging Example