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Recharging Lead-Acid Batteries

Keeping lead-acid batteries properly charged is the key issue to maximize the performance and lifetime of the battery. Overcharging will lead to loss of water in the electrolyte and subsequent corrosion of the uncovered parts of the battery plates, whereas the lead-oxide (PbSO4) that is not removed from the battery plates due to systematic undercharging will start to christaline eventually making part of the plates inactive. Both negative effects will result in irreversible damage of the battery plates.
The way in which the recharging process is performed is directly related to the reliability and lifetime of the battery system. Ideally each battery should be recharged with a battery charger, which is optimized for this specific battery. Especially the absorption voltages and float-charging voltages should be applied according to the battery's data sheet. Even for relative small battery systems as used on board of vessels, the extra cost for adaptive high-quality battery chargers is easily compensated by the longer lifetime of the batteries.

General Recharging Scheme

When a lead-acid battery is recharged, electric current is supplied to the battery electrodes to perform the discharge reversal electro-chemical process. Lead-sulphate in the positive electrodes is converted to lead oxide, and on the negative electrode, is converted to metallic lead, the sulphuric acid electrolyte goes from dilute to more concentrated. The lead-acid charging process is somewhat lenient as long as overcharging, overheating, and excessive gas production is avoided.

The recharging process is usually divided into four successive phases:

  • bulk charging: in the first stage the battery is charged with a constant current. The current will be in the range of 20% of the battery's Ampere-Hour rating (C/5) and should not cause overheating (e.g. 10A or lower for a 50Ah battery). The battery voltage will slowly increase, but should never exceed the type-specific voltage above which the electrolyte starts producing excessive gas. As soon as the battery voltage reaches this level, the charging with a constant current must be stopped. The battery recharge level is then about 80% of the nominal capacity.

  • absorption charging: in the next phase, a constant voltage is applied to the battery electrodes. The rate at which the battery continues to absorb charge in this mode gradually slows down. The charging current will decrease until the battery is fully charged. The voltage applied at this stage depends on the battery type and is typically 14.2 to 15.5 V.
    At the end of this phase the charging current drops below 1% of the battery's Ampere-Hour rating (C/100; e.g. 0.5A or lower for a 50Ah battery). At this state the 95% recharged state is normally obtained.

  • equalization charging: this phase is optional and should only be performed for non-sealed wet-cell batteries. Equalizing reverses the build-up of the chemical effects like stratification up on the plates. During the equalization phase a controlled 5% overcharge to equalize and balance the voltage and specific gravity in each cell by increasing the charge voltage (5% over nominal voltage). During this phase heavy gassing will occur (at about 2.4V cell voltage @ 25°C). This phase should be supervised carefully. Equalization has completed once the specific gravity values no longer rise during the gassing stage (which is hard to check!).

  • float charging: during this phase the charging voltage is reduced to typically 12.8V to 13.2V (about 2.2V per cell @ 25°C), and held constant at this value to obtain and maintain a fully charged battery state. The applied voltage is chosen such that the float charging can extend indefinitely without damaging or stressing the battery.

The correct values for the absorption and float-charging voltages and especially for the currents and voltages applied during the equalization phase depend on the type of battery to be loaded. Moreover these parameters are different for different battery models and manufacturers.
The basic chemical reaction occurs between the conductive lead grid, the various oxides and active materials, and the electrolyte. Depending on the reaction rates, the electrolyte will decompose in to gaseous components. The biggest danger to a battery is water loss, ultimately resulting in the escape of hydrogen and oxygen gasses from the electrolyte during recharging. This electrolyte decomposition into gas must be kept under control by correct selection of the voltage values applied to a battery during charging. This is the reason why different manufacturers recommend different charging voltages for their batteries.

Ideally the battery charger should apply the correct charging parameters (voltages and currents) according to the battery's data sheet or according to the manufacturer's recommendations.

Recharging Wet-Cell Batteries

Non-sealed lead-acid batteries should not be recharged with high charging currents because in contrast to the sealed versions, non-sealed batteries will show excessive gassing during rapid recharge.
Charging at 15.5V will give a 100% charge. Once the charging voltage reaches 2.583V per cell, charging should be stopped or reduced to a trickle charge (float charge). Note that wet-cell batteries must "bubble" somewhat to obtain full charge, and to mix the electrolyte. Float voltage should be about 2.2V per cell, or about 13.2V for a 12V battery.

sealed batteries

Where non-sealed lead-acid batteries have the situation of excess gassing during rapid recharge, the sealed types do not. This lends itself to higher rates of recharge without damage or reduction of life. Complete charge times of less than 1 hour are possible. Nevertheless most battery chargers will limit the charging current in order to minimize the power dissipated in the charger.
When float charging at 25°C, the cell voltage should be maintained at a range between 2.2 and 2.4V. Exceeding 2.4V may cause accelerated grid corrosion. The temperature affects what the optimum charge voltage should be. For higher temperatures, the chemical reactions are more active and therefore the voltage should be reduced to or below the 2.3V range. For lower temperatures, the voltage must be increased to a range exceeding 2.4V to obtain completed charging.

Recharging Gel-Cell Batteries

Generally the charging current (bulk charge) for gel-cell batteries should not exceed the "C/20" rate or 5% of the capacity. The charging voltages (absorption and float) should be 20% lower than the voltages for wet-cell batteries.
If overcharged, voids can develop in the gel, which will never heal, causing permanent loss in battery capacity. The charging current and voltages must be strictly limited to the manufacturers specification. To obtain a reliable full charge state, special battery-specific charge controllers may be required.

Recharging AGM Batteries

Nearly all AGM batteries are "recombinant" meaning that the oxygen and hydrogen recombine inside the battery. These use gas phase transfer of oxygen to the negative plates to recombine them back into water while charging and prevent the loss of water through electrolysis. The recombination is typically 99% effective, so almost no water is lost.
Charging voltages are the same as for any wet-cell battery. There is no need for any special adjustments or problems with incompatible chargers or charge controls. And since the internal resistance is extremely low, there is almost no heating of the battery, even under heavy charge and discharge currents. Some AGM batteries don't even have a maximum charge and discharge current limit specified.
The charging of AGM batteries can be done by applying a constant-voltage, where the charging voltage is held to a range of 2.3 to 2.6V (at 25°C) and the current is allowed to vary. This lends itself to higher rates of recharge without damage or reduction of lifetime. Complete charge times of less than 1 hour are possible (C/1 rate). Nevertheless most battery chargers will limit the charging current in order to minimize the power dissipated in the charger and the cables. Since the voltage drops over the cables may be significant for high charging currents, AGM battery chargers should have an extra pair of wires for "voltage sensing" directly at the battery connections.
AGM batteries may be equalized twice a year by following the manufacturers specifications.

Charge Controllers for Marine Power Systems

A charge controller is a regulator that connects an electrical power source (e.g. power line, alternator, wind generator or solar panel) to the batteries. Regulators are designed to keep the batteries properly charged at peak without overcharging. Monitoring meters for charging currents and battery voltage are included with most types.
Especially for marine installations, multiple power sources will have to be integrated into the battery recharging strategy. One feasable solution to obtain this is to use a high-quality charger that can work on an DC input voltage of 10-16 volts (e.g. a solar charge controller) and to adapt the available power sources to this voltage range by high-efficiency AC/DC- and DC/DC-converters.
sail080f_B.jpg Modern charge controllers are based on the same technology as used in switch-mode power supplies. They can be easily designed to accept a wide range of input voltages while retaining an optimal charging performance. Based on the same technology, AC/DC converters can be designed to adapt the wide range of line voltages (typ. 80-250V) and frequencies (typ. 40-200Hz) in use worldwide, to the required DC input range for the battery charger.

Such high-quality battery chargers allow to program the characteristic parameters of each of the recharging phases such as bulk-load current, absorption, equalization and floating voltage. They will also be temperature controlled, adapting the recharging parameters to different environmental conditions. In this way the optimal charging algorithm can be used for the connected battery system. In order for temperature control to work properly, a temperature sensor must be available, which must be thermally connected to the battery before the charging is started.

In the data sheets of battery chargers the "Bulk-Absorption-Float" algorithm is usually described as "I-Uo-U"-algorithm referring to the characteristic parameters of each of these charging phases (bulk-charge current, absorption- and float voltage). Usually these chargers do not automatically include an equalization phase in a their normal recharging algorithm. In some models, equalization can be initialized manually as an extra recharge phase.
If the on-board power system consists of a multiple-battery pack, a charger with multiple outputs should be used to recharge such a system. This enables that each battery will be recharged individually with the optimal recharge rate. In such a system the temperature control will work reliable only if each battery has its own temperature sensor.

Some vendors of high-performance battery charges are:

Alternatively a separate AC-DC converter (isolator) fed from the land-based power line, can be used to generate a resilient on-board 12V supply, which can feed the solar- or wind charge controller.

Some vendors of high-performance AC-DC converters are:

Notice: high-load AC-DC converters may exhibit high inrush currents (e.g. Powersolve 1500W model has up to 45A @ 230VAC). Most marina or pier power installations will not accept such high peak currents (typical value will be 16A @ 230V).

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