Troubleshooting Your Pack and BMS

As a major seller of BMS units to the growing community of battery builders, we’ve heard from hundreds of people who are having problems with their build. We know pack building can be frustrating, so we put together some troubleshooting steps and additional notes related to battery building to help those of you who find yourself stuck or frustrated.

First, understand that we test each unit that ships out our door. We do this for several reasons. It gives you, the battery builder, a known good operating unit from which to start, thus allowing you to not waste any time thinking the BMS is bad. It isn’t the BMS. We know that with confidence. We also do this so that when some of you burn the units up by connecting them incorrectly, and then send them back to us for a refund, we can identify rather quickly that you burned the unit up, and that it wasn’t a faulty BMS unit.

Second: There is a reason Tesla thought about – and then abandoned – the idea of recycling lithium batteries for further use. Let me put it bluntly: used/recycled batteries are the number one headache for any serious pack builder. Unless you are really in there testing (and matching) the internal resistance of each used cell, you are only setting yourself up for failure. Used cells will cause you more grief than anything else you will come across in this field. If you build your packs with used cells, prepare to get acquainted to the daily fatigue of wondering why your rig isn’t working properly. Failing cells cause BMS circuitry to do things that might look like failure of the BMS, and such oddities will continue until you identify and replace the bad cells. If you are building packs with used cells, you are a masochist. Period. End of story.

We buy used cells for tinkering and experimenting, but when it comes to building something useful, we always build with new cells.

Charge and Balance Cells First: When building a pack, charge your cells all to the same voltage before making your series connections. We often build our parallel groups out of newly-charged cells, and then balance them by connecting them all together as one giant parallel group. We leave them connected overnight so they have a chance to completely balance. The professional pack manufacturers use large “balancing boxes” that have copper plates on the top and bottom. They charge all the cells at once, and then let them balance out in the balance box overnight.

If you prefer not to build your own balance box, get yourself some alligator clips with good wire, and clip all the negatives together on one end, and all the positives together on the other end. This will balance your cells. Just be sure to start with nearly equal voltages. Otherwise, you might experience large current movements as voltages try to balance out. Do like us and build your parallel groups first. We then clip to the nickel strip.

BMS Troubleshooting: No output voltage from BMS. You most likely have a broken weld or connection at one of your series connections, or at one of the ultimate ends of your pack. We use a plastic tool to pry at all of our welds to try and find anything loose. If you can’t find a broken weld, you may need to completely remove the load from your circuit. Try reconnecting the load, and also try charging the battery through the circuit. Sometimes a BMS simply needs to be reset through brute force.

BMS Troubleshooting: Output voltage low by a few volts. You most likely have hooked up your balance harness incorrectly. The balance harness has one black wire, and several red wires. If you count them, you will find that there is a red wire for every positive cell terminal (or parallel group) in your pack. The black wire goes to your pack’s ultimate negative end. The red wires MUST be connected in order. The first red wire (the one adjacent to the black wire) goes to the positive end of the first cell or parallel group – which is the same cell that we just identified that also has your pack’s ultimate negative end. This first red wire goes to the positive end of that first cell. It does not go on the positive at the other end of your battery. That is called the ultimate positive, and we will be getting to it later. Now that you have made your first red wire connection, you may continue to connect red wires IN ORDER to the remaining positive terminals. Just follow your series connections through the entire pack, and connect each successive red balance wire to each successive positive cell (or group) terminal. The final red wire will attach to your pack’s ultimate positive end. See how that worked out all nice and tidy?

If you had these hooked up incorrectly, it often doesn’t help to figure it out at this stage. Most often you’ve burned out the BMS, and it is genuine garbage. Throw it away. We don’t want it back.

The large Blue Wire: The blue wire is a negative wire that should be connected directly to the ultimate negative of your battery pack.

What about the Main Positive Wire? The BMS is like a switch in a circuit. It doesn’t have a positive wire. It sits in the negative side of your circuit, and current flows through it. We don’t include a positive wire with your BMS, and that is on purpose. You will need to supply your own positive wire, and connect it directly to the ultimate positive end of your battery pack. Most people will then attach the other end of that wire to some sort of connector. We include an XT-60 connector in most of our BMS kits. Whatever connector you choose, you will attach the negative black wire of the BMS to the negative port of the connector, and a positive wire directly from the connector to the ultimate positive end of you battery pack.

Send us your feedback. We enjoy helping diagnose battery pack issues, and we’d like to know if our notes helped you find a problem in your build.