Parallel solar panels: with or without two charge controllers?

[wdc]

Hello, this is my first post, I had one 40W 12V solar panel connected to a cheap PWM 20A charge controller and this to my battery, then I bought two extra 50W 12V solar panels to add to the system in parallel (I wanted to increase current to allow the battery to charge faster), the three panels are almost identical in their specs (40W: Voc=21V Isc=2,56A, 50W: Voc=21,4V Isc=3,06A) so I went and connected them in parallel, but then I read a website that said that the "ideal mixing" for this cases is the 40W panel to the original charge controller and the two 50W panels parallel connected to a new charge controller and both CCs to the battery. Is there any benefit to this approach? or should I just let the panels as they are? thanks in advance!

(I didn't include battery specs because I don't think they are relevant to this matter right now)

w.

ps. I don't know if this is in the right forum so please move it if it doesn't.

[SunEagle]

Since you are using a PWM type controller wiring the panels in parallel is probably the best circuit.

When using different panels wired in parallel the Vmp's should not be more than 5% different from each other. Otherwise the lowest Vmp panel will "set the voltage" going to the CC and you lose a little efficiency of the total panels.

If you add up the Imp values of all your panels they should not exceed the Amp rating of the CC. My guess is that a 20amp CC should be able to handle the 140watts of panels you have.

[wdc]

Since you are using a PWM type controller wiring the panels in parallel is probably the best circuit.

When using different panels wired in parallel the Vmp's should not be more than 5% different from each other. Otherwise the lowest Vmp panel will "set the voltage" going to the CC and you lose a little efficiency of the total panels.

If you add up the Imp values of all your panels they should not exceed the Amp rating of the CC. My guess is that a 20amp CC should be able to handle the 140watts of panels you have.

thanks, I read in the forum that one can not measure Vmp and Imp and that's why one have to rely on Voc and Isc to estimate the first ones, is that correct? thanks again in advance.

[SunEagle]

thanks, I read in the forum that one can not measure Vmp and Imp and that's why one have to rely on Voc and Isc to estimate the first ones, is that correct? thanks again in advance.

Usually the panel specs are on the back of the panel. The printed specs should be on each panel and you should be able to see the Vmp & Imp along with the Voc & Isc. If not then the "mp" values are a little less then the oc & sc values but it is hard to pin down without the exact values.

Based on your Voc values of 0.4 volt difference I would expect the Vmp values to be also very close to that 0.4v difference.

[wdc]

Usually the panel specs are on the back of the panel. The printed specs should be on each panel and you should be able to see the Vmp & Imp along with the Voc & Isc. If not then the "mp" values are a little less then the oc & sc values but it is hard to pin down without the exact values.

Based on your Voc values of 0.4 volt difference I would expect the Vmp values to be also very close to that 0.4v difference.

thanks, yes Vmp are 17,6V and 17,5V (40W and 50W) and Imp are 2,28A and 2,78A each but these are cheap chinese panels and I don't trust chinese manufacture very much.

do you know some way to measure that adding these two 50W panels really increases the current delivered to the battery? thanks again in advance.

[SunEagle]

thanks, yes Vmp are 17,6V and 17,5V (40W and 50W) and Imp are 2,28A and 2,78A each but these are cheap chinese panels and I don't trust chinese manufacture very much.

do you know some way to measure that adding these two 50W panels really increases the current delivered to the battery? thanks again in advance.

The best total charging amps the 3 panels can get you is; 2.28a + 2.78a + 2.78a = 7.84a but in reality you might get a little under 7 amps at NOON.

[wdc]

The best total charging amps the 3 panels can get you is; 2.28a + 2.78a + 2.78a = 7.84a but in reality you might get a little under 7 amps at NOON.

thanks!, one last question if I may, how much AH should I look for in a AGM deep cycle battery? right now I'm using an used truck 70AH battery but it's 5 years old so I know it won't last very much and I'll have to replace it. Here in Chile we don't have high quality solar gear and almost all the batteries are chinese cheap ones, there is a Trojan one 50AH (the model is 22-AGM) but it's soooo pricey for my taste (about 260 US dollars). I'm thinking something in the range from 50AH to 75AH, this battery will be used to light 40 12V 10W led bulbs, not all at once, just about 12 at the same time, they're my house ceiling lights. I use thick cables and the house is no so big so not long cables also.

[SunEagle]

It will be hard to calculate your battery needs based on you just saying "40 12v led bulbs about 12 at a time". Having the number of watt hours you would use each day is better. YOu would have to calculate that value from the amount of watt each led uses and for how many hours each day they will be used.

As for sizing your battery based on the charging amps. An AGM can usually handle a charge between C/6 and C/12 where C = battery Ah. So with maybe 7 amps of charging you could get a 12v battery between 40Ah to 84Ah. But if you get a battery that is too small for your loads you will drain it every night and reduce it's life. So that 70Ah will get you about 210 watt hours daily (70Ah x 12v x 25% = 210wh) if you only discharge it 25%.

[wdc]

thanks again!, is there any risk if a buy a bigger battery (let's say 100Ah) and my panels are too small to charge it to the top every day? or is better to buy bigger and that way minimize the risk of over-discharge every night?

[SunEagle]

thanks again!, is there any risk if a buy a bigger battery (let's say 100Ah) and my panels are too small to charge it to the top every day? or is better to buy bigger and that way minimize the risk of over-discharge every night?

Having a bigger battery will help reduce how much you drain it each night. But batteries are strange.

They need to get the proper amount of charging amps. Too little and they stagnate by sulfating their plates. Too much and they cook off all of the liquid. Both of those actions can kill it off and lower their lifespan.

[wdc]

Having a bigger battery will help reduce how much you drain it each night. But batteries are strange.

They need to get the proper amount of charging amps. Too little and they stagnate by sulfating their plates. Too much and they cook off all of the liquid. Both of those actions can kill it off and lower their lifespan.

thank you very much SunEagle, now I know better what to do.

[SunEagle]

You are welcome. Let us know how you make out with your system. Please ask more questions if you need help.