# How bms works?



## bblocher (Jul 30, 2008)

From my limited understanding all the cells are connected to the BMS system. There is probably many different ways to do this but the cells are compared to each other. If they vary in voltage they are connected in parallel (directional to prevent a direct short from the series main connection) and this then pulls power from one cell which has more power and gives it to a cell that has less power.

Cells will usually become unbalanced when they are discharged or charged at higher rates. After which they will self balance, so it's not a preventative solution as it is a correcting solution.

Brian


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## madderscience (Jun 28, 2008)

A Battery Management system can be relatively simple or very sophisticated of course, and what needs to be done will vary based on application and the type of battery. 

As an example, For AGM and maintenance free lead acid (where overcharging will permanently damage the batteries) a Battery Management system will primarily try to balance out all the cells at the top of a charge cycle, since each individual cell will have slightly different capacity than all the others and so each one will reach full charge at a slightly different time than all the others. Stopping too soon (when the first battery reaches full charge) would leave all the other batteries not fully charged, and stopping after the last battery reaches full charge would overcook the others.

To do this balancing, each cell (or battery in the case of your typical EV) will have some kind of sensor device attached to it that can sample voltage and decide when the battery is getting fully charged and then shunt further charging current around the battery to avoid overcharging it. This may also involve telling the charger to cut back on its output somewhat. 

A management system might also cut back charging current if temperatures get too high. It might also have the smarts to detect a bad battery and warn the operator. You could also imagine that the system would track the charge/discharge voltage profiles of each battery or cell over time and analyze the data to determine which battery is weakest/strongest etc and notify the operator of this before cell failures occur so that preventive measures can be taken. The list goes on, you get the idea.

Note that some people avoid the balancing issues of charging a long string of lead acid batteries by building a setup with individual chargers for each 12v battery or pair of 6v batteries. each charger will shut off when its individual battery gets topped off. This has its own drawbacks though including what happens when a single charger fails and more hardware.

There are BMS systems available off-the-shelf to DIY converters from manzanita micro and others. 

You can also get a battery /Monitoring/ system which doesn't actively manage each cell or battery but does give you visibility into what each one is doing so while it doesn't automatically balance things you can at least know if the pack needs attention (a weak battery or imbalanced pack) without having to manually check each battery. This setup could be as simple as a digital panel voltmeter attached to each battery, or there are off-the-shelf systems that do this. A monitoring system would be most appropriate on a flooded lead acid pack since active management is not really necessary. The batteries can handle overcharging (within reason, as long as the required maintenance is performed) so individual cell management is overkill, but it is still good to know if the pack is getting out of balance and needs to be equalized or if a weak cell is developing and the monitoring system would tell you this.


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