Making electrical connections with busbars

Once you’ve got your battery cells inserted into the VRUZEND terminal caps and the caps are all snapped together, you can begin making your electrical connections using the busbars supplied in your VRUZEND battery assembling kit.

Making connections with the busbars is really easy. Before you begin though, make sure you understand how series and parallel electrical connections work. If you want to brush up on the subject, read this article. Connecting the cells with busbars is not difficult, but this is the step where the danger of short circuits is most plausible. Make sure you also understand what a short circuit is and how to avoid it.

Assuming you understand series and parallel connections, then you already know where you need to place your busbars. Now we’ll talk about how to place them.

Start with a clean work area, free of any distractions or unnecessary tools. You don’t want any extra bits of metal hanging around that could accidentally short circuit your battery.

Lay out your pile of bus bars next to your battery. Make sure you don’t have any nuts screwed onto the VRUZEND terminal caps – you want the threaded posts to be bare. You can leave the nuts in the bag for now, you won’t need them yet.

Begin my making your parallel connections across each of your first two parallel groups. Simply lay the bus bars over the ends of the threaded posts, one after the other.

If you made a standard rectangular-shaped battery, your parallel groups will all be in parallel rows, which will make this step clear and easy to follow. You’ll want to start on the side of your battery that has the positive end of your first cell group facing up.

Theoretically you could lay down all of your parallel connections right now, but it’s better to wait and do them one set at time, making the series connections that connect each two parallel groups before moving on to the next parallel connection. This is a safer method because it leaves less exposed metal on the battery surface. If you accidentally drop a busbar further down the battery where you haven’t made connections yet, you’re less likely to cause a short circuit. While it doesn’t matter on the first side of the battery (since the other side isn’t connected yet), when you flip the battery over to work on the second side, you’ll want to do each two parallel groups at time for safety’s sake.

Now that you’ve got your first two parallel groups connected, go ahead and start your series connections. Pay attention to which terminals you’re connecting though. If the first two parallel groups are cell groups 1 and 2, then you’ll want your series connection to be between the positive terminal of cell group 1 and the negative terminal of cell group 2. That way you are leaving the negative terminal of cell group 1 open to be the negative terminal of the entire pack. If you started with your battery upside down, then you might connect the negative terminal of cell group 1 to the positive terminal of cell group 2. That’s not what you want.

To create this series connection, place one busbar between each set of adjacent cells from cell groups 1 and 2. If you did this correctly, you will have connected group 1’s positive terminals to group 2’s negative terminals.

Theoretically you could use only one busbar anywhere in the cell group to make this connection. That would create a functional electric circuit as long as it connects the two cell groups in series. However, the current that could be supplied by the battery would be limited since all the current would have to flow through a single busbar. Instead, you should use at least one busbar for every cell in your parallel groups. So if your parallel groups each have five cells, then you should use at least 5 bus bars in every series connection.

Once you’ve laid down your busbars in series, you can now tighten the nuts onto the threaded posts. Start by holding a nut in your fingers and gently lowering it into place on the first threaded posts. The nuts are small and are easy to drop, so be careful. Once you’ve got the nut in place, get it started on the threaded post by gently spinning it clockwise between your fingers. When you feel it catch the threads on the post, you can switch to a handheld 5.5 mm socket driver. Tighten the nut down firmly. Alternatively, you could also use a 5.5 mm socket on an electric drill or electric screw driver, but be careful that you don’t apply too much torque. Turn the clutch on the electric drill or driver way down to avoid stripping out the threads on the threaded post. While it takes a little longer, a handheld socket driver is the best method, and it gives you a perfect feel for the proper amount of torque to apply.

You want to tighten the nuts down firmly, but not so hard that you’ll strip out the threads. A good hard twist with a handheld socket driver is plenty. The goal is to compress the plastic in the cap just slightly. This provides some back pressure on the nut, keeping it from vibrating loose.

Continue down the first two parallel groups, adding nuts to each threaded post and tightening them down.

When you’ve finished the first two parallel groups, move on to the next two parallel groups. This should be cell group 3 and 4. You’ll lay down their parallel busbars and then their series busbars in the same way. The series busbars will connect between the 3+ and 4- terminals. Next tighten the nuts down on those two parallel groups. Then continue on to finish up this side of the battery, doing two parallel groups at a time and then completing the series connection between those two parallel groups before moving on to each successive set of two parallel groups.

Once you’ve finished all of the connections on this side of the battery, you’ll flip the battery over to work on the other side. At this point you need to be very careful, because this is where it is possible to start causing short circuits. The back side of the battery is now fully connected, so working on this side of the battery has the potential to cause short circuits if you create a series connection between the wrong groups of cells. You always want to create series connections between two cell groups that are not connected in series on the other side. For example, on the first side of the battery you created a series connection between +1 and -2. That means that on this side of the battery, you will not create a series connection between cell groups 1 and 2 (because doing so would create a short circuit). Instead, you’ll create a series connection between cell groups 2 and 3, specifically between 2+ and 3-.

You’ll know you’ve made a mistake if you start to lay down a busbar and suddenly see sparks. That indicates that you’ve made a short circuit. If that happens, don’t panic. Focus on removing the busbar or other metallic object (nut, screw driver, etc) that is causing the short circuit. Whatever the offending metallic object is, it will likely begin to heat up within a second or two. For this reason, it’s a good idea to keep a wooden object like a toothpick or broken chopstick around, just in case you need to pry something up. It saves you from holding onto a hot metal object with your finger, and the wood won’t cause any other shorts like a flat head screwdriver might.

But don’t worry, I don’t think you’ll cause a short circuit. You’re working slowly and carefully, so you’ll be fine.

Once you make your parallel connections on the 2+ and 3- terminals, you’ll make your series connections between the two terminals and then bolt them down. Now you can move on to the 4+ and 5- terminals, where you’ll make parallel and then series connections between the two terminals. If it makes you feel more confident, you can lay a piece of paper over the first few terminals on which that you’ve already completed the series connections. This will shield them while you’re working on each successive set of terminals and further reduce the risk that you’ll drop a bus bar on them and potentially short circuit the terminals.

Now you’ll continue on, connecting every two successive cell groups in parallel and then in series. Remember, double check the opposite side of the battery to ensure that you are making series connections in locations that are not already connected in series on the opposite side of the battery.

Once you’ve finished all of those connections, the only terminals that are left unconnected should be the negative terminals of cell group 1 and the positive terminals of the last cell group. This is where your charge and discharge wires and/or your BMS wires will connect. You’ll want to make sure these terminals are each connected in parallel, either with bus bars or with the wire clamps. I usually do both, adding the busbars first and then placing the wire clamps on top. Both of these terminals should only be connected with parallel connections – they should not be connected in series to any other part of the battery.

And that’s all there is to making your busbar connections. At this point you’ll probably be looking at either adding a BMS or adding a balance connector if you aren’t using a BMS.


About The Author
Micah Toll is a mechanical engineer, lithium battery builder and ebike educator. He’s written multiple books including DIY Lithium Batteries (an Amazon #1 Bestseller!) and The Ultimate DIY Ebike Guide (an Amazon #2 Bestseller!). When he’s not tooting around Tel Aviv or Florida on his ebikes, you can probably find him reading, writing, running or vegging out on the couch.