Battery Configuration

The batteries are a critical part of the project; without batteries that last for 6 months, the 2 man crew will have to go up and replace them every time this occurs.  
In order to choose the correct batteries, several types were investigated; Lithium Ion, Lithium Ion Nanophosphate, NiCd, Sealed Lead Acid and NiMH. This can be seen in the characteristics , Pugh Chart, and narrowed choices comparisons.  In this comparison, all the attributes that we wanted the batteries to have were listed and weighted from 1 to 3.  Then we assessed each type of battery compared to the Lead Acid battery, which was previously used.  

The batteries chosen from the Pugh Chart are brand, A123 Batteries, which are lithium ion nanophosphate technology.  Below is the spec sheet for the batteries provided  by the A123 company.  

 

As shown in the spec sheet, these batteries are ideal for this project due to the lightness, capacity, short charging time and low temperature operation.  

The above graph shows the I-V characteristics of the batteries; it shows that this battery type will stay at a constant flow rate until it reaches it's maximum capacity, then the curve drops off.  This is important for a power system because this means that we will be getting a constant voltage until the batteries are almost dead.  

The below graph shows that even at low temperatures, the batteries will discharge at a constant rate, which is also important for this project since a majority of the time the batteries will be in freezing temperatures.  

The minimum voltage calculations are as follows:
The following equation was given by the A123 company in the expanded data sheet:

(# of cells in series) x (recommended discharge cutoff voltage,cell) = Cutoff Voltage, string

3 x 1.6V = 4.8V ~ 5.2V should be our cut-off voltage to be safe for the 9.9V source.
4 x 1.6V = 6.4V ~ 7V to be safe for the 12V source

The trickle charging voltage calculations are as follows:
The following equation was given by the A123 company in the expanded data sheet:
(# of cells in series) x (Recommended float charge Voltage, cell) =
Float charge voltage, string

3 x 3.5V = 10.5V Float Charge
4 x 3.5V = 14V Float Charge

For our prototype, we are using a 9.9V source to cut down on cost; we also did calculations for a 12V source in case the user needs to switch to a 12V source for a different sensor.  

Katie Gallo          kgallo@uvm.edu
Tom Lanagan     tlanagan@uvm.edu