What is Battery Pack?
A battery pack is a set of any number of (preferably) identical batteries or individual battery cells. They may be configured in a series, parallel, or a mixture of both to deliver the desired voltage, capacity, or power density. The term battery pack is often used in reference to cordless tools, radio-controlled hobby toys, and battery electric vehicles.
Components of battery packs include the individual batteries or cells, and the interconnects which provide electrical conductivity between them. Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detect the end of charging.
Interconnects are also found in batteries as they are the part that connects each cell, though batteries are most often only arranged in series strings. When a pack contains groups of cells in parallel there are different wiring configurations that take into consideration the electrical balance of the circuit.
Battery regulators are sometimes used to keep the voltage of each individual cell below its maximum value during charging so as to allow the weaker batteries to become fully charged, bringing the whole pack back into balance.
Active balancing can also be performed by battery balancer devices which can shuttle energy from strong cells to weaker ones in real-time for better balance. A well-balanced pack lasts longer and delivers better performance.
For an inline package, cells are selected and stacked with solder in between them. The cells are pressed together and a current pulse generates heat to solder them together and to weld all connections internal to the cell.
Why use battery packs?
Battery cells are like eggs. Cells come in fixed voltages and capacities. If you need more voltage, you can deal with multiples of the cell voltage. You can’t get half an egg, and you can’t get half a cell, at least in voltage.
Cell capacities do vary, but voltages don’t. All NiCad or NiMH cells are 1.2 volts nominal, lead-acid is 2.0 volts nominal and the various lithium technologies are about 3.6 volts per cell.
If you need more voltage you have to add them in series, if you need less voltage, you need some kind of voltage regulator or DC/DC converter. If you need more current than a single cell can supply you may need to put cells in parallel. If you need more capacity to give a longer run time you may also put cells in parallel.
Many times, the physical configuration makes it more attractive to use many small cells rather than a few large cells, since a large block is harder to fit than several small subunits.
How many amp-hours do I need?
Cell capacity is rated in amp-hours or milliamp-hours. The symbol for capacity is C. This is amps times hours. Divide by hours and you get amps, divide by amps and you get hours. For example, a 5-amp hour battery is the same as a 5000 milliamp-hour battery.
If you want to discharge in 10 hours, you can get a current of 5/10 = 0.5 amps. If you need 100 milliamps current, then you can run for 5000/100 = 50 hours. Often a discharge or charge rate is given proportional to C. So a discharge rate of C/5 means C/ (5 hours), or the constant current to fully discharge the battery in 5 hours.
The calculation of run time versus current is a rough estimate but is accurate under the right conditions. The faster you discharge, the lower the capacity of a battery. This trade-off depends on the battery chemistry and construction.
Usually, the capacity of a battery is quoted at a C/20 discharge rate. So a 12 amp hour battery sealed lead acid battery will actually put out a steady 0.6 amps for 20 hours. However, if you discharge the same battery at 12 amps, you would expect to run an hour, but you will only last for 22 minutes.
Also, if you want to run at 10 milliamperes you will get less than the expected 1200 days since self-discharge of the battery will limit your run time. Different battery chemistries differ in this respect. Lead-acid batteries are probably the worst at the rapid discharge end of the scale. NiCads and NiMH are much better.
How do you determine the current that your system draws?
The best way is to measure with a current meter and an adjustable power supply (usually the current meter is built into the power supply). Set the power supply to the highest voltage that the system is rated at and measure the current, then set the power supply to the lowest voltage that the system is rated at and record that current.
Adding a measurement halfway between the two will give you an idea of where the lowest power consumption point is (power is voltage times current). The idea is that you want to design your pack so that the voltage swing of the batteries is adequate, and where the power consumption is the least.
Some systems will show approximately constant power consumption no matter what the battery voltage is, and some will have a sweet spot where the power is lowest. If a variable power supply is not available, chart the current versus the voltage of the battery during a discharge cycle.
If making actual measurements is not possible, use the system datasheet, or the “boilerplate” sticker on the back to find the rated wattage or input current. This will usually give you a high estimate, or a peak value.
Advantages of Battery Pack
An advantage of a battery pack is the ease with which it can be swapped into or out of a device. This allows multiple packs to deliver extended runtimes, freeing up the device for continued use while charging the removed pack separately.
Another advantage is the flexibility of their design and implementation, allowing the use of cheaper high-production cells or batteries to be combined into a pack for nearly any application.
At the end of product life, batteries can be removed and recycled separately, reducing the total volume of hazardous waste.
Disadvantages of Battery Pack
Packs are often simpler for end-users to repair or tamper with than a sealed non-serviceable battery or cell. Though some might consider this an advantage it is important to take safety precautions when servicing a battery pack as they pose a danger as potential chemical, electrical, and fire risks.