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Looking to design a system to recharge batteries for high school robotics.

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ehfastring

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Hey folks, I am a mentor for a high school robotics team. Some of the kids on the team wanted to design a system to recharge the batteries we use on the robots with solar panels mounted on the roof of our high school build space. As a mechanical engineer who has never ventured into solar before, I am not sure where to start.

We are looking to charge lead acid 12V 18AH motorcycle batteries. We would ideally like to have up to 8 batteries that we hook up to charge and stay topped off between uses. The most common use case would be that we would drain a battery over a 1 hour timeframe, and put it on to charge and pull one of the other batteries that is already topped off of the charger.

Our budget would be up to $1,500 (maybe more if there were a compelling reason). We have a multitude of those same batteries that we could use a reserve bank if it would make more sense to charge them and then have more energy banked for when we needed it.

What sort of equipment would be required to accomplish this and what could we hope to accomplish?
 
Solar panels and mppt charge controller that can handle 12v sla batteries.

Problem is you may need a charge controller for each battery, since each battery will have a different charge when you plug them in.

First Robotics?

Another option is to get an off-grid growatt, instead of the mppt charge controller, generate 120v, and plug your chargers into that.
 
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$1500 is not a lot and it needs to be safe / student friendly.


My "first" reaction is to get something turn key like a Pecron E2000LFP ($1100 now, on sale for $850). That's basically a 2kw lifepo4 battery, integrated charger, inverter, blah, blah. All it needs are some solar panels (400 - 1200w) and you can be operational. Best of all, it's portable and if you don't have solar, use the included power brick to charge it.

It's very very very hard to build from scratch anything better and safer!!

After you have something working, then you can expand and build something bigger or smaller and teach the students.
 
DIYrich said:
Solar panels and mppt charge controller that can handle 12v sla batteries.

First Robotics?
Click to expand...
Yes FIRST Robotics.

Do you have any suggestions for equipment for solar panels and charge controller? Also, are there guides for wiring (series vs parallel, wiring size, etc.?)
 
ehfastring said:
Do you have any suggestions for equipment for solar panels and charge controller? Also, are there guides for wiring (series vs parallel, wiring size, etc.?)
Click to expand...
Will you need to recharge all 8 batteries daily? More than once daily? If you give a little info on your power needs a system can be properly sized.

If daily:
8x 18Ah x .5 (50% discharge capable = 72Ah

Guessing 5 solar hours:
72Ah / 5h x 14V charging = 202W of solar panels at 100% efficiency (figure 75%) so 250W-300W

300W / 14V charging = 22A solar charger

Something like this:
houston.craigslist.org

New REC brand 290 watt Solar Panels - electronics - by owner - sale - craigslist

New REC brand, 290 watt solar panels with black frames. $110 each. Few remaining for sale. We also have other brand / wattage panels and related equipment in stock. Click "more ads by this user"...
houston.craigslist.org

SCC on the cheap and cheerful side:
 
Is there any interest in pursuing/teaching Solar as a new subject/project?
If so you could make a small system suitable for a shed or small cabin. Clubhouse?
You could have lots of fun with 12v DC stuff (think automotive and RV LED's, fans, switches, fuse boxes, etc.)
And if they really want to get into it you could add an Inverter for 120v AC stuff, as well.
Tie-in with their electricity course, if they have one.
Sounds like fun! The sky's the limit!
 
ehfastring said:
Yes FIRST Robotics.

Do you have any suggestions for equipment for solar panels and charge controller? Also, are there guides for wiring (series vs parallel, wiring size, etc.?)
Click to expand...
I will let other guys suggest rv type equipment. You want either an inverter that doesn't require a battery, or an inverter with a 12v battery.
 
A few additional questions. So if we want to charge 8 batteries, would we do:

2X 300 watt panels wired in parallel, run to the charge controller linked above, output in parallel to the 8 batteries? Would it be better to have a 30 amp charge controller? Is there any sort of suggested power distribution system for hooking up 2 solar panels or 8 batteries, some sort of bus for connecting multiple items?
 
ehfastring said:
A few additional questions. So if we want to charge 8 batteries, would we do:

2X 300 watt panels wired in parallel, run to the charge controller linked above, output in parallel to the 8 batteries? Would it be better to have a 30 amp charge controller? Is there any sort of suggested power distribution system for hooking up 2 solar panels or 8 batteries, some sort of bus for connecting multiple items?
Click to expand...
One charge controller can charge 8 batteries in parallel. However, getting them in parallel is the problem. They all have to be at the same voltage level when you put them in parallel. Yours will be at random various states of discharge (voltage), so you will need a separate charge controller for each battery. When you put batteries in parallel with different voltages, an unlimited amount of current will flow from the higher voltage battery to the lower voltage battery. Not a good idea.

The easier option is to use your existing battery chargers that run on 120v. They are designed to charge individual batteries with individual connections.

The project then becomes converting the 2x 300 watt panels from DC to AC.

Charge Controller to convert panels from power to charge battery

Victron Energy MPPT Solar Charge Controllers with Bluetooth

Victron Energy MPPT Charge Controllers offer bluetooth connection with your Smart phone or other bluetooth-enabled devices. 5 Year Warranty.
www.altestore.com www.altestore.com

Battery 12v or 24v (use one of your old 12v SLA batteries)

Inverter to convert 12v DC to 120v AC
signaturesolar.com

Victron Phoenix Inverter 12/1200| VE.Direct | 1200VA Output| 12VDC Input | 120VAC Output

Signature Solar provides solar panels, off-grid solar systems, grid-tie, and hybrid systems. Quality solar inverters, bifacial solar panels, complete solar kits, solar batteries. Featuring brands such as EG4 Electronics with their solar battery, LifePower4 and EG4 LLifePower4 and EG4 LL
signaturesolar.com signaturesolar.com

Or
Combined charge controller and inverter:
signaturesolar.com

Growatt 3kW Stackable Off-Grid Inverter | SPF 3000TL LVM-24P

The Growatt 3kW Stackable Off-Grid Inverter offers scalable power solutions with high efficiency, perfect for independent energy systems and expandable configurations.
signaturesolar.com signaturesolar.com
Battery: 24v

Panels are additional.
 
45North said:
If I read the OP right they already have 8 batteries that they use for robotics projects.
Click to expand...
I assume they already have a number of chargers already that operate off 120V AC. Unless they just came from under a rock. Also assume he would like them to learn something besides how to spend money.
 
Given we have an abundance of older batteries that we don't use for competitions could we do this?

2x 300watt panels (like those on craigslist above)
wired parallel into
Victron SmartSolar MPPT 100|50
Output parallel into
4 older same 12V lead acid batteries we dont use anymore
Hooked in parallel to
Inverter linked above
Plugged into
2 AC battery chargers we currently use
Topping off and charging up 6 of our current year batteries

I know there are certain levels of inefficiency that we get from going DC->AC->DC, but wouldn't it allow both top off charging as well as rapid charging of batteries (at least until the 4 older batteries are depleted)?
 
ehfastring said:
4 older same 12V lead acid batteries we dont use anymore
Click to expand...
Hopefully deep cycle battery of sorts with a bit of capacity left. What size? Also 18Ah?

You will have a LOT of power with 2x 290W panels. Loss going to old batteries and then conversion to AC and then conversion to DC. But convenience of having 2 chargers will be nice. You will only be limited by your 4 old battery capacities if charging outside solar hours.
 
MPP 1012-LV-MS, a couple used 200w panels, a long power strip, and a cheap LFp?

Put it all on/in a plastic shed so the school doesn't have to deal with holes and permits and such.
 
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As a former teacher, I would ask the teacher in charge of the program and the administration if they’ll accept specifics of the project. My bet is they will not accept the risk.

I don’t see them being excited about drilling a whole through the roof to accommodate wiring nor be excited about getting permits to mount them up there.

There’s quite a few well intentioned projects at school that get started but never finished or are personality driven and exist until that person moves on.
 
chrisski said:
As a former teacher, I would ask the teacher in charge of the program and the administration if they’ll accept specifics of the project. My bet is they will not accept the risk.

I don’t see them being excited about drilling a whole through the roof to accommodate wiring nor be excited about getting permits to mount them up there.

There’s quite a few well intentioned projects at school that get started but never finished or are personality driven and exist until that person moves on.
Click to expand...
We have already walked the roof, there are several penetrations that we could run wires through. The teacher is onboard and we have a good working relationship with the principal. Hopefully we can get the go ahead.
 
ehfastring said:
I was concerned about going over the 50V safety mark with the high school kids. What are shading issues?
Click to expand...
Shading a panel can mess up production of the entire string. Even shading from a vent pipe.

Make sure the charge controller you buy can start at the low voltage of one panel.
 
ehfastring said:
We have already walked the roof, there are several penetrations that we could run wires through. The teacher is onboard and we have a good working relationship with the principal. Hopefully we can get the go ahead.
Click to expand...
This seems like a doable project for the kids then. Many good ideas mentioned and many ways to do this.

I feel my district would have looked for reasons not to allow installation, You’ve got yourself a good district.
 
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