Proposal for Power System Upgrade

James Jefferson KB0THN
24 September 2000

Right now the battery the 7Ah battery and the 10watt panel are a good match. On sunny days the battery voltage will get up to 14.1 volts, at which point the panel disconnects. Under normal sunny conditions the battery is usually charged by noon. The problem arises when there a multiple cloudy days in a row. The battery gets partially recharged, but overall the charge keeps going down.

Battery voltage on a sunny day Click to Enlarge

Battery voltage on a cloudy day Click to Enlarge

I think we need to upgrade both the storage battery and the solar panel. I think we should at least double the size of our PV panel (preferably tripple) and greatly increase the battery capacity.

We should get AGM (Absorbed Glass Mat) batteries which are designed for solar applications. They are basically a hybrid between a gel cell and a deep cycle -- and the have the best characteristics of each. Windsun.com says "A newer type of sealed battery uses "Absorbed Glass Mats", or AGM between the plates. This is a very fine fiber Boron-Silicate glass mat. These type of batteries have all the advantages of gelled, but can take much more abuse." The battery is ideal for our use because it will not freeze, no matience, no charge / discharge current limits, and low self discharge.

Three suitable batteries would be:

1. SunExtender PVX-1248 AGM Sealed Battery 9.41"L x 5.22"W x 9.35"H (to top of terminal) 12 volt 48 Amp-Hours @ 20 hour discharge rate.
   $88.00
2. SunExtender PVX-1260 AGM Sealed Battery 10.25"L x 6.60"W x 9.35"H (to top of terminal) 12 volt 60 Amp-Hours @ 20 hour discharge rate.
   $105.75
3. SunExtender PVX-1285 AGM Sealed Battery 12.91"L x 6.75"W x 9.25"H (to top of terminal) 12 volt 85 Amp-Hours @ 20 hour discharge rate.
   $142.75

I have chosen such large batteries because low temperature change the capacity of the battery. At -22F a storage battery will only provide about 50% capacity. (1) The current station enclosure will not accomodate any battery much larger than what we are currently using. I therefore suggest that we either burry the battery or install a second pole mounted enclosure. If the battery was below the frost line it would never be de-rated more then 20%. If a suffiently large hole is created we might also be able to install other temperature sensitive equipment in it. PVC pipe comes to mind as an excellent solution because a watertight housiung can easily be built.

There are a variety of different solar panel manufacturers that offer ~30watt panels. We are currently using a Solarex MSX-10 panel which provide 10 watts output. During November Minnesota on averages recieves about 50% less power then normal. This means that for the system to work through low-light month it PV output needs to double what is expected.

Three suitable solar panels would be:

1. Solarex MSX-30, 30watt glass panel (uses same mounting scheme as current panel, power cable attached)
   $229.00
2. Unisolar US32, 32watt thin film panel (inexpensive, unbreakable)
   $187.00
3. Siemens SP-36, 36watt glass panel
   $232.00

1. Air 403 Wind Generator, 400 watts.
   $459.00

Our current solar charge controller can handle up to 3.5amps of input, which is more then any of these panels provide. It would not need to be upgraded. Most wind generators include built-in charge controllers.

Our most inexpensive solution using the technology I have outlined above would be a combination of the PVX-1248 48 Ah battery and the US32 solar panel. These items could be aquired for $275 plus shipping. I feel that the combination of the PVX-1285 85 Ah battery and the US32 solar panel will provide the best value. This will cost $329.75 but will tripple the solar output and provide 12 times the power storage capacity.

References:
1) http://www.windsun.com/Batteries/Battery_FAQ.htm