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PlanMyPower Deep Cycle Solar Batteries

Theory on Deep Cycle Solar Batteries

Deep Cycle Batteries are the Fuel Tank of every OFF-Grid Solar system. The workings, and the understanding of it however, seems to be much more complicated than just the filling up of your vehicle’s fuel tank. On the contrary, if you would compare the energy density of Fuel and a normal Lead Acid battery you will be surprised at the different  power ratios.

If we take 1 Liter of Petrol/Gasoline approximate weight 1 kG, the energy stored in that 1 Liter is approx. 13 kW.h .  In contrast a 105Ah battery at 12V , of approx. 27KG , can only deliver 1300W.h, Thus approximately 50W.h/kg.  This means you need 260kg of battery power to deliver the same energy of 1 Liter Gasoline, Nearly 10 x 105A.h batteries!!

I mainly used two sources to whom I need to give credit  for writing this page.  The First is the e-book called “ BOOK-ENERGY-Unlimited-EN.pdf” , which is a product of Victron Energy on and  “The Boat owner’s Mechanical and Electrical Manual “ at's_Illustrated_Electrical_Handbook.pdf

I am however going to attempt to write the theory on the working and understanding of Solar and power backup deep cycle batteries with a bit more simplicity.

There are various types of batteries which fall under the name of being Lead Acid, although the manufacturing process, the life time and the usage of them differs somewhat, the basic “Battery Chemistry” remains the same

The Deep Cycle battery types under discussion are:

What happens inside the deep cycle solar  battery Chemistry?

Discharge Process of a Deep Cycle batteryFigure 1 The Discharge Process of a deep cycle solar Battery

Discharge Phase of the deep cycle solar battery

Recharging Phase of a Deep Cycle Solar battery

Figure 2 The Battery Recharge  Process of a deep cycle solar

Re-Charging Phase deep cycle solar battery

This is the basic charging and recharging phase of a Lead Acid battery. Now we can look further at other aspects which influence the battery- of which the most important surely is to determine what most influences the life expectancy of a battery

Influences on Battery Life expectancy

Shedding caused in Deep Cycle Solar BatteriesFigure 3 Shedding causes materials to form at the base

All Factors affecting failures in Deep Cycle batteriesFigure 4 All the factors affecting Life Time reduction present in a deep cycle solar battery

Now that we have a basic idea of the working of a Lead acid battery, understanding why there are different versions of batteries, and how to prolong the life time of batteries becomes a bit more understandable, let us discuss further on this topic:

Differences between a Car battery & Leisure or Deep Cycle battery

The Internal Build of a Deep Cycle Battery versus the Car Battery BuildFigure 5 Difference between the Car battery (Top) and deep cycle solar Cycle battery (Bottom)

Differences With-in Deep cycle type batteries

OK, so understanding that, now you can understand that the design of deep cycle batteries would go around decreasing whatever would influence the effects of shedding and corrosion. I do not refer so much to the effect of Sulphation- that is not a spin-off that affects, but much more as a result of neglect.

One thing I do not understand is why they refer to deep cycle batteries as different names. In my opinion if the battery is not manufactured according to the specifications of a traditional “car” battery type, but have cycling ability it is a deep cycle battery, yes from there on, we will get different grades. I.e.- Car battery in a motorbike. And then Deep cycle battery in a Motorcar- of which you get various qualities. But now they refer to it a Semi Cycle, Leisure type, Semi traction, full traction, etc. Why not give it a Deep Cycle – D type, Deep cycle AA+ type etc. because in affect we do talk about a different kind of battery don’t we?

I am not going to be battery manufacturer specific here, but generic, to focus only on explaining the concepts. For now, I cover only the Lead acid family group.

Let us look of methods used to increase battery Lifetime, as well as try to keep battery performance in check

Compartmental units

Deep Cycle Battery- internal Cross Cut explinationFigure 6 Cross cut view of Compartmental deep cycle solar battery

 AGM deep cycle solar batteries  

Lead Alloy related deep cycle solar batteries

The definition of an Alloy is that it is a mixture of metals. Under these fall the batteries Lead Calcium, Calcium Calcium. Lead tin lead Selenium.  Why? Well the functionality remains the same but, the added alloy materials in the deep cycle battery combination, makes the plates Harder. So the plates are not lead only, the plates are more durable and easier to process.

If the plates are harder, they are less prone to corrosion, as well as less prone to Sulfating, it thus allow more charge cycles - simply because the plate can handle more.

The disadvantage though, is that the harder plate cause higher internal resistance, this in turn requires a higher Charging voltage in order for the charge process to be reversed and to be effective

Gel deep cycle solar Batteries (VRLA)

Gel deep cycle solar batteries often referred to as Maintenance-free batteries and also are lead acid batteries but a Silica dust is mixed with the electrolyte. Some more modern batteries such as the so-called Lead Crystals, use a Silicon Di-Oxide   as the electrolyte.  This mixture of added silicon creates a Gel - a slow moving material. Compared to the Flooded, it offers a main advantage that gassing does not occur so easily. For this reason these batteries are Semi sealed, they still do have a “safety Valve relief - so if the pressure would build up, the valve opens and release hydrogen, which cannot be replaced. These batteries are then often referred to as VRLA Valve Regulated Lead Acid batteries. If the first phase of gas is formed at the electrolytes (positive), this gas tends to migrate to the negative plate and re-integrates into the system

One can the clearly see the reduction in the corrosion affect as a result of this

This battery range unfortunately has the effect that it is prone to thermal runaway, or sudden failure of the complete system

It is easy to see that the Gel process, takes more space for the same performance and dynamic responses are also slower.  For this reason gel batteries cannot be used as Car starter batteries and are used where slower charge/discharge processes are required. A bit Like battery storage for solar systems

The so called Lead Crystal batteries, which hold SIO2 as the electrolyte, offer advanced cycle abilities up to 3100 cycles  but at a higher price though (cost per cycle is critical for all battery evaluations).

Combination Options

Power Density of deep cycle solar batteries

From the discussion above, one can see that different battery types will have different power density capacities.  I discussed the pros and cons of different battery technologies; let us see how they compare in kWh per kg:

The effect of temperature on a deep cycle solar  battery

Deep Cycle Battery capacity Is usually specified at a temperature of 20 degree C.  Deep Cyclle Battery temperature and the affect is a bit of an oxymoron- a contrast in itself.  Due to the fact that the battery process is a chemical reaction and all chemical action perform faster at higher temperatures, batteries have a higher capacity at higher temperature.  Thus if a battery capacity is rated to be 100AH at 20C, at 30C the rating will be in the area of 105A.h but at 0 degrees around 85% of the rated value.

However, at the higher temperatures, due to the accelerated chemical process, everything happens faster, thus oxidation as a result thus, Corrosion, and Shedding happens faster.  

Temperature in batteries rise both during the Discharging and Charging phase, but much more in the Charging phase, as a result, damage to batteries occurs much faster during fast and rapid recharging  process.

It is estimated that for every 10 degree increase in battery operating temperature, the battery lifetime is reduced by 50%


The intention of the page is to give you a basic idea of the operation of a battery, as well as a short description of the working of each

One can see how the complexity of each battery directly influences the cost of such a battery  and as a consequence, also the expected life time

It is clear rapid recharging is the single biggest killer of any battery - with gel especially it can cause a thermal runaway

In short, whichever battery is used, if you maintain within operational parameters of discharging/Charging and temperature, life will be increased

Copyright : Dr Gawie van der Merwe; PlanmyPower (PTY) Ltd ©

Battery type

 Volume Wh/dm^3

Weight Density Wh/kg

Starter (car Battery)


50 W.h

Semi traction ( Thick Wall)


47 W.h

Traction Wet Spiral thick wall


40 W.h



32 W.h

VRLA AGM battery


35 W.h



25 W.h

Hoppecke Solar Flooded


30 W.h

Various  Deep Cycle Batteries, the Unit on the Left Is a typical Front connection VRLA Gell battery , where as the oneMonbat Solar Deep Cycle battery on the right in contrast is a Thick Plate Flooded Lead Acid , electrolHoppecke Deep Cycle Solar Battery yte circulating Deep Cycle Solar battery

Table 1- Various Power Density comparisons of deep cycle battery types