What's wrong with having an extra battery in the trunk?

There are many workshops in eastern countries that offer installation of an additional battery in the trunk. In Western Europe there is significantly less, but they also do this.

There are three different ways to connect the external battery:

• Connection in parallel to the main battery + separate BMS for additional battery
• Connection in parallel to battery cells. This requires a lot of additional wires (96, if I’m not mistaken), but the native Leaf BMS is used.
• Connection in parallel to the main battery, but without any BMS. One car owner with this type of external battery connection said that after a year of use everything is fine and there is no considerable difference between the battery cells... but personally it seems to me that this is just a good way to burn your car! If the first two options still have the right to life, but the option without BSM is in the category of “never do that.”

These are three typical options, and many people are satisfied having one of the solutions. But I don't like them at all. Here's why.

1. You need to remove the main battery

The least of the problems is that you have to run the cable out of the main battery, making a hole in the battery enclosure and connecting it to the terminals behind the main battery contactors (connection place is inside the battery). And to do this, the battery must be removed. You can’t do this in a home garage: the battery weighs about 300 kg. But this isn't the worst problem because... Typically, owners take their Nissan Leaf to a workshop, and within a week they receive a finished car with an extra battery in the trunk.

2. Irreversible changes

And the worst problem is that the changes are irreversible: if necessary, you can’t just take it all and remove it, the changes become a part of your car.

Is it possible to connect an additional battery without having to remove the main battery?

I was impressed by a 6 years old video. In this video a man connected an external battery directly to the high-voltage bus of the car. He made some kind of adapter with high-voltage plugs. Yes, looking ahead, I will say that he burned the pre-charge resistor, and his car was unable to charge anymore. But, this was only a prototype, not the final solution! And keeping in mind that it was only prototype, I suppose it is a very beautiful solution!

I wrote a separate article about what a precharge resistor is and how it can be burned, you can read it for better understanding.

As for connecting the battery to the high-voltage bus, here are 2 videos from Leaf Xpack. The first is about how he connected an additional battery. And the second is a drive test of the prototype.

And the second video - prototype test-drive

What are the advantages of the decision to connect to the Nissan Leaf high-voltage bus?

As I already mentioned, the main advantage is that everything can be reverted to its original state, as if nothing had happened. You don't need to make a hole in the HV battery. To restore the original car state you just need to remove the HV adapter.

The second advantage is modularity! Switchable batteries can be modules that can be removed and installed at any time, upgraded to a larger capacity, and have several of them. If you are stuck somewhere, and you run out of charge, just pull out the module and charge it from the outlet, or ask a friend to bring you a spare module from home.

The third advantage is that you can do it yourself. There is no need to remove the 300 kg high-volt battery. And you can make all the connections yourself (of course, if you understand what you are doing. After all, there is 400V, and unlike 220V from a 400V outlet, it is many times more deadly).

What are the disadvantages of connecting the external battery to the Nissan Leaf high-voltage bus?

Yes, there are many disadvantages:

• You can't use a standard charger. The additional battery will have to be charged separately. (There are ways to cope with it, read below)
• The precharge resistor may burn out. This happens when the machine turns off, but the external battery remains connected to the high-voltage bus. This is just an important stage in the transition from a prototype to a real product. An additional electronic is needed. That electronic device will turn off the additional battery when the car power will be turned off.
• It requires sophisticated electronics. Complexity doesn't mean that it is expensive. In my understanding, a cheap microcontroller that costs $10 is enough. The controller is connected to the CAN bus and turns the additional battery on and off. It's difficult to write firmware. Someone has to do this, after which there will be a ready-made solution - an electronic board that will manage the external battery. And this will already be a simple and reliable solution.

Which external battery can be connected?

First option

Leaf Xpack, the author of previous YouTube videos, assembled a homemade battery from 18650 cells. And he has a very interesting solution: 2 halves of 200V each, which he connects in parallel when charging, and to power the machine they are connected in series through a relay, the output will be 400V. I think it is very labor-intensive to assemble the battery with a large number of small 18650 cells, but the option is interesting, especially the series-parallel switching for charging.

Second option

You can use 96 cells with the same chemistry as the main Nissan Leaf battery. This is Nickel Manganese Cobalt (NMC). It is also called Ternary battery. You can buy new NMC cells from China. Moreover, not individual cells, but 4SP1 blocks (4 cells connected in series). You will need 32 such blocks. And they come in different sizes and different capacities. You can assemble either a small lightweight battery that can be pulled out, or you can assemble a large and heavy one that needs to be screwed to the trunk (remember about safety). You definitely need a BMS that will equalize the voltage of individual cells. You can use the original BMS from Nissan Leaf, or you can find a third-party one. But I could not find a Bluetooth BMS for such numerous cells (96 cells), you will probably need 3-4 individual BMS. If you know any Bluetooth BMS for 96 cells, please post the link to the comments!

The advantage of this method is that the battery can be charged together with the main battery. And it makes the solution very similar to the solution when external battery is connected in parallel with the main Nissan Leaf high voltage battery. It's similar,  except for the fact that the external battery can be easily dismounted, easily disconnected, and there is no need to make holes in the enclosure of the main battery.

Third option

This option is more exotic, but I also want to mention it, because... it also has its advantages. You can use a 12V, 24V, 48V...72V battery of any chemistry. At least a cheap 12V lead batteries or a 72V battery from an electric motorcycle, or make some chip solution from two CATL modules 4p1s with a total voltage of 29,6V.

Potentially, the battery can be detachable: the battery can be charged from a home solar power station and then put in the car in the morning. The point is that 400V cannot be charged from a regular solar controller. But 12V or 24V is fine. 72V is also possible if you buy a special controller).

In order to convert 12V -> 400V you need a step-up DC-DC converter + some electronics that will monitor it. And here the problem of potential equalization is automatically solved: if in the second option (400v external battery with the same chemistry connected to the HV bus) you still need a precharge resistor (Yes, an additional battery is also capable of creating huge currents, even if it has small capacity). In the version with an additional converter, this same DC-DC will monitor the output current and limit it.

There is no ready-made solution for the converter yet, but there are Chinese modules capable of making 400V from 12V at a power of 1000V (if they are not cheating), so they can be used as a first version. Bought Dc-DC modules may have to be redesigned to support higher input voltages, and to do this you will either have to resolder the transistors, or take only the transformer from the Chinese board, and do the rest on your own board.

Where to begin

• An adapter is needed to connect to a high-voltage line. You need a connector on the battery side + a plug on the car side. You can buy two cables from Ebay, cut them and make an adapter.
• You need a board that processes CAN bus commands. There is a project by Dalla (Daniel Öster) - CAN bridge, which on the fly modifies messages between the car and the battery, which allows you to install a battery taken from a new model of the Nissan Leaf (the manufacturer does not support this). The board can be bought on Aliexpress. And significant part firmware code is already there - Dalla's firmware have a code that intercepts battery messages, you just need to learn how to catch the necessary messages and send a signal to the external pin of the controller, which will control the relays / contactors.
• You need a contactor + precharging resistor. You can probably take the entire block from the same Nissan Leaf. In the version with DC-DC, I think that you can just connect it through a cheap 10A relay (please, don't connect the output of DC-DC directly to the HV battery without any relay), the currents there will not be so large (1000 W / 400 V = 2.5A). It is more difficult to find a relay specifically for 400V (220V is not recommended because there are smaller gaps between the contacts).
• You need a contactor control board. It receives a 3.3V signal from the microcontroller and applies a 12V voltage to the contactor coil.

Let's share information

I was inspired by the Dalla experience and the Leaf Xpack experience and got the idea to make such an external battery for myself. I don’t know how much time and effort it will take me. I will keep you informed. Please write the comments: suggest solutions, point out my mistakes and tell me about your successes!