A battery refers to any device that stores energy for later use. Water pumped from a well and up a hill to a tank can be considered a kind of battery, since the energy used to pump the water is stored as potential kinetic energy at the top of the hill and can be recovered by opening a valve and letting the water come back down.

Having said this though, the common use of the word battery is an electrochemical device that converts chemical energy into electricity using galvanic cells where a galvanic cell is a device consisting of two electrodes of different metals or metal compounds (an anode and a cathode) and an electrolyte solution (usually an acid). Each cell produces a nominal voltage (say 1.5V) and battery is often two or more of those cells in series (meaning the positive side of one cell is connected to the negative side of the next cell and so on) to make a 6V, 9V or 12V battery.

For off-grid homes understanding your battries is CRITICAL so we will be spending alot of time here to avoid some very costly mistakes as off-grid batteries are the most expensive part of the whole system due to their short life span compared to the other components.

Battery Efficiency

Batteries do not make electricity, they store it. As chemicals in the battery change, electrical energy is stored or released. In rechargeable batteries this process can be repeated many times. Batteries are not 100% efficient as some energy is lost as heat and chemical reactions when charging and discharging.

Most of the loss in charging and discharging batteries is due to internal resistance. The lower the internal resistance, the better. Also internal resistance is not constant so the more charge that goes into a battery the more the battery does not want to charge.

Typical efficiency in a lead-acid battery is 85-95% and in alkaline and NiCad batteries it is about 65%. You should figure as a general rule about a 10% to 20% total power loss when sizing batteries and battery banks for your storage needs. E.g. if you calculate you need 500A of storage then you should allow for 600A due to losses.

Battery Types

There are a variety of batteries that are used with solar systems, but all are what are called deep-cycle batteries. A deep-cycle battery is a lead-acid or nickle-iron battery designed to be regularly deeply discharged using most of its capacity. In contrast starter batteries (most automotive batteries) are designed to deliver short, high-current bursts for cranking the engine, thus frequently discharging only a small part of their capacity. Using an automotive battery for storing your power will kill it very quickly.

For energy storage the best solutions right now are AGM (Absorbed glass mat) batteries or Edison Cell batteries (Nickle Iron).

AGM batteries differ from “flooded” lead acid batteries in that the acid electrolyte is held in glass mats, as opposed to freely flooding the plates. You can expect a life span of 2 to 10 years depending on how you discharge them.

Edison Cell batteries use an alkaline electrolyte of Potassium Hydroxide and nickle iron plates instead of lead. They are vastly more environmentally friendly and:

  • Have en estimate life of 65+ years.
  • Can not be damaged by overcharging or deep discharging.
  • Have a higher self-discharge rate than an AGM if left in a charged state.
  • Have a much higher initial setup cost than AGM batteries as each battery typically only produces 1.5V of storage voltage.
  • Are not as good at providing rapid surges of current as AGMs, although the technology is improving all the time.


The lifespan of a deep cycle battery will vary considerably with how it is used, how it is maintained and charged, temperature and other factors. Batteries can also destroyed without ever being used in less than a year because they were left sitting in a hot garage or warehouse without being charged. Check the manufacturing date of any batteries you purchase for this reason as you could get caught out with inferior product.

There are some typical expectations for AGM batteries if used in deep cycle service, but due to these factors  it is almost impossible to give a fixed number of how long you can expect your batteries to last, but around 2 to 10 years is standard. Our system on our home is now three years old and the batteries no longer hold any significant charge and are being replaced by Edison Cells.

As a general rule of thumb, if you actually drop an AGM 12V battery below 12V (fully charged a 12V battery is actually about 12.4V) many times you will shorten the lifespan from 10 years to around 3. Edison Cells do not have the same issues and can be expected to give you 65+ years even if you treat them badly.

We purchased our Edison Cell batteries from from China as they are the only military quality manufacturer we could locate. The original Edison Cell battery factory in the USA, made by Edison, was purchased by a competing lead-acid battery manufacturer and promptly shut down.

Battery Capacity

When you purchase a battery it is rated at a specific capacity measured in Amp-Hours. This figure tells you the maximum power output from the battery for a period of one hour. The less you draw from the battery, the longer the supply lasts. For example a 250AH battery could provide 10A for 25 hours, 5A for 50 Hours or 100A for 2.5 hours.

Battery capacity is also reduced as temperature goes down and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year outdoors in the cold, then the reduced capacity has to be taken into account when sizing the system batteries.

Temperature and Batteries

The standard rating for batteries is at room temperature which is 25oC. At approximately -27oC battery capacity drops 50%. At freezing capacity drops by 20%. Capacity is also increased at higher temperatures. At at 50oC battery capacity would be about 12% higher. As a general rule of thumb for every 10oC over 25oC battery life is halved (this is actually not as bad as it seems as the battery will tend to average out as the battery is generally not always very hot nor always very cold).

The voltage required to charging your batteries also changes with temperature. You will need about 16.4 volts at -40oC and 13.8 volts at 50oC. This is why you should have temperature compensation on your solar regulator if your batteries are outside or subject to wide temperature variations. Some charge controls have temperature compensation built in and 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 does not work well.

Adding another complication is that large battery banks make up a large thermal mass. This means that because they have so much mass, the batteries will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. For this reason external temperature sensors should be attached to one of the positive plate terminals (and covered with insulation on the terminal). The sensor will then read very close to the actual internal battery temperature.

Series and Parallel Battery Connection

When you purchase your inverter to supply your solar power station, they will be rated for a specific input voltage of either 12V, 24V, 36V or 48V. In order to meet the voltage input requirement of the inverter, you typically will need to connect multiple batteries together in a specific way known as Series connection.

Series connection means you connect the Positive terminal of one battery to the Negative terminal of the next. Each time you do this, the voltage than can be supplied by the battery array is increased by the voltage rating of that battery. So if you were to connect three 12V batteries in series, you would get 12 + 12 + 12 = 36V as your output voltage.

You increase the storage capacity of your system by connecting the batteries in parallel, which means the negative terminal on one battery connects to the negative terminal on the next and so on. This does not increase voltage supply but instead increases the possible current that can be supplied. So if you had three 12V that were rated at 100AH each, you would be able to supply 12V for 100 + 100 + 100 = 300AH.

There is no limit to how many battries you can connect together. For a normal off-grid home like ours where we run all the typical middle-class Australian family appliances with two kids and two adults using a 48V supply inverter, I would recommend aiming for 500Ah. Using 12V 250A deep cycle AGM batteries as an example, this would mean connecting 4 batteries in series (12 + 12 + 12 + 12  = 48V) and then two of these sets in parallel to give 250 + 250 = 500AH as a miniumum. Better would be 750AH of storage as this gives some leeway for cloudy / low solar array output days.

Battery Charge and Equalisation

AGM batteries are made of a multiple of individual galvanic cells, the sum of the voltages of which make a battery of a given output voltage (typically 12V in our case). If you could measure the voltage output of each cell seperate to its neighbour, you would find that in an ideal world each cell would have exactly the same voltage. In the real world there may be differences and if this difference exceeds more than about 0.2V then the battery could be dying or you may need to Equalise the battery cells.

Equalisation refers to a process where the battery is heavily discharged and then overcharged to “fizz the plates” on the cells and hopefully correct the cell voltage balance (do not do this to Edison Cells, it is not required). Many solar regulators have this as a scheduled event that occurs monthly to ensure that the batteries are kept in the best state, from my research on AGM batteries this is not a good idea. Only ever equalise an AGM battery if your batteries are running out of power very quickly (i.e. they are on their way out anyway). In this case an Equalisation charge may give you some more life. Some manufacturers recommend an equalisation charge once a year. My best advice is to talk to the battery manufacturer as see what they recommend.

These chart below is a ready-reckoner for the current state of charge of 12V AGM battery. These voltages are for batteries that have been at rest for 3 hours or more. For batteries that are being charged the voltage will not tell you anything, you have to let the battery sit for a while.

For longest life batteries should stay in the green zone. Occasional dips into the yellow are not harmful, but continual discharges to those levels will shorten battery life considerably (for 24 volt systems multiply the values by 2 and for 48 volt systems multiply by 4).

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

Cycles vs Lifespan

A battery “cycle” is one complete discharge and recharge. It is usually considered to be discharging from 100% to 20% and then back to 100%. However, there are often ratings for other depth of discharge cycles (the most common being 10%, 20%, and 50%).

Be careful when looking at ratings that list how many cycles a battery is rated for unless it also states how far down it is being discharged as there can be marketing spin to make a battery appear better than it is. For example a batteries be be advertised as having a 20 year life but in the fine print that rating is only at 5% discharge. That same battery could have less than 5 years if cycled to 50%.

Depth of discharge (DOD)

Battery life is directly related to how deep the battery is cycled each time. This is called Depth of Discharge or DOD. If a battery is discharged to 50% DOD every day it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD it will last about five times as long as one cycled to 50%. The most practical number to use is 50% DOD on a regular basis.

Battery Charging

Battery charging takes place in three stages: Bulk, Absorption, and Float.

Bulk Charge: The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts on a 12V battery. There is no “correct” voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.

Absorption Charge: The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2V to 15.5V.

Float Charge: The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level, typically 12.8V to 13.2V, to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it’s main purpose is to keep an already charged battery from discharging.

Battery Charging Currents

Most flooded batteries should be charged at no more than the “C/8” rate for any sustained period. While some battery manufacturers state a higher maximum charge rate higher charge rates can result in high battery temperatures and/or excessive bubbling and loss of liquid.

“C/8” is the rated battery capacity divided by 8. For a 220 AH battery, this would equal 26 Amps. Gelled cells should be charged at no more than the C/20 rate. Some AGM batteries may give a charge rate of C/4, however since very few battery cables can take that much current I don’t recommend you try this at home. To avoid cable overheating stick to C/8 or less.

When assembling your solar power plant, the solar regulator will take care of the correct charge voltage for you changing from bulk to absorption to float as needed. You will normally have to program the charge current according to the storage capacity of the battery, although some will give you the option to set the capacity of the battery and the regulator will then set the C/8 rate automatically.