Unique Application

[florida cracker]

I have questions about a unique application. I am a teacher and my students are working on a solar charged, electric powered, go kart. It is a competition through a local college (have older thread asking different questions about it https://diysolarforum.com/threads/introduction-and-advice-on-solar-charged-go-kart.40588/ ) . There are limitations on the battery capacity we can use (720wh). We are running an electric motor (ME0909) that is capable of drawing 300 amps for up to 30 seconds and 100 amps continuously. We want to try lithium batteries to run in series to run motor at 36v.

The battery option would/should allow us to do this is the 12v 18ah battery (12.8v x 18ah = 230.4wh). While looking up options for that size battery I have found two different problems, most of them have the F2 connectors which I assume would not handle the amps we would want to run through them.

I also found a recommendation that due to the individual BMS on the batteries, they would not charge properly wired in series. Logic being that the first battery would charge fully and the BMS would then shut down further charging. My understanding is that because they are wired in series, they should all be charging at the same time, at the same rate, similar to how one single battery would charge.

Am I correct in assuming the F2 connectors are a no-go and is there a problem with wiring in series?

 

[KITROBASKIN]

Giving this a bump, hoping a knowledgeable member will respond.

 

[BentleyJ]

Tried to respond yesterday but the server maintenance disrupted things and I didn't find your post again till now.

I don't understand why there is such a massive mismatch between the size of the motor and the small amount of battery storage that is allowed.

1) At 100A that's 4 min of run time assuming the battery is drained down to 20%. What's the point?

2) 100A discharge from a 18Ah battery is 5.5C which is quite high. Would need special high current cells or maybe LTO but they have lower voltage so take up more space for equivalent storage capacity.

3) The flat spade connector is maybe good for 20-25A and I'm being generous.

This application is going to require a custom made battery maybe some 18650 high current cells with heavy bussing between cells. A custom build would also allow you make a single 36V battery with the requisite number of series and parallel connections and then one BMS could handle cell balancing and protection functions.

 

[ejfluhr]

Given that it is a competition, I wonder if challenges involve speed vs. distance requiring careful battery management.

Even though the motor can draw that much, they may not need to drive it that hard. It seems improbable to get a simple battery setup to drive that much power; even a reasonably performing battery probably needs to be custom-built. If not, maybe some power tool batteries could be used. For example, 3 x 6Ah x 36v Ryobi batteries or some combo made from 48v batteries may work.

Presumably any setup will be power-limited so hopefully they can program the motor controller appropriately.

 

[Warpspeed]

In any kind of motorsport, the first thing you always do is look at the vehicles that are currently winning, and try to figure out why.

Second thing is read the rules very carefully, and unless it says a thing definitely cannot be done, and it seems like it may give you an advantage, test the idea out and see.

A silly example:
The rules might say it has to have four wheels. So you figure that four wheels in a diamond configuration might offer better aero.
Aero may be much more important than cornering speed, or maybe not.

If you win with something a bit odd, everyone will bitch and complain, but the rules did say four wheels, and if has four wheels, you just laugh !
Try to think outside of conventional solutions if you can.

Maybe one bright idea in twenty might work, but one radical idea might be enough, if its really good.

 

[MisterB1959]

as its ( i assume) also a science/engineering project, why not build your own battery using cells as part of the project. its nice to win but i am sure the real objective is to educate students, so why limit that opportunity by 'purchasing' ready made solutions?

it shouldnt be that difficult to build your own battery, to the shape that suits its positioning, to the max power level allowed.

 

[florida cracker]

Given that it is a competition, I wonder if challenges involve speed vs. distance requiring careful battery management.

Even though the motor can draw that much, they may not need to drive it that hard. It seems improbable to get a simple battery setup to drive that much power; even a reasonably performing battery probably needs to be custom-built. If not, maybe some power tool batteries could be used. For example, 3 x 6Ah x 36v Ryobi batteries or some combo made from 48v batteries may work.

Presumably any setup will be power-limites so hopefully they can program the motor controller appropriately.

This is the thing, Speed controller is programable so amperage can be limited. Race is comprised of two heats of two laps around a 1.88 mile loop (3.76 miles). running at speed it should only take 8-9 minutes to complete, Heats start at 30 minute intervals so if you run in 8 minutes, you have 22 minutes charging time. Custom built batteries are not permitted.

We have run this contest for the past two years coming in second and fourth with SLA 12v 30ah batteries and the motor running at 24v, to get more speed, we either need to make a radical change in gearing or set it up to run at 36v.

Tried to respond yesterday but the server maintenance disrupted things and I didn't find your post again till now.

I don't understand why there is such a massive mismatch between the size of the motor and the small amount of battery storage that is allowed.

1) At 100A that's 4 min of run time assuming the battery is drained down to 20%. What's the point?

2) 100A discharge from a 18Ah battery is 5.5C which is quite high. Would need special high current cells or maybe LTO but they have lower voltage so take up more space for equivalent storage capacity.

3) The flat spade connector is maybe good for 20-25A and I'm being generous.

This application is going to require a custom made battery maybe some 18650 high current cells with heavy bussing between cells. A custom build would also allow you make a single 36V battery with the requisite number of series and parallel connections and then one BMS could handle cell balancing and protection functions.

Custom batteries are not permitted and the whole idea behind this is to do more with less I guess. Working on reducing rolling resistance along with milking the most efficiency out of your electronics is the name of the game.

Given the parameters we have we wanted to go with lithium if at all possible. I found a few lithium batteries with conventional connectors but they come with statements telling me they can't be run in series, only parallel. Wondering why when most other lifepo batteries don't seem to have that designation.

Can anyone think of any reason they would not be able to run in series, something in the charging protection part of BMS??

 

[Q-Dog]

This is the thing, Speed controller is programable so amperage can be limited. Race is comprised of two heats of two laps around a 1.88 mile loop (3.76 miles). running at speed it should only take 8-9 minutes to complete, Heats start at 30 minute intervals so if you run in 8 minutes, you have 22 minutes charging time. Custom built batteries are not permitted.

We have run this contest for the past two years coming in second and fourth with SLA 12v 30ah batteries and the motor running at 24v, to get more speed, we either need to make a radical change in gearing or set it up to run at 36v.



Custom batteries are not permitted and the whole idea behind this is to do more with less I guess. Working on reducing rolling resistance along with milking the most efficiency out of your electronics is the name of the game.

Given the parameters we have we wanted to go with lithium if at all possible. I found a few lithium batteries with conventional connectors but they come with statements telling me they can't be run in series, only parallel. Wondering why when most other lifepo batteries don't seem to have that designation.

Can anyone think of any reason they would not be able to run in series, something in the charging protection part of BMS??

You really need to ask the battery manufacturer why they don't allow series connection. Possible the BMSs or the internal wiring can't handle the throughput?

 

[florida cracker]

You really need to ask the battery manufacturer why they don't allow series connection. Possible the BMSs or the internal wiring can't handle the throughput?

Sellers have no idea, will only tell me the manufacturer told them that.

I saw one site that mentioned it was because the first battery in the series to get power charges before allowing other batteries to charge. First battery senses it is charged and BMS shuts charging down and does not allow other batteries to charge. That sounds ridiculous to me but I can't think of another, better, reason

 

[Q-Dog]

Sellers have no idea, will only tell me the manufacturer told them that.

I saw one site that mentioned it was because the first battery in the series to get power charges before allowing other batteries to charge. First battery senses it is charged and BMS shuts charging down and does not allow other batteries to charge. That sounds ridiculous to me but I can't think of another, better, reason

Go straight to the source ... contact the manufacturer. Do they perhaps have a user manual online?

 

[Sanwizard]

This is the thing, Speed controller is programable so amperage can be limited. Race is comprised of two heats of two laps around a 1.88 mile loop (3.76 miles). running at speed it should only take 8-9 minutes to complete, Heats start at 30 minute intervals so if you run in 8 minutes, you have 22 minutes charging time. Custom built batteries are not permitted.

We have run this contest for the past two years coming in second and fourth with SLA 12v 30ah batteries and the motor running at 24v, to get more speed, we either need to make a radical change in gearing or set it up to run at 36v.



Custom batteries are not permitted and the whole idea behind this is to do more with less I guess. Working on reducing rolling resistance along with milking the most efficiency out of your electronics is the name of the game.

Given the parameters we have we wanted to go with lithium if at all possible. I found a few lithium batteries with conventional connectors but they come with statements telling me they can't be run in series, only parallel. Wondering why when most other lifepo batteries don't seem to have that designation.

Can anyone think of any reason they would not be able to run in series, something in the charging protection part of BMS??

Take a look at the ebike battery world, especially the Surron models.

Also, many LiFePo4 12V batteries can be connected in series. You need to check with the manufacturer though. I believe SOK and Battleborn are two brands that explicitly say they can be series connected.

 

[justgary]

Do you have to be able to charge while you are racing? If not, flip a switch and charge the batteries in parallel.

In your case I don't think it is going to matter if you charge in series since you can top (parallel) balance all of them before the race, then top balance them again after the race. They aren't likely to get out of balance in a few hours. Manufacturers don't want to have to warranty batteries used in series since they can easily unbalance over time. The real question is whether the BMS in each one can handle the output current.

So if someone builds a battery for you, it just has to be available for other teams to purchase as well?

I read your other thread to catch up. I would definitely look at a bicycle type tire rather than a go-kart type tire to reduce rolling friction and weight. Mount them with some positive camber to help with turns.

 

[florida cracker]

I'm trying to wrap my head around possible reasons why connecting in series would/could be bad.

This battery sounds like it checks all of the boxes until you get to the part of the description that tells me not to connect in series

12V 18AH Lithium Replacement Battery for CB19-12 - MightyMaxBattery

Buy now 12V 18AH Lithium Replacement Battery for CB19-12. Mighty Max Battery is the name you can trust for all your SLA, AGM, LiFePO, and Power Sport batteries.

www.mightymaxbattery.com

 

[Q-Dog]

I tried their chat. Looks like no one available but there is the contact info. Tell them whta you are doing, maybe get a sponsorship for batteries.

 

[florida cracker]

I tried their chat. Looks like no one available but there is the contact info. Tell them whta you are doing, maybe get a sponsorship for batteries.


View attachment 181554

Tried chat earlier, manufacturer told them not to connect in series and thats all they know. can't tell me who manufacturer is either. Trying to run through the contact us feature, lol

Do you have to be able to charge while you are racing? If not, flip a switch and charge the batteries in parallel.

In your case I don't think it is going to matter if you charge in series since you can top (parallel) balance all of them before the race, then top balance them again after the race. They aren't likely to get out of balance in a few hours. Manufacturers don't want to have to warranty batteries used in series since they can easily unbalance over time. The real question is whether the BMS in each one can handle the output current.

So if someone builds a battery for you, it just has to be available for other teams to purchase as well?

I read your other thread to catch up. I would definitely look at a bicycle type tire rather than a go-kart type tire to reduce rolling friction and weight. Mount them with some positive camber to help with turns.

Yes, we charge while we run as well, you can feel the kart speed up when the panels are getting full sun.

It obviously uses more amps than it produces while running.

 

[Q-Dog]

Max continuous current on that battery is 15 amps? Too small for what you want to do, no?

 

[400bird]

Power tool batteries or ebike batteries are my recommendation. They are designed for higher discharge currents than most other small capacity batteries. From what I am picturing, the 18Ah battery you are looking at is designed for a UPS (low current)

The benefit to either of these batteries is that you can have spare sitting around waiting to pop in for the second race, no need to recharge between races. You can just drop in a nice cool battery for best performance, and not worry about high cell temps after a quick, high current discharge/charge/discharge cycle.

I did respond yesterday, but it was lost in the forum update. The entirety of that post was about how the spade connectors on those batteries aren't up for 100amps let alone 300.
Power tool batteries probably aren't either, but they'll do more than the F2 spade terminals.

 

[Hedges]

8 minute run means 7C discharge rate. First thing to look for in a battery is the max continuous discharge rate.
LiFePO4 is a safer chemistry. EV grade cells I think may support 3C or so; DIY people usually get reject cells with spec 1C.

Other lithium chemistries carry more fire risk. Some I think offer high discharge rates, used in racing toys, etc.
These would weigh less, could be an advantage. Although your limited capacity battery should weigh some fraction of the driver, diminishing weight reduction available.

There are lithium jump-starter packs which can deliver high current, supposed to be for a very short time. Somewhere between 300A cranking for seconds and the steady operation spec of other batteries, you might get an 8 minute solution. Suggest a temperature sensor that cuts load.

AGM starter batteries weigh more, but can deliver high current. Can't charge terribly fast, although some say they accept 5C at least part way. You drained at 7C, need to charge at 3C. AGM will take multiple hours to properly and fully recharge. Swap batteries?

Odyssey, 40 lbs, ~ 500 Wh, ~ 4000W output under "cranking amps" condition.

ODS-AGM42LMJA (PC1200MJT) ODYSSEY Extreme Series Battery | ODYSSEY® Battery

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Supercapacitor?
Maybe not:

This one, 54V 150F for $800 works out to 60 Wh, 1/10th of your battery.

Maxwell super capacitor power bank 54V 150F supercapacitor battery power bank 782890829052 | eBay

Maxwell super capacitor 54V 150F ModuleAssembly of 20pcs 3000F 2.7V super capacitor. Quality standard :ISO 9001:2000;ISO 9001:2008;ISO 14001:2004.

www.ebay.com

Power tool batteries or ebike batteries are my recommendation. They are designed for higher discharge currents than most other small capacity batteries. From what I am picturing, the 18Ah battery you are looking at is designed for a UPS (low current)

Good point.

My Homelite wacker & blower have 60V 4 Ah battery.
Some tools take multiple batteries.

Dual 80V, 2.5 or 4 Ah batteries for this mower. Add a speed control, and its motor might power the cart.

80V Brushless Cordless 21 In. Self-Propelled Lawn Mower - Tool Only

Amazing deals on this 80V Self Propelled Lawn Mower - Tool Only at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

Do you have to be able to charge while you are racing? If not, flip a switch and charge the batteries in parallel.

Instead of a literal switch, connectors and wiring harness. Plugged into charger, they are in parallel or have independent chargers. Plugged into cart, they are in series. This takes care of balancing between batteries as well.

 

[florida cracker]

Max continuous current on that battery is 15 amps? Too small for what you want to do, no?

Not sure, I've been trying to learn this as I go. First year really doing anything different with the batteries so having to learn about how current runs through the system. We were using this battery connected in series to 24v and getting to 20+ mph https://www.vmaxtanks.com/V30-800-Deep-Cycle-High-performance-AGM-Battery-_p_46.html


Swapping out batteries or using capacitors are not permitted and all charging/running gear must be mounted on the kart as it runs. Panels and charging system.

 

[Q-Dog]

The LiFePO4 you linked to has 15 amps continuous capacity. I would expect the BMS to shut down if you try to run 100 amps continuous through it. Lead acid/AGM battery does not have a BMS.