DC-DC converters not good for rechargeables - Page 1
Forum section: electronics / general

author: JazzHarper
author_rank: Contributor
timestamp: January 05, 2016, 09:10:11 pm
content: My wife got a few of these LED candles a few months ago, to use as holiday decorations. Each unit takes two R14 (C cell) batteries. I thought that it might be a good use for rechargeables, so I put some NiCd cells in them. What I did not realize is that these devices contain an aggressive little DC-DC converter which drawsincreasingcurrent as the battery voltage declines. That works well for alkaline batteries, as it keeps the LED at constant brightness until the cells are fully exhausted, but it can be fatal to rechargeables. It put several of my NiCd cells into an unrecoverable polarity reversal. Next year, we will remember to only use disposable batteries in these candles.Seems like DC-DC converters are popping up in all kinds of products. Be careful where you use your rechargeable cells.

author: Stonent
author_rank: Super Contributor
timestamp: January 06, 2016, 03:13:55 am
content: I wasn't aware NiCd was still used outside of cheap chinese solar garden lights.

author: Rick Law
author_rank: Super Contributor
timestamp: January 06, 2016, 05:23:53 am
content: Finding the hand written note on the chip after I wrote the first reply, I realize it too may be a boost and thus nothing different. Rather than wasting readers’ time, I decided to reedit this before too many read this.I like the simplicity of these kinds of 2xAA stuff, so I have experimented with it before using a Chinese made chip. It creates a dirty square wave.In the past I experimented with boosting before but at3xAAto 12V, and preventing it from draining to reversal. I decided to use Lithium protection board and it works well. I was boosting to drive a 1Watt LED at 12V with 3xAA NiMH. I had 3xAA in 4xAA container, using the protection board for 18650 (18mm wide) and putting that slanted in the 4th AA slot.With the lithium battery protection board, the 3xAA in series cuts out at 2.5V so if one cell is stone dead, the other two cells have to add up to > 2.5V and 2xAA cant do that for long with load. It does eat up some juice. The one I used was from FastTech 2Amp version. I tested that with some cells that was driven negative but revived. I drained them till light goes off a few times. Each time, the protection circuit prevents any cells in that series from going negative.With the 2xAA stuff, I found the PR4404 which is designed for this purpose. The elegance caught my interest. I experimented with a similar Chinese made chip (which after finding my note makes me think it too may be a booster). The CL0118B. It takes 2xAA (1.2V to 1.5V AA), takes in 370mA and put out 185mA.[Pictures below was included when I thought the author was looking for making that circuit, now it is just relic. Might as well show it.]To give you some additional info on the chip, I decided to show it. This is not a impractical light, but made just for experimenting. I do use it instead of reaching for a flashlight:Picture one: the circuit with the needed inductorThe PWM chip is the lone TO92 device on the board.Picture two: The circuit is so small it is hidden behind the battery spring plate.EDIT2:Found my hand-written note: "[Chip is] CL0118B. Max output 185mA at 370mA draw. 33uH inductor. Very dirty SQR Wave." This actually could be a booster too.

author: JazzHarper
author_rank: Contributor
timestamp: January 06, 2016, 07:01:12 am
content: Quote from: Stonent on January 06, 2016, 03:13:55 amAmazon is still selling bundles of them, particularly in the C (R14) size, under the SunLabz brand. They also sell NiMH that look identical. In any case, mypointwas that an innocent-looking device like this LED candle can destroy NiMH or NiCd cells before you know it, if you aren't aware of its current-sucking characteristic. By the time the LED starts to dim, the cells are damaged. Boosters are hidden in cheap products, now, and they don't play nicely with rechargeables.

author: Rick Law
author_rank: Super Contributor
timestamp: January 06, 2016, 05:54:42 pm
content: Quote from: JazzHarper on January 06, 2016, 07:01:12 amYeah, that is a good point you raised.I didn't get your point when I first read your postand thought you were seeking a solution. So I was quick to try to be a hero and show my solution. It was only after I found my notes on the chip I proposed that I realized my initial reply on solution was way off-mark. The re-edit merely stopped me from looking too silly.That said, boost or not,no_cutoff + serialized_rechargabledoesn't mix well. Even with cutoff, low drain will force it down to the cutoff every time. Those garden solar-recharged decorations are deceptive in battery cost. I like that blinking butterfly in the garden in the dark, but they lasts no more than two years. By second year, I am lucky even if it retains 50% of the lighting (battery capacity). I think I must have spend at least 3X the $ for batteries over one with a primary-cell and with LED equipped with a good late-night turn off.To prep for winter, I pulled out the solar-recharge batteries from all my garden displays. After this winter, the solar-recharge LED house-number display "rock" will be left battery-less. No one should be looking for my house in the dark anyway. The blinking butterflies and blinking grass hoppers will stay as NiMH/NiCD killers.

author: station240
author_rank: Unknown
timestamp: January 08, 2016, 06:44:55 am
content: Well yeah, as the battery voltage drops, the current draw needs to increase to get the same LED voltage and current. No idea why they 'forgot' to add a low battery voltage cut out.I've got a couple of these solar powered lights with the boost converter in. Well to be more accurate the PCBs out of them, after I ratted them for the solar cells.So far out of 2 PCBs with a 4 pin YX8018 flat TO92 type IC, and an inductor.One has 560uH inductor, the other 170uHI think the lack of output capacitor is a crude way to have pulsed LED drive, for higher apparent brightness. The lack of input cap is just being cheap, and also helps to kill the battery.Datasheet for the IC can be found herehttp://www.datasheetbank.com/YX8018-Datasheet-PDF-Shiningic-Solar-LED-Driver-4Pincomplete with inductor chart, plus the 9 different way to build the circuit