How to choose the best Lipo battery for FPV

Does Larger battery allows for longer flight time?
Unfortunately the increase of flight time isn’t proportional to the increase of battery size.As the battery gets larger, the increase in flight time becomes ineffective. Eventually it will reach a point where it just doesn’t gain any more flight time with bigger battery (even lose flight time). This is mainly caused by the weight of the battery. Also note that the heavier your copter gets, the less agile it will be.
There are techniques you can use to help with selecting FPV battery – creating graphs and mathematical model in Excel,
which I found really useful when comparing various products of the same kind. I recently bought some 4S batteries for my FPV quad , like Turnigy Nano 4s batteries and Tattu 4s batteries
I will use these as an example. This technique should be useable for any multicopters, including the 250 mini FPV quad.

List All Batteries and Create Graphs

First, list all the batteries with different capacities, brands of the same cell number. You can include their weight, price, etc in your table. For example I was looking for 4S lipo battery for my FPV and here is the table I created.

Listing batteries and data

From these data, you can create some very interesting and useful graphs, for example I usually do
Density, which is capacity per gram (= capacity / weight)
Value, which is capacity per dollar (= capacity / price)

From the above graphs, if I am only going for best performance and not worry about the price, I would definitely go for 2650mah, 3000mah, or 3300mah FPV batteries.
These graphs tell you some insights into which battery has the highest price/performance ratio, but it doesn’t tell you whether this is the best battery for your RC aircraft, quadcopter or tricopter. To do that, we need to create a mathematical model.

Build Mathematical Model to Estimate Flight Time

By using this model, you will be able to calculate the flight time of any battery. Of course, this is just an estimation, but it will give you an rough idea what battery to go for, and save you from spending too much money and time to try each type.First of all, you need to get one battery first, and collect data using it. Basically the data we need is flight times under different loads. You can collect some other data as well to help you identify what is the max load your FPV can take, for example, the throttle value, current etc.
For example, I used a 3s 2200mah lipo battery, and tested the flight times under different load weight (0g, 110g, 220g, 340g, 405g, 515g – all these weights are physical items I can find in the house that’s why the uneven numbers). This as below is the data I collected from 6 test flights.

I always land the FPV when the voltage alarm beeps, which is when the voltage reaches 3.5V per cell. The column “Actual Capacity Used” is not really needed, but I collect it just to make sure my data is valid. I got that from my charger, when I charge it fully at 4.2V per cell, I know what capacity was used in the previous flight.?I then work out “mah / second”, which is the speed of power consumption, and we can draw a graph from this.

So here is the model. “Cap at 3.5V (86%)” is the effective capacity that can be used during a flight.?I am making an assumption that the voltage drops down to 3.5V when 86% of battery capacity is used. “mah/s” is calculated from the above equation, we can work out the mah/s for each FPV battery, depends on their weight.

And now the estimated flight time is roughly equals to effective capacity divide by mah/s. For example for the 6000mah battery, the effective capacity is 5160mah, and the speed of power consumption is 5.75mah/s, so the flight time would be 898.16 second which is nearly 15 mins.

That’s the analysis I always do when buying FPV batteries. There are a lot of assumptions above, so also do your research on the flight time, ask people for their experience, to verify your theory.