Lead-Acid Batteries – All you need to know

This article covers all you need to know for the lead-acid batteries in the context of solar, wind or off-grid use – from the different types and applications, through the various factors that have effect on battery life and capacity, to the basic maintenance activities that can prolong battery life.

Battery Types

We have the following major types of batteries, classified by application:

  • Starting – These batteries are usually used for starting engines, hence they are called SLI (Starting, Lighting, Ignition). This is the type of battery that you have in your car and these batteries are designed to provide a large starting current for a short period of time. In order to be able to do that, they need a very large surface area of the lead plates that are inside, so the plates are usually made from lead foam or sponge with a very fine structure. While performing a great job for starting engines, these batteries perform very poorly if they are frequently discharged to less than 80% of their capacity. Under these conditions, the lead sponge that is forming the plates disintegrates and falls to the bottom of the cells and eventually short-circuits the battery. These batteries can give more than 1500 cycles if used as per their design (frequent discharge with no more than 5% of the capacity) and as low as 30 cycles if they are deep-cycled or drained to less that 30% of their capacity.
  • Deep cycle – Also called traction or fork-lift batteries, because they are used to power these machines. These batteries are designed to be frequently discharged to rates of 75% or more of their capacity. Instead from lead sponge, their plates are made from solid lead, so they can survive the frequent charge / discharge cycles. Compared to the starting batteries, they can provide less starting power, however they can provide it for much longer time. These batteries can survive hundreds of cycles when discharged to 80%, however it is most cost effective if discharged to around 50%. It is possible to use a deep cycle battery as a starting battery, however it needs to be at least 20% over sized, to compensate the lower starting power that a similar sized deep cycle battery will have, compared to a starting battery.
  • Hybrid – Sometimes called marine or semi-traction, these batteries are situated somewhere in between of the starting and the deep cycle batteries. Their plates can be composed from a lead sponge, however it is much thicker and will last longer than the one used in the starting batteries. A typical semi-traction battery for this type can handle 300 cycles at 75% DOD (Depth Of Discharge), however the performance of the batteries from this class can greatly vary depending on the manufacturer and the type of plates used.

Lead-acid batteries can also be divided based on their construction:

  • Flooded – Also called wed batteries, this type of lead-acid batteries is the oldest from all and even after hundred years it is still the most cost-effective. The electrolyte in these batteries is a mixture of 30% sulfuric acid and 70% water. They are perfect for well ventilated areas (because they produce hydrogen when charged) where extreme temperatures are not a concern.
  • Gelled – The electrolyte in these batteries is turned into a jelly like substance with the addition of silica gel. This prevents the acid from spilling, if a cell or the battery itself is damaged. The disadvantages of this type of batteries is that they need to be charged slower and with a lower current, compared to the flooded or AGM batteries. If this is not taken into account, dangerous voids can form into the gall this reducing the battery capacity and shortening it’s life. They also tend to loose water in hotter climate and this can eventually lead to the malfunctioning of the battery.
  • AGM – Absorbed Glass Mat batteries have the features of gelled batteries, but lack some of their disadvantages. The electrolyte is contained in the mat and therefore it will not spill if the battery is damaged. Since the mat is only saturated at about 95% with electrolyte, there is no free liquid to expand and therefore the AGM batteries cope with freezing much better. Another neat feature in AGM batteries is that they are designed to recombine hydrogen and oxygen into water inside the battery, therefore greatly limiting the water loss. Also the charging current can be the same as in flooded batteries and the discharge rate is usually lower than 3% per month. Having said that, the cost of an AGM battery is 2 to 3 times greater than the cost of same capacity flooded battery, which will make a great difference for some.
  • Sealed – These types of batteries are not really sealed, as the name implies, because they need to have vents which allow the charging gases to escape. Sometimes they are also referred to as maintenance-free batteries, however maintenance-free batteries are not usually leak proof. This type of lead-acid batteries are frequently designed to last only a month after their warranty expires and they can die prematurely from water evaporation, if cycled too frequently.

Battery Capacity

The capacity of the batteries is calculated in Amp-Hours or AH, which means one amp per one hour. If you have a fully charged battery with capacity of 100 AH and you connect a load that drains 5 A, then the battery will last for about 20 hours. The capacity of most deep cycle batteries, unless specified otherwise, is calculated with the assumption of a 20 hours discharge cycle. You have to keep in mind that if you drain the battery faster that what it is rated for , you will get less amp-hours from it and vice verse. This means that if you have a 100 AH battery, and you hook up a 20 A load to it, it will last for less than 5 hours and you will be able to drain around 90 AH before it gets flat. Also for the same battery, if you hook a 2 A load, it will last for more than 50 hours and you should be able to drain around 110 AH. Of course these figures will vary depending on the battery type and manufacturer, but you get the idea.

Peukert’s law describes the process in detail, but in essence it means that due to the internal resistance of the battery, the more amps you pull from it, the greater the losses. You should be careful with batteries that are rated for much more than 20 hours, as this might turn out to be just a trick from the manufacturer so he can put a higher AH capacity on the battery. Before buying a battery always try to find information for the rate of discharge and size the battery accordingly, as you will be able to discharge  higher capacity battery at a greater rate than a smaller capacity battery.

Voltage and State of Charge

The voltage of a fully charged 12 V battery with no load attached is usually 12.7 V (it can vary from 12.6 V to 12.8 V) and the charge of each cell should be around 2.14 V. The voltage of a flat 12 V battery should be around 10.5 V and it should not be discharged any further, or it may never recover to it’s full capacity. While the following battery voltage readings will give you an idea of the state of charge, you need to keep in mind that they will vary slightly for the different battery models and manufacturers: 12.7 V – 100%; 12.5 V – 90%; 12.41 V – 80%; 12.31V – 70%; 12.19 V – 60%; 12.05 V – 50%; 11.9 V – 40%; 11.74 V – 30%; 11.57 V – 20%; 11.3 V – 10%; 10.5 V – 0%.

To prolong battery life, you need to avoid regularly discharging the battery to less than 40%, still seldom discharges to 30% or even 20% are OK. If you go lower than 20% on regular basis, then you are certainly impacting battery life, while draining the battery flat even occasionally might not allow it to ever recover to full capacity.

Another thing that needs to be taken under account, when speaking of voltages and state of charge, is the battery self-discharge. While it can be just 1% per month for AGM batteries, it can be as much as 15% per month for some industrial batteries or for older batteries that approach their end of life. This can be very harmful for batteries, especially if they are stored and not charged for extended periods of time. The best way to store a battery without charging it, is to charge it to full level and then remove the electrolyte. Still, the storage of such dry charged batteries should not exceed 2 years, especially in hot climates, where the batteries self-discharge faster. The best way to store a battery is to keep it at float charge, even when it is not used. This will eliminate self-discharge and will prolong battery life.

How temperature affects batteries

The temperatures to which the batteries are exposed can have significant effect on their capacity and life. The battery parameters are only valid when the battery is at 25 C – for example a battery rated as 100 AH at 25 C will have a capacity of 112 AH at 50 C, 80 AH at freezing point and 50 AH at -25 C. So the rule is that lower temperatures decrease the battery capacity and higher temperatures increase it. However this comes at a cost – while higher temperatures increase the capacity, they decrease the life of the battery or the number of charge / discharge cycles it will be able to survive. And vice versa – while colder temperatures decrease the capacity, they increase the life of the battery. While a battery will loose 50% of it’s capacity when subjected to -25 C, it’s life will be extended by approximately 60%. Then the effects of heat are also something that needs to be taken into account as battery life will be cut in half for about every 10 C over 25 C.

Over time the effects of colder and hotter temperatures cancel each other out, provided that there are no extremes and proper battery maintenance is performed. Obviously you will want to avoid having the electrolyte freeze or having too much water evaporate.

Charging voltage also changes, depending on the temperature of the battery. For example if it is 13.4V at 25 C for a certain battery, it needs to be 14V at freezing and 15V at -25 C. This is why all more sophisticated charging controllers have thermal sensors, some built into the body of the controller and some have add-on sensors that can be attached to the battery itself. Obviously the ones with built-in sensors will work better if they are subjected to the same weather conditions as the batteries.

Another thing that needs to be taken into account when speaking about temperature effects on batteries it the thermal mass. Due to the mass and the build of the batteries, their temperature does not immediately match the outside temperature at which they are subjected. This is especially  true for large insulated battery banks, where the temperature of the batteries can vary with no more than 10 C, while outside temperature varies with 30 C during day and night. So in order to take the most accurate readings for the actual battery temperature, you will want to connect the add-on temperature sensor from your charge controller to the positive plate terminal and then insulate it from the outside weather conditions.

 Cycles, Depth Of Discharge and Battery Life

Battery life is directly proportional to the DOD or Depth Of Discharge. A DOD of 80% means that a battery that was 100% full is discharged to 20% of it’s capacity. A battery cycle is the process of discharging a fully charged battery to a certain DOD and then charging it back to 100%. The number of battery cycles and the DOD are directly proportional and are the major factors that determine the life of the battery. The life of all truly deep cycle batteries is given in battery cycles and depth of discharge – for example a typical deep cycle battery can be rated at 500 cycles at 80% DOD.However, if you only discharge the same battery to 50% of it’s capacity (or 50% DOD) you will get approximately 1000 cycles. If you discharge it to 90% of it capacity (or 10% DOD) then it may last 5000 cycles. Obviously the figures will vary with the different models and manufacturers, but the basic rule will remain – 10% DOD will give you 5 times more cycles than 50% DOD, which will in turn give you 2 times more cycles than 80% DOD.

Having said that, it is neither practical nor enconomically sane to order a truck load of batteries, only because you want to keep them at most at 10% DOD, so they can last longer. In the long run, practice shows that the best solution will be if your system is calculated to utilize the batteries to an average of 40% to 50% DOD. You also need to keep in mind that regularly discharging the batteries to only 5% DOD  or less, could also shorten their lide (compared to 10% DOD),  as the lead dioxide that builds up on the positive plates does not build up even on the surface.

Battery Charging

From flat to full, the battery passes through the following charging stages:

  • Bulk charge – At this stage the battery is charged until it gets to 80% of it’s capacity. Charging voltage is not really significant at this stage and is usually around 15 V. It is more important that the current does not exceed the safe limits for the battery – usually this is “C/8” for flooded batteries or 1/8 of the rated battery capacity. If the battery is rated at 100 AH for a 200 hour discharge cycle, then the safe charging current will be 100 divided by 8, which is 12.5 A. Gelled batteries require some extra attention as they usually need to be charged with no more than “C/20”.
  • Absorption charge – At this stage the battery is gradually reaching it’s full capacity and the charging current is slowly decreasing as the internal resistance of the battery is increasing. The typical voltage for this stage is between 14 V and 15.5 V.
  • Float charge / Trickle charge – At this stage the battery is already fully charged and it’s main purpose is to prevent the battery from self-discharging. The charging voltage is lower and ranges from 12.8 V to 13.2 V, in order to reduce gasing and extend the battery life.
  • Equalizing charge – This charge prolongs the life of flooded batteries and is only applied about once per month. The voltage is about 10% higher than the full charge voltage and is applied from 2 to 16 hours. During the equalizing charge all cells are charged equally and the produced gas bubbles mix the electrolyte, thus preventing stratification.

Battery Maintenance

In order to be able to squeeze out the most of your batteries, you need to perform regular maintenance checks. The following checks should be performed anywhere between once per month to once per year, depending on the local climate (hot weather will require more frequent water level checks for floating batteries) and the battery usage (golf cart batteries drained to 80% DOD each day and then fully charged during the night will obviously need to be checked at least once every two months):

  • Water level in flooded batteries – the plates need to be covered by water and you need to have at least 4mm of water above the plates. Remember, only add distilled or deionized water – never add acid, unless some of the liquid from the battery was spilled.
  • Hydrometer checks for not sealed batteries – the specific gravity in each cell should be 1.265 for fully charged battery and 1.13 for a flat battery – anything else might indicate a problem. Also remember to measure the gravity at least half an hour after charging, in order to get accurate readings. Charging until the battery makes bubbles also helps to mix the acid in the battery and therefore get accurate reading with the hydrometer.
  • Equalize the flooded batteries once per month.
  • Maintain the temperature of the batteries close to 25 C whenever possible.
  • Keep the vent caps on the batteries while charging – this will reduce the water evaporation.
  • Never fully discharge a lead-acid battery – they don’t have a “memory” and the full discharge will greatly reduce their life.
  • Avoid buying batteries and then storing them for long periods of time. Try to buy them before you actually start using them or if you buy them in advance, keep them on trickle charge.
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