Charging directly from solar panels

[Newton]

Hey all, I'm thinking of making a simple fold out 4 panel system that will sit on top of the bed slope. for when I take trips deep into the woods.
my questions are:
-Will I need a battery buffer between the panels and inverter that the 120v tesla charger will plug into? Like if a cloud passes by, would it be bad for the truck or charger to just turn on and off like that?
-if so how small can the battery be?
-I know very little about electricity,
How fast would the truck charge if each panel is 300watts? That would be 1.2kw/h meaning it charges at about 1kw per hour?

The panels would be 4 total about 30x77 inches each. I would add 2 more but they would cover the vault panel. Here's a little pic. I would just manually fold it out.

Any info would be appreciated! Thanks

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[ajdelange]

How fast would the truck charge if each panel is 300watts? That would be 1.2kw/h meaning it charges at about 1kw per hour?

Let's start with this question.

With the technology you can buy today a "panel" yields about 1/4 kW at 240 VAC. You might get a bit more but let's use that as the math is easy and you call always adjust the answer down by 10% if you think systems will be better by 10% when you do the project.

Four panels at 1/4 kw = 1 kW when the normal to the panel is pointed directly at the sun, there is no haze and no clouds and the sun's elevation is greater than, let's say, 30°. But let's hold off on that aspect for a minute. Feeding 1 kW to the trucks charger will send about 900 W to the battery. The CT's will probably require 350 - 450 Wh/mi. At 400 you would get something like 2.25 miles per hour of charging per hour under the optimal conditions we've discussed. By dividing the kWh collected from a panel by it's ideal circumstances performance you can calculated the effective number of ideal hours in any time period. Over the last year I averaged 3.33 hours per day. That means that on average I would be able to add about 7.5 miles per day from a 4 panel system. On the best day of the year I got just over 7 hr. That was in June (when the summer solstice occurs though the best day wasn't at the solstice). That would have added 15.8 miles to a CT. In December of last year on the best day I got 2.66 hrs and the average for that month is 1.61. These numbers come from a system that experiences pointing loss of about 5% so you might be able to improve on them by having your rig track the sun or manually tracking it. You will also do better or worse than the numbers I gave depending on whether you live in a place with more or less sun than I get. NREL publishes maps and tables of insolation. You can consult them to get a rough idea as to what you can expect. Keep in mind the truck will tax you as much as 12 miles per day for phantom/vampire drain (battery energy used to run the computers and modems when the truck is parked). You can do things to reduce this somewhat. Thus, and I'm always telling people this, the proposed Tesla built in array or a project similar to the one you are contemplating might cover your phantom drain with a bit over in a sunny clime but you had better not arrive at your campsite with 10% SoC and rely on solar to get you back to a SC 100 mi away after several days camping. OTOH were you to stay 10 days you could lose as much as a quarter of the battery to phantom without such a solar system but lose nothing to it with one.

-Will I need a battery buffer between the panels and inverter that the 120v tesla charger will plug into? Like if a cloud passes by, would it be bad for the truck or charger to just turn on and off like that?
-if so how small can the battery be?

That's a very good question as it leads to the broader question of what goes between the panels and the truck.

I'll start by pointing out that the minimum charge current you can set the current cars' chargers for is 5 amps. At 220 that's 1.1 kW. At 240 its 1.2 kW thus a 1 kW 220 - 240 V system does not meet the car's minimum requirement and, while I am not sure of the consequences of this I think the car's charger would probably shut down. OTOH 5 A at 120 is only 600 W and to charge at a 1 kW rate at 120 V would take 8 A. So at first blush it looks as if you would need a 120V inverter capable of handling 1 kW (8 A),

Do you need a battery? Yes, I rather think you do largely because I can't think of how a grid independent solar system would work without one. On a cloudy day a solar array may be producing near full power at one moment and 10% 15 seconds later. You need something as a flywheel. If the grid isn't there this has to be a battery. If Tesla does indeed offer a solar tonneau option (ans they would be crazy not to given the number of people who think they will be driving free if they have one) it will tie directly to the cars battery and it will serve as the buffer. A completely external system can use the car's battery as a buffer too but it would mean constant cycling of the UMC and charging system in the car which, while it is designed to handle power interruptions is not designed to handle a couple a minute.

There are, of course, a million ways to skin this cat. The essential components are
1)A solar array
2)A battery controller (does MPP tracking on the panels and converts panel voltage to battery voltage level
3)A battery
4l)Inverter (converts battery voltage to 120 VAC)

The car's UMC then plugs into the inverter.

The picture below shows the system you want (but the one pictured isn't as big as you want)
'

The box on the right is a Jackery Explorer 1000 and it contains all 4 of the items listed above. A Tesla can be plugged into it (using a special adapter that fools the UMC into thinking the Explorer is a house) and will charge the car at up to 1 kW (this model's limitation). When solar panels are connected, as they are in the picture, energy from them goes to the car if it is drawing current. If it isn't that energy goes into the battery. The Jackery box is a miniature off grid system i.e. just what you would have for an off grid cabin but scaled down.

So that picture shows exactly what you want except that these are 100W panels capable of producing at most 200 W and, at the time of the picture, producing more like 110 - 120 W because while it is obviously a sunny day it is hazy. Thus if I start with a full battery and plug the car into it will get 0.1 kW from the panels and thus take .9 kw from the battery. As it's a 1 kWh battery it will be completely discharged in 1.11 hours. At this point it will shut down.

Clearly what you want is a product just like this from Jackery or another manufacturer that will accept 1000 amps of panels rather than just 200.

To get more insight into this note that we express the battery size in Wh. Thus for this battery C = 1000. Watt Hours. If we discharge the battery at some rate r*C it will be fully discharged in 1/r hours. For example, if r = 0.9 the discharge rate is 0.9C we are drawing 0.9*1000 = 900 W and the batery is discharged in 1/.9 = 1.11 hr. If we discharge at 0.9 C but charge at 1.0 C the net drain is (0.9 - 1)C and the battery will actually charge. But if we can only charge at 0.8C the battery will discharge at (0.9 - 0.8)C and discharge in 10 hours.

Summary:
1)Probably not worth doing but potentially a fun project and can probably cover phantom drain
2)You need a small off grid solar system. It would be nice to be able to buy this as a package but if not you can assemble something just as if you were doing this for your hunting lodge
3)Unexpected conclusion from being forced to think about this and a great conclusion so thanks!: The Jackery Explorer 1000 is effectively an electrical Jerry can that holds, for my X, 3.5 - 4 miles worth of "gas".

 

[Ryan95738]

I would definitely find a way to tie into the trucks battery this will cut down on losses and get more out of your charging rig.

 

[ajdelange]

I would definitely find a way to tie into the trucks battery this will cut down on losses and get more out of your charging rig.

You have the engineering and programming skills? You are willing to void the warranty? You are willing to do this to get 9 miles per day instead of 7?

 

[Dids]

Hey all, I'm thinking of making a simple fold out 4 panel system that will sit on top of the bed slope. for when I take trips deep into the woods.
my questions are:
-Will I need a battery buffer between the panels and inverter that the 120v tesla charger will plug into? Like if a cloud passes by, would it be bad for the truck or charger to just turn on and off like that?
-if so how small can the battery be?
-I know very little about electricity,
How fast would the truck charge if each panel is 300watts? That would be 1.2kw/h meaning it charges at about 1kw per hour?

The panels would be 4 total about 30x77 inches each. I would add 2 more but they would cover the vault panel. Here's a little pic. I would just manually fold it out.

Any info would be appreciated! Thanks

If as Elon said it would be cool if CT had a solar cover then chances are that the expansion connection and management system is already included in the truck. Probably all you have to do is add the additional panels because why build a truck that has solar panels on it then not make it so it's easy to add panels. Plus I don't know why you would convert DC power to AC then convert it back to DC to charge the battery... So no invertor needed. Fast charger supplies DC @ 480v.

 

[Newton]

Awesome info, thanks
ajdelange

I made a small systems for my balcony.
A 300w panel, cheap charge controller and cheap inverter 1000w.
It charges a couple deep cycle lead acid batteries( scavaged from an electric moped). but can only run my gaming laptop, or tv/ps4 for about 4-5 hours before the battery gets is low. than it charges from the solar in about 4-5 hours when no load is on it.

I also have a couple lifep04 lithium battery packs using a123 26650 battery cells from a hybrid bus.
these things are pretty cool. much lighter than the lead batteries obviously and they can allow 50amp max continuous discharge
I was thinking of making a decent sized battery from theseto replace the lead acid that isnt performing as i hoped.
Perhaps if it allows I could than use this for the CT

side note, heres a guy that made a little portable charger for his tesla https://sfbay.craigslist.org/sfc/ele/d/san-francisco-aesc-lithium-48v-3kwh/7130518455.html

 

[ajdelange]

-I know very little about electricity,

I made a small systems for my balcony.

It seems you are the opposite of the typical poster on such forums as these. You know more than you think you do. Usually it's the other way round.

 

[Newton]

"Dontcha know every one here is innocent" -shawshank
"Dontcha know everyone here is a phd"
-internet

 

[mlee168]

I believe there might be a DC to DC charger built in. If you are able to tap into that, that might work. No idea what the min volt and amp will be required.

 

[ajdelange]

Built into what?

 

[Newton]

Built into what?

I think he means a separate DC to DC that the vault solar will use... maybe just connect my panels to that

 

[docwisdom]

Solar just doesn’t seem to be worth the cost and hassle. I mean it’s the equivalent of plugging in to a 110v charger for like 5 minutes, right?

 

[Bigvbear]

It seems you are the opposite of the typical poster on such forums as these. You know more than you think you do. Usually it's the other way round.

"One of the greatest challenges in this world is knowing enough about a subject to think you are right, but NOT enough about the subject to know you're wrong" -Neil deGrasse Tyson

 

[Newton]

Solar just doesn’t seem to be worth the cost and hassle. I mean it’s the equivalent of plugging in to a 110v charger for like 5 minutes, right?

doc, i think 120v at home charges at like 5 miles per hour

edited becuase a mistake...
It really dosnt seem worth it unless your letting it sit for days at a time.
I think ajdelange calculated about 2.5 miles per hour in optimum conditions. so 8 hours In direct sun for about 20 miles minus "phantom drain" so maybe 15?
seems wierdly low as elon said 15 for JUST the vault panel. but 10 to 15 + 15 is still 20-30 miles charge just sitting there all day

For what I would use it for that's actually fine as it would probably sit for 3 or more days doing nothing while I camp or hunt or whatever. Although it would most lo likely be in the shade

Once I get the truck, maybe I'll do it and make a video to show people the real numbers.

 

[ajdelange]

I think he means a separate DC to DC that the vault solar will use... maybe just connect my panels to that

If toneau solar panels are indeed offered there will, of course, have to be appropriate electronics built in to accommodate them. This can be done in several ways. One of those would be to have the toneau panels equipped with a microinverter feeding AC to the existing charger. Were it done this way it would then, of course, be a simple matter to add additional similar panels to the system simply by paralleling them with the AC bus. Musk did, IIRC, talk about auxiliary panels at one point for camping. If they wanted to be really nice they would allow people to plug any panel with an AC output that matched the system's requirements into this bus and here's where I think we hit a wall. They could, of course, sell a compatible microinverter or license someone else to sell such a unit and then allow anyone to attach any panel to the system as long as it used their microinverter. Or they could build a few microinverters in (perhaps offered as an option) and then allow any panel that met certain DC specs to plug into those. As usual there's a lot of might/could/should built into this but it is certainly technically feasible. Write to Musk if you want this. He's put stuff in cars in response to user Tweets before.