DIY Tracked Vehicle...battery?

[daveweber34]

Hello Gentlemen and Ladies,

This is a fantastic website that you have here. I have been pouring over the information that you have here for the last several days but I still feel the need to post some questions for my own piece of mind. While many of you here may be used to dropping $1,000 on batteries, I am not and getting some confirmation on what I was planning to do will definitely help me rest better.

I modify stuff...kind of like you do...but different. Right now I have three kids aged 6, 4, and 2 and I modify some Power Wheels (Battery Powered Ride On or BPRO) for them to play with and keep me busy. I have decided to do a "big" build now that my wife is out of residency and I have time to spend in the shed again.

Has everyone heard of the Ripsaw tracked vehicle? No? Google it and check it out. It is flippin' sweet.
[youtube]http://www.youtube.com/watch?v=tlcenWPzsUU[/youtube]




I have many of the parts and components that go into this already ordered or in hand. In no particular order:

The body and the point-of-no-return item (welder):

I will be welding up a frame and suspension to go underneath the kids BPRO body.

The track (purchased, in transit):
View attachment 3


The motors (Purchased. I am using four of these, two each side): Let me elaborate on these more. Ever heard of the EV Warrior electric bicycle? Well, maybe this is the wrong place to ask that, I hadn't. In all my research on this, I stumbled across it. Here are the specs on this motor (from Robotmarketplace.com):




The motor controller (in hand): A Sabertooth 2x60. It has two independent channels, one for each track. Each channel is capable of a sustained amp draw of 60 amps (120 sustained amps in total) with a burst draw of 120 amps per channel (240 total peak amps) for a few seconds.




At 24v (where this is going to be run at), the motors are listed at 1.55 hp. I am going to be running four of these. 1 hp = 745 watts.
So, each motor has a peak output of 1154 watts (1.55 hp x 745 watts). 4 motors x 1154 peak watts = 4616 peak watts.

I need to figure out what kind of peak amp draw that is. Since: volts x amps = watts. I can solve amps because I know the approximate voltage (24v) and total watts. So, the peak amp draw will be around 192 amps. That is inside the specs of my motor control for burst output, so I should be ok.

What I need to know about is the batteries for this. I have never used anything other than SLA/AGM batteries for these BPROs. I would really like to go a different route with this though because of the amp draw this can have and the weight SLA batteries would add.

I've talked to a another couple of guys, but I want to confirm it here. I was thinking about using some LiFePo4 Headway #40152, 15ah, 3.2v cells. To be specific, running 8s to yield ~24v. Now, I need to plan for a continuous current draw of 120 amps because that is what the DC motor controller can do. So, I should be aiming to limit the draw from the batteries to 2C, correct? If that is the case, I should be looking at building a 8s4p 60ah pack, correct? Would running a 8s3p 45ah pack and possibly drawing over 2C cause problems?

BMS? Check. I definitely plan on getting them. For the current draw that this could potentially do, I think I'd do a 50/100 BMS on each 8s pack and let my motor controller limit the current since that is built in feature it has. Also, just for your information, this motor controller has a built in regeneration feature where it recharges the batteries anytime a motor is commanded to slow down or reverse. Also, it has a lithium low voltage cut off at 3.0v (for the pack...it auto senses the number of cells in series).

Where are some reputable buyers? I am new to this. Anything else someone cares to comment on or suggest? I am open to anything you have to say on this.

If I am posting in the wrong place, I ask for forgiveness. Since this is a custom build, it doesn't really fit in the ebike, emotorcycle, ecar category so I placed it here.

Thank you in advance for your assistance.

 

[liveforphysics]

I would switch to motors making the power at a lower RPM. Gearing stages add loss/complexity/failure modes.

I assume it's only tracked not a half track? You could save more complexity by just running a single drive motor per side. I would bolt the track drive sprocket right to the side of a hubmotor like hubzilla.

Can you share where you found the track and pricing? Looks like very nice light duty track.

 

[Hillhater]

Can you share where you found the track and pricing? Looks like very nice light duty track.

Looks like industrial "Flextop" slat conveyor as made by Intralox.
Available in various types,widths and materials etc, including Stainless ( but $$'s !)
http://www.conveyor-manufacturers.co.uk/Gallery/slat-conveyor-photos.html

 

[spinningmagnets]

Geared hubs can be found from 6-inches in diameter to 8-inches or so, is it possible that something like that would be a possible option to directly drive the track (as LFP suggested)? They can be had for under $100 each and 350W at 24W (depending on a variety of factors).

 

[daveweber34]

Thank you for the reply. This will be a fully tracked vehicle, not a half track. This will mainly be a kids ride on toy so I am looking to make it work on a "relative" budget. I always need to weigh the cost vs. performance on what I'm doing. If money was no object, I would definitely be interested in doing more motor than I have right now, but that is not the environment I have for this build. For less than $200, I had to snap those EV warrior motors up. Some of the robotics sites where they build some tracked vehicles really spoke highly of them. Having said all that, I think I'll keep those hubzilla motors in the back of my mind if I ever want to upgrade it when the kids get older.

I haven't figured out the gearing yet, but I most likely will be running a jack shaft to bring the speed down and increase torque. As I do not know what to expect at the beginning since I've never build a tracked vehicle, I am not aiming for the sky. I am going to shoot for a final gearing that gives this between 5-7 mph. My kids have driven modified power wheels that go much, much faster so I feel safe at that speed to start. Plus, I estimate this will be around 400lbs when everything is all said and done. This isn't some light weight.

Okay, you wanted to know about the track. It is repurposed tabletop chain. That is what some of the stuff used for conveying goods around is called. I found this one at an online liquidation business for what I consider a good price. Here is a link to where I purchased this from eBay: http://www.ebay.com/itm/360685467000?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 By my estimate, I will need between 12' and 15' per track. This particular one is 7.5" wide, 1/8" thick, standard carbon steel (a lot of tabletop chain is stainless steel), made to flex for curves. It is plenty heavy duty enough for this application at 3.7lbs/ft. Here is a picture of what 30' of it looks like laying in my shop.


I would like to try dealing with the EV Warrior motors and DC motor controller I have if that is okay. I would like to get confirmation (or a rebuttal if I am wrong) that I am going about this battery stuff correctly. While I have plenty of SLA and regular lead-acid automotive experience, I have none when it comes to lithium based batteries.

From my reading, LiFePo4 is pretty much the safest chemistry for lithium based batteries. Now, I am not saying these are idiot proof, just that on a relative scale they are about the safest lithium battery around that can be reasonably purchased, especially compared to "lithium-ion" (which I understand is a broad category). I read some of those threads in which people's houses burnt down and even lost pets and I want this to be on the up-and-up. NOOOOOO short cuts here to save a dime.

I have a 24v Sabertooth 2x60 amp brushed DC motor controller. It is capable of a sustained 60 amps/channel (120 amps total) with bursts of 120 amps/channel (240 amps total). Based the spec sheet, the overall wattage of the motor system is 4600w @ 24v. So, the motors can draw right around 200amps at full load. Have the kids drive right into a tree and I'm sure deadheading the motors could spike the amperage to 240 no problem.

With all that in mind, I was thinking of building a battery system with the Headway #40152, 15ah, LiFePo4 cells. Running 8 in series (8s) to yield ~24v. Then, placing three or four in parallel. So, the battery system would either be 8s3p 45ah or 8s4p 60ah. At this time, I was leaning towards 8s4p 60ah so that I would limit the constant current draw to 2C for the battery system. Also, each individual 8s pack would have its own BMS. I have seen several that run at 50amp constant with 100amp bursts for each 8s pack. This would allow the battery system to supply more current than my motor controller can handle...which is good (right?). Then, my motor controller can limit the current like it is designed to do.

 

[dogman dan]

Sounds like that much of the headways, 3 or 4 p, is on the right track for that much controller.

I'd consider them to be about 5c cells in real world use, so 45 ah of them could supply about 225 amps. Initially, you could just tone down your controllers enough to run 2p, limiting power to 100 amps or so for the initial experiments.
2000w might be all the kids need, depending on their age. A pot in line with the throttle signal could serve to limit the throttle, thus limiting amps to any setting you choose for that outing.

Too many amps for too long duration might melt the motors, so work up to the full power gradually, making sure the brushed motors aren't overheating too much to run a huge battery to it's full depth of discharge. You'd feel silly if you bought an extra thou worth of batteries that only smoke the motors.

 

[gogo]

Don't be too quick to dismiss SLA for this application. If you can mount them low and the ballast would be a design feature, they could be the value choice. They are capable of putting out high peak amps and don't need an expensive BMS. You could try some 35AH and if runtime is not enough either swap in more, or design in larger batteries.

Is this an all-year toy, or seasonal? How long at a time would it be used?

 

[daveweber34]

I plan on starting out slowly on this once it is all put together. There will be bench testing too before it ever hits the floor. I also have an inline watt meter that I will use for one of the motors. I have found this helpful from doing Power Wheel modifications because it will really give me hard numbers to work off instead of just trusting a spec sheet.

As for the batteries....I have given considerable thought to SLA batteries but I keep coming back to run time/performance and weight. Doing some back of the envelope calculations, I think the tracked vehicle will be 400+ lbs empty. It can seat four kids, ~50 lbs each or more and total weight for this could be 600+ lbs. Cutting the battery weight by 30 or 40 lbs is a strong argument by itself. Plus, the lifepo4 will deliver close to peak performance so much longer during discharge than SLA does (at least that is my understanding).

Can someone point me in the direction of a reputable dealer who can help me fill my needs for this? I have never done lithium anything before so I will be starting from scratch on this.

Thanks,
Dave