Battery Confusion

Archive SparkFun Site Questions/Comments
1

This may a stupid question, but I can’t find an answer anywhere. Anyway, I’ve grown interested in batteries and I’ve been researching all the different types and their characteristics and whatnot; but I am still a little confused on the basics. Mainly when a battery is discharging and the relation between it voltage and amperage.

When a battery discharges not only does it lose charge (making its amperage go down), but its voltage also drops. So, say the battery is used in some electric circuit, losing charge is fine until it has none left, but electrical circuits run at specific voltages, so once the voltage drops too far wouldn’t the circuit stop working; and does the voltage drop when there still is charge in the battery?

Also, everything I read about charging/discharging batteries always focuses mainly on the voltage level per cell, which leads to another two questions. 1) Why do they focus only on voltage and not amperage? and 2) If you have a battery pack, is there any way to measure the voltage per cell other than taking apart the pack and measuring each cell individually?

Thanks in Advance!

2

Yes, when the voltage drops too far the circuit will eventually stop working. Exactly how it stops working depends on the circuit. In some cases, part of the circuit may work but other parts won’t.

The cell will continue to discharge even if there is no load since there is a certain amount of internal impedance in the cell that leads to self discharge. That’s why if batteries are left on the shelf for years, they’ll eventually be dead. If you have a battery pack you will need to probe it to get the individual cell voltages. Some sophisticated chargers work this way: by charging the individual cells to avoid possibly overcharging one if the entire pack is done at the same time.

The focus is on voltage (actually it should be on voltage AND temperature) because the most common rechargeables, NiCd and NiMH exhibit a voltage change when fully charged. They also have a noticeable temperature change at charge completion. Current is also important in that trying to charge at too fast a rate can overheat the cell, leading to damage.