Power supply, etc.
OK Guys. I've been thinking about the type of battery we need for DB-1. We know the voltage, 12V. But, what about capacity (i.c. - How long do we want DB-1 to operate on a single charge). Codecage got me thinking about this when he mentioned his concern over trace width on the power board PCB he designed. Measuring current draw by the motors at various speeds should do the trick. Attached is a plot showing test results from an experiment titled Five Speed Motor Test.
The "No Load" curve is with the drive wheels propped up. The "Full Load" curve is with the drive wheels on the floor (DB-1 moving). Tests were performed with Go Bilda 5201 53:1 Gear Motors, Cytron MD10CR3 controller boards and a 12V power supply with digital current readout.
Once we know how much current the motors draw, we can calculate the width of the trace required to handle the current safely.
The attached plot shows Trace Width vs Current. Most PCB manufacturers specify 1 Oz. copper on FR4, two sided PCBs. If we look at the 1 Oz. trace, we see the trace width is just under 0.5mm for the full load current draw of 1.36A taken from the Five Speed Motor Test. So, any trace width wider than 0.5mm should be sufficient. To be safe, choosing a trace width of 1.0 - 2.0mm gives us redundancy and allows the use of 2,3, or 4A fuses keeping in mind that the FSM Test plot is based on both motors running at the same time and there is one fuse on each motor and one fuse on the 12V circuit. For example, a 2mm trace width on the power board 12V circuit would have a 4A fuse and each motor fuse would be a 2A.
Based on the motor test we need to decide the type of battery and capacity to be used. If anyone wants to weigh in on this, I welcome the input. There are two choices that I believe are the best fit for this project. They are LiPo (Lithium - Polymer) and Pb,acid (Lead, acid "Gel" cell).
LiPo is an interesting chemistry. It produces 4.2 volts per cell at peak charge. A three cell (3S) battery charges to 12.6V in approx. 1 hour using a charger designed specifically for charging LiPo batteries with balancing plugs. The balancing plugs allow the charger to charge each cell individually, matching them at the end of the charge cycle. It is a safety feature that ensures no cell is under of over charged. There are lots of capacity choices for 3S LiPo batteries but a reasonable size might be 5000 mAH.
Gell cell is a lead-acid battery with a twist. The electrolyte is infused in a gel like substance making the battery safer to use than standard lead-acid. Also, the case is sealed to avoid spillage or release of gas. Other than charging, the battery is maintenance free. The gel cell is heavier than LiPo but this may be helpful if DB-1 has traction problems. A 7 AH (6"x 4"x 2.3") battery is available that fits the opening just behind the motors. I recommend using a charger designed to shut off automatically when full charge is reached. I also recommend a charging current no higher than .5A (500mA) be used to protect the cells.
I haven't decided which one to pick but either one can be used with a LM2596 DC to DC Buck Converter to provide 5 volts to the micro-computers and sensors.
Interesting question. And lots of underlying questions too 🙂
Not sure if you've looked into cost. My quick scan tells me there can be quite a cost differential. GelCells being much cheaper. I suppose there is some variation based on quality of the batteries. Quality costs, but not necessarily in the long-run. One thing I've not seen, is many of the straight LiPo batteries. Mostly the safer LiFePo4s. There are many of these advertised as GelCell replacements (equivalent dimensions).
For 12 volts at 7 aH:
GelCell - $40
LiFePO4 = $50 - $200
We will need to know more about the power requirements before a shopping trip.
Excellent feedback. Lithium Iron is another choice we should consider. I use LiPo batts in electric RC airplanes. I have a built in timer on my transmitter that beeps when it is time to land. This ensures the battery voltage does not fall too low (9V). If a 3S LiPo battery is allowed to go below 9V the battery can be damaged and rendered un-chargeable. I am not familiar with LiFe. Does it have a similar requirement? If so, we may have to install a timer on DB-1.