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I've relented!

Many have asked for this proven design criteria for solar charging systems. It is all too often found that systems are under-designed and the current available from the solar panels is too low to effectively maintain the battery system on a solar site.

Featured here is an Excel97 spreadsheet, developed over time, that has everything to ensure a fully successful solar charging system.

These calculations ensure a working system, not a cheap system. Before embarking on such a project please weigh up the costs of putting in that little extra such that the only visits to site are maintenance visits, not because the system has failed yet again! Cheap systems are costly, not just in money, time, and effort, but in dented reputations too.

Determining the energy requirements.
This is where a lot of mistakes are made. There is a place to enter the basic quiescent current of the equipment. This is the maximum the equipment draws in standby state. Averages are often used which could lead to system failure as they tend to err towards getting the battery size down to reduce costs.

The next step is to determine what each input and output, being active, adds to the overall current drain. In large systems this may not appear to be a major part but in smaller systems it is crucial. In large systems, however, it could assist in ensuring systems are correctly designed.

After the I/O structure is dealt with the radio system (if any) is catered for. These, unless incredibly busy, don't add much to the overall requirements but must, however, be taken into account. Should the system be deemed to grow a high figure may be well worth considering here.

Finally there is an area to enter the current drains of miscellaneous equipment. This would be ancillary equipment attached to the supply. This must include all equipment, including sensors, that draws on the power source.

With all these figures entered the basic hourly and daily energy requirements are known. The hourly is required as this is energy required from the panels during sun hours, only the remainder being available for battery charging.

Autonomy.
This is an often misquoted term. It specifically refers to the amount of time (specified in days in our case) that the system will remain live with no input whatsoever i.e. time till death during no sun. This figure must be chosen carefully as it, again, is often chosen towards the lesser so as to reduce the battery size. An example would be choosing a 3 day autonomy in an area that may not see sun for 2 weeks, this is a recipe for disaster.

Once the energy requirements and autonomy are entered the basic battery size can be determined. This is calculated and rounded off to the next amp/hr.

Energy Requirements
Although battery technology has advanced they are still imperfect items and have an efficiency rating. Standard Lead-Acid batteries are fairly high at 80-85% but NiCd and L-Ion can be as low as 70%, please ensure the correct value is entered here. A simple method of calculating this is

%Eff = 1000 ÷ C/10 charge time

An example would be a NiCd requiring 14 hours to charge at C/10 would result in 1000 / 14 = 71.5%.

Number of days to charge vs Sun hours per day.
These are highly important figures as the number of days to full charge is very dependant on sun hours available. The sun hours should be chosen for mid-winter sun hours available - nothing more. Again, watch for the pitfalls of averages.

After this figure is entered the number of days for recovery is now chosen. This figure is also very important as it affects the number of panels needed. There are limits as shown by the two criteria below.

If the average weather pattern is 1 day of sun out of every two weeks then this must be set accordingly. During this calculation it can often be seen how small systems fail as the recharge time allowed, e.g. 1 day, strains the battery system by exceeding the maximum charge currents allowed (usually C/3).

If the site experiences high levels of sun then the maximum number of days to full charge that should be chosen is shown. This is the maximum allowed to ensure the battery is charged at a current greater than that which could cause sedimentation, this occurring when charge currents are kept below C/10. This figure should not be exceeded if the system is to remain reliable, again a failing of the cost conscious system.

Once these figures have been entered the actual panel requirements are now known. The calculator, based on typical panels available, computes the number of panels required based on the currents available from each.

Also available is a grey day tolerance. This shows the percentage grey allowed (100% = total dark) to maintain the system in a precarious balance.

Further figures are available such as the Value For Money (VFM) indicating the probable best options for panels keeping the cost realistic without compromising too much on reliability.

Fudging the figures
Now be honest, after all this, you did not expect to see a paragraph on this now, did you? The fudging we're referring to is not in order to make the system less reliable but where to ensure the costs are kept to a minimum.

Two figures are highly important and careful consideration can make all the difference. Ensure you have calculated the energy requirements properly. Ensure you have a sufficiently sized battery, and if you require a large one try costing out two smaller ones, you may be pleasantly surprised. If you require a very short recovery time then it will be noted the panel values are shown in red indicating a high or low charge. Go for a smaller or larger battery as required to return to the charge range allowed (C/10 to C/3).

If you have a system not functioning properly this calculator could show you where the problem lies. Often it is too few panels for the size of battery (over catered autonomy). Try reducing the battery size till the charge currents are correct and see if the autonomy is still acceptable, chances are you will not have to spend nearly as much to make the system reliable.

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