Constant current (CC) charging initially allows the full current of the charger during the BULK stage to flow into the battery regardless of the battery state of charge or the temperature until the battery terminal voltage reaches a pre-set steady state. The battery is now in a state of charge of >80%.
Constant current (CC) charging requires the initial charge current to be limited to a % of the battery’s capacity to avoid unnecessary gassing.
NOTE: Manufacturers publish different current limits for the BULK charge phase of a CC charge curve:
- 13% of the C20 (15%C5) rating for flooded deep-cycle
- 15% of the C20 (20%C5) rating for general-purpose GEL deep-cycle
- 25% of the C20 (30%C5) rating for Semi-traction industrial deep-cycle AGM batteries
- 25% of the C20 (30%C5) rating for Semi-traction industrial deep-cycle GEL batteries
- Up to 30% C20 (35%C5) rating for high rate and thin plate AGM batteries
The charge output then switches to a constant voltage ABSORPTION phase, during which the charging current tapers down due to the decreasing potential difference between the charger-output voltage and the battery terminal voltage, as described earlier in the section on constant voltage charging. The current will taper down until it falls to a low pre-set limit. This low limit - usually no more than 1% to 4% C20 of battery capacity, or a % of the initial charge current in the BULK charging phase – will be maintained for either a pre-set period (time wins over voltage) or by looking for voltage stability where the charge is terminated when the voltage fails to rise over a measured period (voltage wins over time).
Finishing phases can be a simple float stage at a voltage below the batteries gassing voltage or a more highly engineered process that involves a special Balance and/or Pulsing mode shown in the graphs below for Discover Dry Cell AGM and GEL batteries.
When using a CC-charge regime:
- The charge current should switch from a high (starting) charge rate to a tapering (finishing) rate when the battery reaches a 95% to 100% state of charge.
- The point at which this switch occurs may be determined by using a mathematical algorithm timer (in smart switch mode chargers) or by sensing the battery voltage.
- The smart timer setting can be determined by calculating the time needed to return 108% to 112% of the ampere-hours drawn out for Semi-traction AGM or GEL batteries. However, this method should not be used unless the previously discharged capacity can be reliably and consistently measured using smart technology.
- The smart timer setting can be determined by calculating the time needed to return 105% to 110% of the ampere-hours drawn out for general-purpose AGM or GEL batteries. However, this method should not be used unless the previously discharged capacity can be reliably and consistently measured.
- The smart timer setting can be determined by calculating the time needed to return 115% to 125% of the ampere-hours drawn out for deep-cycle flooded batteries. However, this method should not be used unless the previously discharged capacity can be reliably and consistently measured.
In theory, once approximately >95% of the previously discharged capacity has been returned during the bulk CC charge stage, a CV absorption stage will be continued at a constantly reducing charge current rate until approximately >100% of the previously discharged capacity has been replaced. In reality, once it attains a theoretical 100% state of charge, it will continue to draw small currents to compensate for standing/parasitic and electro-chemical losses to achieve the needed overcharge.
An electric circuit is formed during charge and discharge when a conductive path is created to allow electric charge to move through the circuit continuously. This continuous movement of electric charge through the conductors of a circuit is called a current and is often referred to in terms of current flow. Just like how the rate that water flows downhill is determined by the steepness of the hill and the debris and grade changes in its way, current flow intensity in a circuit is influenced by the voltage potential and the resistance in the circuit.
The force or pressure creating the flow in a circuit is called voltage. Voltage is a measure of the potential energy differential that must always exist between two points if there is to be any flow between the points. Without potential, there is no voltage.
Current flow moves from one point to another through various points of friction. This opposition to motion is called resistance. The intensity (flow amount) of current in a circuit depends on the voltage (downhill pressure) and the resistance against that pressure in the circuit to oppose or resist the current flow. Just like the voltage, resistance is a quantity relative between two points. For this reason, the combined quantities of voltage or resistance are often spoken of as being “the voltage drop” between two points or “the total resistance between two points.”
To speak to the charge and discharge process, we need to be able to make meaningful statements about the quantities of voltage, current and resistance in the charge and discharge circuits so we need to be able to describe them:
Unit of Measure Symbol Units Abbreviation
Volts (Potential Pressure) V or U or E Volt V (U)
Current (Intensity of flow) I Ampere (Amps) A
Resistance (resistance to flow) R Ohms or (Milliohms) Ω (mΩ)
Periods of inactivity can be extremely harmful to lead-acid batteries. When placing a battery into storage, follow the manufacturer’s recommendations and/or the recommendations below to ensure that the battery remains healthy and ready for use.
The most important things to avoid:
- Freezing. Avoid locations where freezing temperatures are expected. Keeping batteries at a high state of charge will also prevent freezing. Freezing results in irreparable damage to battery plates and containers. NOTE that if a battery is kept completely charged, the chances of freezing are minimal.
- Heat. Avoid direct exposure to heat sources, such as radiators or space heaters. Temperatures above 80°F / 26°C accelerate the battery’s self-discharge characteristics. NOTE that heat causes more damage to a battery than cold ever will so keep your battery storage area as cool as reasonably possible.
- NOTE that storing your battery on concrete will not damage your battery!
Step-by-step storage procedure
- Completely charge the battery before storing
- Store the battery in a cool, dry location, protected from the elements
- During storage, monitor the specific gravity (flooded) or voltage. Batteries in storage should be given a boost charge when they show a charge of less than 75% or approximately 12.40 volts for a 12-volt battery.
- Completely charge the battery before re-activating
- For optimum performance, equalize the batteries (flooded) before putting them back into service. Refer to the Equalizing section for this procedure
How fast will an AGM or GEL battery lose its charge sitting in storage?
AGM and GEL batteries will have a self-discharge or internal electrochemical “leakage” of between 1% and 15% per month, depending on storage temperature. This internal or self-discharge rate will cause the battery to become sulphated and fully discharged over time. High temperatures accelerate the process so that a battery stored at 86°F (30°C) will self-discharge twice as fast as one stored at 68°F (20° C).
Discover AGM or GEL batteries will naturally discharge at approximately:
- 1%-2% per month when stored at 8˚C/46˚F
- 3%-4%% per month when stored at 20˚C/68˚F
- 5%-6% per month when stored at 30˚C/86˚F
- 10%-12% per month when stored at 40˚C/104˚F
By comparison, a quality flooded deep-cycle battery will have a self-discharge rate at least two times greater than AGM or GEL batteries.
- Check and charge, if required, your Deep-cycle AGM or GEL batteries every 2 – 3 months.
- Check and charge, if required, your Deep-cycle wet Flooded batteries every 2 – 4 weeks.
Can I store my battery on a concrete floor?
A hundred years ago, when battery cases were made of porous materials, such as wood, storing batteries on concrete floors would accelerate their discharge. This is no longer a problem as modern battery cases, made of polypropylene or hard rubber, are sealed against external leakage, which causes the discharge. However, the top of the battery must be kept clean and dry. Temperature stratification within large batteries could accelerate the self-discharge if the battery is sitting on a cold floor in a warm room or is installed in a submarine.
Can I store my battery in my garage during the winter, or will it freeze?
Provided the batteries are maintained in a fully charged status, batteries can be stored in temperatures as low as -25°F without freezing.
What do I do if I have to store my batteries outside for winter?
AGM and GEL VRLA DRY CELL batteries can easily be stored in sub-freezing temperatures as low as -30°F/-35°C or lower as long as they are fully charged before storage. The same goes for flooded batteries.
The self-discharge rate of fully-charged AGM and GEL batteries is very low in these conditions, and they will not require charging for many months. Flooded batteries have higher self-discharge rates, so they may require top charging sooner. If your AGM or GEL batteries do freeze, they most likely will not recover. To attempt recovery, bring them inside and let them sit until the temperature of the inside of the battery reaches the temperature of the room it is in (approx. 20°C/68°F). Then, charge the batteries normally. DO NOT ATTEMPT TO CHARGE THEM UNTIL THE INTERNAL TEMPERATURE IS EQUAL TO ROOM TEMPERATURE. You may have to cycle the battery several times before deciding on its final condition.
If a flooded battery freezes – because it has excess water volume – it will probably freeze solid, expand, break the plates and even crack the case. A flooded battery that freezes is not typically recoverable.
Table 1 - Electrolyte Freezing Point
| Digital Voltmeter | State of Charge | Hydrometer Reading | Electrolyte |
| Open Circuit Voltage | Approximate | Specific Gravity | Freezing Point |
| >12.65 | 100% | 1.265 | -75°F (-59.4°C) |
| >12.45 | 75% | 1.225 | -55° F (-48.3° C) |
| >12.24 | 50% | 1.190 | -34° F (-36.7° C) |
| >12.06 | 25% | 1.155 | -16° F (-26.7° C) |
| >11.89 | Discharged | 1.120 | -10° F (-23.3° C) |
Why do Discover VRLA DRY CELL AGM and GEL batteries have an excellent shelf life?
Discover uses higher quality superior grids, premium separators and pure electrolytes, which prevent high self-discharge rates. Impurities in the lead alloy, separators and electrolyte cause tiny currents inside a cell, eventually discharging the battery and shortening its shelf life. Premium inter-cell welds block the slight cell-to-cell currents that cause self-discharge.
- Pure lead-tin grid alloys incorporate proprietary additives that increase the surface area of the plate, which ultimately helps to retard corrosion and extend the life of the grid.
- Thicker industrial semi-traction grids (>3.5mm) achieve more corrosion resistance than the thinner grids typical of general-purpose deep-cycle AGM or GEL batteries.
- Discover DRY CELL AGM and GEL CELL batteries are protected against deep discharge because they are precisely “acid-starved”, and in the case of Discover’s DRY CELL types are “extremely acid starved.” This means that the battery uses the power in the acid before it uses the power in the plates. Therefore, the plates are never subjected to destructive ultra-deep discharges. More technically speaking, the quantity of electrolytes must be tightly controlled so that the battery attains the expected design life (under normal operating conditions), taking into account the rate of recombination and the corrosion of the positive electrode.
- Discover uses more active material per amp hour of battery capacity, which ultimately results in a “more” acid-starved reaction meaning that even when the battery is discharged deeply (the sulfuric acid in the electrolyte is used up and absorbed by the negative active material in the process of discharge), the active material utilization is kept to a minimum. More technically speaking, the active material is over-dimensioned to ensure that all of the active material is never completely charged or discharged in early discharge and charge cycles. Depending upon the part number, Discover EV Semi-Traction Dry Cell AGM and GEL batteries contain as much as:
- 77% more active material than flooded deep-cycle batteries
- 33% more active material than general-purpose VRLA deep-cycle batteries
- Discover DRY CELL and GEL CELL batteries contain ultra-premium dual-insulating PVC or PE separators (depending upon model), which will not break down in service.
- Discover DRY CELL AGM batteries containing woven organic capillary glass mats or fibreglass mat separators (depending upon the model) at the ideal compression and ideal saturation to achieve the best balance between capacity utilization and recombination efficiency. The dual insulating separators also help to prevent separator misalignment and treeing or shorting at the bottom and sides of the plates. The glass fibres are embedded into the plate, which helps to retard active material shedding.
To prevent the permanent loss of gases so that the recombination process has time to take place, each cell can hold approx. 1.6 pounds per square inch (psi) of pressure without venting. (the battery has a VALVE-REGULATED lead-acid design).
Large batteries with very large cells will bulge somewhat as this normal pressure builds. This is especially true in higher temperatures because the polypropylene case is pliable. Therefore, a certain amount of bulge is normal. The valves only let the gas out, never in.
A partial vacuum (sucked-in appearance) can form within a sealed battery under various circumstances. Battery temperature and ambient pressure play a role, but the recombination and discharge reactions are predominantly responsible. After charging ends, the recombination reaction continues until most oxygen in the battery is consumed. The total volume within the battery decreases slightly during a discharge. Deeply discharged batteries often have a “sucked-in” appearance. Batteries with large cells may display this appearance even when fully charged. If a battery bulges severely on a charge, it may indicate a blocked valve or an overcharged situation. When a battery is in this situation, it should be removed from service.
A sucked-in appearance is quite normal. A sucked-in battery should be charged, but the appearance can safely be ignored if it remains sucked-in after charging. However, a load test would be prudent if only a single cell displays or lacks this appearance. A partial vacuum can form within a sealed battery under various circumstances. Battery temperature and ambient pressure play a role, but the recombination and discharge reactions are predominantly responsible.
After charging ends, the recombination reaction continues until most oxygen in the battery is consumed. The total volume within the battery decreases slightly during a discharge. Deeply discharged batteries often have a “sucked-in” appearance. Batteries with large cells may display this appearance even when fully charged. To prevent the permanent loss of gases so that recombination has time to take place, each cell can hold approx. 1.6 pounds per square inch (psi) of pressure without venting. (the battery has a VALVE-REGULATED lead-acid design).