Activating Plan B

There is a saying - "we ought to prepare for rainy days". This is especially fitting when it comes to solar energy harvesting in Singapore. Not only rainy days but cloudy days as well.

Singapore is a tropical island country surrounded by sea located near the Equator. Most of the days are sunny, hot and humid except at the end of the year in November and December which is so called the monsoon period when heavy rains and thunder storms are common in the afternoon.

I have 3 x 30W Mono crystalline panels, 1 x 42Ah and 3 x 9Ah SLA batteries connected in parallel for a total of 69Ah capacity. I am running a 3W LED lamp for few hours at night, a 15W Solar fan running 8 to 10 hours overnight, a 6W LED strip running occasionally and a 2W handphone charger.
With my batteries, I should have 828Wh (69Ah x 12V) if fully charged, so they should be able to power my loads continuously for about 32 hours or 4 nights without charging if I run them 8 hours per night.



Under normal weather condition, that is mostly sunny with occasional one to two days of rain and cloud in between, my setup should be adequate. However, the reverse happens during this 2 weeks whereby most of the days are raining and cloudy with 1 to 2 sunny days in between.
Eventually after almost 2 weeks of working, my system sounded the first low voltage alarm at around 6.30am Saturday morning. I looked at the battery voltage and it was at 10.4V. I quickly switch off my fan to prevent the batteries from over discharge. The batteries were at 11.9V the night before.

Now I have 2 options. One is to expand my solar capacity to cater for rainy and cloudy days. Another is to use an AC battery charge to charge up the batteries.

If I opt for the first option, I think I would need to at least double my panel capacity from 90W to 180W, which means it would impact my ROI (Return on Investment) for at lease a few years. The other disadvantage of this option is that it would be over capacity during sunny period.


So I opted for the second option which is my plan B. Then I started to look at SLA battery chargers. The price ranges from S$18 to almost S$50. I am reluctant to spend extra money on charger and also have the fear that poor quality charger might damage the battery. I also read that we should not use charge meant for charging car battery to charge SLA battery, otherwise, it may cause gassing and explosion.

Then I remembered I still have an 10A PWM Solar charge controller laying around after I switched to the MPPT controller. This would be an ideal charger as it is designed to charge SLA batteries. I also have a Philips power adopter which I use to charge my Philips electric shaver.

The Philips power adopter output is at 17.2V which is well within the input range of the PWM charge controller. I then connected the output of the Philips power adopter to the panel input of the PWM charge controller and observed that the red panel LED lighted indicating that it is receive power from the "solar panel". I measured the voltage of the "panel" input and it read 11.89V. I measured the battery output and it read 11.86V. The charging green LED also blinking indicating that it is charging the battery.

The Philips power adopter is rated at 9W. I do not know how much charge it will put into the batteries. But so far it has been able to maintain the voltage above 12V with current weather condition of frequent rains. With that little extra electricity it consumes, I think it is worthwhile to go for the second option and implementing Plan B. I have decided that I would always use the AC charger to charge the battery if the voltage is below 12.6V.

With this plan B, it gives me the peace of mind that I will always have enough energy to power my loads in all weather condition and can sleep soundly without being wake up in the middle of the night by the low voltage alarm again.

Update: The Philips power adopter with it's 9W power is not enough to buffer me through a few consecutive days of cloudy sky and heavy weekends usage during the Christmas period. I ended up ordering a 2A battery charger from DealExtreme which should tide me through longer rainy days. The learning point is to get a charger that is sufficient to charge your battery than what you will consume for the whole day.