KROAK S300 1200A 400F Super Capacitor Jump Starter \\ test / review / take apart

[@johntate5284]

So, when you hook up capacitors in series, the voltage increases and the capacitance decreases. Since these are 400F 2.7 VDC super caps, when you connect the 5 in series, you actually end up with 13.5 VDC at 80F, not 400 as this company advertises. Also, FYI, a lead acid battery is technically dead (100% discharged) when the terminal voltage 80%of the full charge voltage (12.6) or about 10.1 volts. Your two batteries with 3.4 and 6 volts are probably not recoverable as they most likely have one or more shorted cells. There is probably a low voltage check that prevents the unit from discharging into these types of severely damaged batteries to prevent fires and explosions. Very interesting to see you actually take the unit apart! Thanks for an interesting and informative video!

[@hansregli8678]

Yeah, wanted to write exactly the same. Cheating, should be labelled 50 F. Also, the old batteries can not be charged when the water has evaporated from at least one cell because all the battery cells are wired in series. First check is always whether the water is still there. And one more thing: Since the sum of the voltages of the capacitors is 13.5 and the manual speaks from charging up to 14.5, the lifetime of the caps is reduced. They should not be overcharged. Bad designed device.

[@redjohn20001]

I have a Sealey E Start 800 capacitor jump starter. When your battery first fails it usually has plenty of energy left to charge it up. Mine sat in the back of my car for twelve months before being pressed into service on a 2.2 litre diesel that had phantom drain continuously discharging the battery and is still starting it until the car goes in for repair. A great bit of kit.

[@Jonathan-L]

What Red John says is true... These gadgets work fine as long as you haven't drained the vehicle battery too much, and as he points out, your best chance of a jump starting with a super capacitor jump starter if the vehicle's own battery is only marginally unable to crank the engine (eg engine won't crank, but car radio still works). I had an experience where my vehicle battery was completely flat, so the super capacitor jump starter I used would not crank the engine. Admittedly, the unit I tried (not a Kroak C300/C320) was only rated at 500A peak (not sure of the capacitor values, but I could tell that there is 4 in the unit based on the shape of the casing) and my engine had 2 big pistons (103 cubic inch Harley Davidson). Four cylinder engines with smaller pistons are easier to crank. But of course, more oomph in the form of a bigger capacity jump starter would have been the solution. Having said that, I had no problems in starting a 1.5 litre 4 cylinder car with the same jump starter. When shopping for a super capacitor jump starter, get one that's rated at least 1000A peak (they're not much bigger or expensive than the 500A ones). The Kroak C320 (not sure how it differs to the C300) is not badly priced (Bangood, Aliexpress or eBay item number: 175754232465) and comes with a carry case. They may seem pricey for what they are, but on the occasions when you put them to use, they are priceless. I'm going to be buying a few units for family members just for a bit of peace of mind.

[@leylandparaglider9051]

This looks the same as the autowit supercap jump starter that I bought last year. It consists of a dc to dc converter that boosts the voltage into the supercapacitors from the flat car battery (provided it has at least some charge in it) and then dumps 15 volts high current back into the car for a 10 second period while you crank the engine. Mine seems to have lasted ok so far, although they are pushing the supercapacitors above their rated voltage which will shorten the lifespan somewhat I imagine!

[@vevenaneathna]

wonder if the original board was meant for 3v caps and then someone in guangzhou decide they could triple their margins by using 2.7v 400f caps. judging by the dimensions of the enclosure it looks pretty obvious that it was meant to be made with ~600-700F caps. This thing is so china.... i bet the people who are smart enough to know the advantages and use cases for caps vs battery jump starters are also smart enough to build their own device. you can get 6x screw top 2.7v 500f for ~25$ shipped or u can buy the pre balanced board mounted version for ~22$ but those are obviously rejects/salvaged caps with super bad leakage . maybe not a big deal for a jump starter tbh. the fact that this thing is 160$ but someone tried to save 5-10$ by underrating the components.... ill pass.

[@lmantuano6986]

yes, the DC/DC boost converter is the key in this device. It will convert the battery's leftover energy, over a number of minutes, into a steady say 14.4v being stored in/by the supercap.. This stored energy will be enough to kick over the starter motor (NOT to recharge the battery as someone has commented or implied). I believe the manual says that a minimum of 6v (?? please correct me ??) is needed for the device to be able to charge the supercap to the useful voltage. I am in the process of building my own with 6x 700F/2.5V caps in series (total 117 Farad - 15.0 Volt) and an Ali 300W DC/DC booster.

[@vevenaneathna]

@@lmantuano6986 the difference between a car battery at 45% capacity (cant start ur car) and 35% capacity, is actually 200$, coz thats how much it costs for a new car battery. but hey atleast you "got her home" instead of having to ask someone for a jump rofl

[@circletech7745]

@@vevenaneathna It probably does run the super caps at a slightly higher voltage than what they were designed for. But does it actually matter? You're probably going to use this jump starter maybe 10 times ever in a 10 year period.

[@icpart]

Haha it is not 400F at all. We have 5 series connected capacitors which is about 80F. I was wondering why caps are charged so fast with low 7A input current.

[@Marks-Garage]

I've got a sealey super cap jump starter and it's a great bit of kit. I always end up having to put it in override mode to get it to start things properly thou. Has started many vehicles over the couple of years I've had it. This looks a little less agro in terms of operation

[@DavidMcLuckie]

It's a Sealey one my friend has and it's still going as well.

[@DaveAber9999]

Glad you used the correct pink manky towel for your anti-static needs rather than any old colour.

[@andrewradford6267]

Am tempted to make a jump start pack as I have a bunch of super capacitors. Interesting video. :P

[@dantronics1682]

I made one a few years back but never tried it, my car have a roof controller which is very very sensitve to any form of spikes and you cant get these controllers anymore so I didnt want to chance it. even though I'm temped to use a smaller battery with the supercap permanently connected.

[@andrewradford6267]

@@dantronics1682 I did play with them a bit but ensuring you have protection and balancing on them was a pain. To not discharge past zero or charge past max voltage. You can get little protection boards for them but haven't played with them yet. I think it's a good addition to a weak battery also permanently or older cars with high draw.

[@dantronics1682]

@@andrewradford6267 they are capacitors so doesnt require balancing and the alternator shouldnt go above 16v but I dont get what you meant by "discharging past zero"

[@markarel2689]

Very good informational video ! Now I know what not to purchase. I was looking at a Schumacher SL1346 Lithium ion . My so has one and I used it today . Works great BUT the fire hazard scares me . I was in the auto business for 38 years and for the last 15-20 years used a heavy old jumper pack . Pain in the butt . As they say “ Back to the drawing board” Don’t know where to go from here ! Thank you for the helpful Video . Saved me from wasting $$$$$$.

[@valveman12]

A great review. The 400F is misleading since the capacitors may be rated at 400F, the capacitors are in series, so the total of around 80F.

[@dantronics1682]

why did they miss advertised it as 400F when they are all in series? 80F isnt going to do much is it? If I had one the first thing I would do is shunt the relay

[@DavidMcLuckie]

That's an interesting point I'll have to check. But it does work. I have used it twice now to jump start the same dead car, charging the unit from cars 'dead' battery (8-10V)

[@shed.projects5150]

I have been using a Genius Boost. "Noko" ( I believe it's a GB70 ) for a few years now, it holds a full charge for over 12 months, and will start at least 10 cars with flat batteries before needing re-charging ( takes about 6 hours to charge it up on 240v mains ) You can not fault it. When you get to your car after leaving it for a few hours or days, and the battery is flat with no one else around, this capacitor thing would be useless . It looks like a load of rubbish to me. If I left my car somewhere outside for a year, with the Noko in the glove compartment, I could just connect it to the battery and start it instantly without any help. Thats what I call a portable jump starter. Nice vid by the way.

[@inoahmann7542]

Lithium jump starters are great, aren't they? I have a Gooloo gp4000. I've jump-started a bunch of box trucks with it, and I jump-started a full-size semi not long ago. I was amazed when the engine started. It was a DD13 detroit diesel. That little jumper box will do more than my 2awg jumper cables. To be fair, the semi batteries weren't completely flat. They had enough juice to make the starter click.

[@c-qc-q2021]

Bear in mind that lithium batteries don't do well in extreme hot or cold. And there's some risk of fire and explosion, to the point that the TSA doesn't allow them in checked baggage and imposes limits for carry-on. So, if you store it in a hot car, you're rolling the dice, or expect it to work in sub-freezing temperatures, you may be SOL. I realize there are loads of EV/Hybrids on the road using similar battery technology with magnitudes greater energy potential, just eyes wide open.

[@inoahmann7542]

@C-Q C-Q What's the Tsa limit? I carried a 20k mah battery bank for my laptop for 5 years. I stopped using it when the boost converter exploded in a hotel room. I couldn't confirm the cells were ok, so I left it behind.

[@babagandu]

I heard the Hulkman is better

[@Simon_Rafferty]

Kroak - bit of an unfortunate name! Looks like it came out of the factory pre-kroaked! I have a DIY alternative (you might have guessed 🙂). I have a 16V, 6 Capacitor bank which came from a popular auction site, with the Jump Leads connected directly to it via an Anderson connector. It has no electronics, relays or magic on the board, just 6 Caps. Connect it up to a charged / charging battery (with a bit of a spark), count to 30 then connect it to the flat battery, run round & start the car (Land Rover). You have to be quick because the amount of charge being transferred is tiny - so the voltage sags fast. You only have a few seconds where the voltage is high enough for the ECU to work. It's nowhere near as good as a decent set of jump leads - but it will fit in your glovebox.

[@dantronics1682]

how long does the caps last before self discharging? I made one years ago but scrapped it before even trying it out, I'm asking about discharge time and current since I have these supercaps and was thinking of permanently connecting to the car battery

[@Simon_Rafferty]

@@dantronics1682 A few hours at best. What they are great for is absorbing or delivering very high currents without any danger of over charge or discharge. If you had something in your car that used high surge currents, like a giant 100kW electric toothbrush - they could be a great option. Most other things though, not so much.

[@singachuotodiy]

@Simon When you connect 6 Caps to Battery, is it need a load resistance ? Is it safe (spark / explode ?) when empty super caps connected to battery directly ?

[@Simon_Rafferty]

@@singachuotodiy The inrush current is quite high, but very brief - limited mainly by the connecting wires. The internal resistance of the Caps is around 10 milliohms, so dwarfed by the wire & connection resistance.

[@dantronics1682]

@@Simon_Rafferty If the inrush current is very hight (v=ir) wouldnt that produce a massive (greater than 50V) spike? I know certain make of car where the roof control is very sensitive to any form of spikes, even a standard 13v jump start would fry these controllers.

[@c-qc-q2021]

My Utrai didn't come with a cigarette plug-cord. Can you confirm that the barrel plug is a 5.5mmx2.1mm and that it is tip-positive polarity (before I buy one)? I'm going to assume the chargers are getting cranked out from the same Chinese factory. Thx!

[@kuyacargo7935]

Nice Video sir👍👌 Thank you. 5pcs 400Farad Super Capacitor🤔 Meaning the actual Capacitance of that jump starter is 80Farad.

[@johnruscigno5738]

Interesting, they only use 5 caps and a boost converter. I think that's because each time you connect a cap in series, the voltage increases but capacitance decreases. So you get more capacitance with 5.

[@joecox9958]

I see your jump battery is not 6V but 12V. can you clarify?

[@bbcellular]

Did it ever work as expected?

[@DavidMcLuckie]

There is a video of me starting a Smart Roadster. But as of yet, nothing bigger has had a flat battery. With winter now on the way this might change. :)

[@bbcellular]

@@DavidMcLuckie Okay, thanks for the reply!

[@paulconner9354]

Your “hallooo” greeting cracks me up

[@stonkeng]

I watched your video as I WAS thinking of getting one of these. One problem I can see is that the output relay type is rated at 30 Amps. I know it is not switching the cranking current but that relay has to carry the cranking current which can hundreds of amps. I don't think that relay will last long. The second smaller relay is probably used to switch the power on only when the polarity is correct. This unit would be much better if it had 6 capacitors.

[@DavidMcLuckie]

Excellent point.

[@tswellersalzer1850]

@DavidMcLuckie This year i bought a "Kunzer Ultracap " Device. I opened it and there are 5 Caps inside. After loading on a Battery the whole series had a voltage >15V, this means more than 3V per Capacitor. And there is no balancing circuit. Will test it on a Motorcycle after 3 Month resting.

[@priv8joker]

can you use this as a car audio capacitor?

[@ched999uk]

Looks like you need a running engine to charge it so it can then jump start another vehicle. Surely all you need if you have another running vehicle you just need some much cheaper jump leads?

[@dantronics1682]

Thinking the same thing and you save £80 as well

[@DavidMcLuckie]

Ahhh no. I've used it now. Dead car, battery 8-10V. Attach Kroak thing to dead battery, starts to charge. Take about 3 minutes to get to 100%. Press and hold the button to start the timer, you get 30 seconds to run around and get in. Once the tone sounds you crank it. Fired right up. Video to follow soon.

[@DavidMcLuckie]

@Dan Tronics - see other reply, charges from 'dead' battery. I am impressed.

[@krimke881]

I've had a handful of those portable starter helpers. They can start anything. But what they don't tell you, is that they only last a handful of times, until the capacity plunges. doesn't matter how magical they are advertised. that is their problem. So When I get a new one, I get two.

[@DavidMcLuckie]

The super capacitor ones? As my friend who has a garage has one and it gets used a lot and is still working.

[@maldonadolemus]

@@DavidMcLuckie What is the brand of the Jump starter capacitor of your friend?

[@FoookYourfeelings]

​@@DavidMcLuckie Have you any experience with the baseus 3000 capacitor jump starter? Bought one without research under desperation.... Hoping it starts a 3.0l diesel.....

[@amijoe1408]

​@@FoookYourfeelingsi had test 1.9 L diesel easily but cant find 3.0 L to jump start..will keep up to date when i found one

[@joecox9958]

would you mind to retest using 6V battery to jump start and measure the cap discharge current? what is the big relay part number? thanks!

[@yupingping3989]

Do you disassemble and analyze for other electronics? Such as PD chargers or DCDC ?

[@DavidMcLuckie]

Only the things I have.

[@AurelianMacau]

My device only charges on 5V. It is not charged either on cables directly from the battery or with a 12V power supply. After a short voltage display, the E03 error occurs. I also tried with a power supply from 12V to 15V, the same result. Is there a possibility of resetting?

[@joecox9958]

I hope you can do retest, jump starting on a 12V battery is meaningless

[@DavidMcLuckie]

What do you mean? Something like 99% of all car batteries are 12V.

[@JWFP3]

I got this on the hopes it might actually be a real 400F, like (5) 2000F in series or something. 80F works though Thanks for the teardown, the first one i tried must just have serious glue holding it in

[@kobolila-yt]

👍 over here, when something Croaks...its dead

[@bigsteviesri]

Dave whats that clamp meter you have got thaught they only worked on ac need to get me one of them.

[@dantronics1682]

most do ac/dc these days, if you are getting one its best to get one that can read inrush current

[@DavidMcLuckie]

amzn.to/3SNPQ46 It's a Uni-T version. I saw Big Clive with one at some point and ordered it there and then. :)

[@bigsteviesri]

@@DavidMcLuckie thanks dave.

[@keithcarpenter9938]

Dude. Why do you not have links to good stuff. You could be driving a Tesla like Will Prowse and telling us how crap it is. I'm getting some diesel heaters soon and I need links.

[@DavidMcLuckie]

Mostly because Will has something like 10x as many subscribers as I do. You already appear to know how crap the powerwall is. ;)

[@larrylane9355]

When the battery voltage is lower than 2.3V, it cannot be recognized and cannot enter the start mode. At this time, you can use the force start function to start the vehicle. Is there a force start mode?

[@c-qc-q2021]

I don't think there's a force start mode on super capacitor jump start devices - at least not on my Utrai - I think that's a *lithium* battery jump start feature. It charges to 100% (or at least 97%, 14.5v), then you push the start button, and the capacitors start dumping their energy, and after a countdown of 10 beeps, the tone changes to continuous, and you turn the key/start the engine, and pray. If your battery is dead (below 8v?) you can alternatively charge the (Utrai) unit through a 2A, USB-A/C cable (mine 35min), 12vDC barrel port, or connect it to another 12v auto battery (best). If you have a difficult-to-start car, you best get a tune-up/ get it fixed, or get a lithium battery version.

[@DavidMcLuckie]

What C-Q C-Q said.

[@kaibonvlog6693]

How much

[@MrSteve1412]

Hi love you videos very informative I have one of those but my one now comes up with E02 when I try to charge it can you help me please Thanks Steve

[@fredflintstone1]

see if it starts a car without a battery???

[@donglue5089]

Hi I wonder if you can help me I Have a diesel heater problem I thought was the pump I replaced it with a new one but the same things still happened The pump makes a ticking noise but does not pump fuel do you think it might be the motherboard that's got a problem I wonder if you have any ideas. Thank you very much. From Don.

[@DavidMcLuckie]

The only pump I've had that ticked, but did not pump fuel, was when I had a 24V fuel pump wired into a 12V heater. It ticked but it wasn't enough to actually move any fuel. The other problem I've had is a longish fuel pipe during priming. It would pump and pump and pump but not draw any fuel up. If I manually sucked fuel up, then put the fuel pipe back on the pump and ran fine after that.

[@Almogamil]

Thanks ❤

[@donglue5089]

From Don glue again Thank you very much. for answering. my e-mail so promptly When I got the heater it worked for about 10 minutes and then suddenly shutdown the pump did work But when trying to start it again I realised There was no power and the fuse had blown Do you think it might be a problem with the motherboard All the best from Don glue.

[@DavidMcLuckie]

It's a odd one I'll give you that. And with these heaters anything is possible. The motherboards are so cheap it would be worth trying it.

[@assassinlexx1993]

With a warm little four cylinder. Ok it will work. Put it to the test with a large diesel engine on a cold morning.

[@MrChrisdavie]

Worked fine on my v8 m5 competition pack on a very cold Scottish winters day when a comfort access fault completely flattened my battery!

[@assassinlexx1993]

@@MrChrisdavie Wow that it worked. I need two large batteries to start her in the cold.

[@MrChrisdavie]

@@assassinlexx1993 I guess the compression ratio with the diesels will make it a bit more tricky, but this was a brand new device and it worked just fine. Lots of electronics in that car too.

[@tommotomlinson3370]

I have an UTRAI 2000A Power bank jumpstartery thing which has been great, can leave it for a couple of months and it still starts 2.5 diesel...not as quick to charge of course but it has a torch and a crap compass 😁

[@wwaldes]

@@tommotomlinson3370 najważniejszy w tym szalonym wynalazku jest kompas.

[@MrSteve1412]

Sorry forgot to add that it has uvp in red as well as the E02

[@patrickday4206]

You should review lithium titanate batteries I built a bank they have incredible charge and discharge rates last 30,000 cycles without cold weather issues like regular lithium ion and much higher c rates and no fire Hazzards from over charging or being punctured

[@dantronics1682]

similar to lifepo4

[@patrickday4206]

@@dantronics1682 not even close about 5 times the charge and discharge rate and about ten times the charge cycles without any cold weather issues and can be charged to 100 percent and drained to zero without damaging cycle life and no fire Hazzards from over charging or being punctured

[@openyourworld]

Ele recarrega celular?

[@LRK-GT]

5S 2.7V Supercaps is *only* 13.5V. Supercaps are particularly sensitive to damage from overvoltage (last I deep-dove the topic ~5ya). This device appears to be engineered to wear out, as it will be seeing 13.6-14.4+V. Also, as-mentioned in other comments, you lose capacitance in Series configuration. So, the 'rating' is obscenely inaccurate.

[@c-qc-q2021]

There are a number of super capacitor jump start devices that look identical and are maybe rebadged: Kroak, Utrai, Buture... but with slight product differences, like a different case (eg hard, soft). At least for the Utrai, it kinda frosts me that the start button isn't modal and starts the charging mode, instead of just the discharge mode... it starts charging as soon as the clamps are connected. Also the capacitors leak real quickly, dropping below 14.5v (min charge voltage) within 15 mins. If it could hold a charge for a month, or even a day, I'd be ecstatic. They all advertise "a life of 100,000 (charge/discharge) cycles". I'm curious what one can expect in real life.

[@DavidMcLuckie]

I was thinking about this the other day. What would be great would be a hybrid device. Combine the super capacitors AND the lithium batteries. You could even have the option of choosing between capacitor jump start (to save the life of the lithiums) or use both for a massive amount of current available for starting.

[@paulpatrick9693]

@@DavidMcLuckie Sealey do hybrid type boost devices

[@wood42shed]

My car drains it's battery in 4-7 days. I haven't been able to track down the parasitic drain yet. With one of these then even if the main battery is down to 10.5V I can charge the jump starter up to say 14V and start the same car, after which the alternator will charge it while it's running. You can't think of these like lithium or lead acid battery based jump starter packs, it's not the same thing at all.

[@4565klaas]

its 80 farad total not 400 farad

[@DaveAber9999]

Looks suspiciously like it! 5 x 400F all in series.

[@mattsan70]

when wired in series the total capacitance is the same as a single cap.

[@DaveAber9999]

@@mattsan70 Not when I went to school it wasn't. Ct = 1/(1/c1 + 1/c2 + 1/c3 + ... + 1/cn). The same as resistors in parallel. Capacitors in Parallel just add up like resistors in series do. Unless electronics has changed in the past 40 years...

[@mattsan70]

@@DaveAber9999 when they are all the same value your formula works out the same. Try it

[@dantronics1682]

@@mattsan70 nope!! 1/(1/f*5), I made one some years back but the self discharge was too high for it to do anything, they make good door stops though

[@wwaldes]

Zalety: brak beznadziejnych 18650 których trzeba nieustannie pilnować, a chińskie są wyjątkowo gówniane. Można szturchnąć prądem ale trzeba go mieć pod ręką aby naładować zestaw kondensatorów. to jest jak baniak do pompowania kół, pompujesz ze sprężarki, zanosisz pod koło i dmuchasz do tego koła aż ciśnienie w banaiaku spadnie. Wady: oszustwo : połączenie szeregowe kondensatorów dzieli pojemność 400F/5. Superkondensatory mają niskie napięcie więc aby uzyskać zapas do bezpiecznych 16V trzeba dołożyć jeden kondensator. Kreatywna księgowość cwaniaków z chin. Pytanie jak długo wytrzymają te superkondensatory ładowane do napięcia wyższego niż ich maksymalne napięcie pracy. Ciekawe jak szybko postępuje samorozładowanie. Tego wszystkiego nie wiemy. "wiaderko z prądem". Myślę że to ślepa uliczka chyba że taki starter kosztował by 50$. Rozwiązanie jest na horyzoncie accu litowo-tytanowe. Wygląda na to że mają kosmiczne możliwości

[@twocvbloke]

Next up there'll be a copycat called Ribbit... :P

[@graywolf2694]

So you could just leave it plugged in all the time

[@jamesemery1]

NO good if your on your own with no way of charging the thing to jump your dead battery. It's fine if you're near to a power source to charge it, but other than that no good for self rescue.

[@dantronics1682]

does chocolate teapot springs to mind?

[@DavidMcLuckie]

Ahhh no. I've used it now. Dead car, battery 8-10V. Attach Kroak thing to dead battery, starts to charge. Take about 3 minutes to get to 100%. Press and hold the button to start the timer, you get 30 seconds to run around and get in. Once the tone sounds you crank it. Fired right up. Video to follow soon.

[@jamesemery1]

@@DavidMcLuckie fair dues looking forward to the video

[@wood42shed]

@@DavidMcLuckie it did seem a bit odd to do a video about a car starter without trying to actually start a car. It's not a battery charger, it isn't supposed to be. Looking forward to seeing it actually working for you. It was good to do the teardown though, that's what I came here for. Thanks for that.

[@wood42shed]

There's a dc-dc boost converter in there which can charge it to 14.5V from a 10V source. So you don't need another good battery or other power source and it's perfectly good for self rescue, as long as a battery with not enough in it to spin the engine over is your actual issue. If it doesn't then start easily with an effectively good battery this won't help you.

[@bighen43]

You got the wrong supercapacitor charger that when you using on dead batteries is no good you need the other one supercapacitor charge bank is better than that one

[@melovescotch]

You could have discharged it into the dead batteries for a real test.

[@dantronics1682]

if the load/battery is less than 10v it doesnt do anything

[@melovescotch]

@@dantronics1682 Even when you press the button manually it doesn't even try to output?

[@pifandrei5743]

Dude sounds like that fifa narator narating the football game on ps4 . 🤣🌈

[@EIRK2046Y]

No balancer for capacitors... OMG.... 80 Farad in total. Crap....

[@Ellifiknow]

Can you stick your tongue on it?

[@raducumilea-is2yi]

It's not a battery charger...so... 🙄

[@yves2348]

You´re doing it wrong. rtfm