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Help needed with portable car battery charger

Hi I have a Guluman portable battery charger, which works a treat for getting our T4 van restarted.
However it's developed a small fault, but quite a major one, namely that it won't turn off.

It has a small on/off button on one side, one of those 'light-touch' on/off switches which also functions as a torch when held down for a few seconds: it seems to have lost its sensitivity meaning a) the torch doesn't work and b) the charger is constantly ON - so as soon as the charger has been recharged it's always on and it runs down in about 24 hours.
I can still recharge the charger from the mains as normal, and once it's fully powered up it still can get our van going. I could leave it on charge indefinitely but obviously that's a massive waste.

So - I could open it up and fiddle with what's under the on/off button BUT since this thing has enough power to start a van (and it's always on) I don't want to risk giving myself an electric shock. Plus I'm no electrician, which is why I'm seeking help here!

Anyone have any bright ideas? Guluman (a Chinese firm...) have no website/contact address or contact details or repair facilities, needless to say. It's a good bit of kit and I don't want to give up on it just yet. I'm sure there must be a fairly simple fix.
Specs: 3.7V 16800mAh USB output 5V/2.1A/5V/1A
There is no risk of "shock" (bodily) harm, because it is only ~16.8V or so, fully charged. It's the voltage that causes X amount of current to flow.

It can instead deliver a lot of current, so don't accidentally short it out with metal tools, which could cause a spark, or melt/pop a component, or the tip of the tool. Be careful prying open the casing, try not to use metal tools for that in case it would slip and puncture the battery or if you must, open it outdoors (or concrete/dirt/gravel/garage floor, etc) where if it were to start a fire, there is no risk of it spreading.

So, yeah you need to open it, no way around that. I'm not sure exactly what you're describing with the "light-touch" switch that functions as a torch, but you need to check whether the switch mechanism is closing the circuit, and that it hasn't broken off the solder joints to the PCB, assuming it is PCB mounted.

If it's not the switch, you'll have to trace and reverse engineer the circuit a bit, which could be beyond your skill level given the statement about shock risk, but basically the start is a visual inspection for things like burnt components or traces, broken wires, cracked solder joints, burst capacitor(s), etc... anything that looks out of place.

All else fails, take a high quality, top-down (and bottom up, if there are components on the back of a PCB) picture of the internals, but it will be too large (as high quality) to post here, so post it on an image hosting site like Imgur and link to it here.

Edit: Is it this product linked below, and has a rubber cover over what is probably a momentary mechanical push switch under it?
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Thanks dave9 - yes it is that one, and you can see the switch in the second picture on Amazon. It's not a rubber cover but hard plastic and the switch is one of those that 'clicks' lightly rather than pushes in or engages with anything; the idea is that holding it down long enough lights the torch/switches it on/off. There are four points where very small screws are embedded (fairly deeply), and from what I can see they need a triangular driver head (which i don't have).
Okay, a non-latching switch.... still have to open it up to see what's what.

Over the years I've collected spare, random/junk screwdrivers. If I didn't have a triangle drive type, I'd turn one into that on a bench grinder... grind a bit, check it for fit in the screw head, grind some more. A file instead of a grinder would work too... or just buy a 1/4" bit set if you have a bit driver.

I had already assumed you have a multimeter and can solder. If not, the repair may be outside your ability... unless you want to buy these things and learn to use them. Both are extremely handy in this age of consumer electronics, appliance electronics, automobile electronics, etc.
Thanks dave9. No, I don't have a multimeter and tend to run a mile from anything electronic, which is why I'm here! hoping that someone can tell me what's wrong so that I can find someone to fix it. I'm (slightly) tempted to start learning to use such things if I felt the repair was something I could manage myself but I'm getting on in years and more likely to try and find someone who has more expertise and can do it for me. I hate throwing things out that could be rescued; this device is a godsend for our van which has a weird habit of refusing to start when the diesel's low or if it's last voyage was a short journey.
Meanwhile bodging myself a triangular screwdriver is much more up my street. Mind you these screws are pretty tiny...
Whoever you find to do the repair, is more likely to be able to diagnose it too since they have hands on it. It will probably cost as much or more for the repair than it is worth... something like that (same capacity) I can find on Amazon (US) for about $65... but if the design is prone to breaking the switch, it's worth less used and broken... just the reality of our disposable society unless you can DIY the repairs.
I've used the tang end of a 3 sided file to remove triangular socket head screws in the past...
Now, I just use a head out of a 100 Piece Specialty Bit Set B^)