About two years ago, I bought a rechargeable 6-LED flashlight, brand Z-Light, model ZL-2225. I chose to buy it in store in Brazil, even to have a more durable product than those found in the informal market.
But I was deceived. A few months ago, the flashlight began to make a frying noise when put to charge and did not work anymore. Since I could not accept such a short life span, I decided to open it up and identify the faults that had occurred.
I dismantled the flashlight, at great cost, because the lid of the LEDs, although threaded, was glued. The parts that comprise it appear in figure 1.
If we look at the LED plate (figure 2), we may note that all resistors are damaged. Not only did they, but the 6 LEDs also burned, exactly because of the carelessness of turning on the flashlight while it was plugged into the power outlet to charge the battery. Just gave a burst and stank electronic component …
The source that carries the battery is FAST (Transformerless Power Supply) type and is shown in detail in figure 3. Figure 4 shows the complete diagram of the flashlight. Instead of the transformer, a capacitor is used in series with the mains. This capacitor (C1) must have a resistor in parallel (R1), to discharge it and avoid the shock between the terminals of the plug.The capacitor limits the current, thus lowering the voltage on the battery.
It is a type of source that does not have galvanic isolation between the electrical network and the stage of low voltage, responsible for the activation of the LEDs and the battery charge. Because of the danger of electric shock, this configuration can only be used in internal and isolated circuits, as in this flashlight.
According to craftinlearning, to charge the internal battery, the flashlight can be plugged into 110 or 220V AC power, there is no key required. The difference is that the charging will be slower at 110VAC than at 220VAC.
After the C1 capacitor, there is a rectifier bridge (diodes D7 to D10), which is connected directly to the battery. It also has an LED charging indicator, as you can see in the diagram in figure 4.
In order to power the LEDs, the battery – which is lead-acid type-is mounted with 2 cells and can therefore reach approximately 4.4VDC. It is that each lead-acid battery cell exhibits a nominal voltage of 2.2V when fully charged. The amount of cells will determine the final battery voltage. For example, 12V batteries have 6 cells in series.
The larger current will be achieved by increasing the size of the plates of each cell. That’s why car batteries are bigger than motorcycle batteries, despite the two being 12V.
Returning to our subject. Each of the white LEDs has a 33 ohm series resistor, which helps to balance current consumption between them. They are connected in parallel, in groups of 3, activated by the slide switch, which has 3 positions: off, minimum intensity and maximum intensity.
After opening the flashlight and testing the circuit energized for the battery charge, I found that there was more than 300VDC over the terminals of it. This voltage is due to the rectification of 220VAC. This indicated that the battery had high impedance and did not limit the voltage to the value of its two cells (which would reach the maximum of 4.4VDC or so). Generally, this problem is caused by the sulfation of the cells.
Sulfation is an oxidation that occurs on positive battery plates, according to references ,  and . Lead sulphate naturally forms in the discharge and is removed during loading. Under normal conditions, sulfation forms small crystals, which makes the process reversible.
But when the crystals are larger, sulfation can become irreversible. The problem occurs when the battery is left uncharged for a long time or is stored for a long period of time with low charge, or when it is overcharged, according to the Pampa Batteries , in a good article.
There are circuits on the internet that promise to carry out the desulfatation of batteries, like MPM Electronics , very complete. I have not set up any of them to know if it works to the satisfaction, but the idea is interesting, could avoid the premature discard of many batteries of lead-acid.
Experimentally, I have already relived several batteries giving 2 or 3 small “shocks” with a 30V source. With the interlocking negative poles, it quickly connected, for less than 2 seconds at a time, the positive poles. Sometimes, it did the process by reversing polarity, but this mode can be dangerous for sources that do not have full protection.
I tried this procedure on the battery of this flashlight, without success. Apparently, the charging frying noise indicates another internal damage.
The problem of circuit simplification
The high impedance resulting from battery sulfation exposes the LEDs to a hazardous situation as they become unprotected during battery charging. Just look again at the schematic of figure 4.
For example, let’s assume the flashlight switch has been turned on inadvertently and the battery is sulphated. There will be no light in the LEDs as the battery is completely discharged.When the flashlight is connected to the mains, the LEDs, which are connected by the switch, will burn immediately because they will receive 300VDC when it should be just over 4VDC. It’s over.
In the flashlight object of this article, in addition to the defective battery, which caused the frying noise when charging, all 6 LEDs burned this way.
Is that there is no voltage limitation for the battery, only the current is limited by C1. Because of this, the battery undergoes enormous stress and can easily spoil, as it actually did.
And the mains plug, two round pins, which is built into the flashlight. A mechanical lock could be developed that would prevent charging if the LED switch was on.
Or, when the plug is exposed, a key must be activated that will necessarily turn off the LEDs (which could be at that point shown by the yellow arrow in figure 4).
But the extreme simplification of the product did not predict this possibility. Or predicted, to sell more flashlights.
The momentary solution to this problem was to properly dispose of the battery and bag the rest, hoping to someday have time to install an improved load circuit and drive the LEDs. Besides having bought another flashlight…
What disgusts me as a consumer is that manufacturers are making increasing use of planned obsolescence.
This technique is due to an accurate knowledge of the behavior of the materials, allowing the tolerance decreases and the safe use of the physical limits of the components. But this can be used in two ways.
The first would be to avoid waste, making products lighter and lowering the costs of the raw material, reducing the final price.
Soon, the magic word appeared: costs. There the eyes of certain businessmen shine and comes the second way of using the knowledge of materials.
An inexpensive product is manufactured with a minimum of components, most at the limit of its capabilities. Any slippage in use has the potential to cause a defect. Or, after some time, several components fail at the same time, forcing the consumer to consume again. Or, the product quickly becomes obsolete, as other “much better” technologies are developed.
All these actions cause a turnover of products that for the industry may be interesting, but for society, not. Because all the money paid for the product stays with the manufacturer plus the shopkeeper, since there is no possibility of resale. Technological evolution is natural, but it seems to be being forced, today, to keep the same countries as ever in the forefront. This is our modern life.
That is why the coefficient of cachaço, so well-known of the engineers, is getting smaller and smaller. This rather cool term actually gives a clear idea of what we are talking about: an engineer who does not feel secure in the quality of concrete in a building increases the size of the beams and columns to even worst case the building never comes below. It was a good custom, at a time when the concrete was made in the work, without much control. It is the same as the intention to make a durable product (which is increasingly rare). Some brands are recognized for their intrinsic quality and durability, while others only excel in appearance and low durability.
What should be commonplace is practiced by a few companies: what is the best quality possible with the product that manufactures, with the price X, that can last much longer than the consumer thinks it should?
It turns out that in the future our children and grandchildren will no longer be able to afford to buy a product and throw away soon after, because the planet already shows signs of exhaustion. Why continue with this insanity?