If you’re the owner of a mobile or off grid solar power system, one of the most obsessive pastimes is determining how much charge you have left in your deep cycle battery bank. This is also known as “state of charge”.
Learn more about deep cycle batteries!
While not totally accurate, the easiest way to determine this is with a multimeter if your solar regulator or charge controller doesn’t have a voltage readout. State of charge does vary a little between a sealed lead acid, flooded, gel and AGM deep cycle battery types and also between brands. Even the weather can play a role
The table below shows the voltage and approximate state of charge for each type of battery.
Note: The figures are based on open circuit readings; i.e. when the deep cycle battery isn’t under load and hasn’t been under load for a few hours. This scenario may not occur very often in a battery based system that’s continually being used; so the best time to take the reading is early in the morning before the sun hits your panels, in the evening as the sun is setting, or when it’s very overcast. If if you take a reading while the battery is receiving charge, it could read anything up to 14.5 volts.
If you take the reading when the panels aren’t exposed to the sun, as there will likely be power being drawn at the time, you can assume that whatever the voltage reading, it’s a conservative estimate. Once all load is removed from a battery, voltage can bounce back up substantially.
|State of Charge||Sealed or Flooded Lead Acid||Gel battery||AGM battery|
Battery depth of discharge
General rule of thumb: the less your deep cycle battery is discharged before being properly recharged again, the longer it will last.
Here’s an example:
A Sonnenschein Solar Bloc 100 AH Gel Battery discharged to a depth of 70%, i.e. with only 30% or 30 AH (amp hours) remaining, will have a lifespan of around 1200 cycles, which is quite impressive. However, if it’s only discharged to 50%, the expected number of cycles skyrockets to around 1700! If a cycle is a day, that adds over 1.25 years to the life of the battery.
Depth of discharge, also known as DOD, shouldn’t be any more than 50% in most deep cycle batteries in order to get the best value for money. So if you have a 100 AH battery, consider the cut-off discharge depth being 50 AH.
Depth of discharge is a very important calculation you should make when choosing the size of a deep cycle battery.
Here’s another example: If you want to power a laptop computer, check the amps rating on the adaptor. It’s likely to be somewhere between the 3 and 5 amp mark. This translates to probably around 2 – 4 amps an hour under normal usage as your laptop won’t be using the full amount at all times. So, based on the lower end:
100 AH battery = 50 AH available capacity/2 amp draw = 25 hours usage.
As mentioned, there’s 4 main types of conventional deep cycle battery – sealed lead acid, flooded lead acid, gel and AGM. To learn more about the difference between them, view our deep cycle battery guide.