Checking the math on 1.65 kwatt off grid solar system

[redneckrob]

Originally Posted by donradcliffe

After investigating a solar panel fire on a friend's boat, I found that it was caused by having more than two panels in parallel on the same controller. A panel developed an internal short. The panels are designed to handle this kind of short with two panels in parallel, but not the 5 he had without additional fusing.

I am struggling to understand this from an electrical engineering perspective. I don't doubt there was a fire on a solar panel on a boat. However, if a panel is adequately fused for a short from two panels in parallel it is adequately fused for a short with n panels in parallel. That's how fuses work, too much current and they blow, removing all current from the protected device. The concept that there might be too much current for a fuse to work just doesn't make sense. If anything you might see the opposite with a problem from too many series panels because the voltage exceeded the fuse voltage and it continued to arc after blowing, but again that's not a problem that really happens in practice. And in practice there are literally many millions of panels in the world operating with more than 2 panels in parallel. They are most certainly designed for this and it is in no way unsafe.

[sesmith]

I believe he meant that those panels were not fused. Pretty common on smaller installations where a dead short isn't a concern on the panel side....unless you parallel enough panels to exceed the current handling size of the wiring and panels, something his friend found out the hard way.

[redneckrob]

Originally Posted by sesmith

I believe he meant that those panels were not fused. Pretty common on smaller installations where a dead short isn't a concern on the panel side....unless you parallel enough panels to exceed the current handling size of the wiring and panels, something his friend found out the hard way.

Well if they aren't fused at all they couldn't handle a short with just 2 panels in parallel. The live panel would feed the short in the dead panel. Either way it doesn't make much sense. Even less sense with reference to the 550 watt panels the OP is using.

And again, literally millions of parallel solar panels in the world with more than 2 panels in parallel that are most definitely not bursting into flame when one panel develops an internal short. So either all those thousands of professionals doing that are wrong or....

[donradcliffe]

I will let Herr Professor explain it to you.

https://www.youtube.com/watch?v=GFjL0-r6LbY

[sailor4life7777]

Hey guys, thanks for all the great input! I purposely oversized my solar panels, but after taking everything in consideration, I am thinking about getting another 24V100AH lithium battery.

If I wire the 3x550watt panels in parallel, do I need a solar charge controller for each 550watt panel? Or would 1 100amp solar charge controller suffice for all three. If I decide to go with 3 solar charge controllers, can I go with 30amp charge controllers (550watt/24V=22.9x1.25=28.65amps)?

Thanks for all the great advice!

[waterman46]

If you read all the posts you will find that due to unequal performance of each panel it is best that each panel has its own controller and these are then connected in parallel.

[sailor4life7777]

Hey guys, for the LiFePo4 24v100ah batteries, they allow me to choose 50A, 80A, 100A, 150A, 200A, or 250A BMS. I'm not sure how to choose the amp size for my BMS. I plan to use this to run a freezer and electric cooking appliances for the most part as well as charging my laptop, phone, and some LED lighting. Are there any downsides with going with the bigger 250A BMS? Price is pretty similar, just slightly more expensive for the higher amp BMS.

Thanks in advance for any input guys!

[Kolchac]

Unfortunately many people, who are giving advice, do not know even the basics.
1. Parallel: voltage does not change, current - summary of the currents.
2. Series: current does not change, voltage - summary.
3. You are interested in power: VxI.
So, if your battery 24V, each solar is V=41V, series=123V, you are going to loose ~115v, or 115v x 13A=~ 1300W, so use parallel. Also you might cook your controller.

Also in the middle of summer, say 30 deg. Latitude your solar will give you only around 50% during ~from 11 AM to 3 PM max. if you have the Sun. Rest hours - from 0 to 50%. So you can calculate what battery you need.

[betwys1]

Originally Posted by donradcliffe

I will let Herr Professor explain it to you.

The 'professor' is not even wrong when he promotes a 10 amp fuse for each 5 amp panel. He TELLS you that if one of FOUR panels goes short, a 10 amp fuse per panel will protect it. Sadly, it does NOT protect a panel going short in a two or three panel parallel configuration.
Better to advocate for a X1.5 max current rating for a fuse in a situation when the load is not 'peaky'. Consider the virtues of contact breakers too. They can run around $10 to $20 each.
(A fuse will hold indefinitely at its rated current)

[Editorial note:" this whole thread is a credit to the questioner and his helpers...."]

[Kolchac]

Originally Posted by Kolchac

Unfortunately many people, who are giving advice, do not know even the basics.
1. Parallel: voltage does not change, current - summary of the currents.
2. Series: current does not change, voltage - summary.
3. You are interested in power: VxI.
So, if your battery 24V, each solar is V=41V, series=123V, you are going to loose ~115v, or 115v x 13A=~ 1300W, so use parallel. Also you might cook your controller.

Also in the middle of summer, say 30 deg. Latitude your solar will give you only around 50% during ~from 11 AM to 3 PM max. if you have the Sun. Rest hours - from 0 to 50%. So you can calculate what battery you need.

Forgot to mention: 140 V is quite significant - it can knock you out.

[Llarian]

Originally Posted by Kolchac

Unfortunately many people, who are giving advice, do not know even the basics.
1. Parallel: voltage does not change, current - summary of the currents.
2. Series: current does not change, voltage - summary.
3. You are interested in power: VxI.
So, if your battery 24V, each solar is V=41V, series=123V, you are going to loose ~115v, or 115v x 13A=~ 1300W, so use parallel. Also you might cook your controller.

This is totally wrong in any modern solar system. It would only apply if your panel is directly connected to your battery, and don't do that.

The solar charge controller (if rated for it) will transform the higher voltage into the correct voltage at a higher current for your 24v battery bank in this example. In no way do you lose 115v.

[Kolchac]

Originally Posted by Llarian

This is totally wrong in any modern solar system. It would only apply if your panel is directly connected to your battery, and don't do that.

The solar charge controller (if rated for it) will transform the higher voltage into the correct voltage at a higher current for your 24v battery bank in this example. In no way do you lose 115v.

Yes, It is possible. You need a device, which has a generator, which creates alternating current 115V, then electrical transformer, which lowers 115V to 24V, then electric rectifier. You also keep in mind the size of the transformer because one of your coils should be able to pass current of 13A, another - 40A, and the system efficiency. Such device will cost $ several hundreds. And for what purpose? To correct an obvious mistake? Most of $20-50 systems just cut voltage.


By the way, I would not advise connecting 115V 13A directly to a transformer.

[FlyingScot]

Maximum harvest and the simplest wiring/fusing is one quality controller per panel.

I wouldn't buy the cheapest components.

[StuM]

Originally Posted by Kolchac

Unfortunately many people, who are giving advice, do not know even the basics.
1. Parallel: voltage does not change, current - summary of the currents.
2. Series: current does not change, voltage - summary.
3. You are interested in power: VxI.
So, if your battery 24V, each solar is V=41V, series=123V, you are going to loose ~115v, or 115v x 13A=~ 1300W, so use parallel. Also you might cook your controller.

Also in the middle of summer, say 30 deg. Latitude your solar will give you only around 50% during ~from 11 AM to 3 PM max. if you have the Sun. Rest hours - from 0 to 50%. So you can calculate what battery you need.

Apparently, you are one of those "many people".

Why would he "lose 115V" if he is using a suitable MPPT controller. It will convert the total incoming Watts at whatever voltage to almost the same number of Watts at the required output voltage. That's what they are designed for. Why would he "cook" the controller if he uses an appropriately rated one?

In the middle of summer at 30° latitude, an appropriately angled solar array will give him a lot more than 50% of rated power.

His battery needs are a totally different issue and should be based on a number of factors other than the solar panel output.

[StuM]

Originally Posted by Kolchac

Yes, It is possible. You need a device, which has a generator, which creates alternating current 115V, then electrical transformer, which lowers 115V to 24V, then electric rectifier. You also keep in mind the size of the transformer because one of your coils should be able to pass current of 13A, another - 40A, and the system efficiency. Such device will cost $ several hundreds. And for what purpose? To correct an obvious mistake? Most of $20-50 systems just cut voltage.


By the way, I would not advise connecting 115V 13A directly to a transformer.

Clearly you know nothing whatsoever about MPPT solar charge controllers.