Is a 16 or 22 hour bot workday possible in the factory, just with a battery pack, and without using umbilical cords, swappable battery packs, or induction charging?
Yes.
For the foreseeable future, the bots will be working the production lines alongside human colleagues. Just as humans take scheduled breaks, so can the bots.
Currently, production lines account for break schedules, and until humans are fully replaced on the line, the lines will still experience scheduled pauses. If a bot keeps working, it will quickly be starved or blocked and will need to wait for its human coworkers to return. Instead, it should use this opportunity to recharge its batteries, just like its human coworkers.
With some help from Jordan of @LimitingThe, I was able to model the bot charge and discharge cycles to see whether it is feasible to keep a bot running during operational hours, just by taking advantage of breaks.
Assumptions:
Charging Infrastructure:
Equivalent to Level 2 charging. That is 220 V, 50 Amp service. As 220 V and higher voltage service is commonplace in factories, L2 will be easy to install. Since the battery pack is only 2.3 kWh, more power is not needed nor recommended. Additionally, a maximum of 7 kW is only needed for a portion of the charging curve, with lower power at the top of the curve, and is included in the model. With this infrastructure, the pack should be able to fully recharge in around 1 hour. However, as the model will show, a max of 2.5 kW is sufficient, or 220V-12A service.
Pack Cooling:
However, the BMS of the battery pack will likely need liquid cooling to charge at these levels. But that does not need to be inside the bot. Since the battery, electronics and actuators are all D/C, the bot does not need any A/C inverters, except for charging. The inverters can be outside of the bot, just like a laptop or EV-bike charging brick. The charging port can also include a coolant loop so this infrastructure also remains outside of the bot to both reduce the bot mass, and to provide extremely efficient cooling.
Shift Schedule:
It is common for there to be 3 breaks over a 9 hour period for 8 hours of work. Two 15- minute breaks mid-morning and mid-afternoon, and a 30-minute lunch break. Every manufacturer is a little different, but this is a typical profile.
Bot Charging:
It is also assumed the bots can self plug into chargers, and may need step-up and set-down time preventing full advantage of the break time. The set-up/set-down time represents the time to walk over to the charging post, plugin, unplug the walk back to station.
Worst Case:
500 W of continuous drain is assumed, which is a worst case scenario. Bots may typically average 250-300 W.
Results:
The attached plots show a 2-shift schedule. The day starts at 8 AM and ends the following day at 3 AM. Each shift as two 15-minute breaks, and a 30-minute lunch/dinner break. There is one hour between shifts.
The bot is running at max power output of 500 W, and the charging is limited to 220V-12A or 2.6 kW. The day starts at 100% state of charge, but is limited to 80% during the shifts.
In the plots, the red rectangles are the short breaks, the yellow are the lunch/dinner breaks, and the blue is the time between shifts. The bot does 16 hours of work over a 19 hour period.
In the first plot, the bot has comfortable margins, assuming zero set-up/set-down charging time.
In the second plot, the set-up/set-down time is 5 minutes, and we can see the bot fully discharges, because of the limited recharge times.
However, in the third plot, the 5-minute set-up/set-down time is mitigated just by boosting the charge speed to a 220V-24A or 5.3 kW service.
Conclusion:
Just by leveraging the break schedules already employed in the factory, it is possible to keep a bot running concurrent with its human coworkers without requiring anything more than L2 chargers.
In some cases, the reserve margins are so large, it invites smaller pack sizes depending on the deployment scenario. A lighter pack will also consume less power.
Using a similar strategy for 16 hour days, a goal of 22 hours of use over 24 hours per bot should be possible, and will be demonstrated in a coming post.