Monday, August 27, 2012

Question For You Electrical Engineers

The recently built telescope garage has no power running to it.  This is a minor concern because the Corvette will spend the winter in there, and I would like to use the Deltran battery minder that I bought last winter.  Also, I would like to recharge the telescope mount power cell, and leave it there, near the telescopes.

However, I do have a 35 watt output (under ideal conditions, of course, so I don't really believe it) solar panel that I bought at Harbor Freight some years ago for some experiments.  (Unfortunate discovery: there is really no money available to fund startup work on ways to improve output of photovoltaics.)

With addition of an alligator clip to cigarette lighter plug converters from Radio Shack, I was able to recharge the telescope mount power cell, although I will have to remember to disconnect it when it is fully charged, for risk of overcharging the battery.  However, when I tried to use the same approach to feed the battery minder, no luck.  I used an inverter originally bought for recharging a video camera while driving to feed the battery minder--and the battery minder would not come on.

I verified that the inverter (originally bought for a 2001 vacation to the East Coast) still works; I was able to take output from the solar panel, run it through the alligator clip to cigarette lighter plug, and then plug a 110 VAC input cell phone charger into the inverter--and it recharged the cell phone.

My guess is that the battery minder has a certain minimum amperage input before it starts working, and the inverter just isn't getting enough wattage from the solar panel to provide that minimum amperage.  Any guesses as to how to figure out what that level might be?  The temptation is strong to buy another solar panel (or a larger one) to put in the window of the telescope garage so that it can operate the battery minder to keep the Corvette properly charged in winter, and the telescope mount power cell the other three seasons.

I have a vague recollection that if you want to combine multiple photovoltaic panels together, the lowest voltage output panel determines the net output.  (Or is it the lowest amperage output panel?)  At a minimum, it suggests that if I were to get another PV panel to pair with this, I might as well get another one of comparable voltage and amperage.

UPDATE: Thanks for the suggestions.  I just ordered the Deltran Battery Tender Solar Charge Controller from Amazon.  It was $21.95 plus shipping--an absolute bargain.  It looks like I can use it in the non-snow seasons to keep the telescope mount power cell topped off.


  1. So are you connecting the inverter to the solar panel? Or to a battery? Generally, you'd want to connect the inverter to a battery, and use the solar panel to recharge the battery, that way you can draw more current when needed, and hope it just averages out.

    Looks like the battery minder needs about 125 watts of power.

  2. It's best to parallel panels of like output, but it's not strictly necessary.

  3. The goal was to use the battery minder to keep the car battery recharged, and not directly connect the solar panel to the car battery because of the danger of overcharging the car battery.

    It sounds like I would need a bit more juice to run the battery minder. But perhaps it would make more sense to rely on the fact that in winter, there is little danger of overcharging the car battery from a 35 watt panel. At best, there are 3 1/2 hours of daylight per day, and much of that won't even hit the panel optimally.

  4. My dad and brother-in-law have an aftermarket controler for their panels that they got from harbor freight. The controler stops charging the battery when the battery is full. You might want to look into that. Then you could directly connect the panels to the vett and not worry about it.

  5. And any portion of the solar panel occluded by a shadow reduces the output of the whole panel by more than the percentage occluded.

  6. Sebastian, you're off by a factor of 10.

    Since Clayton is not really needing to recharge the battery (I assume when the Corvette is driven and and parked for the winter, its battery will be fully charged), it will probably start out in float mode--probably drawing no more than a watt or two.

    And Clayton, if you're starting with a solar panel, you'd be better off getting a charger that's designed to take "12v" solar panel as its input; these are a little more expensive than the AC-powered one you got. I got this one to control the output of a 15W solar panel on my boat; West Marine has this much cheaper one-- $29.99 on their web site-- that I also looked at, but can no longer remember why I went with the more expensive unit (unless it was not recommended for AGM batteries, perhaps, which of course would not be an issue with your car battery.)

  7. I should also say that you're totally right not to skip the controller. 2.5 amps of charging current when you only need (swag here) 100ma of float-charge current means a lot of electrolyzing going on. Even 2 hours a day of that kind of abuse really adds up if you're talking all winter.

  8. Deltran makes a solar battery tender that is designed to avoid the regulation and overcharging problems of using solar directly to maintain a battery.

    I leave finding the amazon link up to you. :-)