Deep Cycle Battery Voltage & State Of Charge

(Last Updated On: March 3, 2021)

What is a Battery?

A battery is a device for electrical storage. Just as a water tank stores water for potential use, storms do not produce electricity; they keep it. Electrical energy is stored or released as the chemicals in the battery change. This process can be replicated several times in rechargeable batteries. Batteries are not 100 percent efficient – when charging and discharging, some energy is lost as heat and chemical reactions. If a battery uses 1000 watts, it can take 1050 or 1250 watts or more to recharge it completely.

Voltage & State Of Charge Of Deep Cycle Battery

If you are the owner of a mobile or off-grid solar energy device, calculating how much charge you have left in your deep cycle battery bank is one of the most obsessive pastimes. This is often referred to as the’ state of charge.’ Most individuals see the battery voltage as a measure of this. While not entirely accurate, if your solar regulator or charge controller does not have a voltage reading, the best way to assess this is with a multimeter. The state of charge varies slightly between types of sealed lead acid, flooded, gel, and AGM deep cycle batteries and between brands. The weather can also play a part.

Battery voltage status of the charging table charging table

For each type of battery, the table below shows the voltage and estimated state of charge.

Note: The statistics are based on readings from open circuits. That is when there is no load on the deep cycle battery, and it hasn’t been under load for a few hours. In a battery-powered device that is in continuous use, this scenario can not occur very often. So, the best time to read is early in the morning until your panels are struck by the sun, at night when the sun is setting, or when it’s very overcast. If you take a reading when the battery is getting a charge, something up to 14.5 volts could be read.

Take the reading while the panels are not exposed to the light, as power is presumably being drawn at the moment. You might conclude that it is a conservative approximation regardless of the voltage reading. Once all the load is removed from a battery, the voltage will dramatically bounce back up.

State Of Charge Sealed Or Flooded Lead Acid battery Voltage Gel Battery Voltage AGM Battery Voltage
100% 12.70+ 12.85+ 12.80+
75% 12.40 12.65 12.60
50% 12.20 12.35 12.30
25% 12.00 12.00 12.00
0% 11.80 11.80 11.80

 Discharge Depth Of Battery

The general rule of thumb: the less you discharge your deep cycle battery before recharging, the longer it will last

An example is here:

Remarkably, a Sonnenschein Solar Bloc 100 AH Gel Battery discharged to a depth of 70 percent, i.e., with just 30 percent or 30 Ah (amp hours) left, would have a lifetime of around 1200 cycles. However, if it only discharges up to 50 percent, the estimated number of cycles will rise to about 1700! That adds over 1.25 years to the life of the battery if a cycle is a day

In most deep-cycle batteries, discharge depth, also known as DOD, should not be more than 50 percent to get the best value for money. So, consider the cut-off discharge depth to be 50 AH if you have a 100 AH battery.

A very critical measurement you can make when selecting the size of a deep cycle battery is the depth of discharge.

Another example is here:

Check the amplifier rating on the adaptor if you want to power a laptop computer. Between the 3 and 5 amp mark, it is likely to be anywhere. This probably translates to around 2-4 amps an hour under regular use as at all times; your laptop will not use the maximum amount. So, on the lower end, centered on:

100 AH battery = 50 AH capacity available/2 amp draw = 25 hours of use.

There are four main types of traditional deep cycle batteries, such as sealed lead acid, flooded lead-acid, gel, and AGM, as stated. See our Deep Cycle Battery Guide to find out more about the difference between them. Find out more about the Tesla Powerwall solar battery.



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