Mid Drive Kit Install Part-2, Matching Connectors from Battery to Controller

April 16, 2016

We always recommend that when you buy any ebike kit, you get the major components (controller, motor, and throttle) all from the same vendor (also, buy a spare throttle). That way the connectors will all match, and the vendor you bought it from can help you trouble-shoot various issues that might happen. If you mix-and-match components from three different vendors, it can get difficult to get those parts to work together, and the vendors won’t be able to help you when a problem comes up.

The battery, however…is a different story. Kit batteries usually just have two thick wires coming out of them for power…a red positive wire, and a black negative wire (there’s also two skinny wires for the charger). Because of this, it is very common for kit builders to get their new battery pack from one vendor, and the rest of the kit from another. Sometimes the electrical connectors don’t match, and this article will discuss some of the issues and important cautions that you need to be aware of, when you are swapping-in new connectors.


What Connector to Use?

If you don’t already have a preference for a style of connector-pair to use, I recommend the popular anti-spark XT90, for ebike systems up to 3000W. Also, instead of buying bare XT90 connectors, I highly recommend buying “pigtails”. That means the factory has already soldered a few inches of wire to the connector, so the builder only needs to form a “butt splice” of two wire ends.

If you do decide to buy bare XT90 connectors, there is only one piece of advice that I strongly recommend; when soldering wire onto the sockets, make sure the male and female connectors are together, so that the heat from soldering will not cause the pins to “droop” in their plastic housings. This way, their connected status will hold the pins and sockets in perfect alignment until the soldering job is done, and the housings have cooled off.

I mention the “anti-spark” version of XT90 connectors (available in black plug or yellow), it is called the XT90-S. Inside the controller, there are electronic components called capacitors, and as soon as the battery is connected to the controller, there will be a sudden in-rush of current to “fill” the capacitors. On 24V and 36V systems, you may not notice any spark, and even if you do, it will be small. On the popular 48V and 52V systems (or any other voltage higher than that)…there will be a noticeable spark, along with a popping noise. That spark will cause corrosion on the surface of the metal contacts inside the connectors you are using.

Another issue is that some battery pack BMS’s (Battery Management System) will shut down to protect the pack, if it senses a slightly higher voltage than what it’s designed to run. The sudden in-rush current can cause a split-second of a voltage spike, which the BMS erroneously reads as a high voltage, and shuts the battery down.

If the controller is ever dis-connected, the capacitors inside will “bleed down”, leading to another spark when you plug it back in. Most modern battery packs have a large connector for the power to the controller, and a smaller connector from the battery to the charger. By having separate connectors for motor-power and charging, the larger power connector doesn’t need to be unplugged to charge the battery (some DIY packs only have one connector for both charging and controller-power, so they plug-in and un-plug often).

The main characteristic of direct current (DC) electricity that we need to be concerned about is the amperage. If you use high amps (the volume of current), it will need physically large connectors, or they will get hot. In the header pic at the top of this article, the Bafang BBSHD ships from the factory with 45A Anderson Power Poles (APPs). Andersons are very common, and they are adequate for this job. However, they also do NOT yet come with an anti-spark feature, and other ebike controllers can be found with any one of a variety of connectors that are not Andersons.

Andersons are not gendered, and it is physically possible for an inexperienced builder to plug them in backwards (if they are not configured properly). This could cause instant controller damage, and likely damage to the battery too. If you use Andersons, make sure to purchase both red and black, so they are not the same color. Also, once you have verified that they are configured properly, bond them with glue or heat-shrink so they cannot come apart. Hastily re-assembling Andersons that have slid apart, in dim light when you are in a hurry? it can be an expensive mistake, and could even cause a fire.

Luna Cycle is growing in popularity as a battery pack supplier for ebike kits. If you buy a pack from them, they already come with a female XT90-S anti-spark connector. It is traditional for ebike systems to have the female sockets on the battery side, because male pins are more vulnerable to having a metal screwdriver or some other tool accidentally touch them, shorting the battery in a huge spark, and possibly ruining your expensive pack. When they are unplugged from each other, the controller is safe, but…the battery is still dangerous, even when it’s just sitting on the bench…

This means that if you buy a Luna battery, you only need to swap-out the controllers’ male connector to an XT90. It is dangerous to work on the connector for an energized high-amp battery (great caution is required, even if you are experienced), but it is very easy and safe to swap-out the male connector on the controller of your kit, even if you’ve never done that before.

I’m going to cover butt-splicing two wires together using a dry crimp method, and then the soldering method.


Dry Crimp on the Controller Side

Lets start with the controller side, because the controller is not energized when not connected to the battery. Doing this adds a layer of safety until you become familiar with proper butt-splicing. Once you are confident with attaching a male connector to the controller, we can move on to the more dangerous battery side of this operation.

In this first pic below, I have cut off the Andersons, and then I have cut the controller power wires to a length to match the XT90-S pigtails. I “staggered” the cuts, and…doing that is not really necessary for a de-energized system, but…it’s just my habit, so I don’t have a brain-drift when I go to the battery side and forget that I’m not on the controller anymore. If two battery wires are exactly the same length, it increases the possibility that a huge spark can occur. Be professional,  you won’t regret it.




The type of wire-cutters shown are a cheap set of “electrical crimping cutters“. The tip has a cutting edge, and the the base of the jaws (where there is the maximum leverage and power) has a squared-off area designed for smashing crimps, which we will see here soon. [I cut the wires on the Andersons to a long enough length to make an adapter out of them later]

A crimp can be cup-shaped, or a cylindrical sleeve, and….they can be made of copper, steel, or aluminum, etc. In the pic below, I am using what’s called an 18-10 gauge crimp sleeve, which barely fits over a 10ga wires’ thick silicone insulation. I made sure the end of the crimp sleeve that has a “bell shape” is pointed towards the joint.

[both wires shown below have 12ga copper wire bundles. The wire on the left has common PVC insulation which is thinner, the wire on the right has thick silicone high-temp insulation. If you are using wire with thick enough insulation that the 18-10 size doesn’t slide over it, the 14-8 gauge size crimp sleeve is slightly larger if needed]

On the right side, I have placed a section of 1/4-inch heat-shrink insulation over the wire (the 2:1 ratio 6mm heat shrink from Hobby King will also work for 10/12 gauge). Like the name says, when it is heated up, it shrinks down. I now only use heat shrink that is marine-grade (designed for boats), and I always get 3:1 shrink ratio, instead of the common 2:1. This marine grade is lined with heat-activated glue, which makes a very water-proof cover for the joint, and helps improve the joints “pull apart” strength (plus providing excellent corrosion resistance from humidity).

Slide the crimp on first, before stripping the tip, which will make it easier. [The fine copper strands look silver because they have been individually “tinned” from the factory]




On the lower right-side are the professional wire-strippers used to remove a 1/4-inch of the insulation from the tips of the wires to be connected, to expose some of the copper wire strands. Just to the left of it is the razor blade I actually use when I think nobody is looking (yes, I have scars on my hands).

[edit: at this stage, I now recommend TWO sections of heat-shrink be used, because sometimes the crimp develops a sharp corner that might pierce only one later]




In the pic above, if you place the exposed wire strand-bundles next to each other “side-by-side” inside the crimp (example on top), the electrical connection will be poor, and the “pull apart” strength of the joint will be weak. Don’t do this.

Whether you are soldering or using a dry crimp, shove the exposed strand bundle tips into each other so all the tiny strands will have a lot of surface contact with each other (example on the bottom of the pic above).




Slide the crimp sleeve over the enmeshed joint. One end of the sleeve is slightly funnel-shaped, so that end should be pointed towards the joint to make it easier to do this.




This is not the proper tool for crimping wires into crimp sleeves (this is the proper tool), and yet…it works great for 10-12 gauge crimps, and is affordable. Use your best “Kung Fu grip” and smash the crimp sleeve until it’s flat. I purchased copper crimp sleeves, but the job of the crimp sleeve is only to hold the enmeshed copper wire stranded bundles together. Steel or aluminum will work just fine, because the crimp does not carry current. In fact, I think steel would hold the crimp better, instead of relaxing over time.




In the pic above, we have the crimp sleeve mashed flat. Now slide the heat shrink over the joint and heat it up.




There are two concerns that are sometimes voiced by builders who have never used a dry crimp on a butt-splice. The first is whether or not it helps or hurts the joints’ conductivity and resistance. If you use a high strand-count wire, and then intermesh the strands on both tips with each other…resistance is very low and conductivity is great. As an experiment, solder two wire stubs together, and also dry-crimp two wire stubs together…then you can measure the Ohms of resistance with a Digital Multi Meter (DMM) to see for yourself.

The second concern is “pull apart” strength. You shouldn’t be yanking on electrical wires, but just to put any concerns you may have to rest…the pic above shows me holding the BBSHD in the air by one 12ga wire that is dry-crimped to the positive power wire to the controller. This is the same exact dry crimp from the last several pics above. Try it on some scrap wire first, and then try to yank them apart.

Even if you prefer to always solder an electrical joint, by having the proper electrician cutters with crimping jaws along with a few crimp sleeves means that…you can do repairs in a remote area with no electricity for the soldering iron.


Soldering a Butt Splice

In the pic below, I have staggered the lengths of the wire-stubs on the XT90-S pigtail. Then I used my razor blade professional wire strippers, to strip the insulation off of the tip of the red/positive wire. I then cut up some scrap wire so I can take several fine strands of copper wire and twist them together, then interlace them into a loop. Instead of wasting good 12ga ebike wire for this, I use some scrap wire I rescued from the trash [if you see an old vacuum cleaner at the curb on trash day, I see an electrical cord with a lot of copper in it. The bigger the vac, the fatter the free wire]




Slide the wire-loop over one 12ga stub, and the heat shrink over the other. Then, just like before [whether you dry crimp or solder] shove the multi-strand tips into each other. Then you position the loop in the center of the joint, and pull the ends of the loop to tighten it over the joint. The loop is not very strong, but…it is “just strong enough” to hold everything together until it’s properly soldered. This simple tip works great to make your soldered butt-splicing jobs easier, and it was suggested to me by endless-sphere member “Amberwolf”.




You can wrap the loop ends around the joint some more and then solder, or…solder first and then snip off the hanging ends. Either way, you will need a high-watt soldering iron with a fat tip. A thin “pencil” style soldering tip will cool off way too fast when it touches a fat wire. I like to put the fat soldering tip under the joint and lift up slightly, then feed the solder downwards. Put a dab of solder right where the soldering iron tip touches the joint, and that will immediately improve the heat connection between the tip and the joint. [don’t do this on carpet, and don’t ask how I know that]

I found this affordable Weller 100W soldering iron [with the thick tip option] in the “stained glass window” section of a crafting warehouse.




As you can see below, when the copper wire in the loop turns a silver color, that means the entire joint is saturated with solder. Make sure to let the joint cool before sliding over the heat-shrink. If you try to slide it over while its still hot, my heat-shrink has heat-activated glue inside, and it will stick halfway to the target. Blow air through a straw if you are doing a lot of these, and want it to cool down faster…




This is the part where I’m supposed to tell you to buy a professional “heat gun” to activate the heat shrink insulation, but…I always use a butane-flame barbecue lighter (which can throw a small flame almost directly downwards or sideways), and everyone I know also uses lighters. Be safe and don’t burn yourself or start a fire, m’kay? (just in case you actually do want to use the proper heat gun, the one I have seen used the most is the $22 Wagner HT1000, which can be found on Amazon, or any one of the big hardware stores)




In the pic below, the male XT90-S “connector upgrade” to the controller is completed. Notice that one side of the plug is squared off, and the other is rounded. This feature “polarizes” the connector so you can’t accidentally plug it in backwards. For instance, if you are in a hurry in dim light [or have been drinking lots of beer…I’m looking at you, Australian ebikers].




Andersons are OK, but….in my humble opinion (IMHO), anti-spark XT90-S’s are better. Lots of chargers use 45A Andersons, so keep these to make adapters.


Connectors on the Battery Side

Batteries are dangerous to work on. Even if you are not physically burned (KFF, for Kentucky Fried Fingers), your expensive battery can be fried in a split-second of carelessness. Even if a battery is low, it still holds a tremendous amount of power. Ebike batteries in particular use high-amp cells, so a short-circuit that is directly connecting the two power wires will cause a huge spark, with an incredibly bright flash that can damage your vision. Maybe your battery pack might even start pouring out lithium smoke! Here’s some VERY IMPORTANT tips:

Only cut one wire at a time. It’s easy to forget…cutting both at the same time will short-circuit them through the cutters.

Stagger the cuts so they are not the same length, as an extra safety measure. Its free, and it can’t hurt…

Cut one side, then complete the crimping or soldering and insulating of that new connector wire…BEFORE cutting the second wire.




In the pic above, I cut one wire of a fully charged battery with metal cutters, but that’s OK as long as you only cut one wire at a time.

In the pic below, I’ve cut the old connectors red/positive wire, and then soldered the red/positive of the XT90-S female connector on…and I am about to activate the heat shrink insulation.




Once the red/positive on the new connector was soldered and covered with insulation, I was free to cut/solder and insulate the black/negative side. The thin twisted red/black wires are for the low-current charger, and the thicker wires are for the full current to the controller. If you add longer wires to the power connection, make the wires and connector larger than stock, so they will run cooler than normal.




[If you need an anti-spark connector that carries more power than 90A/3000W, I have read of good results using AS150’s (Anti-Spark, 150A). XT90-S’s can fully seat 12/10 gauge wire, and AS150’s can accept 9/8 gauge. A 9ga wire has double the copper mass cross-section of 12ga]

Now at this stage, you should be able to plug-in and bench-test your new kit, and that way you can sort out any issues before you finish going through the full installation. Speaking of the kit installation, this is a 3-part series for the BBSHD, but…part-one was written so that it could be used as a stand-alone tutorial for removing the bottom bracket cartridge, regardless of the mid-drive model you like. Here, in part-two…I wrote it so that it can also be used as a stand-alone tutorial for mating connectors between any common controller and battery.

Part-three is be everything else you need for getting your BBSHD running, and you can read that article here


Written by Ron/spinningmagnets, April 2016

Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas

  • Ozzie

    Great information. Thanks so much for putting it down so we can all leverage your knowledge. When do you think you’ll have ‘Part 3’ done? I’ve already started my build, but I may put it on hold until I read part 3 (just so I don’t screw-up anything).

    • spinningmagnets

      Give me a week and a half. I’ll likely be done sometime before Friday night May 6th due to my work schedule…