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I have a 20 watt panel and a 120 watt mppt controller. Is it feasible to connect a 100watt panel in parallel with the 20 watt panel to give me a120watt equivalent? Also I have a 12v gel battery and a normal lead acid starting battery. If I connected these two in parallel would I need a diode between them as the gel battery has a higher full charge voltage compared to the lead acid battery, or would it be ok to just connect them?
Madness Guru Joined: 08/10/2011 Location: AustraliaPosts: 2498
Posted: 08:15am 04 Dec 2017
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You probably would get a better result by putting them in series providing the combined voltage is not higher than the maximum for your charge controller.
Connecting them in parallel will probably result in less than 100W if there is much difference in the maximum power voltage of the 2 panels.There are only 10 types of people in the world: those who understand binary, and those who don't.
davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499
Posted: 09:03am 04 Dec 2017
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I suspect that putting them in series might get you about 40Watts. How would the extra current that could be provided by the bigger panel pass through the smaller panel?
I have a 150W 72 cell and 2 - 250W 60 cell panels all in parallel. The 150W 72 cell panel contributes slightly less power output due to the higher Vmp.
George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308
Posted: 10:56am 04 Dec 2017
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Pretty sure putting panels in series results in a total output of the lowest panel multiplied by number of panels. In this case, as said, 40W.
You may be able to parallel them depending on respective voltages. I have a string of 250W panels in parallel with a string of 190's and get a good combined output because the voltage of each array is only 16V difference pmax. I'm sure there is some small loss but I am able to run both arrays from one inverter and max it out most of the day.
I really wouldn't worry about the 20W panel with the 100. you could test them in parallel but even so, you'll be lucky to get about 15W from it in reality and it may actually drag the big panel down if there is much of a difference.
Spend $20 and buy another pwm controller and have the small panel charge the little battery and the big panel with the Mppt on the lead acid.
What do you want to do with them? If it's anything practical that needs any grunt, upsize the whole thing. You can get used panels 250W on flebay for $50 and a 20A MPPT controller for 50 as well. That will give some real power and if you add another panel you will not only get more total power but more power longer in the day.
IMHO, max power is one thing, how long you are generating useful power is the bigger consideration. I like longer strings and over powered inverters for that reason.
hotwater Senior Member Joined: 29/08/2017 Location: United StatesPosts: 120
Posted: 03:27pm 04 Dec 2017
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Just how much did you pay for that controller? Given the system you have now I suspect it isn't a real MPPT controller. Am I correct in assuming both panels are 12V. MPPT literally has no advantage with 12V panels as the input and output voltages are so close.
I built the mppt controller from a silicon chip designed kit, and I am confident it is a mppt unit. Using my 20watt panel it certainly charges the battery quite quickly with measured current into the battery above expectations. My gel battery is a 90ah geltec not a small one. I was thinking by using a power diode I could prevent the gel battery from charging the lower voltage lead acid battery,if it is a problem. I was aiming to power my 350watt TV for short periods during a blackout for example. Last Thursday we lost power for one and a half hours on a very hot afternoon with air con running.i expect the power company is getting a higher price by selling it to vic or sa. It is mainly interest that is driving me as I am an electronics hobbyist. To power the TV costs about 10c an hour from the grid so it certainly is not a financial benefit. I have ordered a 600watt(1200peak) 12v inverter.
George65 Guru Joined: 18/09/2017 Location: AustraliaPosts: 308
Posted: 12:21am 05 Dec 2017
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A 20W panel is going to give 1.5A charging at best. On a 90 Ah battery, that's almost a float charge. If you think it's coming up quickly, you must not be taking much out of it to begin with.
If your controller is MPPT, you should be able to run a 24V ( 36V) panel into it and charge a 12V battery. For $50 you could get a LOT more power from a used 250W panel and have the capability of running a fridge or something similar through the day if there was a longer term blackout. Edited by George65 2017-12-06
The controller can be used for 24v and a 240 watt panel but it would require component changes. I don't see the need to go any bigger at this stage. Even with the small panel and a 150watt inverter I was able to run my internet modem and recharge our tablets etc. As we live in a semi rural area and depend on a pressure pump for our water, that is a nuisance when the power goes. A system to make that work would be very expensive and a small generator would probably be more useful. My battery was pretty well charged, however I was just trying to illustrate that the max power point was being used and I was getting the full capacity from the panel.
yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166
Posted: 05:15am 05 Dec 2017
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Hi Bristan8,
Should be fine for the panels.
paralleling the batteries will be a PITA unless their voltage specs match closely.Edited by yahoo2 2017-12-06 I'm confused, no wait... maybe I'm not...
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