Off-Season Battery Cart Design

Here is my most recent design, a battery cart. We have always laid our batteries out on table top, but that takes up a lot of space in the pit. We were recently donated aluminium, so what a great way to potentially use it (and practice welding). I shot for a very small footprint in the pit holding 6 batteries. It is only 18.5" x 13.5" and just over 2 feet tall, doubling as a seat :smiley: or shelf.

Click here for the CAD Files
If you have any questions or comments, please feel free to ask away.

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Looks good! I love the idea of planning for the battery cart to be a seat or shelf - that’s often the case in our pit :slight_smile:

The only advice I’d give (from experience) is that unless you’re going to sew little straps to the batteries, leave more room above and below them so that you can grab their handle features without smashing your knuckles or pulling on the battery leads. Right now, it looks like you’ve got the SB-50 connectors right in the way.

Are the chargers going on the shelves? If so then this looks great.

Great idea, I could just offset the Anderson connection back a bit farther from the front on the bottom angle bracket. I will definitely change that, thanks!

On the back inside wall of the cart (behind the batteries) the 2, triple battery chargers will be mounted to a plate of some kind, wood or polycarb, and the leads will run underneath the batteries to where you can see the Anderson connections.

A few comments:

  1. I’ve always heard that you should charge these batteries upright, for proper operation of the vents (which are on the top of the battery). I’d love for someone to point to any sort of official documentation on proper charging procedure, I haven’t been able to find one that addresses battery orientation. Without that, I would recommend increasing the angle to at least 45 degrees, versus the nearly horizontal orientation now.

  2. It’s hard to tell from the picture, but consider usability. How easy/hard is it to remove the battery without pulling on it by the wires?

  3. What charger(s) are you using, and where do they go? You may want to include those in the CAD - they will likely have status lights on them that you’ll want to be able to see, and they may have controls (for example, adjusting the charging current between 2A, 4A, and 6A).

  4. Have you done a center of gravity analysis of the cart? When stacking batteries up, there’s always the possibility of tipping it over. It’s at least worth a look, especially since most of the weight (the batteries) appear to be on one side of the cart.

  5. I would not recommend using this as a seat at competition. The wheels could make that dangerous, you don’t want legs dangling against the batteries, and you want to do an analysis of the load rating - how much weight can safely be put on top of it before things start to break?

  6. I can’t tell from the picture what type of wheels you want to use, but they look small. Larger diameter wheels may be preferred, as you’ll run into large cord protectors at some events that you’ll need to roll this over. And while I can’t tell from the picture, I would recommend putting fixed wheels on one side and casters on the other, for ease of movement. I personally find it easier than 4 casters, as it doesn’t wander side to side as you wheel it around.

  7. Consider raising the height so the top board is a comfortable working height. If you beef it up some, you can get a really good working surface there that you won’t be afraid to pound on when needed (It can be sturdier than the table you get in the back of the pits!). You mentioned wanting a small footprint, but depending on the rest of your pit setup it may be useful to double the width and put in a storage area next to it for parts or tools. Having a nice big work surface on this would let you get rid of the table in the back of the pit, freeing up room for other stuff as well!

As an example of some of these ideas, here’s what my team has:

On the left you can see the electrical cart. There’s an area on the bottom where we charge batteries, with chargers right above it (LED’s easily visible, and cables numbered to match!). Lockable storage on the right side of the cart, as well as a drawer above the batteries. Some working room on top, as well as a rack for wire storage. On the right side of the picture is our mechanical cart. This has 8 drawers for storage, along with a butchers block top that lets us really beat on it. The top lifts up (hinges and gas springs make this easy, and holds it in place when it’s in the up position!) for larger part storage underneath (sheet metal, round, square, rectangular, and hex stock, etc - there’s about 2" of depth, covering the entire top of the cabinet). We also have a vise permanently mounted to one corner of the top, which is used at every event.

The tabletop is a nice, handy feature that will be very useful in the pits.

What size are the casters? If they are too small, you may have trouble rolling the cart on sidewalks and getting it over door thresholds when you take it to competitions.

Consider moving the battery trays back so that when the cables are plugged into the charger, they don’t protrude from the front (by very much). Otherwise, when people squeeze past it in the pits, they will brush against the battery cables and possibly catch on them. This will also get the CG more between the front and rear casters.

Do you have some feature to keep the batteries from falling out as the cart is pushed around? We have damaged batteries when the cart hit a seam on the floor and the cart tipped forward causing them to fall out and hit the floor.

Is the total loaded weight such that two people could lift it into the back of a truck?

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I do like the size and dual-use features, but it doesn’t take many battery leaks before you wish you’d charged them upright. Upright charging may or may not make them less likely to leak, but with some thoughtful design, it does reduce the size of cleanup, as you don’t have one battery leaking on to the one(s) beneath.

We lost two batteries on consecutive days this summer to a new charger with a faulty float state (since replaced). The second battery swelled up so that it wouldn’t come out of the carrier easily. Having the battery in a carrier on the shelf made it much easier to take the carrier to a well ventilated area (outdoors) away from people to take it apart, and simplified keeping the acid off of everybody’s skin.

THat’s going to make it tougher to see the LEDs I suspect. Something to consider.

The casters are only 2" diameter, but this was done to keep the CG low to make it easier to move around. I didn’t consider the cord protectors and such, but in my experience, even the robot cart (which has 6" wheels) has trouble getting over them.

I think for transport a bungee cord running down the front of the rows would be sufficient, and just to remove them while in the pits. Simple and easy not adding any complexity. If that doesn’t work then we have to figure something else out for the next iteration.

Solidworks says the loaded weight is around 130 lbs, so realistically around 140-150. We get our full toolbox in the trailer and I think that weighs almost 500 lbs, so this should be alright.

To be honest, I didn’t think of that until you said something, but it would force pit crew to check each battery with a battery beak before putting it into the robot. Bad design with good consequences??? I’ll look into putting the chargers on the shelf and making the whole thing thinner to compensate. Thanks for the advice.

One more suggestion… build in an extension cord! While usually you have a plug in the corner of your pit at competition, many off-season events you may take this to may need an extension cord. You can wire it in permanently, and include features on the side to allow you to wrap it up for transport. We’ve had this on our electrical/battery cart since our first iteration of one about 10 years ago.

As someone who has handed off many batteries to the drivers, that honestly just sounds tedious to check each battery until a good one is found. The pit crew also won’t be able to take batteries off the charger to put new ones on during downtime without manually checking if it’s done.

+1. If you get a heavy duty (12 gauge) cord with power bar, you can also use the battery cart as a place to plug in other pit things without the safety inspectors getting too mad about daisy-chaining extension cords.

+1 on the permanently attached power bar. At DiscoBots, we plugged in all sorts of stuff like chargers for the drills.

You might want to think about going to at least 9 slots. We used to have a 6 slot but we soon went to a 12 slot. You never know, you might find yourself having to do 5 round robins then 3 final matches only to find yourself with not enough charged batteries.

Thanks 141 for letting us borrow batteries in 2017!

Have you ever ridden a skateboard? Small wheels are usually a giant pain.

Have you considered more of a hand truck design? You would put two large, fixed wheels on it with handles and a flat base. When standing there, the wheels would be just slightly off the ground, giving you stability and helping to prevent it from rolling around. Then you grab some handles and tilt it back onto the wheels to move it around. This would also help prevent batteries sliding out of the cart while moving it.

In my dreams! I’ll talk to the coach…

From a look at this cart, I have a suggestion…

Biggest issue I see with this design is the caster location.
These are laid out in a similar fashion as the cart from Andy Mark P/N am-2797…

Last year we built this cart with two of the their battery chargers

The casters were in the same general location as yours. The cart was very unstable when moving and once you unloaded a few batteries the cart was extremely easily to tip, if it didn’t do it on its own. As you move the cart around the wheels pivot under the cart and cause instability.

The best solution was to move the casters outside the weight where their rotation would not allow them to pivot under the cart.

I took off the original casters and put the cart on an adjustable caster base, very similar to

This system worked very well. I has never tipped even with all the batteries removed. you just step on the lifters on the end and the cart rolls easily. Drop them down and the adjustable rubber footed posts keep it quite still.

Since you are building the cart you can make caster pods on the corners, like many of the shop vacs …

Looking like a good project, I agree with having a view port to see the charger status lights. Much quicker to narrow down which battery is worth using the beak on…

If you like shoot me a PM and I will post some pictures of the cart.

Hope this helps,