Alex's Workshop Powerwall

Hello,

After reading a lot of good information I decided to start project #1/5: to outfit our 5th wheel camper RV with 4x165W PV panels and 2S2P lead acide batteries for 208Ah of usable 12V. The system was designed to be self sustainable up to 1500W continuous draw (mainly to be able to use A/C and microwave albeit not at same time).

This project is done and b/c of the weight of the setup, I discovered the 18650 wonder cells.
So project #2/5 was born, equip my off the grid workshop with PVs and a Powerwall. Today it's using a 9kW gas generator to power power tools, battery chargers etc. The workshop is a ~40x40 portable class room with full HVAC (albeit not central HVAC) and 240V input mains.

The plan is:

Build a solar powered battery as cheap as possible (while learning in order to move onto the house project)

I want to be able to pull continuously 7.5kW (30 amp on 240V) circuit inside for HVAC, the panel is rated up to 200A.

Recharging will be via solar PV panels. I only spend about 4 hours a week, if that. I may spend up to 3 hours at once but most time I only average 2 hours. Capacity is therefore more important, and recharging time is less of an issue.

So far I have purchased ~1100 cells out of which ~600 have been tested with only 2 being bad and maybe 40 more salvageable. The average capacity as measured on a 1A charge/draw with Opus4S is around 2150mA. For now I plan on sticking with 4S 12V system (although 7S like Pete's seems very appealing).

I'm building 80P modules which will be arranged in 4S packs. I plan on using 3 such 4S80P packs for a total of 980 cells. Im not certain on which inverter to use although there's a nice used 8k SMA unit on eBay that's 10 yo but functionning.

Continuing to use the genset would obviously be the cheapest way to go but if I keep the project scalable and transferable I should be able to move the setup to the house. What issues do you foresee with this plan?

Alex

7.5kW on a 4s system is alot! Thats 506A!!

Are you sure you thought this out? I wouldnt go under 48V system aka 14s with that amount of power going out of the bank. And on that its still 150A so you do need beefy cables and more importantly i would go with atleast 14s150p if its 2nd hand laptop cells we are talking about.

But if you stay with above i sugest 4s500p atleast so the cells can cope with.

(Most test at 1A and 1A is in most cases what those cells will do without issues. They can and will do much more but... If so you need to have tested them at that)

But no mather what i await to see ur progress!

Thank you daromer for your post and your insight !

daromer said:

7.5kW on a 4s system is alot! Thats 506A!!

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LOL, you are absolutely correct ! Sorry if I wasn't clear, 7.5kW is my end goal. Pulling continuously 150A as I've done with the RV's 12V setup is the maximum I'd consider going.

daromer said:

Are you sure you thought this out? I wouldnt go under 48V system aka 14s with that amount of power going out of the bank. And on that its still 150A so you do need beefy cables and more importantly i would go with atleast 14s150p if its 2nd hand laptop cells we are talking about.

Click to expand...

Yes and no... LOL. I'm envisionning 2 steps for this project.
The first phase will actually use the RV's system pannels, MPPT and wiring. I'd be looking into replacing the 260# of Lead Acid batteries with ~100# of 18650 as the real reason for doing this. But if you think about it I can always park the RV next to the building and use it as a mobile Solar genset. But I'll be limited to 1500W and 120V (i.e. no HVAC). This would allow me to test the quality of these cells and help me take some strategic decisions before phase 2...

The second phase (once I will have found and processed another 2500 cells) will be to go with a 48V system as you mentioned. I'd like to stay at C0.5 for the cells but this will require even more cells.

My vision right now is to house the system in an enclosed 30' trailer I already have and therefore moveable/transportable (no permitting needed ;>) ).
I already measured I can fit quite a bit of panels on it but would also house deployable racks with more panels in order to accelerate the recharge. I'm thinking 10x300W to start with on the trailer roof, expandable to 30 (5 racks of 4 panels).
It would of course be booby-trapped in case someone wants to steal it... LOL

daromer said:

But no mather what i await to see ur progress!

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Thank you. I'm very motivated! I think this is the forum I'll use to post my questions and progress.

In general 1A max is good number on 2nd hand laptop cells but i would consider going lower if you can!

10 panels is massive trailer! Thats like 1.8*8Meter trailer (depending on size of them)

I agree with Daniel. What you save on the inverter, you will spend on connections and cabling. Then there's the cable losses to contend with and the associated heat involved. 48volts is the sweet spot where you can still find good pricing on a ton of different options on inverters. It's considered low voltage and safe voltage.

Consider a 14s pack.

That's where I'm going as well. I'm looking at 30amps at 240 volts output to start with. It's roughly 6 kw output. (30A X 240V X .707 power factor)/.90 efficiency = 5656 watts dc

5656watts / 42V (14s at 3volts) = 134.7 amps

3 volts is worst case, but you will want your lower cutoff voltage to be 3.35 volts to protect the batteries.

I would definitely round up to 150 amps capability on your battery cables.
The gage will depend on the length of run you have between inverter and battery as well as the charge controller and battery if they are separate components.

In conduit, the minimum cable size you would have is 1/0 copper if you keep the runs short.

As far as the 7.5 kw of PV goes, you seldom see the nominal output from the system, so many systems are designed with more PV wattage than the charge controller can output. In the spring and fall at times you might get close to nominal output, but the extra is clipped. It all depends on the capability of the charge controller. But 7.5 kw solar panel charging batteries at 6kw seems about right depending on your sun. You can do a rough calculation using pv watts to find out what your location and mounting angle will
Output.

And always fuse the cable for what it is capable of carrrying.
Safety first.

Hi egam,
Thank you for your post. I just got a bit of progress on this project by just purchasing 24 Canadian Solar panels of 315W each (~7.5kW nominal).

Since your project has many similitudes with mine, I'll follow yours closely.

What kind of solar controller do you plan to use?

silverse2m6 said:

Hi egam,
Thank you for your post. I just got a bit of progress on this project by just purchasing 24 Canadian Solar panels of 315W each (~7.5kW nominal).

Since your project has many similitudes with mine, I'll follow yours closely.

What kind of solar controller do you plan to use?

Click to expand...

You are about a year ahead of me buying solar panels. I just saw a magnum energy ms4448PAEthat looks promising.

MS4448pae is a hybrid/charger inverterthat allows you to go off grid as well as be grid tied.

It can allow for redundancy in the system.


I Think you would need a dump load like a hot water heater to take the extra power. Look at Mike or peters free hot water videos for more info on that.

It depends on what your daytime load is, but this seems to be a pretty good match for DIY battery packs. It needs about 400 amp hrs at 48 Volt to be stable in anoff grid situation. I need to do more research.

you can use up to 4 of these in a system and also grid inputs as well as grid tie inverters including micro inverters.

I have shading in the morning on the southeast corner of my roof from the neighbors tree. Also, I use 50 kw hr of power per day.

It would allow me to grow my system and not wear outthe battery too much.

I am leaning toward grid tie inverters: eitherenphase or solar edge. They are safer. The solar edge is more efficient, but the enphase system is much simpler.

If you are cost conscious, then the string inverter with bypass diodes on each panelare the way to go. provides some safety. I have not researched this much.

egam said:

silverse2m6 said:

Hi egam,
Thank you for your post. I just got a bit of progress on this project by just purchasing 24 Canadian Solar panels of 315W each (~7.5kW nominal).

Since your project has many similitudes with mine, I'll follow yours closely.

What kind of solar controller do you plan to use?

Click to expand...

You are about a year ahead of me buying solar panels. I just saw a magnum energy ms4448PAEthat looks promising.

MS4448pae is a hybrid/charger inverterthat allows you to go off grid as well as be grid tied.

It can allow for redundancy in the system.


I Think you would need a dump load like a hot water heater to take the extra power. Look at Mike or peters free hot water videos for more info on that.

It depends on what your daytime load is, but this seems to be a pretty good match for DIY battery packs. It needs about 400 amp hrs at 48 Volt to be stable in anoff grid situation. I need to do more research.

you can use up to 4 of these in a system and also grid inputs as well as grid tie inverters including micro inverters.

I have shading in the morning on the southeast corner of my roof from the neighbors tree. Also, I use 50 kw hr of power per day.

It would allow me to grow my system and not wear outthe battery too much.

I am leaning toward grid tie inverters: eitherenphase or solar edge. They are safer. The solar edge is more efficient, but the enphase system is much simpler.

If you are cost conscious, then the string inverter with bypass diodes on each panelare the way to go. provides some safety. I have not researched this much.

Click to expand...

Wow egam, our energy needs and thoughts are so alike ! What I really like about the Magnum is that's made in USA. However for one system I can buy 3 x PIP4048MS as seen here:
http://www.ebay.com/itm/142416623252

I'm willing to pay up to 50% more for American made, so it would definetly be on my shopping list if the price was closer. I need 2 inverters and I just cannot afford 2 Magnums.

Ok... is anyone using a PIP inverter/charger in the US?

Yes there are. Mostly with transformer to go from single phase to split fase 120 - 0 - 120. The pip is single phase.

Indeed, mpp solar for us market are also one phase (110/120V). I don't want to use a transfomer, efficiency would suck

Turns out a transformer will be necessary and required to power any 220v appliances.
So I've been working on my powerwall (7S100P) after several try outs and practice with smaller packs (7S40P).

I sourced a great deal on #10 AWG stranded copper wire. It is said to carry up to 55 Amps. But that also means I'll be limited to a maximum power of 7 x 4.2 x 55 = 1617W

Seems low doesn't it?

From 28.9V down to 23.2V, my first powerwall has a capacity of 5.5kWh!

Nice. What meter is that?

jdeadman said:

Nice. What meter is that?

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Thanks ! Amazon best seller

Wow Awesome and even with the shunt. Very cool