Your Robot Lawn Mower Battery – Is It powerful Enough?
Why is this even a question? … Aren’t all robotic mowers basically the same? No!
At robotzoo, we know a lot about lawn mowing robots. Read this, and so will you!
We’ve been selling and servicing robotic mowers in Australia for more than seven years now.
If you read our other articles, you will see that we have good advice to offer.
We know that there are good reasons why some mowers are cheap, and others are costly.
It’s not just to do with the brand (although that can play a part). It is also about:
- The physical size of the mower and its mechanical components;
- The number and complexity of electronic components (ie. the circuit boards);
- The sensors, software, and firmware that run on those electronics;
- And then there’s the battery!
How does the battery affect the cost of the mower?
It goes without saying that a more powerful battery costs more than a less powerful one.
The robot lawn mower battery itself could account for as little as 5%, or as much as 30% of the cost to make the robot mower.
Imagine seeing two mowers selling at the same price…
But one has a battery that is twice as (or more) powerful than the other.
Naturally it would make sense to choose the one with the better battery.
Why choose the one with the better battery?
Our business has examined a lot of different lawn mowing robots over the years.
Large and small, fancy and simple. And we’ve found that there’s one critical component.
Large and small, fancy and simple. And we’ve found that there’s one critical component.
The battery often determines the efficiency, performance, and reliability of these devices.
Specifically, it’s the relationship between battery voltage and power.
Let us explain:
First, it’s essential to first understand electrical power in practical terms.
Battery voltage, current and power are all related.
Power in an electric circuit is determined by the formula:
Power (Watts) = Voltage (Volts) × Current (Amps)
Higher Voltage delivers greater current, for longer durations
Analogy
Think of a waterfall, where Voltage represents the height of the fall, and Current represents the volume of water flowing over it.
The combination is what makes it powerful.
Voltage determines the potential of the current the battery can push through the mower’s motor.
A robot lawn mower battery with a higher voltage rating can produce more power to the motors.
And a battery with higher voltage can contribute greater power output, for a given level of current.
Why Voltage Matters in Automated Lawn Mowers
It’s true that automated lawn mowers revolutionise lawn care.
These tools save lots of time and manual effort.
But as with all tools, it’s important that they are fit for purpose.
So they need to have the power to get the job done properly.
With automated lawn mowers, the impact of voltage on power is about how effectively the mower can perform the all-important tasks of:
- cutting dense grass;
- climbing inclines, and;
- running for extended periods.
Voltage Matters for Performance
The voltage potential of the mower’s battery plays a critical role in its performance. Eg.:
- Motor Power: A higher-voltage battery allows the motor to deliver more power. This is important for mowers working on uneven terrain or cutting thick spongy grass. A low-voltage mower may struggle, or even stall, leading to uneven cuts.
- Runtime Efficiency: Voltage also affects how efficiently the mower uses its battery capacity. While higher voltage means higher energy consumption, it translates to more efficient performance. The mower’s motors can work at their optimal speed, and especially torque, to give them greater cutting power.
- Battery Size and Weight: The lithium battery packs for these mowers are made up of banks of individual cells. These 18650 cells are about the size of an AA cell. If a bank has more cells, it will have higher voltage. It will be bigger in size and weight.
The 18650 Lithium-Ion cell compared to the standard AA cell
Size matters. What they don’t tell you about how battery packs work.
1. Safety in numbers
Modern lithium-ion batteries are an engineering and mass production marvel. They are made by combining standardised cells. Each cell is small, just a little bit bigger than an AA size battery. Individual cells are about 3.675 Volts (nominal, 4.2 V maximum), and maybe 2.6 Amp hours. And they are packaged to provide safety, because each cell is small enough that damage to a single cell will not create runaway risks for the whole battery pack.
The floor pan of the early Tesla models contained over 7,000 18650 cells.
2. Managing the pack
The runaway risks usually occur when the whole combined battery pack is subject to a malfunction, eg. In the regulating Battery Management System (or BMS), which controls the rate and limits of charge delivered to the cells in the pack.
putting cells together in a bank
The following voltages are achieved by wiring a bank of cells together (in series), eg.:
- 3 cells together will deliver 11.1V nominal, or 12.6V max, and 8.4V when ‘run flat’
- 7 cells together will deliver 25.7V nominal, or 29.4V max, and 19.6V when ‘run flat’
As a battery gets used, it uses up voltage, ie. the voltage potential runs down. Note that ‘run flat’ is equivalent to:
- running down the maximum potential by about one third, or
- running down the nominal potential by about one quarter.
And over time, with multiple use, the maximum voltage potential achievable after recharging will gradually become less and less. According to Wikipedia, the 18650 cells are usually rated for around 300 discharge/charge cycles.
The Battery Management System protects all the cells in the pack
putting banks together in a pack
Further to the above each bank can be wired (in parallel) within the pack, to deliver the following for example:
- 2.6 Amp hours from one bank of cells
- 5.2 Amp hours from two banks of cells
- 7.8 Amp hours from three banks of cells
A more powerful battery pack will have more cells, and therefore will cost more to produce. But we show in the following section that it is worth it.
A ‘7S3P’ pack will contain 21 individual 18650 cells
The chemistry of battery cells, banks and packs
What happens when a single cell, within a bank, in one of these robot lawn mower battery packs dies?
Well, this is a key question, and it’s something they don’t tell you.
The individual 18650 cells have a safe operating voltage range, from 2.7V to 4.2V:
- Above 4.2V is the danger zone. The cell may overheat and risk catching fire.
- Below about 2.5 Volts is the dead zone. A chemical reaction can occur that will permanently damage (kill) the cell.
The BMS is supposed to ensure that these limits are not exceeded, when charging and/or discharging.
But things can go wrong. And when they do, it’s usually because of a faulty BMS.
Example
Take our 11.1V bank with only 3 cells in series for example.
If just one cell dies, the bank won’t even be able to generate the minimum ‘run flat’ voltage, even if it is fully charged.
Then the whole bank is dead.
What’s more, the entire pack can only generate the lowest voltage potential produced by any one bank.
That is regardless of how many banks make up the pack.
Let’s repeat that:
A battery pack can generate a voltage potential equivalent to the lowest that can be generated by any one bank in the pack.
In our example pack of 11.1V, no matter how many banks of 3 cells there are in the pack, then the pack will be dead, if one cell dies.
That would represent a huge waste.
Conclusion. Why we prefer to use powerful battery packs, with more cells
We hope that our explanations, analogies and examples help to explain why size matters when it comes to batteries.
And why it’s a false economy to try and save on costs by using smaller battery packs.
We prefer to use larger batteries in our devices because they:
- reduce waste, in the event of any battery cell failing;
- extend the life of the battery over its whole life cycle;
- afford the device more power each and every time it runs;
- help ensure a more reliable and maintenance free experience overall.
What does robotzoo do?
robotzoo offers the best robotic lawn mowers Australia can get.
We ensure that you will get sharp edged results from your robot.
Whether you have been looking at Husqvarna, Worx, Gardena, or others, you can compare our models.
If you are looking for user guides for some of these models, try here.
And have a look at the challenges they can overcome.
We’re sure that you will find that our mowers compare very favourably.
If you’re not sure if robot cleaners are ideal for you, shoot us a question.
We’d love to chat.
Want to Find Out More?
Would you like to discuss the suitability of our devices to your situation?
please just fill out the enquiry form.
Alternatively, if you would prefer to speak to us first,
please call us on 1 3000 ROBOT. (1 3000 76268)
