In Seattle, building solar rig for Buring Man

[gribbly]

Hello all, first post - new member in Seattle!

I'm hoping to talk through designing a solar rig for Burning Man. We've been running a ~5kWh solar rig using SLA batteries for years at Burning Man, this year we want to upgrade to lithium (the SLA batteries are getting very old and we don't want to buy new lead acid).

I don't have much experience with Lithium, so I'd appreciate any guidance and sanity checks!

We calculate our power needs at 11kWh/day. Most days will be less than this, but we'd like to be comfortable on peak days.

The cost of this project is shared across ~40 people, so I don't think budget will be too much of a constraint (although obviously cheaper is better!)

I would love advice on constructing the battery bank especially (I feel pretty good about the panels/charge controller side of things)

Here are questions I have about the battery bank:

1. I think I should go for 24 or 48V batteries, so the wiring can be lighter. Is that the right approach? Seems like 24V is a sweet spot between cost and efficiency? I was thinking of maybe using something like these batteries: https://www.amperetime.com/products/ampere-time-24v-100ah-lithium-lifepo4-battery
2. I don't know how to think about a BMS. Do I need a BMS in addition to the charge controller?
3. Would those Ampere Time batteries do OK in desert heat? We can shade them from direct sun, but ambient temps will be hot (100 degrees+ probably).

What else should I be thinking about?

 

[John Frum]

The popular diy cells here are 280ah.
280 ah * 3.2 volts per cell * 16 cells * .9 depth of discharge = 12902.4 watt hours
Those cells will weigh ~200 pounds.
You need to add a bms to make a battery.

Do you need an inverter?

 

[gribbly]

Yes we'll need an inverter.

I wasn't planning to DIY the battery, but buy something like the AmpereTime battery in the link and wire them up in a bank.

That said, I'm curious about the DIY option... I'd like to hear more!

 

[John Frum]

Yes we'll need an inverter.

What continuous watt rating?

I wasn't planning to DIY the battery, but buy something like the AmpereTime battery in the link and wire them up in a bank.

I don't usually recommend putting batteries in series to get 48 volts.
This use case might be the exception though depending on weight considerations.
The problem with putting batteries in series is they tend to drift out of balance.
Here is a 48 volt battery.
You would need 3 of these to make your power requirements.

Ampere Time 48V(51.2V) 100Ah, 5120Wh Lithium LiFePO4 Battery with 4000+ Discharge Cycles & Built in 100A BMS

5.12kWh | 10 Days Uninterrupted Power Supply: Ampere time 48V 100Ah Lithium battery can power your home for up to 12 days without using any utility power (Ensure a daily electricity demand of 0.5kWh), allowing you to avoid grid outages. 4X Charging Speed & 1/2 Space Saving: Equivalent to 4pcs...

www.amperetime.com

Here is a less expensive battery of the same capacity.
I think its a better battery and its available from Current Connected who are also in the PNW.

SOK 48V 100Ah PRO Server Rack Battery - User Serviceable | Current Connected

SOK 48V Server Rack Battery: 4000+ cycle life, UL1973 & UL9540A certified, UL9540 pending, Built-In Smart BMS, 10 year warranty & support!

www.currentconnected.com

That said, I'm curious about the DIY option... I'd like to hear more!

They also have the big blue prismatic cells I mentioned in a previous post.

EVE 304Ah | 3.2v LiFePO4 Cells | Grade B (In Stock) ~ Current Connected

Price is per cell - This is the lowest price for this capacity cell we will ever offer. Cells show no signs of swelling. Factory QR Codes are In-tact. Guaranteed to pull over 304ah Capacity. Testing reveals capacity is 308~310ah. See Data Log Covered for 1 year under our warranty policy...

www.currentconnected.com

I'm sure Pepper is a fine upstanding cat.

About Us ~ Current Connected

Current Connected, LLC. At Current Connected, we are more than just a Solar and Energy Storage Company. Our core mission is to promote energy independence with renewable energy solutions. As a family-owned and operated business, we ensue our customers are taken care of like they are a part of...

www.currentconnected.com

 

[gribbly]

What continuous watt rating?

We've been running with a 3000W for years, currently thinking of going to 4000W. But really I need to run the numbers and figure it out properly.

Thanks for the links to Current Connected, I wasn't aware of them!

I don't usually recommend putting batteries in series to get 48 volts.

I was thinking of having the battery bank be 24V... hang on I'll attach my WIP system diagram:


This assumes each 24V battery has a built-in BMS.

I also need to comfirm models/specs for the charge controller and inverter.

Am I on a sane path here?

Something I'd love to hear more experienced opinions on is if I should choose 24V or 48V for the bank. I could get 48V batteries instead, but I'm not 100% clear on the pros/cons of that decision.

 

[John Frum]

We've been running with a 3000W for years, currently thinking of going to 4000W. But really I need to run the numbers and figure it out properly.

Thanks for the links to Current Connected, I wasn't aware of them!



I was thinking of having the battery bank be 24V... hang on I'll attach my WIP system diagram:
View attachment 97338

This assumes each 24V battery has a built-in BMS.

I also need to comfirm models/specs for the charge controller and inverter.

Am I on a sane path here?

Something I'd love to hear more experienced opinions on is if I should choose 24V or 48V for the bank. I could get 48V batteries instead, but I'm not 100% clear on the pros/cons of that decision.

4000 ac watts / .85 inverter efficiency / 20 volts low cutoff = 235.294117647 service amps
235.294117647 service amps / .8 fuse headroom = 294.117647059 fault amps
That means 2/0 awg pure copper wire with 105C rated insulation and a 300 amp fuse, preferably class-t.

4000 ac watts / .85 inverter efficiency / 40 volts low cutoff = 117.647058824 service amps
117.647058824 service amps / .8 fuse headroom = 147.058823529 fault amps
That means 4 awg pure copper wire with 105C rated insulation and a 150 amp fuse, preferably class-t.

Both of those wire ratings are minimums and I generally recommend to go up a size.

So 4/0 awg with 400 amp fuse for 24 volts and 2 awg with 200 amp fuse for 48 volts.

100 amp hours@24 volts will weigh ~55 pounds a piece.