Laptops For Less Blog

The Secrets of Battery Runtime [Part II]

A bigger rechargeable battery does not mean a longer runtime for your portable device. Just because some people think attaching a larger battery to their laptop may be a good idea, this doesn’t make it so. In some cases, a larger capacity battery will actually decrease a device’s runtime. There are other things to consider when determining a battery’s runtime. Capacity is not the determining factor.

In a previous article we discussed two factors that shorten or cause unexpected downtime. These factors were declining capacity and premature voltage cut-off. In this article we will explore increasing internal resistance, and elevated self-discharge. These two factors affect nickel, lead, and lithium based batteries and their runtimes.

Increasing Internal Resistance

A battery’s capacity defines its stored energy. Internal resistance determines how much energy releases at any given point in time. Solid high-quality rechargeable batteries provide high current on demand. However, any battery with high resistance comes crashing down under heavy loads. A battery may have sufficient holding capacity but when there is a heavy load the result will be a [potentially big] voltage drop. A voltage drop triggers a device’s “low battery” indicator which in many cases will will stop the device from properly functioning.

Batteries with high internal resistance usually perform well on high current devices as well as low current devices like flashlights, portable mp3 players, or alarm clocks. On the other hand, high current digital equipment draws heavy bursts of electrical current. Basically, batteries with low internal resistance are able to provide high current on demand. Rechargeable Batteries with high internal resistance will cut off equipment when battery voltage collapses.

High internal resistance eventually renders a battery useless. There is little or no delivery of energy even though there may be energy present in the battery. Cycling will not help the situation either. The battery is no longer salvageable
and the condition is permanent. [See the graphic below].

Nickel-cadmium batteries deliver high electric current on demand and offer very low internal resistance. Alternatively, nickel-metal-hydride batteries start with elevated resistance. The battery’s resistance readings will increase rapidly after about 300 to 400 cycles of use.

A Lithium-ion battery has a higher internal resistance than a nickel-based rechargeable battery. Lithium ion batteries use cobalt and manganese as positive electrodes. The cobalt system in the battery increases the internal resistance of aging. The manganese keeps resistance throughout the life of the battery but eventually loses its capacity through chemical reaction.

Lead-acid batteries have difficulty providing a high sustained load and respond well to short current bursts. This is because lead-acid batteries have a very low internal resistance. Eventually, the battery’s internal resistance increases
through sulfation and grid corrosion.

Elevated Self-Discharge

All batteries will experience self-discharge. Lithium-Ion batteries self-discharge approximately 5% in the first 24 hours and 1-2% per month afterwards. Add a protection circuit to the battery and the discharge increases by another 3% a month. Protection circuits assure that the voltage and current on each cell does not exceed a safe limit. [The graphic below illustrates a battery with high self-discharge.].

Lead-acid batteries self-discharge the least. They decrease only 5% per month. This sounds great, right? Unfortunately, the lead battery has the lowest energy density and is terrible for portable devices. Lead and lithium-based batteries do not increase their self-discharge in the same manner or rate as nickel-based batteries. Nickel-based batteries self-discharge the most. They lose 10-15% of their capacity within the first 24 hours after a charge. They continue to self-discharge 10-15% per month for the life of the battery. Self-discharge peaks right after a battery is charged and levels off. The discharge increases with age, high-cycle count, and elevated temperatures.

Battery self-discharge increases with higher temperatures. An increase of only 18 degrees Fahrenheit doubles the rate of self-discharge. Avoid leaving a rechargeable battery outside or in a hot vehicle. In fact, older nickel-based batteries tend to lose the most of their energy during the day. They can self-discharge without any actual use. Nickel-metal-hydride rechargeable batteries self-discharge after approximately 300-400 cycles. Nickel cadmium batteries typically last over 1000 cycles before self-discharge is most noticeable and performance is affected. Crystalline formation can damage separators. This damage allows packs to cook while the battery is charging. A high cycle count accelerates swelling in the cell. Both of these conditions increase self-discharge. Self-discharge is unavoidable and there is no way to reverse it.

As you can see, these two factors will affect your rechargeable battery. Use the tips mentioned in this article and you will increase your rechargeable batteries runtime.

LaptopsForLess.com has a huge selection of batteries in stock, at very reasonable prices [often a fraction of the cost of manufacturers’ own brand replacements]. We stock; Laptop Batteries, Power Tool Batteries, PDA Batteries, Camera/Camcorder Batteries, Cell Phone Batteries and much more. If an electronic device contains a battery, we’ll stock that battery!