Laptops For Less Blog

The Secrets of Battery Runtime [Part I]

Does a bigger rechargeable battery mean a longer runtime for your portable device? Have you ever wondered if your laptop will run longer if you have a bigger battery attached to it? For the most part, the answer is “yes”, but is it practical to carry around a 16 lb battery pack to run a 5-6 lb laptop? Not likely. We cannot look at capacity alone. Other factors determine a battery’s runtime.

If you have used any sort of rechargeable battery then you have likely experienced unexpected downtime or short battery life. In this article we will examine two factors that affect nickel, lead, and lithium-based batteries; declining capacity and premature voltage cut-off on discharge.

Declining Capacity

As a battery ages the amount of charge it can hold gradually declines. The same holds true for batteries used on a continuous basis. The more you use a battery, and the older it gets, the less life it has. This makes sense, right?

Imagine a battery. Divide the battery into three sections. The bottom section is rock content. This rock content was energy and is no longer useable. The middle section is the available energy for use. The top section is an empty zone and is for energy refills.

The rock content present in a nickel-based battery is a crystalline formation. We call the crystalline formation “memory”. A nickel-based battery restores to a full discharge within one volt per cell if caught early. If one waits too long, say four months or so the repair becomes more difficult. A full repair is not likely. To avoid “memory” owners should deep-cycle their battery every one to two months. The most common nickel-based batteries are nickel-cadmium and nickel-metal-hydride batteries. Medical devices and power tools use these battery types.

Lead-acid batteries degrade in performance because of sulfation and grid corrosion. If a lead-acid battery does not get a fully saturated charge, sulfation will occur. Sulfation is a thin layer that forms on the negative cell plate of the battery. Avoid sulfation by cycling or topping the battery with charge.

Poor performance also occurs when a battery is overcharged. If a lead-acid battery is overcharged the positive plate acquires grid corrosion. Stop grid corrosion by maintaining the proper charge of the rechargeable battery. Larger portable and wheeled devices use lead-acid batteries

For lithium-ion batteries cell oxidation is your main enemy. Cell oxidation occurs naturally as the battery ages. Under normal use a lithium-ion battery will last about 2-3 years. Cycling will not restore a lithium-ion battery. Only cool storage minimizes lithium-ion battery performance degradation. Cell phones, laptops, and other mobile computing devices use lithium-ion rechargeable batteries.

As rated, new batteries deliver 100% capacity. A solid rule of thumb is to replace a battery when its capacity drops below 80% of its nominal rating.

Premature Voltage Cut-Off

Did you know that batteries do not use all their stored power during use? Some devices push rechargeable batteries to an early cut-off. These devices require high current bursts and cut off before the battery’s designated end-of-discharge
voltage. This creates a surplus in unused energy.

The early cut-off is often apparent in batteries with elevated internal resistance and during warm temperatures. This also occurs when cells “short” in multi-cell battery packs and “memory” in nickel-based batteries. Remove the load, the voltage recovers, and the battery appears normal.

Some portable devices with high cut-off voltage do not utilize all available battery power and leave energy behind. After discharging a battery under the conditions mentioned above a battery analyzer may pick up residual capacity readings of 30% or more unused energy.

Imagine a battery divided into quarter sections with a water spigot in the lowest quarter of the battery. Open the spigot and release energy from the battery. The energy drains until the top three quarters of the battery and just under the spigot is empty. Underneath the spigot lies untapped energy.

In part two of “The Secrets of Battery Runtime” we’re going to explore increasing internal resistance and elevated self-discharge. These two factors affect nickel, lead, and lithium based batteries and their runtimes.