18650 DIY powerwall 48v

[OffGridInTheCity]

How did you determine 1) the pack was bad and 2) which cell specifically? Was the pack self-discharging? and cell wise was it low voltage compared to the others? or what exactly.
Good for you for diagnosing / tacking time to figure it out

I ask because....
@Wolf had a suspect pack as well a while back but had difficulty pinning it down to a specific cell - so we're not sure what happened exactly.

In my own case, pack #54 is in a 2018 battery from second-hand NCR18650A(s) and is over 2,100 cycles and started self-discharging earlier this year. This is my 1st case of pack failure and of course it's the battery that's physically difficult access / blocked in. So I'm adding ~25ah charge every 3-4weeks to keep the pack in proper voltage range till I get time to tackle it.

 

[kje]

How did you determine 1) the pack was bad and 2) which cell specifically? Was the pack self-discharging? and cell wise was it low voltage compared to the others? or what exactly.
Good for you for diagnosing / tacking time to figure it out

I ask because....
@Wolf had a suspect pack as well a while back but had difficulty pinning it down to a specific cell - so we're not sure what happened exactly.

In my own case, pack #54 is in a 2018 battery from second-hand NCR18650A(s) and is over 2,100 cycles and started self-discharging earlier this year. This is my 1st case of pack failure and of course it's the battery that's physically difficult access / blocked in. So I'm adding ~25ah charge every 3-4weeks to keep the pack in proper voltage range till I get time to tackle it.

Yes the cellpack was self-discharging.

1) With no charge/discharge on the battery and just the bms on for a few weeks, I discovered cellpack #5 was lower in voltage than the rest. The bms is using some small amount of power so all the cellpack is loosing some voltage, but cellpack #5 was drifting away.

2) I desoldered all the fuses on each cell in cellpack #5 and it's now resting for some days. I assume one or more cells will be lower in voltage than the rest after resting. At least this happened last time I did service on the battery.

 

[kje]

After resting for 3 days I measured voltage on every cell in cellpack #5. Every cell was 3.67v except one with 3.57v and one with 0v. If I change both of them I think the cellpack is good to go.

 

[OffGridInTheCity]

After resting for 3 days I measured voltage on every cell in cellpack #5. Every cell was 3.67v except one with 3.57v and one with 0v. If I change both of them I think the cellpack is good to go.

Finding one that's 0v might very well be the 'smoking gun'. I'm hoping to find such clear evidence when I finally get time to work on pack #54.

 

[kje]

I was lucky and picked up a solar system for free:

Solaredge SE 5000 inverter, 30 x 135w Stion solar panels, and some Solaredge optimizers and racking.

I consider using this in an offgrid system and not use the se 5000.

I have 29 working 135w panels
Voc: 77.4v
Isc: 2.6a

Is 6p4s a good solar panel configuration for Axpert Max?

This configuration would be
309.6v (max voltage for Axpert is 500)
15.6a (max current for Axpert is 18a)

Is this correct?

Or is 5p5s better? (387v 13a)

 

[Korishan]

DUDE!!! That's awesome!! Gettin free stuff is dope!

Personally, I'd recommend going for series, and then parallel. Unless you have a bunch of panels that are mismatched on output. Definitely use a Solar Power Meter to check each panel, not just wattage, but also voltage and current. If all the panels are basically the same, no worries. If there's a big variance, >10% I guess, then you might want to see about making smaller strings and then parallel those.

If the max Solar input for the Axpert is 500, then 6s is the max, 5s would give you 385 if the panels can truly output 77V, which isn't likely.
I would go 6s1p, and then have 5 strings. At that point I'd parallel them together.

The reason is primarily for possible shading. If you don't have an optimizer for every panel, then don't even go that route, tbh. You can't really use them effectively and efficiently on just some panels.

You would also need to get some Blocking Diodes rated for 5A. This allows you to parallel strings and if one string lags behind the others voltage wise, it won't bring the others down. This is how I'm getting 4kW out of 2 strings (12s). Pretty nuts how well they work.

You want to make the voltage as high as possible without going over the chargers input max as this will make the wiring better. 12awg is cheaper than 10awg. And yeah, you'd go with a minimum of 12awg. You also need to make sure to wire that is rated for 1000V. You don't need arcing through the insulation and causing issues.

So, you'd run 5 strings on 12awg to a combiner box. A breaker for each string (this is the other reason for individual strings, safety). The Blocking Diodes can be attached either before or after the breakers, doesn't really matter. Tho, I'd be partial to putting them after the breakers. To me, just feels safer, but that's just speculation.
Then combine them all together to a common bus. This can either be a large plate of copper, or you can use those solar combiner blocks. You are dealing with 400+VDC, so anything open/exposed I'd not recommend, personally I'd go with the combiners.
And finally, from that point, I'd run 10awg to your controller.

You could merge two strings together at their mounting point on the rack, and then run a single pair back to the combiner box, but this would make it a little more difficult to isolate strings easily. Not saying it's not possible, or not recommended, just stating a little more legwork to turn off strings if needed to in a hurry or whatever.

 

[kje]

DUDE!!! That's awesome!! Gettin free stuff is dope!

Personally, I'd recommend going for series, and then parallel. Unless you have a bunch of panels that are mismatched on output. Definitely use a Solar Power Meter to check each panel, not just wattage, but also voltage and current. If all the panels are basically the same, no worries. If there's a big variance, >10% I guess, then you might want to see about making smaller strings and then parallel those.

If the max Solar input for the Axpert is 500, then 6s is the max, 5s would give you 385 if the panels can truly output 77V, which isn't likely.
I would go 6s1p, and then have 5 strings. At that point I'd parallel them together.

The reason is primarily for possible shading. If you don't have an optimizer for every panel, then don't even go that route, tbh. You can't really use them effectively and efficiently on just some panels.

You would also need to get some Blocking Diodes rated for 5A. This allows you to parallel strings and if one string lags behind the others voltage wise, it won't bring the others down. This is how I'm getting 4kW out of 2 strings (12s). Pretty nuts how well they work.

You want to make the voltage as high as possible without going over the chargers input max as this will make the wiring better. 12awg is cheaper than 10awg. And yeah, you'd go with a minimum of 12awg. You also need to make sure to wire that is rated for 1000V. You don't need arcing through the insulation and causing issues.

So, you'd run 5 strings on 12awg to a combiner box. A breaker for each string (this is the other reason for individual strings, safety). The Blocking Diodes can be attached either before or after the breakers, doesn't really matter. Tho, I'd be partial to putting them after the breakers. To me, just feels safer, but that's just speculation.
Then combine them all together to a common bus. This can either be a large plate of copper, or you can use those solar combiner blocks. You are dealing with 400+VDC, so anything open/exposed I'd not recommend, personally I'd go with the combiners.
And finally, from that point, I'd run 10awg to your controller.

You could merge two strings together at their mounting point on the rack, and then run a single pair back to the combiner box, but this would make it a little more difficult to isolate strings easily. Not saying it's not possible, or not recommended, just stating a little more legwork to turn off strings if needed to in a hurry or whatever.

Thanks. Yes I was really happy after demounting everything (30 panels and racking) from the owners roof. It was a very big job. I did use fall protection.

One panel doesn't have junction box, so I have 29 panels and can't have 6s1p and then have 5 strings. I miss one panel for that (6×5=30).

Is 6s1p and then have 5 strings, the same as 6s5p?

So you recommend 6s and as many strings I have panels for? (6s4p?)

Is this the correct blocking diods? https://www.ebay.com/itm/175522760045

 

[Korishan]

Is 6s1p and then have 5 strings, the same as 6s5p?

For solar panels, no they aren't the same. Well, sort of. When you put solar panels in parallel, it's not just increasing amps. They also share in when there's shading and both are hindered. If you have a 6s4p and one panel gets shaded, for what ever reason, then the entire array suffers. You could go from 4500W down to 1000W easily. The panel level bypassing/blocking diodes work only for the panel itself, not the others in parallel.

You would be far better off 4p6s. You'll get the most power output possible that way. Also, you would need to have a LOT of combiners to make them parallel. This introduces not only extra costs, but also a lot of extra possible failure points. You do not want that in something like this that can carry arc welding capable voltages. So it also makes it safer.

Blocking Diodes, like these:

Solar PV Diode Connector Male Female IP67 Waterproof Panel Cable Blocking Diode | eBay

Feature: 1. Item Type: Solar PV Diode Connector. GOOD WATERPROOF AND DUSTPROOF: The product carefully select materials and adopt high strength waterproof ring design to provide good waterproof and dustproof effects.

www.ebay.com

or these:

Transitron Stud Mounted Diode - Wind and Solar Blocking Diode - 27 mm Long | eBay

Find many great new & used options and get the best deals for Transitron Stud Mounted Diode - Wind and Solar Blocking Diode - 27 mm Long at the best online prices at eBay! Free shipping for many products!

www.ebay.com

When you say:

One panel doesn't have junction box

do you mean it's missing the Solaredge optimizer or it physically is missing the small black box on the back of the panel that connects up the cells and has the pigtail wires coming off?
If just missing the optimizer, you could just not use them and go straight connection. And then sell the optimizers to make some money to spend on other parts.
Optimizers are really only beneficial in areas where shading is frequent. If they are put out in an open area and likely of trees, leaves, or anything else other than clouds will shade them, then the optimizers aren't really doing any favors. Just adding an extra layer of complexity and possible failure points.

 

[kje]

For solar panels, no they aren't the same. Well, sort of. When you put solar panels in parallel, it's not just increasing amps. They also share in when there's shading and both are hindered. If you have a 6s4p and one panel gets shaded, for what ever reason, then the entire array suffers. You could go from 4500W down to 1000W easily. The panel level bypassing/blocking diodes work only for the panel itself, not the others in parallel.

You would be far better off 4p6s. You'll get the most power output possible that way. Also, you would need to have a LOT of combiners to make them parallel. This introduces not only extra costs, but also a lot of extra possible failure points. You do not want that in something like this that can carry arc welding capable voltages. So it also makes it safer.

Thanks Korishan, but I don't understand. Is it possible for you to show with pictures? What is the difference visually with the cables of

1) 6s1p and then have 5 strings
and
2) 6s5p?

Blocking Diodes, like these:

Solar PV Diode Connector Male Female IP67 Waterproof Panel Cable Blocking Diode | eBay

Feature: 1. Item Type: Solar PV Diode Connector. GOOD WATERPROOF AND DUSTPROOF: The product carefully select materials and adopt high strength waterproof ring design to provide good waterproof and dustproof effects.

www.ebay.com

or these:

Transitron Stud Mounted Diode - Wind and Solar Blocking Diode - 27 mm Long | eBay

Find many great new & used options and get the best deals for Transitron Stud Mounted Diode - Wind and Solar Blocking Diode - 27 mm Long at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Thanks

When you say:

do you mean it's missing the Solaredge optimizer or it physically is missing the small black box on the back of the panel that connects up the cells and has the pigtail wires coming off?

Yes. But I could maybe fix it myself?

 

[Korishan]

Thanks Korishan, but I don't understand. Is it possible for you to show with pictures? What is the difference visually with the cables of

1) 6s1p and then have 5 strings
and
2) 6s5p?

What does S and P stand for? What is Xs, Yp or XsYp with X and Y being numeric digits?

What does S and P stand for? What is Xs, Yp or XsYp with X and Y being numeric digits? This is the notation for the configuration of the battery. S means Series, P means Parallel. XsYp means X cells in series (to get the desired voltage) and Y cells in parallel (to get the desired capacity). If...

secondlifestorage.com

Every solar panel has 2 wires. If you connect them in parallel, you have to use a combiner. You can use a 4-to-1 combiner, or 3x 2-to-1 combiners. Problem is, every connection adds a point of failure as that's another connection point.
These are combiners:

You could use a combiner box panel like this:

But you generally put these at the end of each string, not on each panel in the string to parallel.

So basically, using battery cells as a comparison, each solar panel is like a single cell. You connect multiple cells to make your voltage. It is better to do a 6s1p, connect 6 panels in series to get your top voltage. Then you make 4 of these. And then after they are all configured, you connect them together as you would connect multiple batteries in parallel. All the Positives are bonded together, all the Negatives are bonded together, and then you have a single Pos/Neg to go to your loads/chargers. This final combination would be what you'd use the box above for.
In addition to that, you'd want to use a Blocking Diode for added safety and efficiency. You'll loose about 1-2V, but that's fine when you're dealing with 400V, drop in the bucket. Just make sure to get a Blocking Diode that is rated for 1.5x - 2x what your max current would see on a single string. In your case, the panels are rated for 2.6A, so you'd go with at least 5A diodes. As cheap as they are, you could go with 10A just for margin/safety.

Then, between your Panel and the Diode you connect to a breaker. This way you can disconnect the solar string to do maintenance, even replacing the diode if it needs to, without risking getting shocked.
Remember, the only time a solar panel is safe to work without much worry is at night time when zero sun is on it. Even dawn/dusk, the string could have 400V on it, even if it doesn't output much current. That can sting really badly.

 

[kje]

It is better to do a 6s1p, connect 6 panels in series to get your top voltage. Then you make 4 of these. And then after they are all configured, you connect them together as you would connect multiple batteries in parallel. All the Positives are bonded together, all the Negatives are bonded together, and then you have a single Pos/Neg to go to your loads/chargers.

Isn't this called 6s4p?

In addition to that, you'd want to use a Blocking Diode for added safety and efficiency. You'll loose about 1-2V, but that's fine when you're dealing with 400V, drop in the bucket.

I thought this was integrated in the connection box of every solar panel, but after a resource I found this statement from Offgrid Garage:

Bypass Diodes: mounted in solar panel connection boxes and will bypass this specific panel if it gets heavily shaded. This diode does not prevent the current from flowing in reverse through the panel (see experiment in this video)

Blocking Diodes: mounted in series of a string of panels to prevent reverse flow of current if one full solar panel string is heavily shaded. This diode does prevent current from flowing in reverse.

Remember, the only time a solar panel is safe to work without much worry is at night time when zero sun is on it. Even dawn/dusk, the string could have 400V on it, even if it doesn't output much current. That can sting really badly.

But the mc4 connectors are safe to connect/disconnect with no load?

 

[OffGridInTheCity]

Isn't this called 6s4p?

Here's a quick diagram of 6s4p where you have 4 separate strings of 6s. For 300w panels this would be in the range of 200v at 9a (per string) combined at the combiner box for 200v @ 36a going to the Charge Controller.

Assuming 24 x 300w panels that's around 200v @ 36a = 7200w going to a single charge controller input. Not sure of you're panels or charge controller but a common design might be 2 groups of 6s2p with 3600w going to each charge controller + and - input. 7200w to a single charge controller input seems high (to me) for common controllers.

I thought this was integrated in the connection box of every solar panel, but after a resource I found this statement from Offgrid Garage:

Bypass Diodes: mounted in solar panel connection boxes and will bypass this specific panel if it gets heavily shaded. This diode does not prevent the current from flowing in reverse through the panel (see experiment in this video)

Blocking Diodes: mounted in series of a string of panels to prevent reverse flow of current if one full solar panel string is heavily shaded. This diode does prevent current from flowing in reverse.

There's Diodes and then there's the phenomenon of lowest voltage panel (or string in parallel) pulling down the voltage of the rest of the panels (or strings in parallel).

If you take the 6s4p (24 panels) above you'll have 6s strings at 200v and 9a paralleled together - e.g. 1 big array. If you shadow ONE panel of ONE string the voltage will drop significantly - say to 150v or 100v on that string - and the other 3 combined (paralleled) strings will be forced to the lower voltage as well because of the 1 string. So overall all you might find you're going from from 200v @ 36a = 7200w to 100v @ 36a = 3600w - e.g. loosing 50% of PV due to 1 little bit of shading.

Thus, if you can do 4 separate strings of 6s to 4 x charge controller inputs - the other 3 strings would not be affected by the ONE panel in ONE string. If you do 2 groups of 6s2p then it's not as good protection against shading but better than 6s4p as only 1/2 of the array will be brought to 1/2 power.

Let's say you have shading on 1 string - then if you can, keep the other 3 strings to their own charge controller(s). If the shading is only for an hour then maybe you don't care as multiple charge controllers/separate strings/separate wiring can be expensive or you don't have room x, y, z.

There's no right/wrong answer - it's just what works for you're situation. I my case, I do 3s4p (12 panel) groups and have shading to 2 of my 4 groups due to the neighbor's roof line but it's only the 1st few hours in the morning and that's OK with me as I have 4 separate controllers - 1 for each group of 12 panels. I do 3s because my charge controller maxes out at 150v - and 3s runs 90-100v most of the time.

I hope this provides some things to think about as you do you're design

But the mc4 connectors are safe to connect/disconnect with no load?

Yes (in my experience) but it's not connect/disconnect MC4 connectors so much as TOUCHING the wires and getting shocked. If 6s = 200v this is high enough to be a serious shock. MC4s are kind of hard to 'touch the wires' but if you're screwing down live wires (with bare ends) in a combiner box or at a charge controller - then it's much easier to get shocked if the panels are hooked in.

One technique is to wire the combiner box before plugging the combiner box wires into the ends of the strings of panels - e.g. no risk of shock from the panels while working on the combiner box

Also, breakers at the combiner box are strongly recommended and if available, turn them OFF before wiring into the charge controller. In my case I have breakers at the combiner boxes AND just before going into my charge controllers - e.g. I can disconnect the incoming PV at the charge controller location to work on them without having to to out to the array.

These kinds of techniques will greatly lower the risk.

 

[Korishan]

Here's a quick diagram of 6s4p where you have 4 separate strings of 6s.

Nah, that'd be 4p6s

6s4p would be 4 panel in parallel, then connected in series with another group of panels in parallel.

This is 2p5s. There's 2 strings connected in parallel that are both 5 connected in series
However, if they were connected this way:

Then that'd be 5s2p. 2 Panels connected in parallel and then connected with 5 in series.

It should be the same way we classify cells in a battery.

 

[Korishan]

I thought this was integrated in the connection box of every solar panel, but after a resource I found this statement from Offgrid Garage:

Bypass Diodes: mounted in solar panel connection boxes and will bypass this specific panel if it gets heavily shaded. This diode does not prevent the current from flowing in reverse through the panel (see experiment in this video)

Blocking Diodes: mounted in series of a string of panels to prevent reverse flow of current if one full solar panel string is heavily shaded. This diode does prevent current from flowing in reverse.

As what OffGridInTheCity states. Just to add. The diodes "on the panels" are only bypassing diodes. They work internally of the cells in the panels directly. They have no affect on the whole string of panels, directly. If a part of the panel gets shaded, then the bypass diodes allow that one section to be bypassed and allow current to continue to flow, albeit at a slightly lower voltage.

Blocking Diodes are generally only installed at the ends of a string of panels if they are to be connected with other strings of panels. The overall voltage of a string needs to remain independent in case one drops in voltage, this is where the Blocking comes in. It keeps the higher voltage string from pushing into the lower voltage string. This causes hot spots on the lower voltage string and can cause damage over time if there's enough power flowing. This could also lead to panel fires.

Ideally, you would want every string of panels to have their own charge controller. This is how you can get the absolutely best performance. Short of being able to do that, and you have to put strings in parallel, then you really should use Blocking Diodes on the strings.

But the mc4 connectors are safe to connect/disconnect with no load?

Yes, but if you are working with bare wires, you do not want those making any kind of short connection, with metal or through you. 60VDC bites as it is. You do not want to find out how 400VDC feels like. Depending on how you got nailed, it could be as little as a burn mark, or as severe as death. Just depends on how you were holding the wires. Altho, safety wise, wear gloves. Cloth/Cotton ones first, and then some rubber ones on the outside. That's the best way to handle them. However, it does make working on the wires more difficult. But we are not saying to not use gloves. Please use them.

 

[OffGridInTheCity]

Nah, that'd be 4p6s

I've always understand PV hookup as different than cells in a battery in "s" and "p" discussions.

For panels, the "s" (series) is the string and the "p" is the combiner box (parallel) hookup of the strings as one doesn't typically parallel panels first and then put them in series. Whereas in batteries one typically does the "p" first (as in build a pack) and then the "s" to get the desired voltage.

But hey, I'm often wrong!!!!! and will take this under advisement as I read posts and make future posts

And I'd recommend a drawing / wiring diagram as a practical technique for panels -> combiner box -> charge controller(s). For example, here's my ground mount drawing - panels and wires labeled so that I could actually hook things correctly!

And most importantly, I hope this discussion helps @kje get to a good level of understanding to proceed with confidence.

 

[Korishan]

For panels, the "s" (series) is the string and the "p" is the combiner box (parallel) hookup of the strings as one doesn't typically parallel panels first and then put them in series. Whereas in batteries one typically does the "p" first (as in build a pack) and then the "s" to get the desired voltage.

Yeah, generally panels aren't connected in parallel first. Though, I suppose it is possible. I just wanted to make absolutely clear what we were all talking about considering the inherent dangers of doing something wrong is with these.

It's also quite possible I could be mistaken in the world of professional panel installations We can all learn new things.

Nice picture of your layout. Easy to read

 

[kje]

Thank you Korishan and Offgridinthecity. This is very helpful. This contributes to a higher level of understanding and I can proceed with confidence.

I found out 20 panels would fit the roof nicely on my cottage. Is 5s4p like in the picture here a good choice? (Not sure yet if 5s4p is the right terminology)

(5s=77.4vx5=387v)
(4p=2.6ax4=10.4a)

 

[Korishan]

Are all panels going to be on the same side of the house facing in the same direction?? Or are the split between different pitches of the roof?
If different pitches, you'll want to get another charge controller to get the most out of your panels. Even with the Blocking Diodes you won't get the full available power.
But, this is a good start to learning your system

 

[kje]

Are all panels going to be on the same side of the house facing in the same direction?? Or are the split between different pitches of the roof?
If different pitches, you'll want to get another charge controller to get the most out of your panels. Even with the Blocking Diodes you won't get the full available power.
But, this is a good start to learning your system

All panels are going to be on the same side of the cottage.

I consider buying another Voltronic Axpert Max2 for this system on my cottage. It has 2 solar inputs

Max. Input Current: 2x18a
Max. PV Array Power: 8000w
Max. PV Array Open Circuit Voltage: 500Vdc

Would this be sufficient?
(5s = 77.4vx5 = 387v)
(4p = 2.6ax4 = 10.4a)

 

[Korishan]

Yup, that'd be fine. Since all 4 strings will be on the same side, no real worries about paralleling them together. Just combine 2 strings and go into one Input, and parallel the other 2 and go into the other port.
Or, take 2 strings and parallel and input into the Solaredge SE 5000, and then a single string into each of the ports of the Voltronic. That'll give you 3 controllers to work with