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To use battries, or not. That is the question!


Wetkit
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For me, battries have been the single biggest stumbling block when looking at a solar system.

The simple reason is cost!!!

 

For a small system that does not need to run 24/7, or use much power, batteries is not that big an issue.

 

Problem comes if you looking at constructing say anything larger than 1kW.

Now you have to start looking at battery configurations, like 12V, 24V, 36V or 48V.

For 12V the currents would be much higher and you would need loads of batteries in parralell.

48V requires lower current, but more battries in series again, 4 x more than with 12V.

 

Then you have to look at backup time, charge time and how low the battries would drain every night.

Everytime a battery is drained and charged again, is called a cycle.

All battery manufacturers state battery drain versus cycles.

The deeper the battery is drained, the less cycles it can perform.

 

Backup time is basically your total Ah rating of your battery bank divided by the hours you going to run from it and the total load.

 

Charging time would be how many hours of sunlight you have during the day to charge your battery bank again.

 

Say your load is 500W and constant 24/7.

Normally we get around 8 hours of sunlight during the day, so your battery bank needs to support 500W x (24-8) = 8 000wh.

With a 12V battery bank you will then need 8000wh/12V = 667Ah.

Now, here comes the problem, that is 667Ah with the battries drained 100%.

To get longer life, you do not want to drain the battries lower than 50%, so this gives you 1334Ah.

With standard 100Ah battries, this would be 13 to 14 battries in parallel !!!

 

The next shocker is that you need to put all of this back in 8 hours or less.

500W x 24h = 12 000Wh

12 000Wh / 8h = 1 500W

This means you would need at least 1500W of Solar panels to power your system.

 

Your solar controller also needs to be extremely beefy.

1500W/12V = 125Amp!!!

 

Please let me know if I have made any wrong conclusions or calculations in the above.

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Hi Wetkit

 

Unfortunately battery behaviour is not a linear function.

Have a look at these documents for some clarity on depth of discharge, cycle life, recharge time, watts per cell. The reason I have posted these links is I work with these products daily.

http://www.odysseybattery.com/documents/US-GPL-AM-003_0906.pdf

http://www.odysseybattery.com/documents/US-ODY-TM-001_0411_000.pdf

http://www.enersys-emea.com/reserve/pdf/EN-GPL-AM-005_0911.pdf

http://www.enersys-emea.com/reserve/pdf/EN-GPL-SG-004_0911.pdf

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