The batter system implemented on the rover should be able to power it and all necessary functions for an operational time that suites its environment. The loads given will be the motor controller and the four corresponding motors, these require 12V and can theoretically pull as much as an amp. The other major load on the batter is the mini-pc which operates at 12V and can require as much as four amps.
When selecting the batter we looked at the area that was going to be its operating space which is third floor Kretchmar. With this in mind the run time we wanted was set for an hour. Taking some measurements of the motors under a simulated load that would be similar to that of the assembled rover each motor drew around .2 amps and the with the possible draw from the computer estimated at 3 amps we decided that the 4000mah 3s lipo battery that robotics club already had was sufficient for starting out but if needed could be upgraded in the future.
There are multiple ways that one can go about upgrading the battery. Probably the easiest would be to just acquire a larger amp-hour capacity 3s pack, certain ones on the market can go as high as 10,000mAh, although the higher you co in amp-hour rating they tend to get expensive quite quickly. Another option would be to go from our current 3s battery to a 4s configuration. The actual process of upgrading is not quite as simple as just plugging the 4s battery in though. To start with the voltage for 4s lipo packs when fully charged is 16.8V. This increase in voltage would require two modifications. Firstly when upgrading, the boost converter will need to be adjusted for the higher voltage so that it will achieve a full charge. Secondly the motor drivers are only rated to 15V, to deal with this we would need a buck converter after the voltage/current meter downconverting the voltage to the desired voltage. It would also be possible to use a 5s lipo battery in the system, this would require a change in the voltage/current sensor along with the changes that come with upgrading to a 4s pack, the bms would not require any modifications as it is rated up to 5s battery packs.
We wanted the rover to be able to charge within a reasonable amount of time, this was basically defined as four times longer than that of its operational uptime. So with a one hour run time that would mean we have to charge within a 4 hour time span. There are two main options for charging; contact based or wireless. We chose to go the wireless route because dealing with contact point that are on the charging station and also on the robot where you have to provide safeguards against shorting them and other hazards. Wireless has none of that but its downside is how fast you can charge. We acquired a wireless power unit with the transmitter operating on 24V DC and the receiver capable of outputting 13.4V 1.6A DC. The required distance between the two coils of the unit is a minimum of 5mm and around 7mm is where you drop to around 13V 1.5A and then around 10mm it ranges around 12v 1.2a and then drops of significantly after 15mm. With a charge rate of 1.5a we should be able to charge the batter in around 2.6 hours.
Flow of power
From the wall we receive 120V ac and convert to 24 dc which powers the wireless transmitter. The wireless receiver outputs (depending on alignment) 12V 1.5A DC into our boost converter. It gets boosted to 13.5V into the BMS (batter management system). The BMS will prevent overcurrent, over discharge, overcharge, and balance the battery. the output of the battery will be monitored with a power and current monitor which outputs a 5V ADC signal. The monitor then outputs directly to the motor drivers and the 12V regulator. The 12V regulator will output 12V 4A and powers the pc.