Attaching Cables to FRC Batteries

I’m just wondering if anyone has a better way of attaching the 6 gauge wires to standard FRC batteries. Historically we’ve used grade 8 10 bolts with two washers and a Nyloc nut as you can see in the attachment. We put two layers of heatshrink around this.

While that method works, they can work loose over time. We’ve tried using two jam nuts (one Nyloc) but that wasn’t any better. We haven’t tried using Loctite with the Nyloc nut but that’s our next idea.

Any better ways?



*How did the terminal get bent like that?

That battery was knocking around in our bin of batteries to be recycled…I just grabbed one for a photo.

I believe FIRST gives you pieces to connect to the battery terminals in the KOP.

I place a standard split lockwasher between the head of the bolt and the washer on that side, and still use the nylon insert nut. My thought being that the lockwasher will help keep the pressure on the connection if it loosens slightly. I’d prefer a Belleville (conical) washer, but never seem to plan ahead well enough to have them on hand when we are putting cables on batteries. I also prefer a bolt with a larger head than the socket head cap screw provides.

However, I find that if you don’t move the battery cable against the terminals (put the cable on at the right angle to connect to the robot, and don’t use the cable as a handle for the battery) they tend to stay tighter.

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I am not sure it really matters. It is not a strength joint. The bolt is not really part of the current carrying circuit. If you follow good practices & never carry the battery by the cables, any metal bolt properly torqued shouldn’t loosen. Lock washers are OK for piece of mind.

I do not see any threads sticking out of the nut. Your nyoc needs full penetration to be useful

We usually use a star washer between the battery terminal and the cable connector, this also seems to help deter rotation.

Best lock-washer type device that I have used in industry is a nord-lock washer. Our local Fastenal carries them.

The video on the Nord-Lock site referenced above is a great introduction to why bolts loosen. It starts about 1:30 into the video. At well over a $1 a pair I doubt we’d be using too many Nord-Lock washers except for the most critical things but it’s sure good for students to know the two reasons why bolts come loose.

Careful with Loctite + Nylon nuts. Loctite can eat plastics (like Nylon) and you may actually end up with a worse failure.

Using lock washers + nylon nuts is a pretty reliable way to make a secure bolted connection. If they loosen over time, you should be able to just snug them back up and get another long period of time before they loosen again. This is all we do, and we don’t have any complaints.


Good idea. After watching the Nord-Lock video it makes sense that a Nylock nut combined with a spring lock washer would be a good combination somewhat simulating what a Nord-Lock X washer does in a single piece.

I knew about Locktite and some plastics but I wasn’t aware it reacted to nylon. Have you had experience with that?

Personally, I prefer to use the supplied hardware, especially on the battery terminals with the groove that retains the fastener across its flats. I tighten it securely (using a wrench—not a Phillips screwdriver, which could cam out—to estimate adequate preload in the bolt) with a small drop of purple Loctite (low-strength, 222MS), which I take care not to apply to the plastic parts. If the fasteners can’t spin, they can’t come loose.

I find that it’s also important to use the right-sized lugs. The Ø0.250 in lugs that FIRST has supplied in the past work fine on the main breaker and power distribution board, but they tend to slide around on the battery. I buy ones sized for a #10 screw instead. (These sit a little lower on the terminal, and thus can’t rotate about the screw because of the edge of the lug in contact with the battery case. That has the advantage of preventing rotation—and loosening—as well.)

Which is usually preferred: compression lugs or regular ring terminals for the PD board and main breaker connections?

I’ve usually used the compression lugs, because the right stud and wire sizes come in the kit already, and because they’re a lot more durable compared to an ordinary ring terminal—you can get two or three solid crimps into the compression lug, versus one with a regular terminal.

The disadvantage is that you need to take a lot more care installing them on the PDB, because of the weak studs. Unlike an ordinary ring terminal, which can be flattened by tightening the nut on the stud to get a good contact area, the compression lugs need to be carefully prepared in a vise to ensure that they are flat across the ring, but bend away to clear the PDB housing appropriately. If you don’t prepare them, and try to just adjust the lugs as you tighten them, you’ll snap the studs off.

(Also, I hate the screw lugs. But if you have to use them, it’s vital that you install them properly. Tighten them so that the wire is clamped hard, then push, pull and twist them to try to unseat the connection. Then tighten them again. You’ll probably find another quarter-turn. A tiny drop of low- or medium-strength threadlocker on the threads, or a tinier drop of cyanoacrylate glue on the edge is probably good insurance.)

The terminals tend to work loose with time because the terminal on the battery and the cable terminal wear against each other and the hardware. This is not the only effect as constant tightening of the hardware cause the terminals to deform making the conditions worse.
Tom (above) uses the same solution we have been using for years. A star washer between the terminals and a star washer under the nut are sufficient to keep everything locked together for the life of the battery. Many will ask about the star washer adding some resistance and this is true. However, the bite of the washer overcomes any surface scum on either of the terminals and when tight produces at least the same surface area of contact that the hardware and terminals produce when new. We also bend over the cable terminal so it lies close to the battery top or side (depending on use) and then cover the entire connection with 3/4" heatshrink. This method is easy for students to assemble as it does not need any great strength when tightening the hardware. It is also very inexpensive as #10 star washers can be purchased in quantity at any hardware store for a couple of bucks. You can substitute a keps nut if you have them on hand.

I’m not sure that I quite believe that, Al. In a properly tightened joint, there shouldn’t be any appreciable abrasive wear, because the faying surfaces of the terminals aren’t ever moving with respect to one another.

(People who pick batteries up by their leads may cause my assumption to be violated, however.)

Do you mean that someone is repeatedly retightening the bolt? (If so, my concern above stands. If it’s not moving, there’s no need to tighten it. If it is moving, it wasn’t tight enough.)

If nobody is adjusting the connection, I could still see this being a problem with heavily preloaded joints in materials that exhibit lots of creep—eventually they’ll plastically deform to reach an equilibrium between the preload stress and the elastic limit stress. But with the materials we’re dealing with, at the temperatures we’re operating at, for the lifespan of the part, it shouldn’t be a factor.

There’s still a flat terminal-to-terminal contact surface, right? (To get a good connection, presumably the washer goes under the nut or the bolt head, not between the terminals.)

The material used for the battery terminal is plastic as it is a very soft material being mostly lead. As such, the tightening of small hardware cause it to not only stretch and creep but also to “dish” in the process. As this occurs, the hardware does become loose requiring teams to tighten as needed. Any movement between terminals cause the wear I described above, again requiring adjustment of the hardware. Any rotational movement between the two terminals also causes the hardware to loosen. The addition of a star washer between the terminal surfaces prevents any movement between the terminals and therefore prevents both the wear and the loose hardware. While small changes in the deformity of the parts still occurs, the spring tension of the star washer facets keeps everything tight. By the time, the spring of the star washers can no longer hold the parts in alignment, the battery should have reached it’s end of life (about 4 years maximum for most batteries.)
To restate, without the lock washer, the terminals will move, there is nothing to prevent it.