Cobra MX51S80V cordless lawn mower review: a powerful machine with battery power that goes on and on

Thanks to its twin batteries, the Cobra MX51S80V cordless lawn mower proves that you can enjoy the freedom of cordless cutting on a big lawn

Reviewing the Cobra MX51S80V cordless lawn mower on a large lawn
(Image credit: Linda Clayton/Future)
Gardeningetc Verdict

Pitched at bigger lawns, the Cobra MX51S80V cordless lawn mower is robustly built with a few neat features. The self-propelling lever makes light work of large lawns, while double battery life means you’ll probably run out of steam before this mower runs out of juice! A big grass box reduces emptying time, while the wide cutting blade makes short(er) work of large areas. Finally, the power boost function ensures this cordless can tackle the most challenging terrains, without slowing your stride.

Reasons to buy
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    Power boost

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    Huge grass box

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    Impressive battery life

Reasons to avoid
  • -

    Awkward self-propelling handle

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Cordless mowers don’t have the power to handle large gardens, right? Wrong. The Cobra MX51S80V cordless lawn mower packs twin 40V lithium-ion batteries capable of chowing through lawns of 500 sq m or more in a single charge. If your garden is bigger than that, get a ride-on (or preferably a gardener with a ride-on). 

To set the scene, our garden is around 1,000 sq m (approximately a quarter of an acre) and I usually use a beast of a ride-on to tackle the majority – a job that takes between one-two hours, depending on grass height. Then roll out our self-propelled petrol mower to get round the parts the ride-on can’t reach, like the front garden, and around the kids’ swings etc.  

For this review I did three cuts. First, with the grass box on, I seriously put the Cobra MX51S80V to work on our lawn when it hadn’t been cut for a month due to May’s continuous downpours – brutal! The second, with the mulch cover on, was a more typical weekly summer cut, and finally I did a side-discharge run three days later, just to be thorough! I was especially keen to find out what impact each method would have on battery life, plus, of course, how the Cobra MX51S80V cordless lawn mower coped with our lawn.

Read on for the results of our test, and if you fancy checking out which other designs made our best lawn mower list, make sure you pop over to our buying guide.

Key product specifications of the Cobra MX51S80V cordless lawn mower

Cobra MX51S80V lawn mower

(Image credit: Cobra)
  • Lawn size: large (at least 500 sq m)
  • Battery: x2 40v 5Ah Li-ion Batteries (Twin 40v) 
  • Drive type: self propelled
  • Cutting width: 51cm
  • Cutting height: 25-75mm
  • Grass bag capacity: 60 litres
  • Charge time: 90 mins
  • Weight: 29kg (without batteries)
  • Warranty: Two years

First impressions of the Cobra MX51S80V lawn mower

Cobra MX51S80V lawn mower in a box on a patio

A big box for a big mower – you'll need two people to move the packaged lawn mower

(Image credit: Linda Clayton/Future)

The box is big, and heavy – definitely a two-man job to move, unless you have a pump truck to hand! At first glance, it’s immediately obvious that the Cobra MX51S80V lawn mower is a heavyweight player, too. The metal blade housing is wide, and the battery mount door opens and closes with reassuring rigidity. The wheels are chunky and robust; no Tonka Toy plastic-ness to see here. 

Getting it out of the box (without breaking it as this test mower had to go back) required back-up assistance but most people can just break out the Stanley Knife and send the cardboard off for recycling. 

Getting started

Cobra MX51S80V lawn mower inside a box

There’s hardly anything to construct once you’ve squeezed the Cobra MX51S80V lawn mower out of the box

(Image credit: Linda Clayton/Future)

Putting the Cobra MX51S80V cordless lawn mower together was a total breeze; screw on the handle, clip the cables into place and screw the handle onto the grass box. Actually, the latter task was a little tougher and called for the electric screwdriver and a bit of brute force (not mine) as the screw appeared to be bigger than the receiving hole! 

I didn’t need to look at the instructions, but for review purposes I can confirm they are very easy to follow. The batteries are supplied partially charged and took 40 minutes to reach full power (there’s an LED indicator on each Fast Charger). Then it was time for the mower to meet the overgrown swampland in my garden and work out whether it warrants a place on our best cordless lawn mower list.

What it’s like to use the Cobra MX51S80V cordless lawn mower

Cobra MX51S80V lawn mower on a lawn

Assembled and ready to mow! 

(Image credit: Linda Clayton/Future)

Once charged, you simply slide both batteries into position – there’s only one way they can go in, and lock into place. I started with the cutting height at its highest – an easy manoeuvre via a huge handle next to the rear right wheel – due to the severe overgrowth situation. 

Next just turn the isolator key to on, press down the ON button and then any one of the four buttons on the main handle to power up. Similar to petrol self-propelled mowers, there’s a skinny secondary handle that pulls up to activate propulsion. Be prepared; it surges forward like a greyhound out of the traps. 

Like most electric mowers, the Cobra MX51S80V lawn mower is blissfully quiet but it’s far gutsier than any others I have tested. The self-propelling is a-mazing, especially going up a slope, but do remember to release the lever as you get close to a hedge or fence, otherwise you’ll bosh right into it bumper-car style. This happened a few times before I got the hang of things. 

To mulch the grass, rather than collect, just hook the mulching guard onto the back. The grass is then double-cut into fine fodder that fertilizes your lawn. It also saves the effort of emptying the grass box, which is a huge bonus considering how heavy a fully-loaded 60 litre grass box is (i.e. very). 

The Cobra MX51S80V cordless lawn mower also features a side-discharge option, whereby a chute clips on one side (with the mulch guard still on). The point of this? Battery power lasts longer because the mower doesn’t have to cut the grass twice, and nor is it lugging around an increasingly heavy box of cut grass. The downside: unless you cut very frequently, every 2-3 days in peak-growing season, your lawn can take on the appearance of a ready-to-bale field of hay. Add kids and dogs to the mix, and your house will end up full of grass, too. 

How easy is the Cobra MX51S80V cordless lawn mower to use?

Cobra MX51S80V lawn mower in action on a large lawn area

Powering through a month’s worth of clover on the first cut

(Image credit: Linda Clayton/Future)

The Cobra MX51S80V lawn mower is exceptionally easy to use. It powers up effortlessly, every time, and thanks to the self-propelling function it stormed over our wilderness. The BEST feature is the power-boost that revs up the cutting speed automatically when the mower hits a thicker/taller patch. You can literally hear the extra effort going in as the motor powers up and there’s no loss in speed or performance regardless of the grass depth. 

Getting to grips with the self-propelling situation took practice, and I’m no novice with this type of mower. When getting to the end of a run – where you need to do a turn – the key is to release the lever handle in plenty of time before you turn, otherwise it will run away with you and potentially slam into the hedge. This 'may' have happened a few times. I chose not to engage the propeller going downhill to preserve battery life but it really came into its own when getting back up those slopes (bear in mind I walked five miles doing one cut, according to my Garmin watch).

Self-propelling lever on the Cobra MX51S80V lawn mower

The black self-propelling lever makes light work of large lawns but isn’t very comfortable to hold

(Image credit: Linda Clayton/Future)

I loved the voluminous grass box, on the grounds that it didn’t need emptying nearly as frequently as I feared. Around 10 times for a 500 sq m area with very long grass, but I’d expect that figure to halve in normal conditions when the lawn is regularly mown. Oh, and it is very easy to dismount and empty, if not so easy to carry when fully loaded.

grass collector box on the Cobra MX51S80V lawn mower

The impressive load capacity of the grass box

(Image credit: Linda Clayton/Future)

The results? On the first, longest and toughest cut, I was bowled over by how unfazed the Cobra MX51S80V cordless lawn mower was – literally nothing stopped it powering on. And if it came across a thicker clump – mainly clover-based – it just upped its game with a boost of power.

Unsurprisingly, given the motor was working at full bore most of the way, it only managed half our lawn on a single charge (so around 500 sq m), but I was mowing for just under 50 minutes (49:46 to be precise) before it admitted defeat. There were a couple of times when the Cobra MX51S80V got blocked and stopped picking up, even though the grass box wasn’t full, but, overall, it did a fine job in challenging circumstances. 

Tellingly, when I dug our Husqvarna ride-on out to finish up (I didn’t have time to recharge the batteries), it couldn’t cope AT ALL. The chute continuously blocked, which meant sludgy piles of grass all over the lawn and a second run-through the next day to pick up the mess. In short, and ignoring all the walking required, the Cobra MX51S80V did a better job than a decent quality ride-on. Impressive stuff. 

The mulcher attachment on the Cobra MX51S80V lawn mower

The mulcher attachment on the Cobra MX51S80V lawn mower

(Image credit: Linda Clayton/Future)

On the second run, with the mulcher, things went more smoothly (predominantly because the grass was more manageable) and the power-boost was barely required. Consequently, the batteries lasted just over an hour (1:03), and of course the grass box didn’t need emptying so the whole process was far faster. Mulching is the future. 

On the third run, with the side-discharge, I nearly managed the whole lawn (remember there’s no stopping and starting to empty a grass box, which impacts battery life), but I did notice a lot more grass on my boots and, once the kids and dogs were let out, they traipsed plenty back inside the house. Grrr. Battery life was similar to mulching (lasting only four minutes longer) and the results were good, but no better than the other two modes. In all honesty, I can’t really see the point of side-discharge, except perhaps some people just prefer this traditional method.

Additional features on the Cobra MX51S80V cordless lawn mower

Twin batteries inside the Cobra MX51S80V lawn mower

The twin batteries ensure the Cobra MX51S80V lawn mower has an impressive run time

(Image credit: Linda Clayton/Future)

There are battery indicators all over the Cobra MX51S80V lawn mower – on the batteries themselves, on the chargers and on the handle – so keeping an eye on juice levels is super easy. I love the full grass box indicator, too. It’s a pretty low-tech, flappy affair: when the flap is flapping there’s room in the box, and when it stops flapping, it’s time to empty. Simple. The handle can be folded for easier storage, and it can also be adjusted to a higher or lower position, which is great for comfort.  

How does the Cobra MX51S80V cordless lawn mower rate? 

Freshly mown lawn using the Cobra MX51S80V lawn mower

(Image credit: Linda Clayton/Future)

Online customer reviews for the Cobra MX51S80V lawn mower are predominantly positive, mostly four stars or above (out of five), with just a few gripes about the uncomfortable self-propelling handle. The trick, I found, is to use your right hand to hold the power button down (the mower stops automatically if released) and your left to lift and hold the self-propelling handle in and out of position. The only negative point I would add is that the Cobra MX51S80V lawn mower did ground out at the front a few times, on only slightly uneven ground. Acceptable when on a very low grass height setting, but this was mid-height so shouldn’t have happened. 

The major pluses far outweigh these few minor bugbears though. For those with larger lawns the biggest selling point has got to be the mega 51cm blade width, which reduces the number of passes you'll need to perfect your lawn ideas – every cm counts when you’re mowing big areas. 

Then there’s the excellent battery power. The Cobra MX51S80V’s twin batteries really deliver on grunt, especially when the grass is overgrown – even though I didn’t quite manage the full 1,000 sq m on a single charge, when you’re covering big areas, a 90-minute break to recharge is more than welcome.

Want more advice on how to mow a lawn and get great results every time? Head over to our expert advice feature. 

About our review – and reviewer

Linda Clayton has been an interiors journalist for 20 years. Since graduating from Cardiff’s School for Journalism, she's happily writing about the latest trends, product reviews and giving her expert design advice for the likes of Gardeningetc, Homes & Gardens, Livingetc, Ideal Home and Real Homes ever since.  

She currently lives in Devon with her husband and two children, and recently renovated and extended the house. The property was previously a commercial nursery and boasts a half-acre plot, giving her plenty of space to road test the latest lawn mowers, hot tubs and garden tools. 

As with all our reviews, the Cobra MX51S80V cordless lawn mower was tested first-hand in Linda's garden, using it just as you would so you know exactly what you are buying. The product was given to us free of charge and we tested it for as long as possible before sending it back to the brand. 

Linda Clayton

Linda fell for the interiors world soon after graduating Cardiff’s School for Journalism and has been happily writing for the likes of Gardeningetc, Homes & Gardens, Livingetc, Ideal Home and Real Homes for two decades. Her current home in Devon was previously a commercial nursery – they grew the plants that garden centres buy. After renovating the house, the garden is now on her radar and she has grand plans to overhaul the extensive outdoor spaces.