Battery Management System

[Tbnrider]

I have a 36 Volt 12AH battery with a built-in BMS.
I assume the BMS performs two main functions; cuts off battery supply when overall voltage is low, and balances the 12 cells in the battery. 
Does it have any other functions?

I would like to know more about the balancing function. For balancing cells, the cell balancers I have some experience with operate by measuring the voltage of each cell, selecting the lowest voltage cell, and imposing a load on each cell with a voltage higher than that until all the cells fall in voltage down to the level of the lowest cell.  This can be done while the cells are being charged or at any time after.

Is this the way the BMS balancer works?  Or does it somehow manage the charging current of each cell during charge, or divert some charge from one cell to another when there is no charging taking place? And does the balancing function work only while the cells are being charged, or is it operative at all times?  (I don't see any way of shutting off the BMS circuit after a charger has been disconnected!)

Is there a circuit diagram available for this BMS?

Roy

[Bikemad]

Hi Roy,

The BMS monitors each of the cells individually and ensures that none of the cells are overcharged too high, or discharged too low.
It also protects the cells from being overloaded by excessive current demand by continually monitoring the current being drawn. If the maximum safe current setting is exceeded, the BMS automatically disconnects the load from the battery to prevent cell damage. Some more expensive BMS units will regulate the current load on the battery instead of completely disconnecting the load.

When being discharged, the BMS will also disconnect the power being delivered when the voltage of any individual cell (usually the weakest cell) falls below a preset safe level.

As far as I am aware, most basic BMS units will only balance the cells when they reach their maximum voltage (this is known as Top Balancing) and therefore will only balance the cells at the end of a full charge.

During the first Constant Current stage of the charging process, the BMS allows the maximum charger output current to flow into the battery until the voltage of any one cell starts to exceed the maximum preset level. (In my experience, this will also be the weakest cell.)

As soon as the maximum cell charging voltage is reached, the balancing of the cells begins, and the BMS automatically switches in a resistive load across that individual cell to bleed off excess voltage to prevent the cell from exceeding the preset maximum safe voltage to allow the other lower voltage cells to catch up. If the cell voltage continues to rise, the BMS will automatically reduce/disconnect the supply from the charger until the resistor drains the cell voltage back down to a safe level to ensure the cell voltage does not exceed the preset maximum safe charging voltage.

The current continues to flow at a reduced rate (or is intermittently switched) into all of the cells until (one by one) they all eventually reach the maximum preset cell charging voltage.
As each of the cells reach their peak charging voltage, the BMS switches in a resistive load on that particular cell (just like it did with the first highest cell) and this process continues until all of the cells are fully charged up the same maximum cell voltage.

At this point, the charger will have reached its maximum cut off voltage, and it will then switch over to the Constant Voltage phase. Eventually, the current will drop below a certain level, causing the charger to cut the power completely.
Some chargers may continue to supply a very low trickle current, just enough to maintain the preset maximum battery voltage.

So, I would say that most basic BMS unit have at least four functions:-

• They constantly monitor the voltage of all cells to prevent any cell from being overcharged.
• They constantly monitor the voltage of all cells to prevent any cell from being over-discharged.
• They constantly monitor the current passing through all of the cells to prevent them from being overloaded.
• They balance all of the cells in the pack to approximately the same voltage at the end of each charging cycle.

More advanced BMS unit will have a lot more functions in addition to the four listed above, but I have tried to focus on the more basic type of BMS like the ones used on the majority of eBike batteries.

Unfortunately GM have not issued any circuit diagrams for the BMS boards used in their batteries.

Alan
 

[Tbnrider]

Hi Alan

Thank you so much for your excellent description of the functions of a BMS.  It made the operation of my BMS much clearer in my mind.

I added a wiring harness to my 12 cell 12AH LiFe battery to allow me to measure the voltage of each cell from a connector outside the case.  So far I have observed that my battery is not completely balanced at the end of a charge.  Most of the cells measure about 3.3 or 3.4 volts, but two measure a bit higher at about 3.7 or 3.8 volts and it is always the same two. Adding a separate balancer to the connector brings all the cells down to about 3.3v.  (At full charge, overall battery voltages vary a bit between 40 and 43 Volts.)  I haven't had a chance yet to discharge the battery to low voltage cutoff and test the balance.

For other types of LiFe batteries I use in my model aircraft hobby, the charger has settings for current and voltage limiting.  Current is limited to the set maximum for the early part of a full charge, then as the battery voltage increases and current drops, the charging voltage is limited to 3.6 Volts per cell. Eventually the charge current drops to near zero and the charger shuts off.

My only remaining questions about the BMS are; 1)  what is considered to be the high voltage threshold of these individual LiFe cells before they are considered to be overcharging, and 2) what conditions will cause the charger to shut off?  (You have already stated that the overall voltage of the battery will do it, but will any other conditions trigger the charger off? One or more cells moving into an overcharge situation?
One or more cells dropping in voltage that will endanger the cell?, failure to reach a minimum overall voltage due to a dead cell? timeout? etc.)

Thanks again for all this very useful information

Roy

[Bikemad]

1) Most of the modern LiFePO₄ batteries are charged to 3.6~3.65V per cell, although LiFePO₄ cells are a lot more tolerant of higher charging voltages than LiMn and LiPo cells.
If you take a look at this article on Powerstream.com you can see from the table that the four test cells have been tested with charge voltages ranging from 3.0V right up to 4.2V.

I suspect your 36V charger will probably have a carefully regulated maximum output voltage of 43.2V or 43.8V (either 3.6V or 3.65V per cell), but if the majority of your cells are only being charged to 3.3~3.4V, your available battery capacity is likely to be greatly reduced.

Have you tried charging your pack with the external balancer still attached to see if the lower cells increase in voltage?

2) Some chargers will have some form of overload protection to protect against a dead short on the charger output leads, but in general they are not "smart" chargers, and will only deliver either maximum current or maximum voltage, according to the load placed upon them.

I'm not sure whether the BMS will prevent charging current from flowing if you have a dead cell with a very low voltage, but it should definitely prevent any further energy from leaving the battery.

As far as I am aware, there is no timeout function on most of the typical "Constant Current/Constant Voltage" chargers as they have no way of knowing what capacity battery they are connected to, or how long it should take to complete a full charge.
Some modern battery chargers are designed to cut-out permanently at the end of the charge to comply with Energy Efficiency Regulations, unfortunately this won't allow an out of balance battery to fully balance itself. 

Incidentally, I recently received a new Zippy 5800mAh 8S LiPo pack from HobbyKing and two of the cells were noticeably lower than the others:

Cell 1:	3.698V
Cell 2:	3.857V
Cell 3:	3.855V
Cell 4:	3.774V
Cell 5:	3.850V
Cell 6:	3.846V
Cell 7:	3.852V
Cell 8:	3.838V

It took over 5 hours on my iCharger to get them nice and evenly balanced at a storage voltage of ~3.85V per cell:

Cell 1:	3.850V
Cell 2:	3.852V
Cell 3:	3.850V
Cell 4:	3.848V
Cell 5:	3.848V
Cell 6:	3.847V
Cell 7:	3.849V
Cell 8:	3.848V

Initially, there was a difference of 0.159V between the highest and lowest cell, but after balancing, there was only a difference of 0.005V between the highest and lowest cells:



You might want to check out some more interesting information on Battery Management Systems.

Alan
 

[Tbnrider]

Hi Alan

Thanks again for all the additional information. I will look up the references.

No I haven't tried charging the battery with an external balancer attached.  The problem is that my balancer will handle only 6 cells at a time, so to completely balance the pack, I have to balance in three setups to get everything balanced to the same voltage level.
 I have a similar problem with my external balancing charger (HobbyKing G.T.Power 10A/200W).  It will charge only 6 cells at a time.

However I will try a charge with the external charger on each half of the battery and see if that makes a difference with the individual cell voltages.

With my LiFe packs for model aircraft, I usually find that even if the cell voltages start out at any level higher than 3.3 volts per cell, under load they quickly drop to 3.3volts and stay at that level for most of the discharge cycle.

Roy