Hi, I’m Mark from EasyLawnMowing. I’ve spent a good amount of time assembling, testing and using the Cobra MX51S40VX cordless lawnmower around my house, and in this article I’ll walk you through everything I discovered—what comes in the box, how to put it together, how it performs on different lawn conditions (wet, long and leafy), and whether it could replace your petrol mower. I’ll also share practical tips on battery care, storage, and exactly what I liked and what I didn’t.
Why I tested the Cobra MX51S40VX
I test mowers to find practical, real-world answers for homeowners deciding whether to switch to battery power. The Cobra MX51S40VX caught my eye because it’s a 51 cm (20″) mower with a steel deck, self-propelled drive, and a twin 40V Max+ battery system. On paper it looks like a genuine petrol alternative, so I wanted to know how it performs when the conditions are far from perfect—wet grass, long growth and leaf-strewn lawns.
My test area is representative of a typical suburban garden: a back lawn of around 450 square metres plus smaller front/side areas, combining to roughly 550 square metres. That size lets me get a good feel for run time and how the mower copes with varied surfaces and conditions.
Cobra MX51S40VX Video Review
To give you a visual representation of the Cobra MX51S40VX, please watch my video review below:
What’s in the box
Out of the box you get the mower chassis with the deck and drive, a 60 L hybrid grass collector (solid top and bottom, mesh sides), a mulching plug, side discharge chute, two 40V Max+ 4Ah batteries and a 4 amp fast charger. A basic manual is included, but assembly is straightforward and I’ll detail that step-by-step below.
- 51 cm / 20″ steel deck mower body
- 60 L hybrid grass catcher (solid top/bottom, mesh sides)
- Side discharge chute
- Mulching plug
- 2 × 40V Max+ 4.0Ah batteries
- 1 × 4A fast charger
- User manual
The mower itself feels substantial. The chassis alone weighs 30.4 kg without batteries—a solid, heavy build that gives confidence in the longevity of the deck and components.
Assembly: quick and tool-light
Assembly is one of the first hurdles for many gardeners. I like products that don’t require a toolbox full of bits—this Cobra is simple. You do need a Phillips (crosshead) screwdriver for attaching the grass catcher handle, but the rest uses quick-release bolts and cam locks.
Here’s the exact order I followed and tips I found helpful:
- Unpack and lay components out on a clean surface so nothing gets lost.
- Raise the lower handlebars by releasing the red catches on both sides. There are three handle positions—pick the one that suits your height for comfortable mowing.
- Attach the upper handlebars to the lower bars. The bolt goes from the inside out; place the washer then screw on the quick release lever. Tighten while the handle is folded down to set the preferred tension, then open and test.
- Hook the handle cable into the small hooks and use the cable tidy clip to run the cable neatly along the handle; that protects it from snagging while mowing.
- Assemble the grass catcher by fitting the handle into the back slot and securing with the supplied screw (Phillips screwdriver needed). Remove transit caps and clip the catcher to the mower using the hooks.
- Insert battery(ies) into the mower and you’re ready to go. Note: the mower accepts two batteries but runs them in sequence rather than combining them to provide higher voltage.
Assembly took me only a few minutes once I’d familiarised myself with the bolts and clips. The cam-lock folding mechanism is positive and feels durable.
Key specifications explained
Before I go into performance, it helps to understand the important numbers and what they mean for everyday use:
- Deck size: 51 cm / 20″ — This is the largest deck in Cobra’s Max+ MX range and makes quick work of larger lawns.
- Deck material: Steel — Steel decks are heavier and more durable than plastic equivalents and give better edge performance. They’re what you’d expect on a petrol mower, so this helps this model replace petrol units for those who want that feel.
- Power system: 2 × 40V Max+ batteries (4Ah each) — Batteries are part of Cobra’s new Max Plus family. Each is 40V and 4Ah. The mower uses one battery at a time and will switch to the second automatically when the first is depleted. This is different to some battery systems that combine cells to make 80V.
- Motor: Brushless — More efficient, lower maintenance and typically longer lived than brushed motors.
- Drive: Self-propelled — The ‘S’ in the model number (MX51S40VX) denotes self-propelled.
- Cutting heights: 7 positions, 25–75 mm — A useful range to cope with seasons and different grassy conditions.
- Grass bag: 60 L — Generous capacity that’s well matched to the 51 cm deck.
- Weight: 30.4 kg (without batteries) — The mower is heavy, which is good for a stable, solid cut but it’s still easy to wheel thanks to large wheels.
- Warranty: 2 years domestic — Standard domestic warranty which is reassuring for a battery-powered tool.
A couple of clarifications that matter in practice: although there are two battery slots, this model doesn’t run at 80V—each battery runs independently. That design simplifies pack design and keeps heat manageable, and it allows you to run on just a single battery if needed, while the mower will auto-switch from one pack to the other as they deplete.
Design and build quality
The overall build quality is one of the mower’s standout features. It’s heavy and feels properly engineered. Here are a few elements I want to highlight and why they matter:
- Steel deck: You get that reassuring weight and durability. The deck underside is smooth which makes cleaning easier and reduces that grass buildup that can slow electric mowers down.
- Large rear wheels: These make a real difference, especially on uneven ground or small slopes. They also improve manoeuvrability around tight beds and paths despite the mower’s 51 cm width.
- Padded, ergonomic handle: The handlebar has a comfortable grip and a starter bar that runs across the handle so you can push and hold from either hand. Vibration is damped and I didn’t feel fatigued even during the longer test runs.
- Cam-lock folding handles and vertical storage: The handle folds neatly and the mower can be stood vertically to reduce storage footprint. The grass box also folds flat which helps when storing in a shed.
- Full indicator on the bag: The solid top with mesh sides is a great middle-ground design—keeps dust from flying up while providing airflow for efficient collection.
Cutting modes — Collect, Mulch, Side Discharge
This is a true 3-in-1 machine: you can collect cuttings into the 60 L bag, mulch with the supplied plug, or use the side discharge chute. Each mode has its ideal use case:
- Cut & Collect: Best for tidy lawns and when you want to remove clippings, or during autumn leaf collection. The 60 L bag proved efficient in my tests and the mower pushes the cuttings to the back well, filling the bag evenly from front to rear.
- Mulch: With the mulching plug in place, the mower chops grass into fine pieces and returns them to the turf. Mulching works best in dry conditions and regular mowing intervals; it’s not ideal when the grass is very wet or when you have a heavy thatch layer.
- Side Discharge: Fastest method for very long growth or when you don’t want to stop and empty the bag. The side discharge channelled clippings effectively without clogging during my tests, even in damp conditions.
Testing methodology — how I put it through its paces
I set out a simple but thorough test plan aimed at replicating common homeowner scenarios:
- Mow my back lawn (around 450 m²) and the smaller front/side areas (additional ~100 m²) to assess run time and coverage.
- Test collect mode with the 60 L bag to check pickup efficiency and how the bag fills.
- Test side discharge, particularly on longer grass, for clogging and discharge pattern.
- Test mulching with the mulching plug fitted, noting performance in wet conditions.
- Do a long grass test to see how the motor responds and whether it can emulate the torque you’d expect from a petrol mower.
- Observe battery behaviour—does it throttle, does it step up power under load, how long before the switch to the second battery?
Conditions were not ideal for mulching: the grass was damp from dew and there were many fallen leaves on the front lawn. That said, those difficult conditions are useful for a realistic assessment because they highlight how a mower handles everyday problems like wet clumping grass.
Real-world performance
Short answer: very good. But let me break down the observations so you can see where it really shines and where you might need to adjust expectations.
Cut & Collect (60 L bag)
I ran the mower on position two at first (second lowest) for a regular cut and found the following:
- The mower cut cleanly and the bag filled from back to front—Cobra has engineered the airflow so clippings are pushed rearwards rather than accumulating at the front. The 60 L capacity is genuinely usable; it filled evenly rather than looking full at the front and empty at the back as some other mowers do.
- Even in dew-wet conditions the mower didn’t leave obvious uncut patches and it handled damp clippings well without clogging beneath the deck.
- Speed felt balanced—neither too slow nor frantically fast. The self-propelled speed felt natural and easy to control.
Side Discharge
The side discharge chute performed impressively. I removed the grass box, fitted the mulching plug temporarily to close the rear outlet, and attached the side chute. With the mower raised one notch to position three to handle longer grass, the unit spat clippings out neatly in a tidy line without clogging.
Pay attention to height selection when side discharging: raising the deck can prevent the mower from bogging down when you’re handling longer growth.
Mulching
Mulching is best performed when grass is dry and growth is frequent. Even though my test day was damp, the mulching plug still produced reasonably fine pieces and redistributed them back onto the lawn. The mower’s internal airflow seems designed to facilitate good circulation for mulching.
Notes for success when mulching:
- Cut regularly and avoid taking too much length off in a single pass; mulching works best when you’re only removing a small quantity of growth.
- Dry conditions are important. Wet clippings clump and don’t break down as effectively.
Long grass performance
One common worry when switching from petrol to battery is torque and performance on long or dense growth. I deliberately tested the mower on some very long grass and listened for the motor response. What I heard and felt was that the motor stepped up power when it sensed thicker, longer material and then reduced output once normal conditions returned. That adaptive power delivery was noticeable and effective.
The mower handled long grass well. With the deck in a mid position (position three), it cut through thick growth without stalling and without clogging the deck. That tells me the brushless motor and airflow design are up to the task for occasional longer cuts—something many homeowners face if they miss a week or two of mowing.
Battery life and charging — what I found
Battery management is central to cordless mowers, so here’s what you can expect and how I used the system to good effect:
- Two 40V 4.0Ah batteries: The mower accepts two batteries but runs them in sequence, not in tandem. When one is depleted the mower switches automatically to the second pack. This sequential design helps keep heat and complexity down and simplifies the charging cycle.
- Run time from my test: I cut approximately 500 square metres and still had a little charge left in the batteries. Given that my test conditions were unfavourable (very damp grass, some long sections, using the self-propelled drive), that’s impressive. For routine fortnightly cuts on a regular lawn I estimate the two supplied batteries should cover 600–650+ square metres.
- Charger: A single 4A fast charger is supplied. That’s capable of charging one 4Ah battery quickly—my experience was just under an hour per pack to reach full. You only get a single charger in the kit, so if you need faster turnaround you can buy an additional charger separately.
- Single-battery use: You can run the mower on a single battery if needed; the second slot provides runtime extension rather than doubling voltage.
Practical battery tips:
- Store batteries in a cool, dry place. Avoid long-term storage with either fully depleted packs or at absolute 100% charge—around 40–60% is ideal for seasonal storage.
- If you own other Cobra Max+ tools, the batteries are shareable and will auto-detect the tool type, optimising output for each device. That’s handy for families who want to power multiple tools (trimmers, blowers) from one battery family.
- A spare battery or a second charger is the best way to ensure continuous work if you have large areas or multiple lawns to deal with in a single session.
Storage and transport
Storage is simple and well thought through. The handles fold down on two cam-lock points and the grass box collapses flat. The mower can be stood vertically on its end, taking up much less floor space in a shed or garage—valuable for smaller storage areas.
Maintenance — keeping it running well
Routine maintenance for the Cobra MX51S40VX follows standard cordless mower practice:
- Clean the deck after use—wet grass especially can cake under the deck; a quick brush or wipe prevents build-up and preserves airflow. The smooth steel underside makes this easier.
- Sharpen the blade periodically. A sharp blade gives cleaner cuts, reduces stress on the motor and improves mulching performance.
- Inspect bolts, guards and cables for wear or security every few months. Tighten quick-release levers if they start to slip.
- Clean the grass catcher and empty it regularly, especially when collecting leaves—wet leaves can be heavy and reduce the effective bag volume.
- Store batteries properly during the winter and charge them periodically to prevent deep discharge if you’re not using them for weeks at a time.
Pros and Cons — my honest assessment
I always aim to be candid when I review tools. Here’s a summary of what I see as the key strengths and minor drawbacks.
Who is the Cobra MX51S40VX best for?
This mower is a great fit if:
- You want a genuine petrol alternative—someone who values a steel deck, robust construction, and professional feel but prefers cordless operation and cleaner running.
- You have a medium to large lawn (roughly 400–1,000 m² depending on how frequently you mow) and you want to mow without the noise, fumes and running costs of petrol.
- You already own or are interested in a battery ecosystem. The Max+ battery range makes it attractive if you plan to add a blower or trimmer and share batteries across tools.
- You value a 3-in-1 solution and like the flexibility of mulching, collecting and side discharge depending on season and condition.
If your lawn is very small or you’re only doing quick light trims, the 51 cm deck may be overkill (Smaller 18″ and 16″ models available). Conversely, if you are a groundskeeper with large acreage, you’ll want larger-capacity batteries or a commercial equivalent.
Alternatives and a quick comparison
Cobra have a few variants in this Max+ line. Important differences to consider:
- MX vs RM models: MX denotes no rear roller; RM indicates a rear roller version (useful if you want a striping effect on your lawn). The model numbers otherwise carry similar feature sets.
- 40V vs 80V systems: The 80V variants will put both batteries to work together to increase voltage (and therefore torque/power), which can be preferable if you frequently tackle very heavy/long grass. The 40V system on the MX51S40VX uses batteries sequentially and is more about runtime than peak torque.
- Deck sizes: Cobra also offer 46 cm and 41 cm decks in the Max+ range, some with rear rollers and some without. Choose a deck width to suit the lawn size and storage constraints—51 cm is fast but larger to manoeuvre in tight spaces.
Cobra Cordless Lawnmower Comparison Table
| Feature | Cobra RM51SP80VX | Cobra MX51S40VX | Cobra MX460S40VX | Cobra MX4140VX |
|---|---|---|---|---|
| Product Image |  |  |  |  |
| Model Description | 20″ Twin Rear Roller | 20″ Standard | 18″ Standard | 16″ Compact |
| Battery | 2x 40V 4Ah Li-ion Max+ | 2x 40V 4Ah Li-ion Max+ | 1x 40V 4Ah Li-ion Max+ | 1x 40V 4Ah Li-ion Max+ |
| Charger | 2x Quick Charger Max+ | 1x Standard Charger Max+ | 1x Standard Charger Max+ | 1x Standard Charger Max+ |
| Motor | Brushless | Brushless | Brushless | Brushless |
| Drive System | Self Propelled | Self Propelled | Self Propelled | Hand Propelled |
| Cutting Width | 51cm / 20″ | 51cm / 20″ | 46cm / 18″ | 41cm / 16″ |
| Cutting Height | 17-75mm / 6 Stage | 25-75mm / 7 Stage | 25-75mm / 7 Stage | 25-75mm / 7 Stage |
| Collection System | Direct Collection | 3-in-1 (Collect, Mulch, Side Discharge) | 3-in-1 (Collect, Mulch, Side Discharge) | 2-in-1 (Collect, Mulch) |
| Grass Bag Capacity | 70L | 60L | 60L | 50L |
| Grass Bag Full Indicator | Yes | Yes | Yes | Yes |
| Rear Roller | Yes (Full-width for striped finish) | No | No | No |
| Grass Combs | Not specified | Front | Front | Front |
| Folding Handles | Yes / Cam Lock | Yes / Cam Lock | Yes / Cam Lock | Yes / Cam Lock |
| Deck Material | Steel | Steel | Steel | Steel |
| Weight (Without Battery) | Not specified | 30.4kg | 24.3kg | 20kg |
Practical buying tips
- If you’re replacing a petrol mower, check how often you typically cut and how much of your lawn is open vs narrow beds—the 51 cm deck is ideal for open spaces.
- Consider whether you want rear roller aesthetics (RM models) or the slightly better manoeuvrability and wider options of MX models.
- If you plan to buy other Cobra Max+ tools, go for extra batteries to share across them—buying a second charger can speed up turnaround if you have large areas to cover.
- Look for bundle deals or discounts from reputable dealers—Cobra kits are often bundled with extra batteries or shears in promotional periods.
Where Should I Purchase This Model
Final thoughts and recommendation
Overall I’m impressed. The Cobra MX51S40VX bridges the gap between heavy-duty petrol feel and the convenience of cordless. The steel deck, large rear wheels and intelligent battery system combine to deliver a mower that performs well on typical suburban lawns and holds its own on tougher patches and longer growth. The 60 L grass bag is generous and collects efficiently, the mulching plug works better than you’d expect in less-than-ideal conditions, and the side discharge is tidy and reliable.
Thanks for reading—happy mowing!
Common questions I get asked (FAQs)
Can I run the mower on a single battery?
Yes. The MX51S40VX will run on a single 40V battery. The second battery slot is primarily used to extend runtime. The mower will operate normally on one battery, and if two are installed it will use them sequentially, automatically switching when one is depleted.
How long will the batteries last?
Run time depends on conditions, terrain and throttle usage. In my real-world test, I cut roughly 500 square metres with a little charge left, despite challenging damp and long grass conditions. For regular fortnightly cuts, I estimate 600–650+ square metres is a reasonable expectation from the two supplied 4Ah batteries.
Is it suitable as a petrol replacement?
Yes—especially if your priorities are less noise, no petrol storage and lower ongoing maintenance. The steel deck and the mower’s performance on long grass make it a convincing petrol alternative for many homeowners. If you regularly need the peak torque that a petrol engine delivers, consider the 80V Cobra variants that combine battery output for extra power.
Can I mulch wet grass?
Mulching is best done on dry grass. Wet grass tends to clump and won’t break down or spread evenly, which reduces the benefits of mulching. In my tests the mulching plug still produced reasonably fine pieces even in damp conditions, but for best results wait for dry weather and cut more frequently.
How easy is the mower to maintain?
Very straightforward. Wipe out the deck after each use, keep the blade sharp, and check bolts and cables periodically. The smooth underside of the steel deck makes cleaning easier than many plastic-deck mowers.
What are the main disadvantages?
Two minor drawbacks: you can’t engage the self-propelled drive without the blade running, which some owners prefer for moving the mower short distances; and only one charger is supplied, so if you need faster pack turnaround you’ll want to buy an extra charger or battery.
Does the mower come with a warranty?
Yes, Cobra supplies a 2 year domestic warranty with this mower in most regions. Always check local terms and conditions from the retailer you purchase through.
As a seasoned expert in the field of garden power tools, I have dedicated over a decade to working with and reviewing a wide variety of lawn mowers. My extensive experience has allowed me to gain a deep understanding of the benefits and limitations of different types of mowers and garden tools.
Over the years, I have honed my skills in writing informative articles and creating helpful videos for various blogs and publications. This has given me the ability to not only recognise what makes a good lawn mower, but also to help you choose the perfect garden tool for your specific needs and requirements.
With my wealth of knowledge and expertise, I am confident that I can provide you with valuable insights and recommendations when it comes to selecting the right lawn mower for your lawn. So, whether you're looking for a battery cordless, electric, petrol, or robotic mower, you can trust in my expertise to guide you towards the best option for your garden.
