centrex wrote:HI Greg
I have been following yours and Peter Perkins information on BMS systems.
I have been doing a fair bit of hunting on the web about the subject but have not been able to find information on the requirements for balancing the cells can you point me in the right direction for information.
I do not have an electric anything I would just like to know the requirements.
If all cells could be guaranteed to have exactly the same manufacturing tolerances and are mounted such that they are all kept at the same temperature then balancing would be unnecessary. But this is the real world. Balancing methods depends on the cell chemistry, but I will assume we are talking lithium. I am by no means an expert, I have only been using lithium batteries for a few months.
With lead acid balancing can be done simply by overcharging the entire pack until all cells are fully charged and then topping up the water. This is not possible with Lithium cells (or indeed sealed lead acid batteries) so a alternative way has to be found. The simplest method is to monitor each individual cell and when it is fully charged, shunt the current around it so that the rest of the cells can also reach full charge. Cedric Lynch uses this method with his cell protectors and is well documented on his Agni motor site. http://agnimotors.com/home/index.php?option=com_content&task=view&id=27&Itemid=39
. The shunted current is wasted as heat, but only on the order of around 1 watt per cell. Cedric's cell protectors were designed for Thundersky LCP cells, but the LFP type cells that we are using have a lower voltage requirement, which is why Peter started his BMS project.
This balancing method works very well on lithium chemistries because of the way they behave when they approach a full charge. During most of the charge the cells stay at around 3.2-3.3 volts each. As the cells reach their full charge their voltage starts to climb very rapidly, particularly when they reach around 3.5 volts. This makes it very easy to identify which cells need to be bypassed. Other possible methods of balancing could involve actively transferring current from fully charged cells to less charged cells, but whether they would be that much more efficient to justify their cost will be down to the end users requirements. For the self builder the first method works well.
So balancing just makes sure that all your cells stay at the same level of charge, but that is by no means all that is required for the long life of any battery pack. Lithium cells will quickly be damaged if they are over or undercharged. So you also need a system to monitor each individual cell voltage to insure you keep the cells within their permitted range. It is no good monitoring pack voltage, you have to do this on a individual cell level, although as a failsafe I have programmed my controller with pack upper and lower limits. The charger will also shut off when the pack reaches its required voltage, but these are fail-safes only. Cedric's and Peter's systems both directly start throttling back the charger so it doesn't overcharge any cell. What you end up with is a battery management system and because it has to monitor individual cells the cost of the system goes up the higher your pack voltage is. You probably would want to add more functions to your BMS system such as temperature, SOC and other things as well.
Lastly, sign up to the Yahoo Thundersky group, lots of good advice and recent discussions in the last week about balancing issues.