There’s a lot of hubbub on the internet about charging Lipo batteries and how dangerous it can be. It can be, but if you take some precautions it’s perfectly safe. Charging Lipos safely typically requires specialty IC’s to ensure they don’t explode. All devices with Lipos (smartphones, tablets, laptops, etc) have these charging chips built in to protect the device and the consumer from an oops. The batteries themselves also contain a protection IC. The IC’s are great but can be over protective. I often get phones and tablets for repair with “bad” batteries that just need a little coercion into working again. The steps I use are outlined here and I’ve never had an oops but you should take your own precautions to protect yourself. Fire proof Lipo bags are a good idea.
All Lipo batteries have a nominal voltage which is printed on it. The most common nominal voltage is 3.7V but a lot of newer devices are using 3.8V batteries (and higher). My examples in this post will assume working with a single cell 3.7V Lipo. The maximum, fully charged voltage of a 3.7V Lipo is 4.2V. If you try to charge the battery above 4.2V you are in a danger zone and it could explode. You can use a charger designed for a 3.7V battery on a 3.8V (or higher) but the battery will not be fully charged (just close). The lowest you should discharge a 3.7V Lipo is 3-3.2V. Most Lipos have built-in protection that cuts off the battery if you try to over charge or over discharge and this is one way devices get into a state where they will no longer charge or power on.
When I get a device for repair and the customer claims it won’t charge or power on, the first thing I check is the battery voltage using a Digital Multimeter (DMM). All devices will have a minimum voltage and current requirement to power on. In my experience with smartphones the low voltage threshold is typically 3.4-3.5V. If the voltage is at or lower than this I charge the battery with an external charger (see below) until it reaches the nominal voltage. If the battery voltage reads zero or close to it, then your battery has gone into protection mode. Sometimes an external charger will not bring a battery in protection mode back to life. In these cases there are several things you can do. The cheap way is to cut the end off a USB cable, expose the power and ground wires, plug it into a USB wall wart and quickly/briefly touch them to the battery positive/negative terminals (about 5 quick touches should do it). As I stated before, you should not charge a battery over 4.2V so why is this ok when USB is 5V? Technically, it’s probably not, but it works and I’ve done it hundreds of times. All this does is apply 5V for a very short amount of time to the protection IC & battery to “wake it up.” A better way is to use a bench top power supply set to 4.2V/100mA and do the same. If the battery still isn’t “revived”, you could disassemble the pack and attempt to charge the battery directly at the terminals. I have done this, but it’s probably not worth it as most replacement batteries are cheap. It’s also possible that the protection circuit board is just toast.
After reinstalling a battery to the device I use a USB Voltage/Current meter to verify the device is in fact charging. Depending on how discharged the battery is and how the device detects the adapter (SDP, CDP, DCP) you should see it pull 100+mA. If it’s not pulling significant current then you have other issues with the device (charge port, charge IC, etc).
There are many different connectors for batteries which can make charging difficult but here are some of the tools I use. For Samsung batteries and similar I use an adjustable plug-in charger like this. The pins slide on a rail so you can adjust them to fit your battery. I’ve used the same one for over a year with no issues. It’s cheap and cheaply made but works well. Some tablets have a single cell batteries with terminals that can be clipped onto with test leads. I just set my bench supply to 4.2V and gradually increase the current from 200mA to 800mA over about 10-15 minutes. Once the bench supply voltage display gets to about 3.9V, I finish charging the battery in the device.
Apple iPhone batteries are special. Each version uses a special proprietary connector that is small and not so easy to work with. It is possible to solder leads where the connector attaches on the battery but it’s a pain. To combat this issue I created a breakout board containing a simple charging IC (MCP73831) and connections for the iPhone 4,4S,5,5S/5C, and 6 (6+ connector wasn’t available at the time I designed it). In the center of the board is a jumper for switching between 5V and the charging IC. If the battery won’t charge you can short the VBAT and 5V pins briefly to revive the battery. The board also has a MicroUSB connector for power input and a standard 0.1” header with all the connections broken out (5V in, Battery, GND, NTC and Gas/Fuel gauge). The iPhone batteries use TI fuel gauge IC’s that keep track of it’s health. I have started experimenting with getting the data from the fuel gauge IC but that’s for another post.
It seems that 2015 is the year of the Lipo for me. Lot’s of experimenting with charging IC’s and talking to fuel gauges. There will be more posts to follow about Lipo’s. I’ve got some fresh prototype boards on the way, but that’s all for now.