I’d say yes to all three of your questions. Even with the weight of a second battery, the benefits of having that third thermal breaker takes quite a bit of current load off the two in the cortex. Especially with the allowance of no-limit 393 motor, current management will be more important than ever. The four batteries you have should be enough, we seem to change the power expander battery far less frequently than the cortex battery.
With 4886a we switch out batteries normally twice each meet. Once in the middle, like after lunch, then right before the quarter finals matches. We do have a charger/decharger, though so we can get the most out of our batteries.
Hmm… interesting… my team normally has to switch batteries every match, otherwise (particularly last year) our motors would overheat really fast. We normally cycle through 8 batteries each competition, using 2 on the robot, then 6 charging at a time.
I would suggest testing a couple days before competitions how long your batteries last on your robot. I know last year our batteries would last around 5 minutes in practice, which then we could figure out how many batteries we needed for the competition. As noted before, there’s teams who just keep the same battery on the entire competition because they managed their power really well.
This definitely depends on which motors are being used for the power expander. Last season I had 4 lift motors on the power expander, and the power expander battery would last twice as long as the drive/intake battery. This year I have 4 drive motors on the power expander, and the expander battery runs out only a little slower than the main battery. It’s definitely worth testing how long it takes to run down each individual battery during normal robot use.
In response to the main post, I do feel like a power expander is definitely worth the added weight of a second battery. The only downsides I see of using a power expander is that it makes wiring a pain, and also there is the cost, but expanders are pretty cheap. I would be a little cautious having only 4 batteries. Depending on how battery hungry your robot is, you may encounter problems. Its worth giving it a test to see if you think it will be okay, but I would personally say that 4 batteries is pretty borderline.
How old are your batteries ? Are they the 3000mAH ones? They should last longer than 5 minutes, do you know what voltage they have dropped to when you change them? The only reason I can think of that would relate motors overheating (or really the PTC tripping) to needing fresh batteries is unusually low voltage causing current to increase above normal.
Hmm… I think ~4 years old, they came with the PIC kits that our school got, so I’m guessing the 2000mAh battery. I did some testing over the summer and noticed that they would reach a max of 7.9 - 8.0 volts and drop down to around 7.0 before becoming unusable. Using the 3-wire motors, I would get around 20 minutes out of them. With the High strength under a lot of stress, around 5 minutes.
Actually I did. Over the summer I tested battery levels vs speeds a lot (since it was the primary issues from our last season). When using AA batteries (~9.5 volts), my prototype-drive-train could move around 30lbs no problem, while with my rechargeable batteries (~7.5 volts) would trip the PIC’s internal PTC all day long. Also I’m not blaming the batteries entirely for the PTC’s tripping, they just were a contributing factor.
EDIT: Before someone says it, yes I know my team desperately need new batteries…