Introduction
A battery is nothing more than a collection of mini chemical factories called cells. A battery cell is an arrangement of two types of metal plates – anode and cathode – soaked in a conductive material (usually acid or a gel mat). When these plates are connected by an electrical circuit, a chemical reaction transfers electrons from the anode to the cathode, producing an electric current.
Basically, it is a chemical reaction within the battery cells that generates the current. Initially, the chemical reaction occurs very quickly, because electrons on the surface of metal plates are very easy to pull off. However, as the reaction proceeds, it takes longer and longer to extract the electrons. This degradation of the reaction is at the root of a charge profile that changes with the state of the battery.
Discharge Profile
Each time the battery is discharged, the cathode becomes a little less willing to let go of the electrons it has just acquired. In so doing, each time the battery is discharged, it becomes a little more unusable. This is one of the reasons why batteries have a life cycle, after which they become inoperable. If the battery is allowed to discharge completely – a “deep discharge” – the cathode becomes even less willing to let those electrons go, further reducing the battery’s lifespan. The usual heuristic rule, more applicable to lead-acid or gel batteries, is to avoid discharging the battery by more than 50% to prolong its life.
Charging Profile
Charging a battery involves reversing the chemical reaction, i.e. storing energy by returning electrons to the anode. This is done, of course, by forcing a charging current. Initially, charging is quick, as electrons can easily stick to the surface of metal plates. After a while, however, the process becomes slower, as these electrons have to be stuck to less accessible areas of the plates, i.e. below the metal surface.
This is why the initial charge of batteries is fast, while full charges take longer. The usual rule of thumb (for lead-acid or gel batteries) is that the first 80% of the battery’s stored energy is charged quickly. The remaining 20% takes longer. Finally, if the battery is already charged and a current is foolishly applied in an attempt to charge it again, the battery may eventually be heated to the point of degrading its condition and therefore reduce its ability to store a current.
Battery Profile
In short, batteries are capable of accepting a maximum charge voltage (beyond which they heat up) and have voltage and amperage charge profiles that vary according to their state of charge. Charging them efficiently therefore requires controlled profiles according to their state of charge. These profiles are specific to the battery technology used (lead-acid batteries, lithium batteries and lead-gel batteries).
An example of a charging profile is shown below, with fast charge shown in the middle (“CC Fast Charge”) and slower charge (“Constant Voltage Charge”) on the right. Immediately after this is a trickle charge (often called a “floating charge” on chargers) which keeps the battery charged. This trickle charge is just enogh to keep the battery charged but low enough to avoid damages.
The 50% to 80% Range is the Sweet Spot
Although unrelated to charge controllers, this is a good time to point out that the 50% to 80% usage range is a good heuristic rule of thumb for a battery system. Staying above 50% discharge preserves battery life. Staying below the 80% threshold facilitates rapid recharging.
The idea is therefore to have a sufficient number of batteries such that their total capacity (in watt-hours or ampere-hours) will ensure that the energy available in the 50% to 80% range is greater than the energy requirements between charge cycles. For example, a daily requirement of 200 ah of energy, dictated by on-board needs and the fact that you don’t want to start the engine more than once a day, will retain enough batteries for a total storage of 667 ah (≈200/(0.8−0.5)).
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References
Monolithicpower.com (s.d.). Battery Charger IC Fundamentals, webpage retrieved in November 2023 at this address.