Weird battery discharge mode in robot, half of the cells dead, half 100% - Page 1
Forum section: electronics / projects

author: dmg
author_rank: Regular Contributor
timestamp: October 04, 2020, 07:10:48 pm
content: Hello,A while back I designed a mini sumo robot, powered by 6 Alkaline AA cells in series (9V). Due do PCB placement constaints, I had no proper place to fit a mechanical power switch rated for the maximum 2A of current this thing can pull from the batteries, so I implemented an electronic switch with a PMOS with this schematic:https://electronics.stackexchange.com/questions/162231/25v-for-pmos-switch-vgswhere, in my case, R1= 100k, C1=10nF, ceramic X7R, R2 = 5.1k, Q1 =NCE4435and instead of using an NMOS to pull down the PMOS's ground, I use a miniature latching push button.There's no extra circuitery hanging from the "input" side, everything is connected to the output side of the power switch, so there's no discharge path for the batteries when the switch is off that I'm aware of, except for leakage currents through the PMOS which should be in the nA to 1uA range, according to the PMOS datasheet.I built two of these robots, played with them back in January, then again in May, and forgot about them. Never changed the batteries, they have the same set since January. In May, they still worked and reported an almost full battery (almost 9V, sustained). I stored both of them turned off, but with the batteries still inside. They should have lasted for a looong long time, but today I wanted to play with them a bit and one was dead. Weird,so I removed the batteries, checked them and three of them are totally depleted, measuring 0.2 to 0.5V, and the other 3 measure 1.5V. I tested, they are still good, they have juice in them and won't drop under load.Any idea on why such an asymmetric discharge may have happened? The off current should be below 1uA, and they've been in storage with the batteries on for almost 5 months. The cells are Panasonic alkaline batteries, nothing special. I will definitelly check as soon as I can to see what's the real off current is, but I can see no mechanism that would increase it enough to explain this level of cell depletion, and I don't know why it has happened in such an asymmetric fashion.The other robot still works but looked slow. I checked, 5 cells at 1.35V, 1 cell at 1.22V.

author: SmokedComponent
author_rank: Regular Contributor
timestamp: October 04, 2020, 11:33:27 pm
content: 2A current draw is too much for AA alkaline batteries. Perhaps some of the cells got damaged and started to leak internally.For such current I'd check NiMH cells (that are rated appropriately) or maybe some RC model battery packs, designed for such currents.

author: dmg
author_rank: Regular Contributor
timestamp: October 05, 2020, 07:09:13 pm
content: Well, actually 2A is peak theoretical maximum at 9V, with both motors stalled at max power for the brief moment (milliseconds) before the motor drivers start limiting the current to the motors to 400mA max per motor, and then the software will (should) detect the stall and the robot tries to escape.Still, coudn't fit a mechanical switch for the normal operating current which is about 500mA when pushing another robot.Also, forgot to mention that the energy generated by the motors isn't reverse-blocked so it's dumped back to the batteries... Or to a TVS that I placed in there just in case. Maybe that's what damaged the cells... I'll get some eneloops for the next time.