Our stairwell now has a functional solar light that gets triggered with an automatic twilight switch. Yet, the circuit is not really working as I had expected, and I wonder if the problem lies in the solar panel, the charger, or the battery.
Initially the panels had been rigged to a Sealed Maintenance Free (SMF) battery I got from a friend. It was an Exide 26Ah SMF - compact,and relatively lightweight at ~8 kg. The trouble was that even after getting charged by the 60Wa panel in over five hours of clear sunshine, it would not get the ~12W LED panel to run for more than a few hours.
Perhaps this was because the battery had got discharged after lying unused for more than a year. So, based on an online tutorial, I pried open the sealed valves and poured in a bit of distilled water in all its compartments. This did not seem to work either, with the LEDs going blank after about 3-4 hours every evening.
At this stage I purchased a dedicated solar battery from Okaya. This one was a 40Ah tubular battery, weighing more than 15kg. Big, heavy and designed for the outdoors. In theory it was expected to hold about 480 Wh of juice (40Ah x 12V). It worked just fine the first night. The lights switched on automatically at dusk (5:30PM) and stayed on until daylight turned the twilight switch off (6:30AM). In effect, during this 13-hour duration it should have used up about 156Wh (12W x 13 hrs).
Again, in theory, there should have been about 324 Wh remaining in the battery (480 Wh - 156 Wh). Yet, within two days, the battery turned out to be completely discharged.
Okaya was a bit tardy when it came to servicing warranties but a service engineer turned up and checked the battery. It was showing an output of 12.31V on the multimeter, but after checking all the six compartmetns with a hydrometer, he declared that the battery had indeed got discharged. Aparantly, the density (specific gravity) of the electrolyte had dropped way below 1.265 for a fully charged battery.
The next day, he came with a charger (16V), plugged the battery to the mains and said that it would get recharged in the nex 24 hours. All the caps were left oepend during charging because he expected it to bubble during the charging period.
Now the solar lighting system seems to be working perfectly fine, even though I continue to be nervous about the voltage drops. A dusk-to-dawn operations brings down the battery voltage to 11.86V, and it looks like the 60Wa panel is not really doing its job of topping up the battery even when there is bright sunshine for ~5 hours every day.
So, finally, the key questions that need answers are -
- How does one measure the juice remaining in a battery?
- If the 60Wa panel is not doing its job, should I get an additional panel?
- Is the LED panel consuming more than the 12W, as claimed by the vendor?
REFERENCES & LINKS
* All about lead acid batteries - http://all-about-lead-acid-batteries.capnfatz.com/
* Lead acid battery charging basics - http://www.powerstream.com/SLA.htm
* DV Voltage Drop Calculator - http://photovoltaic-software.com/DC_AC_drop_voltage_energy_losses_calculator.php
* Video - Using a hydrometer to test batteries - https://www.youtube.com/watch?v=SRcOqfL6GqQ
- A lead acid battery cell is fully charged with a specific gravity of 1.265 at 80° F.
* Maintaining SMF batteries - https://dmohankumar.wordpress.com/2012/08/28/how-to-maintain-maintenance-free-battery-tech-focus-5/