Hacking a Cell Phone Battery

Figure 5-4 SparkFun and generic LiPo chargers

CHAPTER 5: Batteries and Power 93

The safest and most reliable way to charge a LiPo is to use one of the chips that exist just for that purpose. These chips are cheap, but generally only available as surface-mounted components. However, there are plenty of ready-made modules available, many of which use the MCP73831 IC. Figure 5-4 shows two of these—one from SparkFun (see the Appendix, M16) and one for just a few dollars from eBay.

Both are used in the same manner. They will charge a single LiPo cell (3.7V) from a USB input of 5V. The SparkFun board has space on the PCB for two other connectors, one to which the battery is connected and the other for a second connection to the battery—the intention is that you connect the electronics that will use the battery to the second socket. The sockets can be either JST connectors as are often found on the end of the leads of a LiPo batter, or just screw terminals. The SparkFun module allows you to select the charging current, using a connection pad.

The generic module on the right has a fixed charge rate of 500mA and just a single pair of connections for the battery.

It is not a good idea to trickle charge a LiPo. If you want to keep them topped up, for say a battery backup solution, then leave them attached to the charger.

Hacking a Cell Phone Battery

Most of us have a cell phone or two languishing in a drawer somewhere, and one of the useful components that can usually be scavenged (assuming it’s not the reason the phone is in the drawer) is the battery. The power supply is another useful item.

Figure 5-5a shows a fairly typical vintage cell phone battery. The battery is 3.7V (a single cell) and is 1600mAh (pretty good).

94 Hacking Electronics

(a) (b)

(c)

Cell phone batteries normally have more than just the usual two connections for positive and negative. So the first task must be to identify the connections on the battery.

To identify the positive and negative connections to the cell, just put your multimeter into the 20V DC range and test each combination of pairs until you get the meter to read something over 3.5V, depending on how well charged the battery is (Figure 5-5b).

The batteries often have gold-plated contacts that make them very easy to solder leads to. Once they have leads attached, you can use a charger like the one described in the previous section. Figure 5-5c shows the SparkFun charger module being used for just that purpose.

Figure 5-5 Hacking a cell phone battery

Caution

When using a LiPo battery, remember that if you discharge them too far (below about 3V per cell), you can permanently damage them. Most new LiPo batteries will include an automatic cut-off circuit, built into the battery package, to prevent over-discharging, but this may not be the case for a scavenged battery.