Lithium Ion Polymer Battery Charging

[hybridsae]

Hey, I am in charge of building a battery bank and charging system for a hybrid vehicle for school. We went with lithium polymer batteries and I am unfamiliar with how they operate and what some of the basics of lithium polymer batteries. I was going to start small scale which is why i'm posing my question here and figured the small scale application theory could be applied to a larger scaled application in the end. But I'm going to ask about the big picture here and see what you think.

Here are my initial questions and was hoping someone could help if they have any experience in this field ---

I am required to have 32 battery cells in series to get a high output voltage, but need to design a charging system for it. Is it as simple as just applying constant current to all these batteries while monitoring voltage and current to make sure they do not break any limits? I explain my question in more detail below ---->

I need to know if charging lipo batteries in series requires you to consider the current limit of one battery or all of them? Example: If you have 32x 40Ah batteries in series will you have to charge at only 1C of one battery so 40A? Is that the limit? Another question about the charge curve --> Do you HAVE to have constant current going through the series batteries until the batteries charge to their maximum voltage? So would you have to alter voltage to the batteries in the above example to keep the current at exactly 40A in order to charge the 32 batteries in series? Like do the lipo batteries in series keep relatively equal resistance in order to maintain constant current themselves while charging to their max voltage? After they reach their max voltage, from my understanding, you then have to keep the voltage constant and keep charging until the current through the batteries drops to approximately 10% of the charging current (40A) so 4A?

Would it be a good idea or recommended to charge so many batteries in series like that?

 

[jpanhalt]

Thirty-two 40AH batteries in series is not a trivial power supply. I am not even sure you can get a single cell LiPo battery of 40 AH. If not, then you are going to have a combination of cells in series and in parallel. Because of their temperature and charging sensitivity, it is common practice to charge cells individually. If that is not possible, consider short strings of cells in series. Charging in parallel creates added issues. Be very careful not to overcharge them. The precautions with LiPo's should not be taken lightly.

Here are some references you might find helpful:

Fundamentals, chemistries, charging, and making packs:
Battery Chemistry Tutorial and FAQ from PowerStream: Custom battery chargers and power supplies for OEMs
**broken link removed**

Specifications (tabs, etc.):
https://www.batteryspace.com/prod-specs/3200.pdf

John

 

[hybridsae]

Hey thanks for the links. They helped a little bit. I now understand a lot better how to charge lipo batteries. 40Ah batteries exist, and eventually our student project requires we build a circuit something like this - **broken link removed** for the batteries. But for now I think we need to start off small scale and design a charger for two 1750mAh 7.4V Li-ion batteries that we have lying around to get a feel for how to build a proper charging circuit and understand more how lithium batteries operate and also how to balance the batteries (hence the noob questions I posted on how exactly to figure out how a charging circuit worked). We need to keep the batteries in series for the purpose of our project and so we are going to start with the two batteries we have (1750mAh) and go from there.