18V solar for charging 12V batteries? - Page 1
Forum section: electronics / beginners

author: asymcon
author_rank: Contributor
timestamp: August 22, 2018, 08:44:21 pm
content: Hey,I have had this question for a while. Can't quite figure out why for are 18V configs used for 12V installations.Most solar cells are most efficient around 0.5V/cell (assumed 50°C surface temp). Typical configuration is 36 cells per panel, which gives around 21V open circuit.The thing is, matched voltage/load for such panels is 18V not 12V.When charging lead-acid at 13.5V, or li-ion at 12.0V, 33% of this potential efficiency is wasted based on the I.V curve of most cells (such as A300 here):https://cs.wmich.edu/~sunseeker/files/A-300%20data%20sheet.pdfPreviously I though that solar controllers act as a dc-dc step-down transformers, but based on results of those I tested, that's not the case.They output the same amps to 12V batteries compared to just the cells.So, where's the catch?How to get maximum output from 18V panel when charging 12V battery?Should I set-up additional buffers closer to matched voltage and then step-down to 12V?Prolly a stupid question, but still...

author: wraper
author_rank: Unknown
timestamp: August 22, 2018, 08:54:44 pm
content: QuoteYou cannot directly attach solar panel to the battery anyway, regardless of the voltage.QuoteThat's some nonsense. To make it linear there will be some huge power dissipation on passing element given that your solar cells are not really tiny.QuoteNow it sounds your measurments were done weirdly. What do you mean by "just the cells"? You cannot attach them directly. You should measure current at the input and output of the inverter.

author: Seekonk
author_rank: Super Contributor
timestamp: August 22, 2018, 09:03:07 pm
content: It became a standard to overcome age, controller, temperature and wiring losses. I've seen panels barely make 15V at power point. I do have a high performing panel that is only 32 cell. It can be difficult to get any gains with MPPT in the summer with a 12V system. Come winter when it is really needed there is some to be gained. Many simple buck struggle to make 85% efficiency due to the almost 1V loss of the flyback diode. At double the input voltage there are gains to be made. I make charge controllers out of small buck modules and modify them to hold at power point voltage, MPPC. This gets you most of the advantages of MPPT with seasonal changes to the set point. These are ideal for 5V applications to boost power. I bought a couple really cheap PWM modules to see if I could attach these to a buck, but haven't gotten too far. The cheap ones float the negative of the panel which makes them more difficult.

author: David Hess
author_rank: Super Contributor
timestamp: August 23, 2018, 04:17:13 am
content: The simplest maximum power point charge controllers use a buck switching regulator so the input must be higher than the output. Ideally the input voltage is twice the output voltage producing a 50% duty cycle in the buck converter for best efficiency but it is not critical.

author: digsys
author_rank: Unknown
timestamp: August 23, 2018, 05:16:25 am
content: As others have pointed out - You need at LEAST several volts above max battery voltage to take into account dull / very cloudy days, otherwise you couldwaste all the power, PLUS the converter switching losses. Some (rarer) MPPTs do buck / boost, but usually for specific applications.On top of that the MPPT needs to know if you're using Lead acid or LiIon and if it's full or empty. ie have some sort of charge profile. If this isn't done,you may just see max voltage on the batteries all the time (not a good thing), and then just waster energy. Not as simple as plug and play.MPPTs are not just a simple DC-DC charger.

author: Seekonk
author_rank: Super Contributor
timestamp: August 23, 2018, 02:08:42 pm
content: Just one of those things like why are there no "B" batteries and why aren't there 8V car batteries (both of which exist). You shouldn't be using buggy whip 12V or 24V panels over 50W. Those panels are so yesterday. You are aware that to equalize a flooded battery the voltage may have to get to 15.6V. A 30V 40 cell grid tie panel won't charge a 24V battery. For practicality, the cells have to be square and that leaves you with limited arrangements like 4X8, 4X9, 4X10 etc.