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Installing the Dilithium Battery Management System in your Electric Vehicle Conversion

Ben Parker

Here's the front view of my battery box before I painted it with both modules in it, before wiring the BMS connections.

Article Highlights

I’ve been frantically working to get the batteries that I’ve had sitting around for a year charged and balanced. I’m using a Dilithium BMS Controller and Satellite as well as a Thunderstruck EVCC charge controller with a TSM2500 3.3kw charger – probably (that’s some foreshadowing). Over the past few days I’ve been wiring up all the BMS.

I’ll do my best to describe the process but there is a lot of documentation out there as well as a really helpful tutorial from the guys at Thunderstruck you can check out here. I’ll mostly focus on the key insights I picked up on the way and the things you really should/ should not do as you’re going about your wiring. 

But first, what’s the point of balancing your battery?

“The purpose of balancing a battery pack is to maximize the usable capacity. Even in an ideal battery pack, all cells will have slightly different capacities and will be at slightly different balance points. The total usable capacity of the battery pack is limited to the lowest capacity cell, less the difference in balance from the strongest to weakest cell.”

- Orion BMS manual 

When you’re using any battery that is composed of multiple cells you want to limit the differences between those cells which occur naturally throughout the life of the batteries. Differences in voltage between cells can not only reduce the overall life of the pack but also lead to fatal consequences when charging the batteries. Let’s give an example, say I have a pack that has 2 lithium cells, cell A has a charge of 3.2V and cell B has a charge of 4.0V, if I plug this into my 8V charger without a balancer the charger could try and get cell A up to 4.2V (the max charge for lithium cells) and in doing so it would overcharge cell B. Overcharging a lithium cell is what can cause it to explode, which you may have already seen happen in a notorious Rich Rebuilds video. About 80% of EV fires/catastrophes occur when charging, needless to say this may be the most important part of your conversion. 

So what does a BMS actually do to prevent that? A BMS is actually fairly simple, it reads the voltages of all the cells in your pack, it notes the differences between them and it discharges overcharged cells. More importantly than that, a good BMS is able to shut off the charger being used to charge the batteries, this could be due to a cell being overcharged or the battery overheating.

This is the whole system wired up including the BMS Satellite and the Thunderstruck EVCC

Here's another picture of the BMS pigtails in the modules, one thing to note is this was without the ground lead wired in.

The Dilithium BMSC (Battery Management System Controller) and BMSS (Battery Management System Satellite) can each measure the voltage from 24 cells (48 in total between the two of them). Each Tesla module has 6 cells wired in series, so with five Tesla modules there are 30 cells to measure. So you’ll need both a BMSC and BMSS to measure the whole pack. Tesla very conveniently has cell tap boards in the front of each module, although their cell tap boards are proprietary and can be swapped with a much simpler cell tap replacement board from Stealth EV or Zero EV in the UK. The cell tap boards have 12 positions (or inputs) but you’ll only be using at most 9 of those, namely the cell taps for cells 1-6, two thermistors, and ground. 

When you buy the replacement cell tap boards they use a 12 pin molex connector, you can buy extras here - I’ve found it useful to grab a few extra in case you need to create test harnesses, or individual module harnesses. They use open barrel crimp pins to connect to the female plug on the cell tap board. The pins lock in there really well so you’ll want to make sure that you’re placing the right wire in the right spot. As meticulous as I am – I’m not the most meticulous – I misplaced a wire/pin a few times which is why you might also want to pick up one of these which allows you to remove the pins once they’ve been set in the molex plug. I promise I’m not trying to convince you to spend more money willy nilly, these are just the things that I wish that I had done at the beginning that I inevitably had to do in the end.

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Here is the Tesla cell tap board pinout for Tesla Model S batteries the cell tap replacement boards use a Molex 0039012125 connector.

The wiring is fairly simple, but you’ve got to make sure you do it correctly. I’ve included a few diagrams of how the system is wired below

Here is a drawing of how a single LTC from the Dilithium BMS wires up to two Tesla Modules wired in series. You'll not the ground lead is attached to the negative most battery in the series group.

Here's the entire system drawn up for a 120V system the white lines from the BMS side represent the entire 12pos BMS leads.

If you’re using the Dilithium BMS with Tesla modules like I am, one BMS pigtail from the BMSC or BMSS will monitor two modules. I’ve added a schematic here below of how the wiring works. Two modules are connected in series and a single LTC monitors both modules. It should be noted that an LTC shouldn’t span across a fuse, you can read more specifically about connections in the Dilithium BMS manual. An apparently common mistake I learned from chatting with the guys at Thunderstruck is when wiring two Tesla modules in series, removing the series connection while having the BMS connected – not even on – can cause an internal short that will fry your BMS. 

Never change or remove series connections while the BMS is still plugged in. 

Stealth EV recommends you use this particular crimping tool: Molex Mini-fit Jr. crimper part# 63819-0901. They also share some alternatives, like the Astro Pneumatic 9477 which is a ratcheting crimp tool. I purchased the Astro Pneumatic crimping tool but found that for the gauge of wire used by the Dilithium BMS it was a bit too extreme and would end up over-crimping the wires. I ended up using this crimping tool instead which is more manual but you have a bit more control over the crimping, it’s also a third the cost of the Astro Pneumatic. 

Because I was passing through the wall of a battery box I decided I’d add some Deutsch connectors outside of the battery box for the BMS so that they’d be weather sealed and easy to connect and disconnect. Here’s where I made my mistake. I wired up the BMS leads for two modules (one box) and had them plugged into the batteries as I was checking fitment for all the HV and LV wiring at the front of the box and creating the pass-through where all the BMS wires would come through. Without unplugging the cell taps from the module I started crimping the Deutsch open barrel pins onto the cell tap wires, two of them touched and shorted then fried the BMS board. I promise, it’s not a fun thing to see smoke coming out of the front of one of your Tesla batteries. Luckily it was only about 12V and didn’t do any severe damage, only just fried the cell tap board. 

Always unplug your cell tap connections before crimping, moving, tapping or rearranging any of your wiring. 

Always verify you have wired things correctly using the test harness provided with the BMS before plugging in any harnesses into the BMS. The BMS can be destroyed without even turning it on, but just by improper cell-tap wiring alone.

Powering the Dilithium Battery Management System

The power for the Dilithium BMS is 12V that can be supplied a few different ways. If you’re using it with the Thunderstruck EVCC the best method is to power the BMS using the switched 12V power output from the EVCC. I’ll be writing a more comprehensive post about wiring and using the EVCC that will include that schematic. For testing purposed I just hooked up the BMS to my variable bench power supply. I highly, highly recommend getting a variable power supply if you don’t already have one. It comes massively in handy when developing your 12V circuit, testing components, etc. this is the one that I use.

Configuring the Dilithium Battery Management System

The Dilithium BMS is configured through a USB serial connection via computer. I’m not gonna re-write all the great detail that’s in the user manual for how to configure your BMS, but I will go through some of the specifics of setting it up for my application. Firstly it was a bit hard to figure out how to install the drivers needed to run the USB to serial connection, here’s the manual for setting up CoolTerm on a Mac

Once you’ve got CoolTerm installed and you’ve verified your cell tap wiring, you can plug the harnesses into their appropriate plugs on the BMS and BMSC. Make sure the data link IMO IMI, IPO IPI wires are connected between the BMSC and BMSS. Then plug the serial port from the BMS into your computer and provide 12V. If you set your CoolTerm settings right and you hit “connect”  it should print out a little message saying the BMS is connected. 

For the Tesla Model S modules I’m using there are a few important parameters that I’m setting. Here are my settings.

There are a lot of configurable controls you can set with the BMS and all are useful, some are crucial. HVC is the high voltage cut-off, this value is the voltage you set as a safeguard to protect your batteries from being overcharged. When using the BMS with a CAN enabled charger (which you definitely should do) the BMS sends a signal to the charger to terminate charging when any of the cells reach the HVC value. Maximum capacity for a 5.3kwh Tesla module cell is 4.2V I’ve got mine set to 4.1 for now just to be extra cautious. 

The LVC is the low voltage cut-off, it’s essentially the minimum voltage a cell should have to be considered healthy. For these modules I’m setting that to 3.2V, although I believe the actual floor is 3V. 

Another useful parameter is BVC which you set to taper off the charging current as another means of protection against overcharging. So you want to set this value under your HVC so that as your cells approach their final charge voltage the charger will reduce the amps going into the battery.

Battery balancing with the Dilithium Battery Management System

As I mentioned in the beginning there are so many reasons why you want your battery pack balanced. The Dilithium Battery Management system gives you the ability to slowly balance differences between cells by discharging cells that are higher voltage than the rest in the pack. The Dilithium BMS can balance cells using the internal shunting resistor. It can only be used to balance thousandths or hundredths of differences between cells. Before using your salvaged Tesla modules in your conversion you’ll want to balance them and a Dilithium BMS isn’t beefy enough to balance major voltage differences. In another article I’ll be writing about how I balanced one module to be within one thousandth of a difference between the other modules in the pack. 

Operating the Dilithium Battery Management System

In standard operation the BMS will be turned on when the ignition key is switched on and also when the ignition key is switched off with the charger plugged in. 

I’m using the Dilithium BMS in tandem with the EVCC Standard and the Thunderstruck Motors 2500 Charger (TSM2500). It’s set up to charge via J1772. Thunderstruck provides a schematic for how this should be wired and I also will be writing up a bit more about setting that up in another post.

Conclusion

Don’t be scared by the amount of wires it takes to set all of this up, it’s actually fairly straightforward. I’d just recommend looking over the Thunderstruck schematic 30x until it’s really clear in your head where things go and focus on one portion at a time, for instance, start with just getting the cell tap wiring correct, then move on to configuration, then finally operation. Also plan for mistakes, build-in redundancies, and if you can use insulated gloves, or insulated tools. Even 24V can be dangerous to mess with. Stealth EV put together a great comprehensive video on BMS wiring and operation that I’d highly recommend to anyone looking to build an electric vehicle conversion, solar power bank, or anything of the like.