To familiarize our students with a revised design process, we decided to design a new battery cart and went through all the steps of the process. The students really like the new design that came out of it comparing to our current battery cart and they want to build it.
I am aware that FIRST has changed the battery teams are allowed to use before. Does anybody know if the size of the batteries in past years are all about the same size? If we know that the size changes, then we will use the maximum dimensions from previous years so it will accomodate most batteries. Does anybody know if we will be using the same battery next year?
First has changed batteries in the past. Initially, (1996) the batteries used were drill batteries and plugged into the drill housing for each drive motor. As things progressed and First adopted the IFI controllers, we started using the motor cycle battery in the size which we are now using. However, over the years, the battery has changed vendor, terminal arrangement, and chemistry (gel vs. AGM), each of which produces a slightly different size. Current batteries expand with heat, so if you make the tolerances small, it is possible you won’t be able to remove the battery as temps flucuate.
As First and their vendors change, so to will batteries. There has been some talk for a year or so, that MK is working on a newer battery. It is more powerful and I expect it to be a little larger then the current battery.
I have seen many battery carts, but designers should remember that hot batteries do not charge properly and although the battery can be used in any configuration, charging is best done with battery terminals up.
Al said it well.
I would just add that having an extra inch or so for fingers to get at the batteries is valuable. If they rattle around, some closed-cell foam can be cut to take up the space yet leave flexibility for the future. (Foam that’s soft but quite stiff, made of a slippery plastic I think is polyethelyene. Usually found in white, it is not styrofoam)
Another thing you could do is to get all the design work done, then hold off on building the car until after kickoff, when you know for sure what the batteries will be.
Or if you’re into assumptions, you could read the list of rookie/veteran parts variables
and assume that since veteran teams are not getting as many batteries as rookie teams, that the same batteries will be used next year. But note that it also says that battery rules are under review.
We will modify our design slightly to charge the batteries in upright position.
We will leave half an inch in each direction to accomodate slightly bigger batteries in the future and swelling during charging.
We will put inserts to take up the slack and change them to different sizes to accomodate batteries of different sizes in the future. I like the idea of using light weight slippery plastics. What will be a good way to secure these inserts that is removable?
We are going to CAD it tonight and build it during Christmas holidays.
The loopy side of velcro should do the trick, and it’s soft enough that if a battery needs to be inserted without any plastic spacers, it would just slide past.
Alternately, depending on how its built, the spacers between batteries themselves can be adjusted. I’m looking for a picture of my team’s station to tell you what I mean…
Al is much more experienced here, so anything I say takes a very distant second to anything he says.
A typical battery that size has a liquid or gel in it–something that can flow within limits–and a series of plates. Essentially, there is a chemical reaction happening. When the battery is recharged, the reaction is reversed. The reason to recharge upright would be to ensure that the plates are fully coated for maximum reverse reaction occurrence, or more speed/better depth of charge. Discharging can be done in any orientation because the reaction will concentrate in the contacted area. Charging, you don’t necessarily want that.
Oh, and charging on the side has another drawback: should a seal break, the acid or other liquid can leak. Charge upright, and it’s easier to clean up than if it spills out.
Oh, and for those inserts: there is a type of pink foam that is fairly dense, closed-cell, I think (not sure of exact type/brand). It cuts easily with a hotwire, great for spacing/organizing.
The document that Tristan has linked to is part of the references I have used in the past. The current battery is an AGM style battery so the electrolyte is absorbed in a mat but can become fluid under certain conditions. I recommend charging upright so that any fluid that might be present doesn’t cover the pressure valves. Should pressure build to the point of venting, loss of fluid is inevitable. For the students handling batteries, if we make it a normal procedure to charge upright, then should they be in position where they are charging some other types, the vents will likely be facing up. If the battery experiences some failure during charging, then all venting will take place where pit crews can see it and react ASAP. We are not sure at this point what failures may have occurred, but a small number of teams have experienced battery failures over the last two years. Some have occurred during charging and the rest appear to be physical damage. Since the battery stores a large amount of energy, physical damage can cause internal heat. The domino effect causes venting, external heat, and leaking fluid. Please instruct your students to never handle the battery by the wiring, always carry with two hands and in the event of a drop, be sure to put the battery on the side until you (or another mentor) can determine if it has internal damage. A few hour test with a load tester like the West Mountain Radio CBA II will show if one or more of the cells have a reduced capacity indicating internal damage.
Thank you to all who responded so quickly and thoroughly in the answers. We were in the middle of finalizing the design tonight and the students wanted to understand why the batteries can not be charged sitting on its side.
Our original design would have the batteries laying on its side with the top facing out when charging in the pit. At other places where space is not a constraint, we will lay the battery cart down on the back which is on wheels so all batteries will be upright.
Now at the pit, we can either lay it on the back and put it under the table or take the batteries out of the holder that carries two batteries and set it upright without the holder in the battery cart. Either way we are going to charge it upright. It may be hard for others to understand what I am talking about. We did manage to finish the CAD model tonight. We are going to tweak it a little, put in the details where the holes are to run the charger wires, power strips etc. We will post it here for you to see at a later date. It is really cool.