Battery planning
Designing the battery for safety, performance, and to work with a BMS.
Plan to install a safety disconnect.
A safety disconnect:
- Will give the EV driver an emergency way of shutting down the EV in case of trouble
- Will give Emergency Personnel, who are extricating a driver from an EV, a way of ensuring their own safety
- Once unplugged, will partially protect you when working on the battery pack
- Once unplugged, will reduce the chance for damage to the battery while you are working on the battery pack
- Once unplugged, will prevent cell boards from completing the circuit and blowing up
Ideally, a safety disconnect must be mid-pack (splitting the pack in 2), as that it where it does the best job at reducing the danger.
Ideally, it should be easy for the driver to open the safety disconnect while in the seat.
The safety disconnect can be a "Big Red Button" that is pushed to disconnect the pack.
Or, it could be an "Anderson" plug with a loop of wire, which completes the battery connection.
Or, it could be the simply a set of "Anderson" connectors, one on the battery and one on the cable to the rest of the vehicle.
Note: a safety disconnect may not be placed within a bank. End a bank before the safety disconnect, and start the next bank after the safety disconnect.
Plan to install a main fuse.
A main fuse will blow in case of a short across the load, or excessive load current.
If installed mid-pack, a main fuse will have the best chance to blow in case of two separate shorts to chassis ground
Regardless, the main fuse must be placed within the battery (not at the far end of a battery cable);
all of the battery current (not just the load current) must go through the main fuse.
The main fuse must be rated for the maximum load current, and for the maximum current that the cables can carry, whichever is smaller.
The main fuse must be rated for full pack voltage; for example, a standard 32 V fuse will not work for a 100 V pack: should it blow, it will arc and not cut the circuit at all, and may actually start a fire.
The main fuse must be rated for DC operation. A fuse that is only rated for AC will not clear the arc after fusing, and it will not cut the circuit at all, and may actually start a fire.
Note: a fuse may not be placed within a bank. End a bank before the fuse, and start the next bank after the fuse.
Any additional set of high voltage loads connected to the battery must its own fuse, rated for the total current of those loads, and for the current handling of the wire, whichever is smaller.
For example, a system with a load that can draw 300 A, and a DC-DC converter that can draw 2 A, should have a main fuse for 350 A that handles the entire pack currrent, plus a fuse for 3 A just for the DC-DC converter.
Typically, a battery consists of a single series string.
In some cases, however, cells need to be paralleled to get the desired battery capacity.
The right way to do that is to parallel cells directly to each other (in what's called a "lattice network").
(If just two cells are paralleled, the term "buddy pairs" is used.)
Paralleling cells increases the battery capacity.
Paralleling cells requires the same number of cell boards as a single series string of the same voltage would require.
Perhaps surprisingly, paralleling cells also increases reliability.
Roughly speaking, that is because is a cell is weak, its buddies will prop it up.
The net effect is that, in the presence of weak cells, a battery with cells directly in parallel with each other battery will be significantly less affected than if it were made up of a single string in series, or, worse, multiple string in parallel.
For an in depth explanation, see section 6.1.1.1.1 of the Li-Ion Book
DO NOT use series strings that are then connected in parallel: that causes a variety of problems, including:
- Reduced reliability
- Increased number of cell boards required
- A single weak cell will shut down the entire pack immediately
Again, for an in depth explanation, see section 6.1.1.1.1 of the Li-Ion Book
We have removed support for parallel strings in the Lithiumate Lite BMS on purpose, because of how badly parallel strings behave.
The Elithion Lithiumate™ BMS can handle battery packs with up to about 160 cells in series.
For technical reasons, and for increased reliability, the BMS views the battery pack as composed of a number of groups, called "banks".
(This does not mean that the pack itself is physically divided in sections: this is only regards the way the BMS sees the pack.)
A Bank is a set of cells wired in series that communicates with the controller through its own communication cable.
So, if the pack is divided into 3 banks, there are 3 cables between the BMS controller and each one of those banks.
This page will guide you in the process of dividing the pack into multiple banks.
|
While it is convenient to divide a pack into banks of equal number of cells in series, that is not necessary, and indeed is not always possible
For example, a pack with 48 cells in series may be easily divided into 4 banks of 12 cells each.
However, if the pack is physically divided into 2 batteries, one with 29 cells in series and the other with 19 cells in series, then it could be divided in banks of 14, 15, 9 and 10 cells respectively.
The rule of thumb is that a bank should have between 8 and 20 cells in series.
|
 
48-cell single pack
divided into 4 equal banks of 12 (top)
and into 4 unequal banks (bottom)
|
As far as the BMS is concerned, each pack is divided into a number of banks:
- There is at least 1 bank, but more that 1 are recommended
- There can be up to 8 banks
- Banks are numbered from A to H:
- The bank number is not programmed in a cell board: all cell boards are completely identical
- The number of a bank is set by where it is connected on the BMS controller
- Bank A is the bank connected to connector A on the BMS controller, and so forth
- Swapping two banks' connectors on the BMS controller will swap their number
- Any bank may or may not be used, banks do not need to be contiguous:
- If groups of cells are physically separated, a bank cannot be across that physical separation: use separate banks in each group of cells
- If groups of cells includes a fuse, a connector or a safety disconnect, a bank cannot be across that separation: use two separate banks, one on each side of that separation
- Fewer banks improve the reliability and the layout by reducing the number of cables
- Fewer banks reduce the material costs and the installation labor
- In the end, the number of banks is usually set by the rule of thumb on the number of cell boards per bank, below
Each bank handles a number of cell boards in series:
- Cell boards are numbered from #0 and up
- The cell number is not programmed in a cell board: all cell boards are completely identical
- The number of a cell is set by where its bank is connected on the BMS controller and by the position of the cell within that bank
- The lowest numbered cell (#0) is the most positive cell in bank A, or, if A is not used, in the bank whose letter is closest to A
- The next cell (#1) is the second most positive cell in that bank
- Etc.
- However, the GUI application allows you to renumber the cells starting from #1 instead of #0, and to renumber them so that the lowest cell is the most negative instead
- Each bank should have no more than 16 cell boards, or at most 25 (31 is the limit set by the software)
- While different banks may have different number of cell boards, it is convenient to have all the banks at similar or even equal number of cell boards in series:
- The time it takes to communicate to all the cell boards is proportional to the number of cell boards in the bank with the most cell boards in series; therefore, spreading the cell boards evenly results in faster communications
- Cell's position within a battery:
- Technically, there is no need to correlate a cell's electrical position within a battery and its position within the banks.
However, to aid layout and troubleshooting, it is best if you number the cells in the same order as their voltage.
|
This table shows an example of a battery pack with 16 cell boards in series.
|
| Bank #
| Number of cells
in series
in each bank
| Range
of cell numbers
of the cells
in each bank
(most positive
to most negative)
|
| A
| 4
| #1 to #4
|
| B
| 5
| #5 to #9
|
| C
| 7
| #10 to #16
|
|
|
This table shows examples of how many banks may be used, and how many cells might be used per bank.
|
| Bank #
| Number of cells in series
|
| 10
| 20
| 30
| 40
| 50
| 75
| 200
|
| A
| 5
| 7
| 10
| -
| 15
| 15
| 25
|
| B
| -
| 6
| -
| -
| 14
| 15
| 25
|
| C
| 5
| 7
| 10
| 8
| 11
| 15
| 25
|
| D
| -
| -
| -
| 8
| 10
| 15
| 25
|
| E
| -
| -
| 10
| 8
| -
| 15
| 25
|
| F
| -
| -
| -
| 8
| -
| -
| 25
|
| G
| -
| -
| -
| 8
| -
| -
| 25
|
| H
| -
| -
| -
| -
| -
| -
| 25
|
|
