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Robomow RS635 Pro SX Review


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  • Excellent lawn finish
  • Gives a ‘just cut’ look all the time
  • Near maintenance free
  • Very quiet in operation
  • Low running costs
  • 3-year warranty


  • Takes time to get settled in
  • Can't do stripes or steep slopes (over 36%)
  • App rather clunky

Key Specifications

  • Review Price: £2999.00
  • Fully automatic mowing
  • For lawns up to 1.25 acres
  • Robohome docking station
  • Twin mulching cutters
  • Optional app programming

What is the Robomow RS635 Pro SX?

Robomow has been in the robot lawnmower business for more than two decades, and the RS635 Pro SX is the company’s latest flagship model. Capable of cutting lawns up to a whopping 5000sqm (1.25 acres), it’s completely autonomous, mulches the cuttings and has its own app.

Over three months last summer, we put the RS635 through its paces – and it didn’t disappoint. It takes a while to set up and settle in to its routine, but the results are neatly manicured lawns all summer long, with no personal effort whatsoever – and much lower running costs than a standard petrol mower.

Related: Best cordless lawn mower

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Robomow RS635 Pro SX – Design and Features

Robomow’s RS-series robot lawn mowers are the company’s premium, fully featured automated lawn choppers; which model you go for will depend on the size of your lawn. The three-strong lineup caters for lawns up to 3000sqm, up to 4000sqm and the 5000sqm RS635 Pro SX we had on test.

At a pound shy of three grand, the RS635 isn’t cheap. Yet ride-on lawnmowers to cover this size of lawn are available for the same price, or even more, and certainly cost much more to run in fuel and annual maintenance. And, of course, you don’t have to spend every Sunday trundling up and down the lawn – the RWS635 does it unaided.

Related: Best grass trimmer
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The RS- series generally boasts stronger motors and faster cutting than its automated grass-munching peers; but we’re not sure that speed matters. The RS635 quietly gets on with the job, nipping out to mow and then returning home to its docking station again for a recharge when the batteries are depleted. Repeat, repeat, repeat.

You can set the actual times of day and days of the week that the RS635 goes to work; this can even be at night if required. It’s super-quiet too, so you’re unlikely to disturb neighbours even if you do opt for nocturnal mowing manoeuvres.

However, it does require at least one day off a week. No, not as a result of EU legislation on working conditions for robots… but the batteries need a day per week to re-balance the cells for long life. Given that even the batteries are covered by Robomow’s three-year warranty, the company must be confident that this cell balancing will keep the fairly bijou 6.0Ah battery pack in peak condition.

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The Robomow uses a completely random mowing pattern, defined by boundaries set by a green control wire that you pin down to the ground around the perimeter of the lawn. When the mower detects the wire it stops, spins around a random number of degrees and heads off mowing and mulching until it hits the perimeter wire on the other side of the lawn.. and so on.

The docking station is placed in line with the wire, so when the machine’s battery capacity dips below 25% it follows the wire back round the lawn and docks itself for a recharge. The docking station needs connection to a large indoor mains transformer, which ensures all power outside is low voltage. If you’re precious about your robot not being out in the elements – although it’s more than capable of being outside all summer long – you can buy the Robomow equivalent of a kennel.

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You can mark off areas to be avoided – such as tress and flowerbeds – with the same loop of perimeter wire, reaching out to loop around islands in the lawn by twisting the out and return parts of the cable together. The Robomow ignores the twisted-pair sections of wire and rolls straight over them. You can also create multiple mowing areas in the app and set the machine to mow each accordingly.

Given that our test lawn was over an acre and had several trees to mark-off, set-up time was lengthy. We used several hundred metres of perimeter wire, which needed staking down every 1.5 metres or so. Well, I say ‘we’ but for £299 Robomow offers a full installation and setup service; we chose that option. It took our installation man Joe the best part of a day to set up, however.

Related: iRobot Scooba 450 review
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On the machine itself are controls for cutting height in millimetres, an LCD panel, four waterproof control buttons and a rain sensor. The combination allows comprehensive setup of the cutting options and scheduling. The app adds several more detailed functions, including this model’s built-in GSM network-based anti-theft feature and manual steering fun, remote programming and remote diagnostics.

The app requires that your Wi-Fi signal reaches the mower base station, of course. That will be a challenge on larger lawns, where you might want the docking station plugged in to an outbuilding away from the house and router. After some faffing with Wi-Fi range extenders and hoping for favourable winds, we managed to get everything connected. Yet a month later, we concluded that the app didn’t really add a great deal to the whole robot mowing experience anyway.

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Robomow RS635 Pro SX – How does it perform?

Our first week with ‘Robbie’, as he was to become known, didn’t see the little fella in its best light. The base station needed moving to avoid a rough bit of terrain when the mower docked; its random mulching pattern caused Mohican-like strips and tufts of grass all over the lawn; and at one point he became caught in a perpetual loop around the trunk of a small tree. Oh dear.

However, in a comeback that The Who would be proud of, Robbie came good. The base station was moved to a flat piece of ground so that he docked into it seamlessly; we marked off the small tree with the perimeter wire; and the lawn finish just got better and better.

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As is the case with robot vacuum cleaners, you have to adopt a slightly different mind-set with robot mowers. While traditional mowers are brought out weekly and create a nice finish, the lawn slowly grows and gets untidy for the rest of the week. The robot mower is more about the long game. It comes out to mow relentlessly, running around very slowly (sub walking pace) for about 90 minutes in every four hours or so, up to 24 hours a day, up to six days a week.

Even our 1-acre lawn only needed a 12-hour per day window, six days per week. The smaller the lawn size programmed, the less time it will need to go out and mow. This slow-but-sure operation gives it a number of advantages over a typical wide-deck petrol lawnmower.

Robbie’s small footprint and smaller cutter deck mean it tracks the terrain well, avoiding scalping high areas or missing the dips. The continual cutting means the lawn clipping mulch is tiny – just a few millimetres – and the mulch disappears into the lawn immediately. Best of all, because it cuts almost every day, the lawn looks perfectly mowed – all of the time. Oh yes. This we like.

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Within a month, an acre or fairly rough paddock was starting to resemble a well-manicured lawn for the first time in our 14-year tenure at the farm. The constant cutting was encouraging low growth, making the lawn look greener by the week. The previously ride-on scalped high spots had greened over and walking the dogs while getting covered in long grass clippings was a thing of the past. Moreover, I hadn’t had to start up a petrol lawnmower or ride aimlessly up and down for four hours at a time for a month! Brilliant.

The RS635 is ridiculously quiet in use – much quieter than even a normal electric mower – so it isn’t going to worry neighbours or wildlife. Plenty of foxes, herons and wild rabbits that would saunter calmly out of its way at our place will testify to that. It’s properly energy efficient too, using an average of 34kWh per month. That is about the same energy as 3.5 litres of petrol. Our ride-on can use most of that just getting started on a cold morning.

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Three months on and Robbie has proved very reliable, too. He’s never once missed the docking station or mowing routine, remained firmly docked when it’s been raining and, thanks to the very small clippings, has rarely needed the cutting deck to be cleaned out. In dry conditions, throughout the summer, the Robomow is pretty much 100% maintenance-free. For comparison, our ride-on mower clocks up at least one major breakdown and a £400 annual service bill every year.

Amidst all this Robbie-love, we had to think seriously hard to find much we didn’t like about the RS635. Okay, if you’re a fan of lawn stripes then you’re out of luck, as Robbie delivers a smooth, all-over cut. If you have very twiddly borders, very steep slopes much over 1-in-3 gradient, or serious rough areas, you’ll still need a strimmer or small normal mower. Yet this is all equally true of having a larger ride-on mower, too.

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By the end of the summer, when Robbie the review sample was called back to Robomow, there was a tear or two shed at chez Stevenson. I had fallen for ‘reviewers curse’ where great products are concerned, uttering that immortal phrase; “dammit, I’m going to have to buy one.”

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Should I buy the Robomow RS635 Pro SX?

While our RS635 Robomow can handle truly epic-sized lawns, we can’t see the smaller versions being any less impressive. Our Robbie wowed us with a fabulous-looking lawn, an all summer long ‘just cut’ finish, and near-maintenance-free, completely autonomous operation. Okay, stripes aren’t in its repertoire, it was a bit of a pain to setup, and the app isn’t anything to write home about.

However, the cutting results blew us away and the running costs are a fraction of the cost of a petrol mower that could handle the same size of lawn. Factor in the personal time saved in not having to actually mow your own lawn (3-5 hours a week in our case), and the Robomow isn’t only a great robot lawnmower, it’s a great advert for the future of robotic automation full stop.


A brilliantly executed, near-maintenance-free robot lawnmower that delivers an outstanding lawn finish and super-low running costs.

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