Cell pre-balancing

Balance cells before building them into a battery

If a battery is built up with cells that start at various SOC levels, the BMS will struggle for a long time doing gross balancing (see the How much balance current white paper for a discussion about gross balancing versus maintenance balancing).

The solution is to pre-balance the cells before using them to build the battery.

To ensure that all the cells are are the same SOC, we force all the cells to have exactly the same OCV (Open Circuit Voltage), because OCV is directly related to SOC.

Any solution that requires an electronic measurement to match the OCV of the cells will suffer from the limited resolution of the measurement, by the fact that in Li-Ion cells, at mid SOC levels, OCV doesn't change much with SOC, and by the possibility that the terminal voltage may have not yet settled to the OCV when it is measured.

Instead, here we instruct you to allow just plain physics to balance cells, with no intervening electronics, to eliminate measurement as a source of error: the physics consist of: 1) direct connection between cells, and 2) waiting for some time for the naturally occurring current to settle.

If you connect all the cells directly in parallel with each other, you will allow their voltages to equalize. Their terminal voltages will be identical (because they are connected in parallel), though, initially their OCVs will be different (because initially their SOC levels are different), which will force charge out of the cells with higher SOC, through the common connection, and into the cell with lower SOC.
Initially, the current will be high (10s or 100s of Amps), but will soon decay exponentially; the current removes charge from the most charged cells, and places it in the least charged ones. Eventually (in a matter of hours to a few days), the SOC of all the cells will be equal, therefore all the OCVs will be equal (and equal to the terminal voltage), and the current will drops to practically zero.
At that point, the cells are balanced.

There are 2 approaches to doing this:

• Mid-balance (passive) pre-balance: simply connect all the cells directly in parallel: in the end, all the cells will be mid-balanced (meaning that they all are at the same SOC, and that SOC is on the order of 50 %)
• Top-balance (active), pre-charge: same as above, but also connect the cells to a charger, set for the maximum cell voltage: in the end, all the cells will be top-balanced (meaning that they all are at 100 % SOC)

Note that cells that are mid balanced will not be balanced also at the top; that is because of variances in cell capacity. So, a pack that is built with mid-balanced cells, will still have to be balanced, again, at the top, after the first charge (that's because the typical BMS does top balancing). This top balance will not take as long as if the cells were not balanced at all, but will still take some time.

The mid-balance approach is simpler. But, again, the BMS will still have to top balance the cells.
Also, when building a battery, you can't mix different batches of cells, because each batch has its own SOC: using cells from different batches will result in a battery that still needs gross balancing.

The top-balance (pre-charge) approach is more complicated, but you'll end up with a battery that is already top balanced, so it will be ready to use immediately.
Also, you can mix cells that were balanced in different batches, because they are all at 100 % SOC.
The top-balance approach is also more effective, because the cell OCV changes more rapidly at 100 % SOC than it does at mid SOC levels. If you stop balancing when the cell OCV is within 10 mV, with top balancing the SOC levels will be within less than 1 %, but with mid balancing the SOC levels may still be unbalanced by 10 %.

Here are some practical ways of connecting cells in parallel in a production environment.

In these examples, we assume the active (top-balance, pre-charge) approach, because it's better.

Build a balancing box

• Build a 5-sided box, of very stiff, non-conductive material, as tall as the cells are tall (2.5 inches, 65 mm) and big enough to hold a large quantity of cells
• Place a conductive surface on its bottom (such as a tinned copper sheet)
• Build a cover for the box, of very stiff, non-conductive material, that has a resilient conductive surface, (such as a thin tinned copper sheet with foam rubber behind it)
• Add fasteners to hold the cover down onto the box
• Add connections to the conductive plate at the bottom of the box, and to the conductive plate on the cover
• Connect those connections to a CCCV power supply capable to deliver at least 10 A (but more like 100 A), voltage limited to 3.6 V (for LiFePo4), 4.0 V (for Thundersky cells) or 4.2 V (for LiPo and Cobalt cells).
• Mount the box on a 45 degree tilt

Use the balancing box during production

• As you receive a shipment of cells, place them in the box, all in the same direction (to match the CCCV power supply's direction)
• Once the box is full, close the cover, so that all the cells are connected in parallel
• The cover will complete the parallel connection between all the cells, with the resilience of the conductive sheet ensuring that all the connections are made reliably
• Wait for a preset time, or, better, until the charger's current drops to 0 Amps
• Remove the cover, and pull the cells out, as they are now ready to be built into a battery

Build a balancing network

• Build a 2-wire network with cables and ring terminals for the prismatic cells
• Connect that network to a CCCV power supply capable to deliver at least 10 A (but more like 100 A), voltage limited to 3.6 V (for LiFePo4), 4.0 V (for Thundersky cells) or 4.2 V (for LiPo and Cobalt cells).

Use the balancing network during production

• As you receive a shipment of cells, place them next to each other, flat against each other, with their polarity all in the same direction
• Use the 2-wire network to connect all the cells in parallel, being careful of polarity
• Wait for a preset time, or, better, until the charger's current drops to 0 Amps
• Disconnect the cells from the network, as they are now ready to be built into a battery