Trying to calculate needed battery capacity

[amiklic1]

I was just trying to calculate something.

Let's say I have one 24W lightbulb, vorking on 12V voltage. If it's on for 4 hours/day and 7 days/week.

It draws 24W/12V = 2A.

2. A * 4hours = 8Ah/day
   
8. Ah/day * 7 days= 56 Ah/week
   
   

I would like to include safety factor of about 20% in this, so it's total:

56 * 1.2 =  67,2 Ah

To give back to the batteries those 67.2 Ah, I need:

67.2 Ah * 12V = 806.4 Wh

Practically, I need my wind generator (125W) to give 10 amps for total of 7 hours (for 70 Ah, exactly), or 14 hours giving 5 amps (again for 70 Ah)

If those were real calculations, is battery capacity of 70 Ah enough for this setup, to gain 7 days autonomy?

 I'm a bit confused with battery capacity ratings. C10, C100..... What's that all about??

It all seems so easy this way!

[scoraigwind]

You should double the amphours if you want the battery to last.  Also if there is no one around looking at the voltage, have an automatic disconnect when it drops below (say) 11.9 volts.

YOu are using it for 28 hours, so something between C10 and C100 would be right.

[SamoaPower]

In one practical word - no.

First of all, you need to give more charge back to the battery than what you remove since the charge efficiency is about 90%. You normally don't want to discharge a deep-cycle battery more than about 25% of it's actual capacity on a regular basis if you want reasonable battery life. If it's a starting battery, a 10% limit is better. I say actual capacity because a battery loses capacity as it ages and is used. A rule-of-thumb for a well-cared for battery, that has not been chronically undercharged, is 8% per year. Mis-treat it and it can be a lot more.

So, a conservative design that, after four years, would still provide you sufficient capacity so that you needn't discharge more than 25% in your 7 day carry-over time, would require a new battery capacity of 412 Ah. I know, it sounds like overkill to power one 24 watt light and many people compromise this.

Actually, it's C/10, C/100, etc. You divide the actual capacity (C) by a factor to obtain a rate in amperes. It refers to a charge or discharge rate, not capacity per se. For example, another rule-of-thumb is that you shouldn't charge a battery at a rate greater than C/10. For that 412Ah battery, the charge current should be limited to 41 Amps.