Battery sizing (aH) for solar panel and PWM controller

[thespirit3]

Hi All,

I'm currently experimenting with a small setup before moving to something larger.

My current configuration:-

1 x PWM Charge Controller (http://cgi.ebay.co.uk/270661620905)
1 x 7AH Gell Cell
1 x 60W Panel

This is used to control a PIR activated 12v fluoroescent tube in the garage.

My question is: will the PWM controller adjust charging rate for the low AH capacity battery? Or when low, will it pass the full current from the solar panel? Obviously I need to keep the charge rate around 0.1C otherwise I risk damaging the battery.

Will a PWM controller (especially a cheap chinese model - see above) do the required magic, or should I size my batteries (or solar panels) to maintain a balance of 0.1C charge current at peak conditions?

Thanks,


Steve

[Sunking]

Steve do not use a Gel battery. For 1 you cannot charge a gel cell more than the C/20 rate, and a PWM controller will destroy a Gel is very short period of time. Stick with either flooded or AGM.

[thespirit3]

Thanks for the warning; I wasn't aware of this.

When replacing this with a larger capacity flooded / AGM - should I size the battery to ensure maximum power output from solar cells won't exceed (for example) 0.1C - or would the charging algorithm from the PWM controller prevent over-charging (in the sense of too much current)?

I've checked through hitorical posts but haven't seen this information. I assume storage capacity (AH) is typically so many times larger than the maximum current output of the panels that charge ratio / rate isn't an issue.

Thanks,


Steve

[Sunking]

OK those are pretty good questions. For lead acid batteries you have to check with the specific manufacture and model to see what the maximum charge rate is. Typically for a flooded lead acid the rate max rate is C/10 to C/8, so if it is a 100 AH battery the max charge current is 10 to 12.5 amps.

However with solar systems charging at the max rates is most often times way too much overkill. Meaning you would have a lot more panel wattage than needed for your daily use. Instead solar panel wattage and battery capacity is based on daily watt hour usage and location.

As a quick example let's assume you will use 1 Kwh per day in Tuscon AZ, and another for Seattle WS using 12 volt battery

Battery capacity is the same for both locations [1000 wh x 5] / 12 volts = 416 AH at 12 volts.

Panel wattage needed in Tuscon is based on winter Sun Hours of 5.6 hours so panel wattage = [1000 wh x 1.5] / 5.6 h = 267 watts. So using a 270 watt panel with a MPPT controller you max charge current is 270 w / 12 volts = 22.5 amps, or roughly a C/20 charge rate

In Seattle the winter insolation is 1.2 hours so panel wattage = [1000 wh x 1.5] / 1.2 h = 1250 watts. So with MPPT charge controller the max current is 1250 w / 12 v = 104 amps or roughly C/4 charge rate which would likely demand AGM batteries for that high of a charge rate.

So as you can see from the two examples location means everything.