Plan - Charger

Selecting and setting a charger to work with an Elithion Lithiumate™ BMS

Charger selection

When using a charger to operate in conjunction with a BMS, the criteria for selecting a charger and setting it up are different than doing so without a BMS; as a matter of fact, it is easier.

Many chargers come with a profile for a particular type of battery. One would think that a charger specifically designed for a Li-Ion battery would be ideal.

Maybe.

The reality is that the BMS is more able to know when to charge the battery (because it knows the voltage of each cell) than the charger (which only knows the total voltage). If both the charger and the BMS compete on controlling the charging process, neither wins:

• If the BMS wins, then the extra cost for a fancy charger with a Li-Ion profile will have been wasted
• If the charger wins, then the battery may be undercharged
• If it's a draw, neither the BMS nor the charger will be able to handle the battery as it wishes to

There are 2 approaches to selecting and setting a charger:

• Use a dumb charger (or a smart charger, but defeat its smarts by setting the voltage higher than the maximum pack voltage)
• Use a CCCV (Constant Current / Constant Voltage) charger (or a smart charger programmed to act as a simple CCCV charger)

(Note that, in either case, the smarts of a smart charger are defeated.)

Each approach has its advantages

• Dumb charger:
  • Balancing will be more accurate, as the charger will be mostly off at the end of charge, leaving long periods without battery current when the cell voltage (and therefore the SOC) can be measured accurately
• CCCV charger:
  • Should the BMS be unable to turn off the charger, the Constant Voltage of the charger may offer a secondary protection, though not a terribly reliable one: see CCCV chargers: a false sense of security

Our recommendation is #1: save your money, and get a dumb CCCV charger, and let the BMS do the thinking. Set the Constant Voltage a bit higher than the nominal full charged voltage of the pack (for example. 3.7 V for standard LiFEPO4 cells, 4.3 V for LiPo cells). That way, the charger will not be the limiting factor to a full charge as cell resistance increases with aging. The BMS will still keep individual cell voltages from going too high. And, when the pack is full and balanced, the charging current will still be high enough to end up with short on periods and long off periods for balancing. Yet, should the BMS not be able to turn off the charger, there will be a little bit of protection.

Charging stages

DO NOT DEVISE YOUR OWN CHARGING STRATEGY!
Do not try to be clever and use the SOC or pack voltage or cell voltages to determine when and how to charge.

THE BMS MUST BE ABLE TO TURN ON AND OFF THE CHARGER DIRECTLY through its HLIM output.
Else, you will not charge and balance the pack, and you risk overcharging.
The BMS knows how to charge and balance your pack. Let it do its work. Do not get it its way.

Fully charging a Li-Ion battery pack with a CCCV (Constant Current / Constant Voltage) charger and a BMS requires three stages

1. Full charge: charger is fully on (full current: CC), until a cell reaches its maximum voltage
2. Balance: charger goes off and on (full current: CC), while BMS balances the cells, until all cells reach 100 % SOC
3. Top off: charger stays on (full voltage: CV), while the current is reduced exponentially down to 0

The Constant Voltage of the charger must be set to CV = number of cells in series * max cell voltage.

Three stages of charging: Full, Balance and Top-off.

For more details, please see FIGURE 6.26 and section 6.2.3 of the book Battery Management Systems for Large Lithium-Ion Battery Packs .

Charger control

You can allow the BMS to control the charger in various ways:

• Through data: use the HLIM bit in the standard CAN messages to turn on and off the charger
• Through a dedicated wire: see the charger installation instructions
• Both, for redundancy.

If the charger is capable of current control, you may also set up a way for the BMS to tell the charger the maximum allowed current, either through the CCL line or through the CCL data in the standard CAN messages.
That is not absolutely necessary, though.