How do battery chargers work?
Batteries and chargers share a symbiotic bond. One cannot function properly without the other. Good battery chargers provide the base for durable well-performing batteries. Many engineers tend to be ignorant of the intricacy involved with power sources. This is particularly the case when charging in adverse conditions.
A common way to identify a charger is through its charging speed. For example, consumer products tend to come with well performing low-cost chargers. On the other hand, industrial chargers are often made by a third party company and include unique features. One of these features being the aforementioned charging in adverse conditions. Batteries can still operate in below freezing temperatures, but certain batteries cannot charge at such extreme temperatures. Li-ion batteries are one such battery. Lead and nickel-based batteries are affected by the cold as well, but they can still charge albeit at a much slower rate.
Focusing on Li-ion chargers, some include a boost feature, a wake-up feature, to allow recharging if the battery “fell asleep” due to overcharging. This sleep condition occurs when self-discharge within a battery brings the voltage to the cut-off point.
Every battery is vulnerable to the effects of self-discharge. Self-discharge is a battery characteristic in which the internal chemical reactions reduce the stored charge of the battery. To simplify this explanation, self-discharge causes batteries to charge to a certain point, but never until they’re full. Essentially they have less than a full charge when they’re put to use. Self-discharge happens due to a number of factors, age and cycling being chief among them. It’s recommended that you discard a battery if a self-discharge level reaches 30% in 24 hours.
To further expand upon the phenomenon that is self-discharge, the results can vary due to battery type. Lithium-metal and alkaline store energy the best (they usually can be kept in storage for years). Lead acid batteries have one of the lowest self-discharge rates and only lose about 5% on a monthly basis.
Regular chargers regard batteries that have reached a sleep state as unserviceable, and the pack at that point is more than likely discarded.
Lead and Lithium based chargers operate on CC/CV, an acronym for constant current constant voltage. This battery can have a charge current that is constant. When the set limit is reached, the voltage is capped. Once the voltage limit has been achieved the battery saturates, and the current will drop to a point where the fast charge function will no longer work.
Batteries that are nickel-based, charge with a constant current and the voltage rises without restrictions. The detection for a full charge occurs when a small voltage drop is observed after a steady increase. For the sake of safety, the charger in question should have a plateau timer to ensure a termination of safe charge if no voltage delta is detected. Since heat may play a large factor, you should also add temperature sensing.
It’s recommended that Lithium-based batteries have a cooler temperature when charging. You should discontinue using a charger or battery if the temperature rises above 10ºC (18ºF) during a standard charge. Li-ion doesn’t receive any trickle charge whatsoever when holding a full charge and doesn’t absorb any over-charge. Keep in mind that you don’t have to remove Li-ion from the charger though if you don’t use it for more than a week, you should relocate the pack to a place with a cooler temperature and recharge before use.
Types of Chargers
Overnight Charger(slow charger) - Overnight chargers don’t have the ability to detect a full charge. The charge continues to stay engaged, and it usually takes about 14 to 16 hours to charge a battery fully. When charging a nickel-cadmium (NiCd) battery, it becomes lukewarm when fully charged. Due to its ability to absorb overcharge, it’s not a good idea to charge a NiMH (Nickel-Metal Hydride) battery on a slow charger. Unlike NiCd batteries, NiMH does not utilize heavy metals (which could have possible adverse effects). Lower end chargers that charge AAA, AA and C cells share a similar method of charging (as do a few toys for children).
Rapid charger – the rapid charger is mostly used with consumer products. It sits firmly between the slow charger and the fast charger, and it takes about 3 to 6 hours for a full charge. As an added convenience, due to its heavy use by consumers, when a battery is full the charger will indicate that it’s “ready”. To avoid issues with faulty batteries, most rapid chargers include temperature sensing features.
Fast Charger – the fast charger has many advantages, but the most obvious are its ability to charge a battery quickly. This is due to the close communication between the battery and charger. Fast chargers typically charge at 1C. At this rate, an empty NiMH and NiCd will charge in about an hour. Some nickel-based chargers automatically reduce the current flowing through so that it can adjust to the lower charge acceptance. This is called a maintenance or trickle charge. Most nickel-based chargers utilize a trickle charge to accommodate NiMh.
Li-ion Charger - is the most simple of all of the charger types. Li-ion batteries have minimal losses while charging. These batteries charge to 70% in under an hour and the remaining time is dedicated to the saturation charge. It’s best not to fully charge Li-ion. Keep in mind that Li-ion doesn’t require a saturation charge like lead acid batteries do, so fully charging them isn’t necessary.
Lead AcidCharger – lead acid has an inability to fast charge. Charging a battery using a lead-acid charger typically takes around 14 to 16 hours. It takes about 8 hours for a lead acid to charge and the rest of the time is dedicated to the saturation charge. Partially charging is fine, but it’s imperative that lead acid receives a full saturation charge to prevent sulfation.
What is sulfation
Sulfation takes place when a lead acid battery does not receive a full charge. While a battery is in use, tiny sulfate crystals begin to form. These small crystals are not harmful by themselves, but during a prolonged period of not charging a battery, the crystals start to transform into a stable crystalline form and begin to deposit on the negative plates. Eventually, large crystals start to develop, and the performance of the battery begins to degrade. Sulfation is very common in starter batteries located in cars that drive in the city. Essentially, a car sitting at idle in traffic cannot sufficiently charge a battery thus prompting sulfation to take place. Another example is electric wheelchairs. 8 hour overnight charging simply isn’t enough to prevent sulfation to settle in. It’s imperative that any device utilizing lead acid receives a full charge for 14 to 16 hours. Otherwise, you’re practically inviting sulfation to set in.
Tips for Buying a Charger
Forklift batteries are constructed differently than regular lead-acid batteries (which are smaller in size). Forklift batteries are what is called traction batteries (heavy duty industrial batteries). These batteries have a composition of high amp hour 2-volt acid cells assembled together. Forklift operating voltages can range from 12 volts, 24 volts, 36 volts, 48 volts, and 72 volts though most fall under 36 or 48 volts. Some forklifts have undergone a conversion in which they’re capable of utilizing multiple traditional lead acid batteries such as 4D and 8D.
Typically, the lifespan of lead acid batteries has a direct relation to the thickness of its positive plates. A few examples include automobile plates which have .040 inch thick plates and golf cart batteries which have .070 to .100 inches thick plates. Forklift batteries are typically .250 inches or thicker. On top of that, they use lead antimony for plate material. Lead antimony increases plate life, but there is an increase in gassing and water loss. Due to this fact, maintenance is mandatory. You should never let water levels drop below the top of the plates. Also, rapid sulfation can be an issue, which can eventually decrease the lifespan of the battery.
Typically, forklift batteries are designed for around 1500 cycles or more. A cycle is a discharge of 80%. This means that the battery only has a 20% charge left before it should be recharged. To get a better idea of the typical lifespan of a forklift battery, if a battery were to undergo a complete discharge/charge five days a week than a battery should last about five years. Of course, that’s only if a forklift is utilized to such an extent. Many aren’t thus lifespans of 15 years, or more aren’t entirely uncommon. In fact, forklift batteries used in a situation where the depth of discharge isn’t very severe, such as solar setups, can last up to 25 years.
Essential facts you should know about forklift batteries: