just to start off!

BB.

Re: just to start off!

If this Item can charge 24 VDC (say really closer to Vmp=30 volts max for charging a 24 volt lead acid battery), there is nothing wrong with connecting to a 12 lead acid battery (of at least 8 AH capacity)...

The "draw back" to a Vmp=30+ VDC panel and charging a 12 volt battery is the old electrical equations for Power:

P=V*I

P=30 volts * 0.4 amp = 12 watts
P=15 volts * 0.4 amp = 6 watts...

Basically, you are "wasting" 1/2 the output of the solar panel--unless you can "chop the wiring in half" and change it to 15 volt output for charging a 12 volt battery (the old series/parallel) question.

Without knowing the actual specifications of the panels--it is difficuilt to determine the optimum connection/voltage/battery combination (or, you can do your own experiments to figure out Vmp for the panels).

This is why a MPPT type solar charge controller is so nice... Basically, instead of "wasting" the difference in voltage*current (between 30 volts and 15 volts in my mythological example)--the MPPT controller acts sort of like a variable "DC" transformer between the panels and the voltage--the MPPT controller "maximizes" the P=V*I for the solar panel and converts that into the maximized P=V*I of the battery bank.

However, this conversion does "waste" some power--and is not practical with currently available MPPT controllers for very small solar panels (like 12 watts in this case).

My guess is that at 24 volts--the panel current starts to drop off towards zero--it probably maximizes around 17-18 volts--which is just fine for charging a 12 volt battery.

-Billl

BB.

Re: just to start off!

sundawg wrote: »

Yes so now i want just the lights too.... Theroetically i could use an 18 vdc dc regulator straight from the panel then.......like this
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2306

That controller appears to have a ~17 volt limit... It is likely that it would be destroyed if connected directly to your solar panel... However, it should work just fine for connecting to a 12 volt battery which is connected to your solar panel.

-Bill

System2

Re: just to start off!

BB. wrote: »

If this Item can charge 24 VDC (say really closer to Vmp=30 volts max for charging a 24 volt lead acid battery), there is nothing wrong with connecting to a 12 lead acid battery (of at least 8 AH capacity)...

The "draw back" to a Vmp=30+ VDC panel and charging a 12 volt battery is the old electrical equations for Power:

P=V*I

P=30 volts * 0.4 amp = 12 watts
P=15 volts * 0.4 amp = 6 watts...

Basically, you are "wasting" 1/2 the output of the solar panel--unless you can "chop the wiring in half" and change it to 15 volt output for charging a 12 volt battery (the old series/parallel) question.

Without knowing the actual specifications of the panels--it is difficuilt to determine the optimum connection/voltage/battery combination (or, you can do your own experiments to figure out Vmp for the panels).

This is why a MPPT type solar charge controller is so nice... Basically, instead of "wasting" the difference in voltage*current (between 30 volts and 15 volts in my mythological example)--the MPPT controller acts sort of like a variable "DC" transformer between the panels and the voltage--the MPPT controller "maximizes" the P=V*I for the solar panel and converts that into the maximized P=V*I of the battery bank.

However, this conversion does "waste" some power--and is not practical with currently available MPPT controllers for very small solar panels (like 12 watts in this case).

My guess is that at 24 volts--the panel current starts to drop off towards zero--it probably maximizes around 17-18 volts--which is just fine for charging a 12 volt battery.

-Billl

what type of specifics are required on the panel.....i did look at the MPPT but they dont seem practical for this small a set up....Also expensive...

BB.

Re: just to start off!

The best "small" MPPT controller is going to be this one (lots of discussions elsewhere here):

Morningstar SunSaver MPPT Solar Charge Controller

It is a 15 amp (output to battery bank) controller that can handle ~200 watts of "12 volt" and ~400 watts max of "24 volt" panels...

Just to clarify the above statement (thanks Niel!): The SunSaver's ability to handle 200 or 400 watts is based on the voltage of the battery bank ("12" or "24" volt battery bank at 15 amps). The input panel voltage, as long as it is (roughly) 2 volts over the battery bank voltage and less than the controller's maximum input voltage, does not affect the charge controller's maximum solar panel wattage.

Still way too large and expensive for your needs. Staying with a simple PWM controller (or equivalent analog) will be just fine.

Regarding your panels--knowing the Vmp (voltage, maximum power), Imp (current maximum power) and Voc (voltage open circuit), is the basics required to figure out the battery and charge controllers needed.

It appears, that they are also plugging these into electric drill chargers and such--just substituting for the "12-18 volt" wall wart (fixed from "wort" which is a beer brewing term--thanks to Mike) transformers (or DC wall worts)... Should be just fine as Harbor Freight has confirmed their tools (probably would work with other similar brands of tools/battery charger sets--but who knows and wants to donate their equipment to test?).

-Bill

BB.

Re: just to start off!

Added clarification note that SunSaver (and most/all? MPPT charge controllers) are limited by output current to the battery bank in terms of maximum panel wattage. If you double the battery bank voltage, the maximum wattage will double (assuming that the input solar panel voltage is high enough to charge the batteries, and that the panel/battery ratings are within the controller's ratings).

Thanks Niel.

-Bill

PS: Thanks to Mike for reminding me that the correct term for Wall Wort is actually Wall Wart transformer...

icarus

Re: just to start off!

The Bluesky series of Mppt controlers have a good reputation as far as I know. I am very happy with mine. http://store.solar-electric.com/sbchco2512vm.html

Tony

System2

Re: just to start off!

Just to let you all know the battery pack lasted 3 hrs running my laptop. it was discharged to 30% when the inverter started to whine. That was from a 100 % full charge. Not to bad.