How do the 12v deep cycle gel battery act in a solar power system? And how to read the data sheet and rating label for the 12v deep cycle gel battery?

When we decide to buy an off-grid solar power system, there are gel battery and lithium battery option for the back up battery.

Here, let me show you an introductory guide for the 12v deep cycle gel battery.

12v deep cycle gel battery have been used almost 40 years and have the advantage of a very low discharge rate. This low discharge rate makes them work good for the back up power.Thanks to the gel , 12v deep cycle gel battery are very resistant to vibrations.

12v deep cycle gel battery is using gel as electrolyte instead of liquid acid, rechargeable no need maintenance. It's regulated by a valve, usually produced by homogeneous dispersion of cryogenic silica in diluted sulfuric acid.

The 12v deep cycle gel battery has no liquid inside, the electrolytes is in gel form and stand solid between positive and negative plate inside battery.

For an off-grid solar or wind power system, there are 2way for charging the 12v deep cycle gel battery. Using solar or wind, or by city grid. We not suggest using diesel to charge the battery, because diesel power not stable, it may have risk for charging the 12v deep cycle gel battery.

Usually, the working mode for an off-grid solar power system is: In daytime, solar charge the battery. At night, battery supply power. When battery power not enough, inverter switch to use grid power or diesel power as back up. When next day the battery is fully charge, inverter will switch to use battery power again.

Here show you the connection drawing:

There are 2way for charging the 12v deep cycle gel battery, by solar panel via solar charge controller and by inverter.

Here is the data sheet of an MPPT charge controller.

And there are 3 stages for the charging mode:
CC (Constant current) CV(Constant voltage) CF(Floating charge)

For example, your system is 12 DC, using 1pc 12v deep cycle gel battery in parallel, and your solar panel is 250w. Then you choose the R4840 MPPT charge controller.

For the R4840 12V system,
CV(Constant voltage)= 14.2V
CF(Floating charge)= 13.8V

The MPPT charge controller will charge the battery in max current according to different voltage. It will track the voltage all the time and give the best current to charge the 12v deep cycle gel battery.

If you are interested, click here to see how the MPPT solar charge controller works.

For the inverter data sheet, the Max AC charging current is 0A-20A. (Depending on the model, the maximum caring power is 1/4 of the rated power).

For example, for the

So for the 5kw inverter Model: 50248/96 (502), max charging current by city grid is 20A.

Charging 12v deep cycle gel battery, just like filling water to the water tank.
For example, you have a 12v 200ah deep cycle gel battery, totally empty, 0 power. (We not suggest to use 100% battery power, here is for better understanding)
And now using a 250w solar panel to charge it in daytime.

The charging current is 20.8A.
Your battery is 12V 200Ah deep cycle gel battery, 0 power.
Then it will takes

It will takes 9.6hrs to full charge a 12V 200Ah deep cycle gel battery via MPPT charge controller.

If you are using city grid to charge the 12V 200Ah deep cycle gel battery via a inverter. The max charging current is 20A.
Then it will takes

It will takes 10hrs to full charge a 12V 200Ah deep cycle gel battery via inverter.

Here is our 12v 200Ah 12v deep cycle gel battery data sheet.
It means if your battery is 100% full, and your discharge current is 20A.

And if your discharge current is 320A.(We not suggest using such big discharge current, it will hurt your battery)

It depends on many factors, including the use frequency and how do you maintain the battery for the 12v deep cycle battery.

The average lifespan of a deep cycle battery is about 3-6 years. We have a customer using 8 years for the UPS system.

However, the lifespan of a 12V deep cycle battery will based on many factors such as use frequency, how it is used and maintained, and the climate in your place. If you live in a hot place, your 12v deep cycle batteries life span would be shorter than in cold place.


When we talk about lifespan of the batteries, we not based on the years, but we based on the cycles.
Just like a car, when you evaluate the condition of a second car, you take notice of the mileage than the production year. This idea is same as the battery.

Here involve a term call DOD(Depth of discharge).

50% DOD means you use 50% power each cycle.
100% DOD means you use 100% power each cycle.(We not suggest to do that)

Then your the 50%DOD life span is twice than 100%DOD.

And here is the data sheet of our deep cycle gel batteries, in 30% DOD, you can use about 1700cycles, 5years. And if you use less DOD, then your battery life span is much more longer.
That’s why in UPS system, the battery lifespan is longer than off-grid system.

Beside the cycle time, what else also affect the lifespan of a deep cycle battery?
The most important factors are:

With the time past by, the battery will get older, and it will bring the higher internal resistance.
That’s why we not suggest to mix new and old battery in same system.

It's hard to predict your 12v deep cycle gel battery life span, barbecue we don't know how you will use the battery. And it also dependent on how well you maintain and monitor the battery. If you do the great job, you will get long life span 12v deep cycle gel battery.

In order to extend your battery life span, below is our suggestion

Your battery performance will go bad if there are dirt and corrosion.

And your battery terminals should keep clean all the time. If not, they will corrode and reduce the current flow to and from the battery, which can lead to reduced performance and eventual failure. 

And store your battery in a cool place, protected from extreme heat and cold.

One of the confusing problem is that you don't know how much battery left in your solar system, if you own an off-grid solar power system. This is also known as "state of charge". We view the battery voltage as a measure of this.

You can see the battery voltage by MPPT controller or the inverter, but sometime not totally accurate. and the easiest way to determine this is with a multimeter.

Here show you the voltage and the % of the battery left.

Note: The figures are based on open circuit voltage.

The table below shows the voltage and approximate state of charge for battery.

Why sometime it goes to 14.5V, but my inverter told me that the battery power is low?
If the battery not supply any power to any home appliance for a few minutes. This is what we call open open circuit.
At this time, the battery voltage is not real. If you use battery to supply power, the voltage will decrease at once.

How to know the real voltage from the 12v deep cycle gel battery?
If your system power is 5kw, connect 3kw home appliance for few minutes, and read the voltage from the inverter or MPPT controller. At this time, this is the real 12v deep cycle gel battery voltage.

Never 100% discharge the 12v deep cycle gel battery.
Because once it goes to 0, it may not recover back. It will damage the battery.

So never fully discharged the 12v deep cycle gel battery.

But inverter will have protection for the battery, no need to worry about over discharge the battery.

Our inverter has setting protection for the battery. It will never goes over discharge or over charge.

We suggest discharging the 12V deep cycle gel battery no lower than 50% DOD, max 80% DOD. For the solar system life span.

It won’t get explode. Because inside the gel battery, there are gel. It very stable and won’t have any problem. 

Gel batteries will not leak out if the casing becomes damaged, so there is a reduced risk of harm coming to the equipment and clean up hazards.

The best way to repair a gel battery is to discharge it as much as possible and then charge the battery slowly.
In your solar power system, the DC voltage should be 24V,48V 96V or 192V 384V.
If one of your battery bank voltage is quite different than other.
You can pull it out, and discharge it as much as possible, and use city grid to charge the battery slowly. Until it same as other battery voltage.
And this is why we should maintain the battery on time. Because if one battery is fail, the other battery voltage will be pull down by this one.
And it will harm the system life span if you not find out this battery problem.

Wang has over 10 years of experience in off-grid projects. And know how to wire the system well. Making every installation step easier.