6.0 Litres of Petrol, 3.0 of Diesel, One Pack

The listing's first bullet is specific about the ceiling: up to 20 starts per charge on engines up to 6.0L petrol and 3.0L diesel. NOCO repeats it in the spec strip on the product image, alongside 12 volt, 20 starts and lithium powered. Same pack, same 1000 amps of peak power, and the diesel figure is half the petrol one.

For most UK diesel drivers that is comfortable headroom, because most UK diesel cars sit well under 3.0 litres. The ones outside the rating are the larger diesel vans and 4x4s. Body shape is not the test, though: the only thing that decides it is whether the engine is over 3.0 litres. If yours is, the GB40 is the wrong pack.

Physically it is a 2.5 lb unit, about 1.1 kg, in an IP65-rated housing with rubberised over-moulding, which NOCO pitches as glovebox sized. One owner who has had the pack for a couple of years calls it a "solid, rugged design" and recommends it as "a handy device to keep in the boot of your car at all times".

Two First-Time Starts and One Struggle

Three of the eight UK reviews on this listing name a diesel, and all three are 2.0-litre cars. The results split.

The most upvoted review here, with 126 helpful votes, comes from a driver whose 2-litre diesel had sat on the drive for weeks during lockdown. The radio worked and the locks opened, but "the starter turned over slowly and then died completely". The pack went on the battery the next evening: a clicking noise while the unit read it, then the Boost button held for the specified three seconds until the white LED came on, and then: "Got into the car, turned the key, and bingo. It started first time."

Worth knowing what that three-second hold is. NOCO calls it manual override, and says it is meant for a battery too flat for the pack to detect, below about 2 volts. NOCO also states, in capitals, that manual override disables all the pack's safety features, spark-proof and reverse polarity protection included, and that it "will not provide any additional power". It is not a try-harder button: if the pack reads your battery on its own, you do not need it. If you do use it, NOCO says to switch the pack off before unclipping the clamps.

The second, from September 2023, involves a 2016 Ford Mondeo 2.0L diesel. The AA had already been out, found the charging system fine and measured the battery at 10 volts. The patrol told him to "drive the car for a good 40 minutes to charge the battery back up". He did not, doing "several short journeys" instead and then leaving the car standing over the weekend, and on the work morning it was dead again. This time he had the GB40, bought that weekend, and the car "started instantly with no problem". A jump start gets the engine running; the 40-minute drive is the part that puts the charge back.

The third is the only one-star review in the set, with 74 helpful votes. That 2.0L diesel "regularly goes flat as something drains the battery". The GB40 failed to start it twice in normal mode. Manual override "just about got it started but sounded like it struggled", though NOCO says that mode adds no power, and the pack showed red and nearly empty afterwards.

Three reports cannot prove a rule, and the three batteries were not in the same state. The lockdown car had simply stood unused for weeks. The Mondeo had gone flat once already that week and never got the drive that would have refilled it. The third was in chronic trouble by its owner's own description, and if that is your battery too, the last section is the one to read.

The one outright failure here is not a diesel. The owner who takes the pack camping had "less luck" with a 2L petrol Mazda SORNed for two years, where "the battery on that was completely gone". His summary is the line to keep: "it definitely works for jump starts but it's not going to provide you with a miracle".

Twenty Starts on Paper, Half a Pack in Practice

The 20-starts figure is the listing's headline. Only two of the eight say what a single start actually cost them, and neither of them gets near it.

The four-star reviewer does not say what he was starting, but his first jump "took 50% of battery pack charge to start car", and a later start after a recharge took significantly less. The one-star reviewer's 2.0 diesel needed two goes in normal mode to reach the same 50%, having nearly emptied the pack the time before.

Against that, the owner who takes it camping started a 1.4-litre petrol "immediately when the car battery went flat, and again the next day, without a recharge in between". Two starts on one charge is the closest anyone here comes to the listing's number.

Recharging is quick, at least. NOCO says three hours. One owner had it "fully charged in a few hours" from the wall, and the Mondeo owner put it at "about 3 hours" from a kitchen USB socket. The listing's charging image puts the input at 10W over USB-C and the output at 2.1A over USB-A. The four-star owner's practical advice is to recharge the pack the same day you use it.

A 24-Inch Cable, No Plug, and a Fiddly Connector

The box holds the GB40, the clamp lead, a microfibre pouch, a user guide and a USB-C to USB-C cable that the listing image labels 24 inches, roughly 60cm. There is no mains plug. You charge it from a phone charger, a laptop or a USB socket you already own, which matters if your only spare charger lives indoors.

The one-star reviewer, who reviewed the refreshed version in January 2025, lists what turned up: "a thin fabric carry pouch (useless), a USB-C to C cable (no charging plug included) and the jump leads, which are short but were fine for me". That review also flags a mismatch. The refreshed unit has a USB-C in and out port and a USB-A out, with no 12V lighter socket port or attachment, while the listing's hero shot still shows a case end moulded 12V OUT. If you are buying this to run a 12V accessory, do not assume that port is there.

Connecting the leads draws the other complaint. You "have to push it quite hard to get it in, and it is not super clear which way round it is supposed to go in", because the plug is round-ish with no markings and you have to match the terminal shapes by eye. The jump leads get a hedged thumbs-up from the same review: they "seem high quality and have a strong grip". NOCO claims patented spark-proof and reverse polarity protection on top of that, and one five-star reviewer calls it a standout feature.

The Torch Is Good, the Power Bank Is Disputed

Two extras ride along with the pack and only one of them is settled. The torch is a 100-lumen LED that the listing credits with seven modes, six of them named on the product image: 100%, 50%, 10%, SOS, blink and strobe. The owner who takes the pack camping calls it "a decent torch built-in" and adds that "you can never have too many of them" out there.

The power bank is where the reviews pull apart. That same owner estimates the pack "can recharge a modern iPhone from almost dead to full approx three times along with occasional use as a torch", enough for a family for a night or two. The one-star reviewer does the arithmetic instead: the battery is labelled 24Wh, which at 5V works out at about 4800mAh before losses, and concludes that is "not enough to give a full charge on most modern phones".

Both cannot be right, and the listing publishes no capacity to settle it, so treat the power bank as a bonus rather than a reason to buy. A third reviewer adds a gotcha: charging a bigger phone over USB-C can make the pack decide it wants charging instead, so it drains the phone rather than filling it. The fix is to charge out of the pack on a USB-A to USB-C cable.

The camping owner's caution is worth repeating: lean on it as a power bank and you wear the cells, and "it may degrade to the point where the primary car start function fails". The jump start is the job you bought it for.

Jump Leads, a Breakdown Callout, or the Bigger NOCO

The rival most people already own is a set of jump leads and a neighbour willing to bring a car round. One owner puts the case against it: with the pack "you don't need to move two cars close enough for the jump leads to reach, or open two car bonnets". At 6am in January, that is the whole argument.

The other rival is your breakdown cover. The four-star reviewer says the pack "takes minutes rather than waiting hours for your breakdown cover to arrive and losing the no callout discount". The Mondeo owner had already had one AA callout and credits the pack with saving a second, and with keeping his first two customer appointments that morning. That same owner puts the original flat down to "what was possibly a faulty OBD that coincidentally lowered the battery voltage" within minutes of use, which is its own warning: our OBD2 scanner review covers that end of the problem.

Inside NOCO's own range the obvious step down is the GB20, and for a diesel it is a trap: its listing rates it at 500A for 4.0L petrol engines and names no diesel figure at all. If your diesel sits between 3.0 and 4.0 litres, buy the NOCO Boost X GBX45 instead, rated at 1250A for 6.5L petrol and 4.0L diesel, with 4.5 stars from 15,013 ratings. Above 4.0 litres of diesel even the GBX45 runs out, and that is where FLYLINKTECH's 5000A pack comes in, claiming up to 9.0L petrol or 8.5L diesel on 4.4 stars from 11,793 ratings.

Who It Suits, and Who Needs a Charger Not a Booster

We score the GB40 4.2 out of 5. That sits below the 4.5 badge it carries from 126,330 ratings, and the reason is not the starting performance. It is the distance between "up to 20 starts per charge" and what these owners measured, plus a box with no mains plug, a 24-inch cable and a lead connector one buyer had to shove hard and guess the orientation on. The write-ups we drew on are the eight Amazon chooses to display, so read their arithmetic as illustrative, not as a survey of 126,330 buyers.

Buy it if your diesel is at or under 3.0 litres, you want the car startable on your schedule rather than a breakdown patrol's, and you will keep it topped up. That last condition does real work: the reviewer who left the pack unused from March 2023 to September 2024 still got a first-time start, and puts that down to the every-couple-of-months top-up.

Skip it if your battery is the actual problem. That same reviewer bought a NOCO Genius 2 to keep the car battery in condition, after which the jump starts stopped being needed: "The NOCO GB 40 is a jump starter only it wont repair the condition of your battery you need a trickle charger for that." Hauling a dying battery over the line every morning is an expensive way to postpone a new one.

For everyone else with a diesel inside the rating, this is the pack that lives in the boot, so long as it goes on charge every couple of months. The boot is a cold place in January, and the four-star owner reckons a pack left to discharge somewhere cold may struggle to hold a charge afterwards. The biggest diesel anyone here names is a 2.0, so the 3.0-litre ceiling is NOCO's number, not one these owners tested in a hard frost. Check the current listing on Amazon for availability.