Larger battery allows for longer flight time. Unfortunately the increase of flight time isn’t proportional to the increase of battery size, which means you are looking at something like this.
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. But some people prefer the stability it brings with heavier weight, some would call it “flies like a tank” :). Apart from capacity, there is also C-rating you need to consider, which I shall explain later.
The trade off between flight time and battery capacity, makes it more difficult to choose which battery should be used, rather than just “pick the largest battery available”. There are techniques you can use to help with this selection task – 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 tricopter, I will use this as an example. This technique should be useable for any multicopters, including the 250 mini quad.
Talking about Batteries, here is a tutorial on how to do LiPo parallel charging.
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 tricopter and here is the table I created.
Max Current Draw and Battery C-Rating
It’s important to note, that the batteries you listed can supply enough current for the motors. Max current supplied by the battery can be calculated by this formula:
max current = capacity * C-rating
First, work out what would be the possible max current draw from your motors. I usually just look at the motor datasheet, which should tell you what the current draw is at 100% throttle, times number of motors, and add some margin for other electrical parts, and you have the possible max current draw from your quadcopter.
My tricopter has a max current draw of around 30A – 35A, and all of the batteries below meet the requirement.
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), and
- 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. Also 4000mah might be a good choice too due to its outstanding value.
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
This is actually pretty fun to do. By using this model, you will be able to calculate the flight time of any battery. All you need to do is to put in the data of that battery in the excel spreadsheet. Of course, this is only an estimation, but it does 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 of course collect some other data as well to help you identify what is the max load your multicopter can take, for example, the throttle value, current etc.
In my example, I used a 2200mah 4S Lipo Battery as a reference, 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 is the data I collected from 6 test flights.
I always land the tricopter 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.
The good thing about Excel is, it provides an equation from a few dots on the graph. This is probably should not be a linear relationship, but the battery weight falls within this range so it’s close enough for me.
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 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.
When I worked out all the flight times, I found this interesting relationship between size of the battery and flight time. That’s exactly what we predicted at the very beginning.
That’s the analysis I always do when buying batteries. I made a lot of assumptions above, so also do your research on the flight time, ask people for their experience, to verify your theory.
Let me know if there is any mistakes in the method, or any better ideas.