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Off-grid solar car charging

pigwet

Member
Jun 22, 2019
50
64
Albuquerque
I recently decided to make an off-grid "portable" solar car charger and finished my project yesterday.

I designed the system to charge my car during the day based on my daily kWh usage (I typically use 10kWh/day and work from home a good portion of the day). The system has a some (required) battery storage (2.4kWh) and a 2.6kW solar array (8 x 320W panels). The power output is sufficient to charge the Model Y with the 120VAC @16A NEMA 5-20 adapter (33% more current than the normal 5-15 adapter that comes with the vehicle). Ultimately this system may end up on a trailer, but for now it is home based and can be disassembled in a few hours in case we move to a new home. I've spent around $7k so far but I may double my battery bank for another $1.8k.

The 24VDC system I made includes the following:
  1. Two 1.2kWh 14.4V LiFePO4 Battle Born batteries, connected in series, with internal battery management system (prevents over/under charge)
  2. 8 x 320W Silfab solar panels (two parallel strings, four panels per string)
  3. A Victron charging controller with maximum power point tracker
    1. The MPPT finds the peak of the current-voltage curve of the solar array input
    2. The charge controller steps down the voltage (from ~150VDC at the solar array to 24-28.8 VDC at the battery) and ups the current (from ~16A at the array to ~80A into the battery)
  4. A 24V 3000VA (2400W) Victron inverter that lies in parallel with the batteries that turns DC into 120V AC
    1. If there's sufficient sunlight, the inverter draws current from the solar array and puts excess current into the battery
    2. If there's a cloud, the inverter pulls current from both the battery and solar array to meet the AC power requirement
    3. If it is dark, the inverter pulls only from the battery (the current battery bank will charge the car for about 1 hour)
  5. A (not required) brain & monitor that dumps all data to the internet and allows you to easily adjust charge settings for all the equipment from one spot
  6. A (not required) battery monitor (the inverter can do clearly, but the additional monitor allows you to measure the two batteries individually)
  7. Two 15A DC circuit breakers and disconnect for the two solar array strings (not your common AC fuses at your hardware store)
  8. Two 150A DC fuses for the monster power cables that connect the battery bank to the charge controller & the inverter
  9. A DC battery disconnect switch
  10. A 20A AC circuit breaker
  11. Ground rod/wiring to ground the metal frames of the array (lightening protection) and to act as a single ground point for the DC and AC sides of the system
  12. Rain tight metal conduit for all outside solar DC wiring
  13. Tons of additional wiring/hardware
  14. Twenty plus trips to Lowes and other random electrical supply stores
I have been working in solar for 15 years, as a semiconductor physicist for space photovoltaics, but this is my first terrestrial system I've done from start to finish. I certainly learned a lot, especially about grounding. Technically it works just fine ungrounded, but certainly need it for added safety/lightening protection.


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Johnny Vector

Member
Jun 21, 2020
222
327
Maryland
What, no rad-hard triple-junction cells? I am disappoint. :p

This is actually amazing. I've been considering building something like this for the storehouse where I park the car at our family's place in Maine. The distance from the nearest power line, measured in dollars, is probably at least 7000, so this works out cost-wise!

I already have a system at the camp, which is on an island and so for 80 years our only source of electricity there was a generator. Now I have a single 275 W panel, a Midnite Solar "The Kid" charge controller, an Outback 2400 W inverter, and two (upgrading to three when I get there this summer) 24V 50Ah Battle Borns. This works fine as long as there aren't 20 people there all trying to charge their iPads. But that's fine because you're at a 1-season camp in Maine surrounded by lake, turn off your damn electronics!

What are you using for brain and monitor, if I may ask? I would definitely need that. And probably an LTE unit to get the data anywhere.
 

CorneliusRox

Member
Mar 3, 2021
108
115
MN
This is pretty awesome. Thanks for sharing the details on it! I'm half tempted to try my hand at this. And a trailer version would be pretty sweet!
 

pigwet

Member
Jun 22, 2019
50
64
Albuquerque
What, no rad-hard triple-junction cells? I am disappoint. :p

This is actually amazing. I've been considering building something like this for the storehouse where I park the car at our family's place in Maine. The distance from the nearest power line, measured in dollars, is probably at least 7000, so this works out cost-wise!

I already have a system at the camp, which is on an island and so for 80 years our only source of electricity there was a generator. Now I have a single 275 W panel, a Midnite Solar "The Kid" charge controller, an Outback 2400 W inverter, and two (upgrading to three when I get there this summer) 24V 50Ah Battle Borns. This works fine as long as there aren't 20 people there all trying to charge their iPads. But that's fine because you're at a 1-season camp in Maine surrounded by lake, turn off your damn electronics!

What are you using for brain and monitor, if I may ask? I would definitely need that. And probably an LTE unit to get the data anywhere.

Yes, sadly no rad-hard cells. The $0.54/W I paid for the Silfab panels (US made - apparently there are still a handful of US suppliers) is "slightly" more cost effective than the III-V cells used for satellites. (The four- and five-junction cells next generation technologies we make surpass the end-of-life performance of our triple-junction cells). And by slightly I mean many orders of magnitude cheaper:).

The "brain" I'm using is specific to Victron components and it is called the Victron Cerbo GX and the monitor is a Victron Cerbo GX. So not compatible with your system. The pair is not really a brain, more a convenient way to display system information and pass it to the internet. The brain really lies in the inverter as it overrides all the subsystem component settings.

I chose Victron components for their reported reliability. There are many new all-in-one mppt/inverters out there on Amazon that are less than $1000 but they all seem to be iffy in terms of reliability and ability to provide continuous power for extended periods of time i.e vehicle charging.
 

Johnny Vector

Member
Jun 21, 2020
222
327
Maryland
Yes, sadly no rad-hard cells. The $0.54/W I paid for the Silfab panels (US made - apparently there are still a handful of US suppliers) is "slightly" more cost effective than the III-V cells used for satellites. (The four- and five-junction cells next generation technologies we make surpass the end-of-life performance of our triple-junction cells). And by slightly I mean many orders of magnitude cheaper:).

The "brain" I'm using is specific to Victron components and it is called the Victron Cerbo GX and the monitor is a Victron Cerbo GX. So not compatible with your system. The pair is not really a brain, more a convenient way to display system information and pass it to the internet. The brain really lies in the inverter as it overrides all the subsystem component settings.

I chose Victron components for their reported reliability. There are many new all-in-one mppt/inverters out there on Amazon that are less than $1000 but they all seem to be iffy in terms of reliability and ability to provide continuous power for extended periods of time i.e vehicle charging.
Yep, I'm definitely familiar with the cost of space-qualified cells! Even though I'm an instrument person, the last proposal I supported includes electric propulsion for a fleet of 12U CubeSats, so the solar panels were a significant part of the cost for sure. I'm back to working instruments now, so I'll probably lose track of future improvements. Anyway...

Are the Victron electronics sealed? That's why I chose the Outback inverter; conformal coat on the boards and o-ring sealed chassis. If you need a fan (I don't, in Maine), they supply one that can blow on the outside of the chassis. I figured anything that had the electronics open to the world would just fill up with ants or spiders in the off-season.

The new system I'm looking at would be completely separate, so could easily go with different components. I'd still want something that can sit unused for 9 months of the year with various small wildlifes seeing it as a prefab cozy house, but it's been 6 years since I built the old one so it's likely there are better choices available now. I'll take a look at the Victron stuff. I thought maybe you had build a Raspberry Pi thingy but if Victron have something I can just buy that's simpler! Thanks.
 
Oct 3, 2020
205
215
Seattle
Very cool. Who needs a Power Wall!?

I've done something very similar (Victron FTW), but mainly as a battery backup solution for my home since solar output is far from ideal in Seattle. Have you considered adding a generator input connection to your house's electrical panel so you can use it in an emergency for small household loads? I can power almost my entire house (minus 240v appliances), including two full size refrigerators, for about 24-48 hours before needing to recharge with the generator (which only takes about 90 minutes @ 1800w). This is using a Victron 24v 2000kVa Multiplus connected to x8 100Ah LiFePO4 cells in series (independent BMS & fuse protected).

In the future, I'll be adding a solar charge controller so I can recharge (slowly) without requiring the use of NG powered generator.
 
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pigwet

Member
Jun 22, 2019
50
64
Albuquerque
Victron are indoor use only. So if it ends up on a trailer I need a really big NEMA weather resistant box that has the ability to vent/blow out the hot air. At least 36"x24"x18". And probably kludge together some type spring/tensioned contraption to dampen the road bumps. Ideas?

I really don't plan on adding a generator. My goal is run my car from the sun (for the most part) and we have a lot of sun here in NM! The backup power is a bonus.

I just ordered 4 more batteries...so totaling $10k now, yikes.
 

mark95476

Active Member
Jun 21, 2020
1,504
880
Bay Area CA
This is an amazing hands-on project! I recall shopping LiFePo batteries a long time ago and they were ~$1k/100Ah (or ~1kWh).

You could buy an entire Tesla just for the battery pack for less. This is one of the reasons why I'm looking forward to the CT.

Victron are indoor use only. So if it ends up on a trailer I need a really big NEMA weather resistant box that has the ability to vent/blow out the hot air. At least 36"x24"x18". And probably kludge together some type spring/tensioned contraption to dampen the road bumps. Ideas?

I really don't plan on adding a generator. My goal is run my car from the sun (for the most part) and we have a lot of sun here in NM! The backup power is a bonus.

I just ordered 4 more batteries...so totaling $10k now, yikes.
 

LoudMusic

Member
Jul 21, 2020
401
500
Arkansas
Victron are indoor use only. So if it ends up on a trailer I need a really big NEMA weather resistant box that has the ability to vent/blow out the hot air. At least 36"x24"x18". And probably kludge together some type spring/tensioned contraption to dampen the road bumps. Ideas?

I really don't plan on adding a generator. My goal is run my car from the sun (for the most part) and we have a lot of sun here in NM! The backup power is a bonus.

I just ordered 4 more batteries...so totaling $10k now, yikes.

That's mostly true. But they also recommend using them on boats, like me! I have three 100/50 Victron MPPT chargers. They are mounted in a weatherproof locker, but they are not in additional water tight boxes. I think if you are mounting this equipment into a trailer that would be plenty weather proofing.
 

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