Flashlight upgrade project (C to 18650) - Page 1
Forum section: electronics / beginners

author: MyHeadHz
author_rank: Regular Contributor
timestamp: May 08, 2020, 03:24:49 am
content: The light:https://www.homedepot.com/p/Duracell-1000-Lumen-LED-Flashlight-with-Batteries-TC060A00/301844171This is a 4x C cell (6v) LED light that I want to convert to 2x 18650 cells (~7.4 volts).I had to put a bit of thick wooden dowel (25mmx57mm) to make up for the extra linear space, and I wrapped it in aluminum foil for contact. Then I took care of the extra diameter space with a simple cardboard sleeve. Everything seems functional and works well, except for one thing- it goes into protect mode if it's on HIGH for more than about 10 seconds. (The light has HIGH, LOW, and STROBE modes- the other two modes work fine.)It's probably because of too much current. What can I do about it? I was thinking about replacing the foil wrap with a contact on either end of the dowel rod and a hole through the middle, and adding an appropriate resistor housed in the dowel rod to limit the current. Is that a good idea, and how would I go about the details to get it to actually work (safely)?

author: Whales
author_rank: Super Contributor
timestamp: May 08, 2020, 05:21:50 am
content: Some numbers make this easier to judge.Good quality LEDs driven well are around 100lumens per watt; in practice more like something in the magnitude of 60lumens per watt. I don't trust that light can actually do 1000 lumens (never trust them) but let's play with that assumption.1000 lumens / 60 lumens watt^-1 ~= 17W17W / 7.4V ~= 2.3AThoughts:- If you use a series resistor or diode then you will need to dissipate watts of power. Very hard to do inside the wrong end of a torch without over-heating your batteries- You can definitely get 18650's that supply 2.3A without hiccup. That's only 1C discharge rate for the average 18650.- You can modify the LED driver board inside the torch to suck less when on high-mode, if you can get access to it physically without breaking anything.Q1: What is actually tripping? The circuitry in the torch or the protection circuits strapped to your 18650's (if you have any)?Q2 (edit): Have you tried measuring current draw on high with C cells versus the 18650's? Often you can measure by leaving the tailcap off. This torch might over-draw at the slightly higher voltage.

author: edpalmer42
author_rank: Super Contributor
timestamp: May 08, 2020, 05:55:44 am
content: If you replaced the batteries with LiFePO4 batteries, the voltage would drop from 7V4 to ~ 6V5 which is much closer to the voltage of 4 fresh alkaline batteries (~6V2). The LiFePO4 batteries don't have as much capacity as regular Lithium, or alkaline for that matter, so you'd have to decide if that would be an issue.I agree with Whales that it would be useful to measure the current draw. But trying to use a resistor or diode to drop the extra voltage sounds like a headache.

author: Rick Law
author_rank: Super Contributor
timestamp: May 08, 2020, 08:14:58 pm
content: Quote from: Whales on May 08, 2020, 05:21:50 amGiven that the OP said it took a few seconds for it to trip, I would venture to guess that it is temperature. If it trips a current sensor, surely something that is sold as an impossible "1000 lumens" is not going to be so extra-designed that it will test for current-limit, then have a timer to trigger it later or when it exceeded certain duration. That would be an MCU and/or extra components.That said, the over-heating is really a symptom of over-volt/over-current.May be the darn thing is just resister ballasted device or a popular LED driver.MyHeadHz, if you have access to the PCB, take a picture and post it. Perhaps some adjustment (like swapping a resister or something) on the driver PCB may just work. Without a picture of the PCB, there is nothing to go on but just guess work.Quote from: MyHeadHz on May 08, 2020, 03:24:49 am