Re: (ET) Solar Cell wattage needed

["David Roden (Akron OH USA)" <roden ald net>]

On 7 Nov 2002 at 21:15, Bill wrote:

> How many solar cell watts are needed to recharge
> in a week? 

Your pack is roughly 220 amp-hours.  Most people don't discharge it more 
than 50% or so, so let's say for simplicity 100 amp-hours.  That would be 
3,600 watt-hours.  You typically need to charge about 5-8% more AH than 
you 
discharged to reach full charge, so we'll say 3,900 wh.

Suppose you used 3 of those little battery maintainer modules (one across 
each battery pair), and by some miracle conditions were absolutely ideal.  
Each panel actually put out its rated 1.5 watts (total 4.5 watts) for 8 
hours a day, for a total of 36 wh per day.

If my math is right (and I admit I'm not at 100% charge myself tonight 
<g>), 
at that rate it would take 108 days to recharge the ET.  By that time the 
batteries would probably have been damaged by sulfation.  And that's under 
the absolute best of conditions.  

But you don't have a tracker to follow the sun across the sky, and not 
every 
day is sunny.  So most likely it would really take at least  twice that 
long.  You might also need some kind of charge controller to boost the 
PV's 
output high enough to make the battery charge fully (not sure about that).

In other words, three little solar battery maintainers aren't going to do 
it.  

But suppose you could count on, say, 32 hours of solid sunshine per week 
(I 
don't know how realistic this is), and you have a tracker to keep the PVs 
pointed at the sun.  Now, if my guess is valid, you can charge your 
tractor 
in a week with one 120 watt pole-mounted panel. Or perhaps you could use 
two 
rooftop 100 watt panels without a tracker.  

In fact, I believe Christopher Zack is using PV -- do I remember that you 
have PVs rated 250 watts, Christopher? -- to charge his ET.

> Also my farm tractor sits outside all winter with its 12v battery.
> Would 1.5 watts keep the battery charged or at least safe from freezing
> through the winter?


A lead battery will self-discharge about 5% per month.  I don't know the 
capacity of your tractor's battery, so I'll take a WAG of 100 ah (might be 
high).

That puts its total wh capacity at 1200wh, and means it loses about 60 wh 
per month, or 2 wh per day.  

Now let's consider a typical February day.  Here in Akron, last February 
7th, sunrise was 7:29am and sunset was 5:45pm.  The sun remains low in the 
sky, though, hiding behind the trees.  So I doubt that we got more than 4 
hours of usable sunshine, if that.  

Again assuming you really did get that much, and derating your PV panel 
50% 
to allow for not having a tracker (solar guys help me here -- is that a 
reasonable derating?), we get 3 wh for our February day.  So it looks like 
it's quite possible that your little PV could make up for self-discharge; 
and actually at low temps you will have less self-discharge anyway.

Now remember that this is with the panel aimed south at the correct 
inclination, and exposed to direct sunlight.  Also realize that days on 
which you don't get sunshine it probably wouldn't keep up.  But it might 
well be better than just letting the battery sit.  And the charge current 
would be so low that I don't see how it could possibly damage the battery 
even if it did overcharge (not likely).  


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1970 GE Elec-trak E15 36vdc
1974 Avco New Idea 36vdc
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