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Discussion Starter · #1 ·
Questions are likely to be way under what you guys are used to but I am very new to this stuff.

I have been recruited to assist a group of high school students in a Solar Go Kart challenge (ESE Instructor in my normal time) and have figured out the wiring for solar panels to Charge controller to batteries (4 12v 15ah lithium batteries) to Speed controller to motor.

Having trouble with batteries. I am using these batteries https://www.amazon.com/Rechargeable-Maintenance-Free-Replacement-Wheelchairs-Trolling/dp/B098NB58RD/ref=sr_1_10?crid=1JVDZ79OM6H98&keywords=12v+15ah+battery&qid=1647955644&s=instant-video&sprefix=12v+15+ah+battery,instant-video,59&sr=1-10 and I can get them to start and run for a bit then it shuts down and my multi-meter shows that two of the batteries polarity seem to be switched....

Have batteries wired to supply 24v 30ah (same wiring as 12v 9ah diagram on top right)
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Not sure what is happening and appreciate any ideas
 

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"I am using..."

"I have...figured out the wiring"

You're supposed to assist, not do.

It's not about your school winning (what your management may want), it's about learning - THEIR learning, not yours.

That includes approaching experts on a forum.

Maybe you should let the kids figure it out....sounds like they have a better chance with their Google Fu than you do.

Seriously. Let them do it from start to finish. Safely. Including coming here as a last resort.

Rip out the wiring, charge the batteries, let them build their car under adult supervision.
 

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These batteries identify as bi-polar. Welcome to the 21st :)

In all seriousness, what do you mean polarity switched ? What's the voltage after they supposedly switch ? You may be triggering BMS protection, like possibly through overcurrent. 15A, and even 30A with two in parallel is very little current at 24v. For comparison, I have a quad that I built with a 60v battery and a 1000W ebike drive - it pulls up to 50A quite easily.

P.S. I agree with Remy's sentiment. Make sure you don't do the work for the kids.
 

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Given the comment about switching polarity...
The diagrams don't show the polarity of the terminals of any of the batteries. I would get the students to determine what they should be in the diagram (should be obvious whether they get it right, but check in here if uncertain), then check the actual wiring against the diagram to see if there is a problem.
 

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Rip out the wiring, charge the batteries, let them build their car under adult supervision.
Honestly, this does not even go far enough! If you were the one who ordered the batteries, send them back, and let the kids work out what sort of battery they will need for this project. I can understand that there are going to have to be some constraints as I am guessing your budget is not unlimited, but I would start with the go kart motor as the known value and work backwards from there.

If you know how much power your motor will draw to meet the design criteria, then you can size the battery accordingly. You will need to brush up on using a power figure (in Watts) to figure out current draw at the rated voltage. Then you size the battery so that it can actually deliver that many amps.

I also suspect that you are tripping the BMS on some of your batteries. Lead acid is going to be a lot more forgiving of abuse if the chassis can take the extra weight. Its also cheaper in the short term, and less picky about how you charge it.
 

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Discussion Starter · #6 ·
These batteries identify as bi-polar. Welcome to the 21st :)

In all seriousness, what do you mean polarity switched ? What's the voltage after they supposedly switch ? You may be triggering BMS protection, like possibly through overcurrent. 15A, and even 30A with two in parallel is very little current at 24v. For comparison, I have a quad that I built with a 60v battery and a 1000W ebike drive - it pulls up to 50A quite easily.

P.S. I agree with Remy's sentiment. Make sure you don't do the work for the kids.
Trying to not do the work but it is interesting (engineering not my forte but old street racer in me is coming out) and I have probably done more than I should have. They have to do a presentation on this thing explaining what each component does, how it does it, and why it is necessary. They are learning the systems but have zero experience with anything like this and I don't think they would have had enough time to get it put together

I do think we have run into Battery protection system and when it happens, we have found that by switching places with two of the batteries it does seem to reset them.

We are limited to 720 Watt-hours as the whole challenge is to get the cart to finish, I ordered the most powerful batteries I could get to fit within the parameters

I think when competition is completed, I will pull all the cables and see if they can get it put back together.... Y'all are correct but we only have one more week until it is time to load it up and go race
 

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Trying to not do the work but it is interesting (engineering not my forte but old street racer in me is coming out) and I have probably done more than I should have. They have to do a presentation on this thing explaining what each component does, how it does it, and why it is necessary. They are learning the systems but have zero experience with anything like this and I don't think they would have had enough time to get it put together

I do think we have run into Battery protection system and when it happens, we have found that by switching places with two of the batteries it does seem to reset them.

We are limited to 720 Watt-hours as the whole challenge is to get the cart to finish, I ordered the most powerful batteries I could get to fit within the parameters

I think when competition is completed, I will pull all the cables and see if they can get it put back together.... Y'all are correct but we only have one more week until it is time to load it up and go race
You should build your own battery using prismatic LFP cells. For 24v you just need 8 in series, that's low enough to be an easy project. You can pick a proper BMS and fuses for it that way too.
 

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Discussion Starter · #8 ·
Finally found out what the problem was with this.... it turns out that the Lithium batteries we ordered were listed as 12v, labeled as 12v, and sold as 12v, but are actually 14.6 volt and the charge controller will not charge them to that voltage.

Another team there lent us their spare batteries and we ended up being able to participate, completed the race (this was our goal) and came in second overall to the team that lent us their backup batteries

Kids learned a ton of stuff about how this stuff works and underclassmen want to go back next year now that they know what to do, lol
 

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14.6V is the top voltage of a LiFePO4 12v Battery. good to know the kids learned tons and had fun it seems. Most 12V chargers made for LiFePO4 will charge to 14.6V. faulty charger. One made for 3s 3.7V cells 11.1V nominal 12.6V max charge.
Job well done, getting kids to want to learn can be a challenge you succeed.
later floyd
 

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Discussion Starter · #10 ·
14.6V is the top voltage of a LiFePO4 12v Battery. good to know the kids learned tons and had fun it seems. Most 12V chargers made for LiFePO4 will charge to 14.6V. faulty charger. One made for 3s 3.7V cells 11.1V nominal 12.6V max charge.
Job well done, getting kids to want to learn can be a challenge you succeed.
later floyd
And that was our problem, our charging system was not made to do that

Kids now have a laundry list of things they want to do before next year to make this thing better, newer, more efficient, electronics is one of them
 

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Discussion Starter · #11 ·
Working on this for next year's competition....

Other than working on bearings and brakes (need replacing on both ends) and lowering center of gravity (things kids can figure out pretty much by looking at it) they are curious about different, more efficient motors. Where could I point them to learn more about what they should be looking for in a newer motor??

Thought I had enticed a local golf cart outfit into helping out with this, but they have yet to reply to my email
 

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Motor efficiency is a bit complicated. Each electric motor will have a unique efficiency curve, so how efficiently it runs will depend on how it is being used. I would turn the kids loose on trying to see if they can find efficiency data for the motor you currently have (failing that, looking up graphs for that type of motor). Then, set them to working out what RPM their motor needs to turn at a desired ground speed. That should give them an idea of the ideal speed to maximize efficiency. If that differs wildly from what speed they are driving in the race, then making adjustments to the gearing, or getting a different motor might be justified. Most electric motors are pretty efficient, however, and simply dropping in a new motor with a higher peak efficiency will not help if the motor is not being run at the optimal RPM.
 
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