You decide to measure the state of charge, and there are rules that apply to all types of batteries. These are to prevent excessive discharge of the 24v battery, which can cause reverse damage to individual battery cells or even produce negative voltage. Overcharging is less clear cut, as in the case of lead acid, it is sometimes required to balance cells or individual cells in a battery pack. However, overcharging can lead to water leakage and positive plate corroding gas, both of which can reduce battery life.

For nickel-based batteries, water loss is the most common problem, again leading to a shortened service life. In the case of lithium chemistry, since the built-in BMS automatically cuts off the current input at a preset voltage, it is usually not possible to overcharge. In some designs, there is a built-in fuse to prevent overcharging. However, this usually renders the battery irreversibly inoperable.

How to avoid battery charging and overcharging?

The decision to charge the battery depends on the environment of use and the degree of discharge. As a general rule for all chemicals, batteries should not fall below 80% DOD to maximize their service life. This means that the final SOC of the battery should be calculated from the point of measurement to the end of its daily operation. For example, if the SOC is 40% at the start of the operation and 70% of its capacity will be used at the end of the operation, the battery should be charged before it is allowed to continue.

To make this decision, the remaining capacity or remaining operating time of the battery must be determined. This is not simple, because the battery capacity is determined by the discharge rate. The higher the discharge rate, the less capacity is available. Lead-acid batteries are vulnerable to this, as shown in Figure 8.

Lithium-ion and nickel-cadmium based batteries do have reduced capacity at higher discharge rates, but they are not as pronounced as lead acid. A FIG. Figure 9 shows the effect of three different discharge rates on the available capacity of the NiMH battery. In this case, 0.2C (5 hour rate), 1C (1 hour rate) and 2C (1/2 hour rate).

In all cases, the voltage curve remains very flat, but at a reduced level, until the voltage suddenly crashes at the end of the discharge period.

Battery charging - Calculate battery charging and discharging time

Calculation of battery charging and discharging times

To determine the discharge time of any battery in a specific state of charge, it is necessary to know the current consumption and battery capacity at a specific discharge rate. Running time can be roughly calculated using a rule of thumb for each battery chemistry.

Knowing the effective capacity at a particular discharge rate will allow you to predict the run time as follows:

Standard capacity of battery (ampere-hour) = C

Discharge current (ampere) = D

Discharge coefficient = D/C = N

Discharge rate (ampere) = NC

Capacity (ampere-hour) at discharge rate D = CN

Discharge time (hours) of a fully charged battery = CN/D

Using the estimated percentage of the charging state, the running time can be calculated:

Run time = % Charging state x CN /(100xD) = hours

The calculation of charge time is complex because it depends on the state of charge of the battery, the type of battery, the output of the charger, and the type of charger. It is necessary to know the charging state of the battery to determine the number of ampere-hours that need to be put into the battery for charging. The speed at which this happens depends on the rating of the charger and how it is charged. Obviously, if the charger has enough output, lithium-ion batteries can be charged from completely dead in a few hours.

Sealed lead-acid batteries with charger output limits will take longer due to voltage limitations and reduced current at the charge stage. After determining the charging status, you can calculate how many ampere-hours you need to reload the battery. Understanding the charger characteristics will help you make time calculations based on the charging rate while keeping in mind the charging mode used.

Another factor is the ambient temperature (weather conditions), which affects the charging voltage and the current consumed by the charger. Higher temperatures will reduce the charging voltage, but will also increase the current consumed. For floating batteries, voltage compensation is required according to temperature.