Re: Portable batter packs and Solar Charging?

From Wear-Hard Mailing list Archive (WH) Maintained by R. Paul McCarty

On 7/12/06,  <> wrote:
...
> I know you can get NiMH ones, I can see two from where I'm sitting. At

Looking at the Xantrex battery pack, it's about the same size as two
RC car 7.2v packs side by side with a casing around it. Hmm...

...

> I can't see any reason you couldn't do it that way, but you'll have to
> do the math yourself as far as what current loads you'll be using, for
> how long, what batteries you use, and how much time you have to charge
> it. It is unfortunate that you can't use DC instead of AC, as the
> inverter will cause some loss. Admittedly, most modern inverters are
> quite efficient, around 85-95%, but it complicates things [1].

OK I did some math today:
No load current draw of the Xantrex Pocket Inverter is 0.25 A * 12V =
3 watts wasted just having the thing plugged in. Voltaic backpack max
output of solar panels is 4 Watts. So in direct sunlight, maybe 1 Watt
would not be wasted in the inverter.

Voltaic shows it takes 8-10 hours to fully charge the battery pack
that comes with the backpack: 10h * 4W = 40Wh, that doesn't match up
the numbers below. Going backward (see below) 15.84 Wh / 10H = 1.584
Watts. Maybe that's what it averages if not angled directly as the
sun--or is my math wrong? or my thinking (quite likely ;-)?

The Xantrex Pocket Powerpack has a 3.3Ah capacity, but it doesn't
specify the voltage ;-) Guessing it's 12V, so 12V * 3.3Ah = 29.6 Wh

The Voltaic backpack battery pack is 2.2Ah * 7.2V = 15.84 Wh

RC car batt back: 3.2Ah * 7.2V = 23.04 Wh

Two RC car battery packs will have 46.08 Wh of energy (is that energy?
not thinking so well right now)

So it looks like two RC car packs would be approx the same amount of
energy as the Xantrex battery pack, and probably about the same size.

46.08 Wh / 4 Watts max charging rate = 11.52 hours of direct sunlight
;-) Double that if only average 2 Watts.

...

> I have a pair of 3000mAH 7.2V NiMH packs, in parallel for 6000mAH.
> That runs into a board that has a 5V 3A regulator and a 12V 1A DC-DC
> converter. I need to upgrade the board to use a 5V 5A regulator that I

How big/heavy/safe/hard to build is your board?

Would it be bigger/heavier/less efficient to have a 12V regulator in
parallel with the 5V regulator instead of regulating then converting?

Same question if you reversed it and had a 12V regulator then a DC-DC
conversion down to 5V.

--
Craig

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