Battery Aging

As batteries age, their service requirements change. This implies a longer charging time and/or faster end (higher current at the end of charging). Electrolytes usually need to be added more often in older batteries. Their capacity decreases, while their self-discharge increases.

What Is A Battery?

The battery is an electrical storage device. Batteries do not generate electricity; they store it like a water tank stores water for later use. As a result of the change in chemicals, electricity is either stored or released. In rechargeable batteries, this process can be repeated many times. Batteries are not 100% efficient, some energy is lost during charging or discharging as heat and chemical reactions. If you use a 1000-watt battery, you may need 1050 watts, 1250 watts or more to fully charge it.
Part or most of the loss is due to internal resistance. This energy is converted to heat, that is why batteries heat up when charged. The lower the internal resistance, the better.
Slow charging and discharging rates are more efficient. If the battery is rated at 180 AH for 6 hours, it can be charged 220 AH for 20 hours and 260 AH for 48 hours. Most of this loss occurs due to high internal resistance at higher amperage rates; the internal resistance is not constant, i.e. it is not the case that the stronger the impact, the greater the resistance.
The typical efficiency of a lead-acid batteries is 85-95%, in alkaline and NiCad batteries it is about 65%. Under optimal conditions, genuine deep cycle AGM batteries can reach up to 98%, but such maintenance is rare. So, when calculating the size of the battery and its container, one should reckon that, as a rule, about 10-20% of the total power may be lost.
Virtually all batteries used in photovoltaic systems, except for the smallest storage systems, are lead-acid types. Even more than a century later, they still offer the best price for power. Some systems use NiCad batteries, but we do not recommend them. They are expensive, just as they are for use because of the hazardous effects of Cadmium.

Types of Batteries

Batteries are divided into two types according to their use (what they are intended for) and according to their construction (i.e. what they are made of). The main areas are car manufacturing, shipbuilding and deep cycle. Deep cycle includes solar energy, backup power, traction batteries, as well as batteries for homes. The main construction types are: Flooded (wet), Gelled and sealed AGM (Absorbed Glass Mat). AGM batteries are sometimes referred to as "starved electrolyte" or "dry" because the fiberglass layer is only 98% saturated with sulfuric acid and there is no excess liquid.
"Wet" batteries may be standard with open covers or so-called "non-serviceable" (this means that these batteries are designed to be out of order one week after the warranty period). All AGM and gelled batteries are hermetic or sealed and "valve regulated", which means that the small valve provides a weak positive pressure. Almost all sealed batteries are valve regulated (usually called VRLA - Valve Regulated Lead-Acid). Most adjustable valves are under a certain pressure of 0.07 to 0.28 kg/cm2.

Battery Life

Deep cycle battery life is highly dependent on usage, service, charging, maintenance, temperature and other factors. It can fluctuate over a very large range.

What Are Batteries Made of?

Almost all of the widely used batteries are lead-acid type. Nickel-Cadmium batteries are also used, but their high cost and expensive utilization do not justify them. Some Lithium-Ion batteries have begun to appear, but they are much more expensive than the lead-acid batteries. At full charge, the acid usually consists of 30% sulfuric acid and 70% water. Most AGM batteries do not even cause a problem with freezing.

Industrial Deep Cycle Batteries

Deep cycle batteries have much thicker plates than automotive (car) batteries. They are sometimes used in large photovoltaic (PV) systems because they provide more storage per battery (very large and heavy).

Plate thickness

The thickness of the sheet (positive sheet) is of great importance due to a factor called "positive grid corrosion". This is one of the three main causes of battery failure. The positive (+) plate gradually decays over time, leaving nothing at the end, it accumulates on the floor in the form of sediment. The thicker the sheet, the more directly it is related to the life of the battery. Under other equal conditions, batteries with a thicker sheet last longer. The negative plate expands during discharge, which is why most batteries have separators made of pressed glass mat or paper. Plate thickness is not the only necessary factor for the long life of the battery, but it is the most important one.

Sealed Batteries

Sealed batteries have ventilation holes that are (usually) impossible to remove. The so-called maintenance-free batteries are also airtight, but usually not leak proof. Sealed batteries are not fully hermetic as they must provide ventilation during charging. If the battery is overcharged too many times, it may lose enough water to cause the battery to fail prematurely.

Battery Size

Batteries come in different sizes. Many have "group" sizes that depend on the physical size and terminal installation. This is not a measure of battery capacity.

Gel Cells

Gelled batteries, or "gel cells," contain acid that has been concentrated with Silica Gel to give it a solid jelly-like appearance. The advantage of these batteries is that the acid cannot be spilled even if the battery is broken. However, there are some disadvantages. One is that they need to be charged more slowly (C/20) to prevent cell damage with extra gas. They should not be charged with regular car chargers, otherwise the battery may be irreversibly damaged. This is not usually a problem for solar power systems, but if an auxiliary generator or inverter charger is used, the power should be limited by the manufacturer's specifications. Most of the best inverter chargers allow limiting the charging current of the battery.
Another disadvantage of the gel cell is the need to be charged at a lower voltage (2/10 less) than flooded or AGM batteries. Overcharging can cause cavities in the gel that never settle, causing a loss of the battery capacity. In hot climates, water loss can be so great that the battery may be out of order for 2-4 years. Newer AGM (absorbed glass mat) batteries have all the advantages of gel batteries (even more) and none of the above disadvantages.

AGM (Absorbed Glass Mat) Batteries

The new type of sealed batteries contains "absorbed glass mats" or AGM between the plates. This is a very fine fiber borosilicate glass mat. These types of batteries have all the advantages of gel batteries, but can carry a larger load. There are also the so-called "starved electrolyte", as the mat is 95% saturated, not completely soaked. This means that the acid will not leak even if the battery is broken.
AGM batteries have some advantages over both gelled and flooded ones, costing about the same price as gelled batteries.
Since all the acid is contained in glass mats, it cannot spill even if the battery is broken. This means that it is not a hazardous material, and has a lower shipping cost. Moreover, since there is no liquid in the battery to freeze or expand, it is practically immune from freezing damage.
Almost all AGM batteries are "recombinant", which means that oxygen and hydrogen recombine inside the battery. They use the gas-phase transfer of oxygen to the negative plate to recombine them back into water during charging and prevent water loss through electrolysis. The recombining is normally 99 +% efficient, so almost no water is lost.
Charging voltages are the same as for all standard batteries, no special adjustments required, no problem with incompatible chargers. As the internal resistance is very low, the battery does not heat up even at very high charge and discharge currents.
AGM batteries have a very low discharge of 1% to 3% per month. This means that they can be stored for much longer without recharging than standard batteries. The Dream batteries can be almost fully recharged (95% or more) even after 30 days of full discharge.
AGM batteries do not have any liquid to spill, so even under severe overcharge conditions hydrogen emission is lower than its maximum value of 4% intended for aircraft and enclosed spaces. The plates of AGM batteries are tightly packed and securely mounted and can withstand shocks and vibration better than any other standard battery.

Even with all the advantages listed above, there is still a place for the standard flooded deep cycle battery. AGMs are 1.5-2 times more expensive than the flooded batteries of the same capacity. In cases where the batteries are installed in conditions without any risk of evaporation or leakage, standard or industrial deep cycle batteries are a more economically viable option. The main advantages of AGM batteries are no maintenance, completely airtight of hydrogen or liquid, non-spilling even if the battery is broken and can withstand freeze. Not everyone needs these benefits.


Technology Absorbed electrolyte in mats Gelled electrolyte Liquid electrolyte
Manufacturing technology Complex Medium Simple
Setting Either Either Ordered only
Service term High (8 to 12 years)
(TPPL 15 years average)
Average (5 to 8 years) Low (up to 5 years)
Sealing Full Partial No sealing
Damage impact Non-critical Partially critical Critical
Water usage None Low Required
Maintenance None Some Required
Gas emission Very low Medium High
Gas emission explosion hazard Very low Medium High
Low temperature resistance Stable Stable Unstable
Starting current High High Medium
Deep discharge Stable Stable Fairly stable
Temperature Effects

Battery capacity (how many hours-hours it can hold) decreases as the temperature drops, and increases as the temperature rises. This is why your car battery fails on cold winter mornings, even if it worked the day before. The standard operating temperature of the battery is 25°C. At about -27°C, the battery capacity drops by 50%. At 50°C it can rise by 12%.

Battery charging voltage changes due to temperature change. It may vary from about 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per cell (13.8 volts) at 50°C. This is why you should have temperature compensation on your charger or charge control if your batteries are outside and/or subject to wide temperature fluctuations. Some charge controls have temperature compensation built in (such as Morningstar) - this works fine if the controller is subject to the same temperatures as the batteries. However, if your batteries are outside, and the controller is inside, it may not work that well. Adding another complication is that large battery banks make up a large thermal mass.

Thermal mass means due to their mass they change the internal temperature mush slower than the surrounding air temperature is. A large insulated battery bank may vary as little as 10°C over 24 hours inside, even though the air temperature varies from 20°C to 70°C. Because of this, external (add-on) temperature sensors should be attached to one of the POSITIVE plate terminals and bundled up a little with some insulation on the terminal. Hence, the sensor will read very close to the actual battery inner temperature.

Although the capacity of the battery is high at high temperatures, the battery life is short. At -5.5 C the battery capacity decreases by 50%, but the battery life increases by about 60%. Battery life is reduced by high temperature: for every 10 degrees above 25°C, the battery life is reduced by half. This can be used for all types of lead-acid batteries, whether sealant, gel, AGM, industrial or other. See the graph of temperature dependence of the battery.

One last note about temperature. In some places, extremely hot or cold, batteries with non-standard acid concentrations may be sold. Acid can be thicker (for cold weather) or thinner (for very hot weather).

Cycles versus Duration

The battery cycle is one period of complete discharge and rechare. It is generally calculated from 100% discharge to 20% and back to 100%. Nevertheless. There are calculations with different degrees of discharge; the most common are 10%, 20% and 50%. Therefore, care must be taken when it comes to the rating written on the battery, i.e. how many cycles it is intended for, unless it also states how much down discharge is calculated.


Battery life depends directly on how deeply it is discharged during each cycle. If it is discharged by 50% every day, it will serve twice as much as if it was discharged by 80%. If the cycle is 10% of the discharge depth, the battery will serve 5 times longer than if it is 50% of the discharge depth. Obviously, there are some practical limitations here. Of course, it is not desirable to have a 5 ton battery pack to reduce the discharge depth. The most optimal option is to operate at a depth of 50% discharge. This does not mean that at some point you cannot make it 80%. When designing a direct system, when you already have some idea about the load, you should take into account the average depth of discharge of about 50%. There is an upper limit, i.e. if the battery is only discharged by 5% each time, it will not last as long as it would last if it was discharged by 10%. The reason for this is that during very small cycles, lead dioxide accumulates on the positive plate in the form of a precipitate, not a flat strip. The graph above shows the dependence of the battery life cycle on the discharge depth. The graph is for AGM batteries, but for all lead-acid batteries the curve will look the same, although the number of cycles may vary.

Battery Voltage

All lead-acid batteries give about 2.14 volts per cell count (12.6-12.8 for a 12-volt battery) when fully charged. Batteries that are stored for a long time may lose their charge over time. This "leak" or discharge can vary depending on the type of battery, age and temperature. It can range from 1% to 15% per month. In general, it is the lowest in the case of new AGM batteries, and the highest in the case of older production (lead-coated plates). This problem is rare in systems where the batteries are connected to any source of charge, whether solar, wind or AC. In any case, one of the main reasons for the "death" of the batteries is the storage for several months in a half-discharged state. Batteries must be constantly charged, even if not in use (or especially when not in use). Even the "driest" batteries (sold without electrolyte to make it easier to carry, with acid added later) wear out over time. The maximum storage period is 18-30 months.
Batteries discharge faster at high temperatures. Lifespan can also fall seriously from high temperatures. Most manufacturers state that the lifespan drops by 50% for every 10°C after 25°C. Life expectancy increases by the same principle when the temperature is below 25°C but the capacity decreases. This is mainly due to the fact that the batteries spend part of their life at high temperatures and part at low temperatures. The average self-discharge of flooded batteries is 5% to 15% per month.

State of Charge

The state of charge, or conversely, the depth of discharge (DOD) is calculated by measuring the voltage and/or the specific gravity of the acid. This may NOT tell you what condition the battery is in (capacity in AH), it can only be calculated by a long load test. A fully charged battery voltage will be 2.12-2.15 volts per cell or 12.7 volts per 12-volt battery. At 50% the reading will be 2.03 VPC (volts per cell), and at 0% it will be 1.75 VPC or less. In the case of a fully charged cell, the specific gravity will be 1.265 and 1.13 or less in the case of a fully discharged cell. This can vary depending on the type of battery and the manufacturer's brand. When buying a new battery, you need to charge it, leave it for a while, and then perform a standard measurement. Some batteries are hermetically sealed, and the hydrometer reading cannot be taken, in which case you have to orient yourself according to the voltage. The hydrometer readings may not be complete, as it takes some time for the acid to mix in the wet cells. If the measurement is taken immediately after charging, you will get 1.27 at the top of the cell, although at the bottom the figure will be much lower. This does not apply to gelled and AGM batteries.

"False" Capacity

At full charge, the battery can withstand voltage tests, but in that case its capacity may drop. If the plate is damaged, sulphated or partially worn out of long use, it may appear to be fully charged, but in reality, it will act like a much smaller battery. The same thing can happen with gelled batteries if they overcharge or have cracks or bubbles in the gel. What is left of the plates can only work normally if at least 20% of the plates have been preserved. Batteries usually do not reach this point, but rather fail for other reasons, but this should be taken into account when your battery seems to be passing the tests normally, while it lacks capacity and under load dies off quickly.

The table below should be used carefully so that it does not show only the surface charge. To measure the voltage correctly, you need to leave the battery alone for a while, or hang a small load for a few minutes, such as a small car bulb. The following voltages are applicable to all lead-acid batteries except gelled. For gel cells you need to subtract 0.2 volts. Note that for rechargeable batteries, the voltage may be different, so do not use these numbers for rechargeable batteries.

Amp-hours – What Is This?

All deep cycle batteries are measured in amp-hours. Amp-hour is one amp in 1 hour, or 10 amps in 1/10 hour, and so on. This is amps times hours. That is, if you use a device that requires 20 amps for 20 minutes, the amp-hour will be 20 (amps) x 0.333 (hours) or 6.67 AH. In the case of solar, electrical and backup power systems (and for almost all deep cycle batteries), the accepted assessment time is the 20-hour rate. This means that during a 20-hour period, the battery is discharged to 10.5 volts, during which time the actual amp-hours it gives is measured. Sometimes 6-hour and 100-hour rates are used for comparison. The 6-hour mode is often used in the case of industrial batteries, as it corresponds to the daily cycle of the battery. The 100-hour mode is sometimes used to show the battery better than it really is, but it can also be used to figure the capacity of a long-lasting battery.

Why are amp-hours specified at different time rates (20 hours, 10 hours, etc.)?


The reason for this is the so-called Peukert Effect. It is directly related to the internal resistance of the battery. The higher the internal resistance, the greater the losses when charging or discharging, especially at high currents. This means that the faster the battery is discharged, the lower the AH capacity. Conversely, the slower it is discharged, the higher the AH capacity. This is possible because some manufacturers and suppliers rate their batteries at a 100-hour rate, which makes the batteries look better than they really are.

State of Charge

Here are no-load typical voltages depending on the state of charge.

(10.5 volts = fully discharged, 25°C). Voltages are indicated for 12-volt batteries. For 24-volt systems, the numbers should be multiplied by 2, for 48-volt batteries, by 4. VPC (volts per individual cell), if you measure and get 0.2 V difference between each cell, you must either equalize the cell voltages, or your battery is in poor condition, or it may be sulfated. These voltages are for batteries at rest for more than 3: hours. In the case of rechargeable batteries, the numbers will be higher, the voltage during charging will not tell you anything, the batteries should be left to sit for some time. Important to know: the voltage measurement is approximate. The best option would be to measure the specific gravity, but for some batteries it may be difficult or impossible. Note the sharp drop in voltage in the last 10%.

State of charge 12-volt battery Volts per cell
100% 12.7 2.12
90% 12.5 2.08
80% 12.42 2.07
70% 12.32 2.05
60% 12.20 2.03
50% 12.06 2.01
40% 11.9 1.98
30% 11.75 1.96
20% 11.58 1.93
10% 11.31 1.89
0 10.5 1.75
Why 10.5 volts?

In this FAQ, we said that the battery is considered dead at 10.5 volts. The reason for this is related to the internal chemistry of the battery. At 10.5 volts, the specific gravity of the acid drops so much that it can do nothing else. The specific gravity of the acid in the discharged battery may fall below 1.1.

Battery Charge

Battery charging goes through 3 main stages: bulk, absorption and float.

Bulk charge. This is the first of 3 battery charging phases. Current is sent to the battery at the maximum safe speed until the voltage reaches the full charge level (80-90%). The voltage at this stage is usually 10.5 to 15. There is no "correct" voltage for bulk charging, but depending on how long the battery and/or wires last, there may be a maximum voltage limit.

Absorption charge. The second of 3 battery charging phases. The voltage remains constant and the current gradually decreases as the internal resistance increases during charging. At this stage, the charger gives the maximum voltage. At this stage, the voltage is mainly 14.2 to 15.5 volts. (Internal resistance gradually increases as it approaches saturation.)

Floating charge. The third of three battery charging phases. After full charge, the voltage drops (usually to 12.8 to13.2) to reduce gassing and prolong battery life. This stage is sometimes called maintenance or continuous charging, as the main purpose of this is to prevent the discharge of the already charged battery. PWM, or "pulse width modulation" accomplishes the same thing. In PWM, the controller or charger senses tiny voltage drops in the battery and sends very short charging cycles (pulses) to the battery. This may occur several hundred times per minute. It is called "pulse width" because the width of the pulses may vary from a few microseconds to several seconds. Note that for long term float service, such as backup power systems that are seldomly discharged, the float voltage should be around 13.02 to 13.20 volts.


Most car chargers are only for volume charging and have a very small voltage regulation range (if any). These can be used to low batteries quickly, but not for long. If the charger voltage is properly regulated for your battery, the battery will keep charging without damage.

Charge Controllers

A charge controller is a regulator is placed between the solar panels and the batteries. Regulators for solar systems are designed to keep the battery at peak charge without overcharging. 

Most modern controllers have built-in automatic or manual adjustment, many have a LOAD output. There is no best universal controller for all cases; some systems may need a controller with expensive elements, some may not.

Battery charging voltage and power

Most flooded batteries should be charged at no more than "C/8" speed for a certain period of time. Although some battery manufacturers state higher charging rates, such as C/3, rapid charging may result in a rise in battery temperature and/or bubbles and fluid loss. ("C/8" divides the battery capacity into 8 in 20-hour mode. For a 220 AH battery, it will be equal to 26 amps.) Gelled cells must be charged no faster than C/20, or 5% of their amp-hour capacity. However, as very few cables can withstand so much current, we do not recommend using this at home. Instead, use C/4 or lower speed to avoid cable overheating.

Charging at 15.5 volt will provide 100% charge for lead-acid batteries. As soon as the charging voltage reaches 2.583 volts per cell, charging should be stopped or reduced to continuous charging. Note that flooded batteries must bubble to ensure a full charge, and to mix the electrolyte. The float voltage for lead-acid batteries should be about 2.15 to 2.23 volts per cell, or about 12.9-13.4 volts for a 12-volt battery. At higher temperatures (above 30°C) the voltage should drop to about 2.10 volts per cell.

Never add acid to the battery just to replenish the spilled liquid. Distilled or deionized water should be used to fill non-sealed batteries. Float and charging voltages for gelled batteries require 2/10 times less voltage than flooded ones to reduce evaporation. Note that many shunt-type controllers sold for solar panels will not provide full charge; check the specifications first. To get a full charge, you need to continue charging even if the battery voltage reaches the cut-off point of most of these controllers. For this reason, we recommend that you use the above chargers and controls.

Flooded battery life can be extended by using an equalizer every 10 to 40 days. This charge is about 10% higher than the normal full charge voltage and is applied for 2 to 16 hours. This ensures that all cells are charged evenly, and the gas bubbles mix with the electrolyte. When fluid does not mix in standard flooded cells, the electrolyte becomes "stratified". You can have a very thick solution inside and a very thin solution on top of the cell. In this case, the readings on the air meter may be far from the truth. If for some reason you cannot balance, leave the battery to sit for at least 24 hours, then use the hydrometer. AGM and gelled batteries should be leveled 2-4 times a year. Follow the manufacturer's recommendations, especially for the gelled.

Brief Facts

Almost all batteries reach their full capacity after 10-30 cycles. A brand-new battery has about 5-10% less capacity than mentioned.

In cases where the batteries are connected in series, parallel, or sequentially/in parallel, the replacement batteries must be of the same size, type and manufacturer (if possible). Age and service life should be the same as for other batteries. Do not insert a battery that is older than 6 months or has completed more than 75 cycles. Either install a completely new battery or a used but working battery.

When using a small solar panel to charge the battery continuously (without a controller), select the panel that will provide the maximum output power at a capacity of 1/300 to 1/1000.

Lead-acid batteries have no memory; rumors that the battery needs to be completely discharged to get rid of that "memory" are completely wrong; they can lead to premature battery failure.

Idleness is extremely harmful to the battery. To buy a new battery and save it for later is a very bad idea. Either buy when you need it or keep it in constant charge.

Only clean water should be used to clean the outside of the battery. Do not use solvents or sprays