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PlanMyPower Deep Cycle Solar Batteries

Battery Backup Sizing Calculations And Suggestions


We are all aware of active Load shedding in South Africa, this is equally followed by the in flood of generally low quality off the shelf “solutions to load shedding” , In a single box, would be a 2 kW DC AC Power inverter, with a build in battery charger and a single deep cycle battery promised to give you up to 6 hours of backup during load shedding


It come as no surprise that these systems often work, but within 2-3 months the deep cycle included battery fail, or the client /user would complain that he is not getting the backup time he use to get (deep cycle battery failure).


The problem with most of these units are common, we get calls daily of people who just want to buy a new battery and it often come as a surprise that we prefer not to sell them a new battery why?


The reason is simple, the new deep cycle battery will also be destroyed within 2-3 months and then the client would expect another new battery “under warrantee”


Deep Cycle batteries are an essential very critical components of any Power Backup 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 batteries for your specific 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, in each scenario, I would make suggestion of what should be the minimum size of the internal battery charger rating - assuming it is a good quality battery charger, which take care of battery temperature, Proper Boost/absorb and float phases, which is not part of this discussion- From experience I can advice that cheap low cost systems, also have a low cost build in charger, which do not take care of the battery in anyway, however this discussion is excluded from this page.


I will discuss the calculations based on a few Examples followed by general guideline-  as follows:


  1. 600 W backup for 4 hours (TV, DSTV, few lights)
  2. 1200 W Backup with 4 hours(TV, DSTV, Computers, Lights)
  3. 600 W hours with occasional load peaks of 2 kW, backup 5 hours (Same as per point 1, but including occasional use of Hairdryer, microwave oven or Kettle
  4. 2 kW Backup for 6 hours (typical office, lots of PC’s , Server, printer and Kettle)
  5. 5 kW backup for 8 hours (Big office, or a big house hold, TV, lights, PC, server, Microwave, Hairdryer etc


As Load shedding do not happen every day, as a result batteries are not discharged every day, therefor it is generally acceptable that deep cycle  batteries used for power backup applications can be discharged down to around 60% , as to on solar backup systems, where cycling will be daily, we do not want to go above 30% DOD (Depth of Discharge)

Example 1 (600W Backup for 4 hours)

This is a typical application , in which people only want to keep essentials running during load shedding. I am not being product specific, as TV ratings all differ and so on. Typical appliances usage are, The size of Backup inverter-charger required is no more than a 1kW unit - a 600W Unit should do , but its a bit on the edge


Estimated power 600W,  if you need to keep same running for 4 hours, battery storage requirements would be


The total battery power you would thus need is



If you have a 12V Power inverter-charger , you thus require


I would thus suggest 4x 102 A.h batteries for your application

   If it is a 24V system, the battery power requirements remain the same, as it would be



As deep cycle batteries are not cycled every day, you do not need to go to the top class AGM, lead Crystal or equivalent batteries, the Calcium calcium, Silver Calcium type VRLA batteries are sufficient for this

 At a DOD of 60%, the deep cycle battery lifetime is expected to be around 350 cycles , considering about 70 Load shedding's per year, a life time (Subject to proper battery recharging ability of the power inverter-charger unit),

We could expect a life time of the deep cycle battery of around 350/70 = 5 Years


Example 2 (1200 Watt Backup for 4 hours)

This is a bigger  application , in which people only want to keep essentials running during load shedding, but with a few more computers, maybe 2 or 3 TV’s etc. I am not being product specific, as TV ratings all differ and so on. The size of Backup inverter-charger required is no more than a 1.5kW unit. Typical appliances usage are


Estimated power 1200W,  if you need to keep same running for 4 hours, battery storage requirements would be



The total battery power you would thus need is



If you have a 12V Power inverter-charger , you thus require


I would thus suggest 7x 102 A.h deep cycle batteries for your application

   If it is a 24V system, the battery power requirements remain the same, as it would be



As batteries are not cycled every day, you do not need to go to the top class AGM, lead Crystal or equivalent deep cycle batteries, the Calcium calcium, Silver Calcium type VRLA batteries are sufficient for this.At a DOD of 60%, the battery lifetime is expected to be around 350 cycles , considering about 70 Load shedding's per year, a life time (Subject to proper battery recharging ability of the power inverter-charger unit),

We could expect a life time of the deep cycle battery of around 350/70 = 5 Years


Example 3 (600 W hours with occasional load peaks of 2 kW, backup 5 hours (Same as per point 1, but including occasional use of Hairdryer, microwave oven or Kettle)

This is a typical application , in which people only want to keep essentials running during load shedding, but also need to occasionally run a Kettle, a hairdryer or a Microwave - all three of these items are very heavy on power.


 I am not being product specific, as TV ratings all differ and so on. Typical appliances usage are, The size of Backup inverter-charger required minimum a 2kW units, that is for a small kettle, a Small Microwave and really small hairdryer…a 3kW backup power inverter is preferable in such a situation

The typical normal usage are :


Add to this other appliances, say for average of 30 Minutes, during the 4 hour period


If we assume a average power draw of around 1700W for power appliance for 30 minute,s the power is thus


Your total energy requirement now becomes



The total battery power you would thus need is



   If it is a 24V system, the battery power requirements remain the same, as it would be



As deep cycle batteries are not cycled every day, you do not need to go to the top class AGM, lead Crystal or equivalent batteries, the Calcium calcium, Silver Calcium type VRLA batteries are sufficient for this

 At a DOD of 60%, the deep cycle battery lifetime is expected to be around 350 cycles , considering about 70 Load shedding's per year, a life time (Subject to proper battery recharging ability of the power inverter-charger unit),

We could expect a life time of the deep cycle battery of around 350/70 = 5 Years


Example 4 (2 kW Backup for 6 hours (typical office, lots of PC’s , Server, printer and Kettle)

This is a typical office application , in which office need to keep PC’s Servers and lights operational, but also need to occasionally run a Kettle,  - all three of these items are very heavy on power. A Minimum of a 3kW 24V Power backup inverter-charger is required here.


The typical normal usage are :


If we assume a average power draw of around 2000W for power appliance


The total deep cycle  battery power you would thus need is



   If it is a 24V system, the battery power requirements are



As deep cycle batteries are not cycled every day, you do not need to go to the top class AGM, lead Crystal or equivalent batteries, the Calcium calcium, Silver Calcium type VRLA batteries are sufficient for this. For this application, I would however suggest the Monobat  180A.h VRLA Gell battery .

 At a DOD of 60%, the deep cycle battery lifetime is expected to be around 500 cycles , considering about 70 Load shedding's per year, a life time (Subject to proper battery recharging ability of the power inverter-charger unit),

We could expect a life time of the deep cycle battery of around 350/70 = 7 Years

Example 5 (5 kW Backup for 8 hours (typical BIG office, clinic , lots of PC’s , Server, printer and Kettle)

This is a typical office application , in which office need to keep PC’s Servers and lights operational, but also need to occasionally run a Kettle,  - all three of these items are very heavy on power. A Minimum of a 5kW 48V Power backup inverter-charger is required here.


The typical normal usage are :


If we assume a average power draw of around 5000W for power appliance


The total deep cycle  battery power you would thus need is




   If it is a 24V system, the battery power requirements are



As deep cycle batteries are not cycled every day, you do not need to go to the top class AGM, lead Crystal or equivalent batteries, the Calcium calcium, Silver Calcium type VRLA batteries are sufficient for this. For this application, I would however suggest the Monobat  180A.h VRLA Gell battery .

 At a DOD of 60%, the deep cycle battery lifetime is expected to be around 500 cycles , considering about 70 Load shedding's per year, a life time (Subject to proper battery recharging ability of the power inverter-charger unit),

We could expect a life time of the deep cycle battery of around 350/70 = 7 Years


Conclusion

The designed /suggested battery banks are probably bigger and more costly than you expected. I can assure you that most suppliers will inform you against the quantity and cut on cost, but the reality is according the battery specifications, expected assumptions and field experience, these examples are not over the board. The only uncertainty is the amount of load shedding's and the durations of same- combined with uncertainty of the future.


Again, I need to highlight the importance of a proper battery charger facility of your inverter-charger system, for instance if your 12V system, only charger your battery to 13.8V, your batteries WILL not last, your inverter- Chargers, have to properly look after your batteries, with a proper cycle of Charging/Discharging equalization etc

Without those charger abilities, your batteries doesn't matter what the investment, WILL NOT survive for 5 years plus


Please fee free to contact us, for proper advice on your choice of battery or system design.  Send us an email to  [email protected]   


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