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The Choice of and design of Deep cycle batteries for Solar Panel Systems and solutions is much more complicated than the design of same for a power backup system, why?

- In a Solar panel systems, cycle -
Batteries get charged and discharged on a daily basis - Deep Cycle Batteries used for solar systems, need to be of a higher grade which allow daily cycling and yet give us prolonged life time

A) An Off grid-

B) True Off-

- Deep Cycle batteries for a solar solutions, have to allow for autonomy, in typical hybrid systems, we looking at the level of autonomy which will carry the user thru any or worst case load shedding conditions
- In true off-
grid situations the deep cycle battery bank might have to carry us thru a 24 hour or possibly a 48 hour period. This is to see thru rainy days.

Deep Cycle batteries are an essential very critical components of any Solar panel solutions system, the under design, over use and under charge of the deep cycle battery cause battery failures, a problem which will keep on repeating itself over in over in the same scenario.

Deep Cycle Batteries are designed according to certain specifications, they have limitations and should be used in accordance to these specifications.

We will herewith present to you considerations to choose the correct deep cycle battery and also the rating of the minimum requirement of deep cycle batteries for your specific solar solutions system application, in order to get the most effective prolonged lifetime out of the deep cycle batteries for any system.

We discuss battery types under a different page and so the rating and quality of battery chargers, the focus here is merely on the design of the deep cycle battery pack. I will do so with the help of a few real examples.

- A 5 kWh per day Small Off grid hybrid Solar system -
typical user inside town who have Eskom to support, but use a solar system to become independent - A 16 kW.h per day Medium size Off grid hybrid Solar system -
typical user inside town who have Eskom to support, but use a solar system to become independent - A 30 kW.h per day Big size Off grid hybrid Solar system -
typical user inside town who have Eskom to support, but use a solar system to become independent - A 16 kW.h per day Medium size FULL Off grid Solar system -
typical user a farm owner - A 30 kW.h per day Medium size FULL Off grid Solar system -
typical user a farm owner

It is very important to understand that in a solar panel system, deep cycle batteries could actually cycle more than once per day. A system should always be designed in such a way that it could not, but in reality the nature of the deep cycle batteries (which is chemically slow in reaction) and the type of unpredictable load,

- I e, today a washing machine is used tomorrow not
- a kettle is boiled more often today-
maybe nothing tomorrow - Today the Sun might shine tomorrow not

This, has the affect that the load is variable and unpredictable . We actually do not know how deep cycle batteries really react on this notion over a long time, so we can only try and limit this cycling of deep cycle batteries and the design accordingly.

Deep Cycle battery data curves is always based on STC (Standard testing conditions) , ie tested at a fixed temperature, tested with fixed constant load etc.

- Solar Panels deliver 5kW.h per day
- Batteries need to be able to handle the 5kW.h charging and discharging per day and preferable not discharge more than 30% (30% DOD)
- On a 5kW.h solar system per day, with a 5 hour charging we can assume 1kW of Solar panel array
- If we assume a 48V system, the peak charging current will be 1000W/48V = 20A
- Thus the smallest battery bank we will allow is thus 100A-
because we do not want to have a recharging rate of higher than C/5 (Capacity /5) , thus if we have 20Ax5= 100A. This is the First test - 5kWh energy rotation, in battery bank, with maximum 30% DOD. Thus our battery bank need to have the ability of 5kW.h/0.3 = 16.6 kW.h battery bank
- This means 16600Watt.hour/48V = 345A.h Battery bank
- For this we would thus suggest 8 x 180A.h Monobat VRLA deep cycle batteries , or 8x 200A.h AGM deep cycle batteries
- In our suggestion, you could can expect up to 5000 Cycles, which means a expected battery lifetime of 10-
12 years - In Reality due to budget restraints -
most people will not go for more than 4 of the equivalent deep cycle batteries, which will give approximately 1500 cycles, which is equivalent to 4- 5 years lifetime ( An estimated R18000 difference in cost for a 6 years lifetime expected increase)

- Solar Panels deliver 16 kW.h per day
- Batteries need to be able to handle the 16 kW.h charging and discharging per day and preferable not discharge more than 30% (30% DOD)
- On a 16 kW.h solar system per day, with a 5 hour charging we can assume 3.2kW of Solar panel array
- If we assume a 48V system, the peak charging current will be 3200W/48V = 66A
- Thus the smallest battery bank we will allow is thus 330A-
because we do not want to have a recharging rate of higher than C/5 (Capacity /5) , thus if we have 66Ax5= 330A. This is the First test - 16kWh energy rotation, in battery bank, with maximum 30% DOD. Thus our battery bank need to have the ability of 16kW.h/0.3 = 53.3 kW.h battery bank
- This means 53300Watt.hour/48V = 1110A.h Battery bank
- For this we would thus suggest 24 x 180A.h Monobat VRLA deep cycle batteries , or 24x 200A.h AGM deep cycle batteries
- In our suggestion, you could can expect up to 5000 Cycles, which means a expected battery lifetime of 10-
12 years - In Reality due to budget restraints -
most people will not go for more than 8 of the equivalent deep cycle batteries, which will give approximately 800 cycles, which is equivalent to 3- 4 years lifetime ( An estimated R48000 difference in cost for a 6 years lifetime expected increase)

- Solar Panels deliver 30 kW.h per day
- Batteries need to be able to handle the 30 kW.h charging and discharging per day and preferable not discharge more than 30% (30% DOD)
- On a 30 kW.h solar system per day, with a 5 hour charging we can assume 6kW of Solar panel array
- If we assume a 48V system, the peak charging current will be 5000W/48V = 125A
- Thus the smallest battery bank we will allow is thus 525A-
because we do not want to have a recharging rate of higher than C/5 (Capacity /5) , thus if we have 125Ax5= 650A. This is the First test - 30kWh energy rotation, in battery bank, with maximum 30% DOD. Thus our battery bank need to have the ability of 30kW.h/0.3 = 100 kW.h battery bank
- This means 100000Watt.hour/48V = 2083A.h Battery bank
- For this we would thus suggest 44 x 180A.h Monobat VRLA deep cycle batteries , or 44 x 200A.h AGM deep cycle batteries or 24 x 2000A.h Hoppecke 2V batteries
- In our suggestion, you could can expect up to 5000 Cycles, which means a expected battery lifetime of 10-
12 years - In Reality due to budget restraints -
most people will not go for more than 16 of the equivalent deep cycle batteries, which will give approximately 800 cycles, which is equivalent to 3- 4 years lifetime ( An estimated R112 000 difference in cost for a 6 years lifetime expected increase)

- A Full OFF-
Grid system, do not have the backup support of Eskom - A Client on a farm will also need 2-
3 days of autonomy - So not only do you need the batteries not to cycle more than 20%, but they must last and support he house for up to 3 days
- The situation of no charging for 2 to 3 days do not happen often, maybe 3-
4 time per year, so we can allow the occasional bit of deeper discharge - It will be ideal to double up or triple up the deep cycle battery bank in reality it becomes to expensive, but engineered this would be the absolute correct way
- We must also be careful . We can also have to many deep cycle batteries batteries for a specific solar panel array bank
- Solar Panels deliver 16 kW.h per day
- Batteries need to be able to handle the 16 kW.h charging and discharging per day and preferable not discharge more than 20% (20% DOD)-
we reduce it to 20%, because we have to compensate for the times in which we need to go down to 50% to assist with autonomy - On a 16 kW.h solar system per day, with a 5 hour charging we can assume 3.2kW of Solar panel array
- 16kWh energy rotation, in battery bank, with maximum 20% DOD. Thus our battery bank need to have the ability of 16kW.h/0.2 = 80 kW.h battery bank
- This means 80000Watt.hour/48V = 1666A.h Battery bank
- The situation
- For this we would thus suggest 36 x 180A.h Monobat VRLA deep cycle batteries , or 36x 200A.h AGM deep cycle batteries
- In our suggestion, you could can expect up to 5000 Cycles, which means a expected battery lifetime of 10-
12 years - In Reality due to budget restraints -
most people will not go for more than 8 of the equivalent deep cycle batteries, which will give approximately 800 cycles, which is equivalent to 3- 4 years lifetime ( An estimated R112000 difference in cost for a 6 years lifetime expected increase)

- A Full OFF-
Grid system, do not have the backup support of Eskom - A Client on a farm will also need 2-
3 days of autonomy - So not only do you need the batteries not to cycle more than 20%, but they must last and support he house for up to 3 days
- The situation of no charging for 2 to 3 days do not happen often, maybe 3-
4 time per year, so we can allow the occasional bit of deeper discharge - It will be ideal to double up or triple up the deep cycle battery bank in reality it becomes to expensive, but engineered this would be the absolute correct way
- We must also be careful . We can also have to many deep cycle batteries batteries for a specific solar panel array bank
- Solar Panels deliver 30 kW.h per day
- Batteries need to be able to handle the 30 kW.h charging and discharging per day and preferable not discharge more than 20% (20% DOD)-
we reduce it to 20%, because we have to compensate for the times in which we need to go down to 50% to assist with autonomy - On a 30 kW.h solar system per day, with a 5 hour charging we can assume 6kW of Solar panel array
- 30kWh energy rotation, in battery bank, with maximum 20% DOD. Thus our battery bank need to have the ability of 30kW.h/0.2 = 150 kW.h battery bank
- This means 150000Watt.hour/48V = 3200A.h Battery bank
- The situation
- For this we would thus suggest 72 x 180A.h Monobat VRLA deep cycle batteries , or 72x 200A.h AGM deep cycle batteries or 24 x 3000A.h 2V Hoppecke Battery Cells
- In our suggestion, you could can expect up to 5000 Cycles, which means a expected battery lifetime of 10-
12 years - In Reality due to budget restraints -
most people will not go for more than 24 of the equivalent deep cycle batteries, which will give approximately 800 cycles, which is equivalent to 3- 4 years lifetime ( An estimated R192000 difference in cost for a 6 years lifetime expected increase)

In Reality the true requirements and design of a Deep Cycle Battery bank for a true off-

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