Disclaimer: I'm not a battery expert yet, it's my first day!
After graduate school I entered industry designing hand-held and portable particle counters and gas meters. That was almost two years ago, and as part of my designs I must consider Lithium Ion Battery Management. Most of the batteries I deal with are the two terminal type with an internal Protection Circuit Module (PCM), like the one shown in Figure 1. They are pure Lithium Ion batteries, not Lithium Polymer, as indicated by the visible cells. The battery in Figure 1, I would guess, is three Lithium Ion cells in parallel.
Figure 1: Battery pack from Megabatteries
My wife owns a Samsung Tab 3 tablet, and sometime last year it stopped charging as well as turning on. I wasn't sure what was wrong, so I put it to the side with the intention of opening it up at some point. I've provided a picture of the Tab 3 from Samsung's website in Figure 2.
Figure 2: Samsung Tab 3 (Courtesy Samsung)
I didn't do any precursory research about the problem because I wanted to practice my troubleshooting skills. So, once the In The Air design challenge was over I opened the tablet to do some probing; I immediately went for the battery since most problems are power related. It was a lithium polymer like the one pictured in Figure 2.
Figure 2: Samsung Battery Replacement (Courtesy Amazon)
Lithium batteries will likely have either 2 or 3 terminals. Usually a +V, GND, and sometimes a temperature monitoring connection (thermistor). If you've got a keen eye, you would have immediately noticed that this battery pack has 5 wires coming from it. I suspect this is just to increase the amount of current that can be drained or sourced to the battery without significantly heating up the wires. That is, two wires are for +V, two wires are for GND, and one wire is for temperature monitoring.
I grabbed my multimeter and measured the battery voltage - ZERO Volts. That definitely CAN be a sign of a dead battery, but for a lithium ion it doesn't necessarily mean the battery is dead. Before designing charging circuits and learning about batteries, I would have assumed if a battery read 0 Volts that it was dead. A reading of 0 Volts can also mean that the battery has been deep discharged and it's output has been turned off. I've discovered through research and experimentation that, mainly, two things can drastically reduce the lifetime of a lithium ion battery: overcharging and discharging too much.
I learned about the discharging trouble one day when I was trying to discharge a lithium ion battery so I could test my product's charger circuit. I had a great idea, I'll run a DC motor off the battery to discharge it at its maximum rated current so I won't have to wait all day for the battery to discharge. A short while later the motor turned off. Great, the battery is discharged! I checked the battery voltage, and it read 0 Volts. I didn't expect that. I figured the battery would discharge until the current dropped enough that the rotor could no longer turn. I thought I would read about 3.0 Volts per cell at this point, but I was definitely getting 0 Volts. Next I thought, "Oh! The battery must have heated up too much, I'll just wait for it to cool down.". Hours later - still no output voltage. So, what's happening? The battery was over discharged or "deep discharged". The Protection Circuit Module (PCM) inside the pack had detected that the battery was going to be discharged too much and basically turned off the battery output to prevent that from happening.
Up until this point, everything is fine. The battery has behaved exactly as it should, that is, turning off to protect itself. The designer needs to account for this possibility by setting a reasonable precharge current for this exact scenario. Any current limiting power supply, if used properly, can be used as a battery charger, so I hooked up the battery, made the appropriate adjustments and the battery charged. I found this very interesting, so I began my research. I found a few interesting articles about left over board charge and the like, but I really don't think any of those postulations are correct. I think it's as simple as the precharge current having been set too low by the designer. Someone at Samsung goofed up on the design? Maybe.
Anyway, until next time ...