A curious case of driver battery drain of a Fenix E28R, identification and fix.
I had this Fenix E28R for over 2 years. It went through countless emitter swaps and optic modification. There are many good design points about this flashlight: the body is really well made with tough anodization, stainless steel bezel and button, well designed clip, smallest form factor of a 18650 light with USB-C charging and TIR optics. But there are also not so nice design aspects: the weird UI (hold for on/off, click for mode advance) and TIR is extremely shallow which exacerbates LED tint shift and the most inconvenient of all: zero (non-destructive) modding potentional. But overall it’s been a reliable and dependable, solid EDC light for me for 2 years.
Recently this light developed battery drain issue. It would drain a fully charged, 3400mAh battery down to 50% in a week, which is very extreme. I took it out of service because I cannot depend on it anymore. The draining is also not safe, since such a high drain rate would release heat somewhere and if trapped in a tight space, can be a potential fire hazard. There is a workaround for that, which is to twist the head to disconnect battery when not in use, but it’s still a hack and if forgot, would completely drain a battery in two weeks. In case of a non-protected battery, it would be destroyed. Not good.
After leaving it in the cupboard for a couple of weeks, I decided to try to fix it again. There is no obvious way to dismantle it. The bezel is pressed in. I had to pray it open every time I want to do emitter swap and there is a limited number of times I could do it before the bezel ring gets too loose and wouldn’t hold in place. There is no obvious way of how the drive can come out. I tried it once before, pushing from the front, twisting from the rear, nothing is budging. Then I thought about Fenix loves to use loctite and glue to fix their light. I got the soldering iron to the highest heat, pressed it on the shelf to heat the entire head up and pushed from the front wire holes. After the head got to about 60C, the glue loosened and the entire driver was pushed out from the rear of the head.
The driver is well designed. With buck LED driving and buck USB charging circuits integrated in two boards in a T-shaped configuration. I measured the resistance between battery + and - terminal and got a reading of 1KOhm - not good. The resistance is way too low. To confirm my suspicion, I measure many other e-switch lights and they are almost open circuit between battery terminals. Something is shorting the battery connections. That’s why it’s draining the battery so fast.
The first thing popped in my mind is, there might be a input filtering cap that has been shorted. After some analysis, there are three ceramic caps in parallel to the battery. The good starting point would be to de-solder them one be one then measure the battery terminal resistance. I found the one closest to the center of the PCB is causing the short. After removing it, the battery terminals measures open circuit.
Reassemble, test, the light is still working. Kept the light overnight with battery connected and no further draining is detected. The driver is fixed.
I did more reading online and learnt that ceramic caps can be broken by mechanical stress on the board, cause short circuit. This all starts to make sense. The battery positive terminal is on the other side of the board and there is no spring, only a very short copper post to connect the battery. When the light gets dropped or dinged in everyday use, or when unloading and reloading the battery, all the forces would be transferred directly to the PCB. This flexing of the PCB after a prolonged period of time caused the ceramic cap on the other side to crack, develop a short circuit. It’s design issue really, but there is nothing I can do other than removing the cap and keep the driver as it is.
While I’m at it, I got tired of having so many 519A 5000K lights in my collection, that I replaced its emitter to a LH351D 4000K. The tint is alright and since the TIR is so shallow and LH351D has well controlled angular tint shift, it beam is actually more pleasant than the previous 519A 5000K in there. (Pardon the terrible soldering job. The wires have been through a lot, lol)
I’m overall quite happy that this light can be fixed again. And anyone having a faulty Fenix: it can be disassembled. You just need to overcome all the glue.