I have a maglite that I am wanting to mod with a charging port to charge a single 18650. I have a DSD charger that I don’t really use and the circuit inside is small enough to easily fit in a maglite.
My question is, how do you wire in an internal charger? Would I need wire it to a switch to use when charging or is it safe for the charger to stay connected to the + - of the battery when using the light?
I don’t have a DSD to tell about parasitic drain but from the looks there isn’t much on the PCB. If you’re using protected cells I wouldn’t worry unless you notice some voltage drop.
Charging batteries inside a sealed flashlight is risky. It needs to be modded to add a blow out vent. The DSD charger can develop problems like overcharging, no longer terminating. There is a safer charger you could use but to be honest I would just suggest reconsidering this.
Rather get a USB-powered Li-ion charging board, with proper chip that implements correct charging algorithm.
They’re $2-$3 off eBay.
Wow, no wonder I came across so many DSD mod threads when trying to find PCB pics. 
Yep, thats the board I would use. They are also available with a micro usb port instead of mini usb. You might find micro usb more convenient since its common for phone chargers. The parasitic draw is low enough to leave connected to the battery, 1uA.
I’ve been ruminating on this for a couple of years. I’d use one of those USB li-ion charging boards as mentioned above (I’ve used 6 or more of them in lights already) and have the -ve wired to the tail cap spring and the +ve wired to the 18650 adapter, so you can remove the tailcap to change batteries if you wish. Think of the adapter as a tube with a thin copper plate at one end and a wire running from that plate to the charger. Only way I can think of that would work.
If you’re hard wiring the cell in, forget all of that 
I don’t think that the parasitic drain on these chargers can be all that high - I’ve left lights with them sitting for 6mths or more and haven’t noticed any obvious reductions in run time.
BTW - you can now get these charging boards for $1 - how nuts is that? When I started using them there were perhaps 2 eBay sellers and they cost ~$4 each, now there are 100s of sellers and they’re less than a buck each. Crazy!
nice, I never knew HKJ had reviewed one of these. Nice to have some data to back up my unscientific gut feelings
I’ve got a couple waiting for projects. At 96c each, I could afford to get some spares. Compared to machining, this is a seriously cheap hobby!
I wouldn't buy the cheapest $1 boards on ebay. Different "tp4056" chip
I've bought from Chip_Partner_Store or ICs_Processors_Store for $1.54. RMM now has them for $1.42
is it ok to use these to charge 2x18650 in parallel?