Our laboratory: research into the causes of fires originating from solar-powered spotlights
A mail-order solar-powered spotlight has been involved in several incidents where it has been identified as the point of origin of the fire. Our laboratory has carried out a complete functional analysis of the product using both new and damaged products in order to understand the reasons behind these incidents and to decide on whether to implement a market withdrawal campaign or a simple update.
Pre-requisite: recreation of the electronic diagram
The product comprises a solar panel, a panel of LEDs producing the light, a printed internal circuit board with electronic components and two batteries for night lighting.
As we did not have the product’s electronic diagram at our disposal, we had to recreate it from scratch in order to understand the architecture and test its operation with an oscilloscope.
Observation: several precautions for use
By studying the diagram and observing its operation with the oscilloscope, we have been able to make the following observations.
The circuit board only manages the “lighting” function according to the amount of ambient light and the selected operating mode, and not the implementation of the lithium-ion batteries’ charge.
Precautions should be taken when storing energy inside lithium-ion batteries to:
- Avoid overloads
- Avoid deep discharge
- Avoid short circuits or intense currents
- Avoid putting two elements of unequal charge in parallel
If these precautions are not adhered to, the battery is likely to overheat, catch fire or explode. Considering the huge amount of energy stored inside (28.5 kJ), its sudden accidental release could cause serious damage.
Protection is therefore provided via a dedicated electronic circuit called the “BMS” (battery management system). This circuit may be an integral part of the device in which the batteries are incorporated (the light in this case) or directly integrated into the battery itself.
As the study of the circuit board revealed that it was not equipped, this protection should, in theory, be incorporated into the batteries themselves, which exist in two formats: with or without integrated electronic protection.
When we opened the batteries we discovered that these models do not feature integrated protection.
Conclusion: an increased risk of damage
The batteries used and found in the product are not themselves protected and the product does not comprise a BMS.
In the case of prolonged exposure to direct sunlight in “OFF” mode, the batteries do not discharge but are constantly charged by the solar panel and are capable of delivering more than 115 mA when two batteries are used in parallel.
The risk of damage to the batteries is undeniable.
On the day of the incident, the marketed product that we examined was not compliant with even the most basic of industry standards for a product intended for outdoor use and incorporating lithium-ion batteries.
The product is dangerous and must be subject to a withdrawal or battery replacement campaign.