On Tue, Nov 16, 2010 at 05:09:57PM -0800, Lew Hodgett wrote:
>
> Cranking motor voltage during starting = 10 VDC
Poor assumption #1: 10V may be getting to the motor, but the load is
across a 12V (or higher) source. Everything above 10V may be getting
lost across wires, switches, etc., but it is getting expended - and will
need to be made up.
> 1,200A x 2 Sec x 10 V = 24,000 W-Sec
> 600A x 3 Sec x 10 V = 18,000 W-Sec
> 300A x 9 Sec x 10 V = 27,000 W-Sec
>
> 24,000 W-Sec + 18,000 W-Sec + 27,000 W-Sec = 69,000 W-Sec
>
> 69,000 W-Sec/3,600/Sec/Hour = 19.16 Watt Hours
>
> Thus 20 Watt Hours is a conservative starting power requirement
> estimate for a medium size diesel engine (60 HP or less).
Poor assumption #2: it would be nice if engines always started in 2
seconds (or even in 9.) There are plenty of times - like bleeding an
engine - when you might be cranking again and again and again, perhaps
30 seconds at a time.
In addition, one of the keystones of electrical and electronic design,
particularly for applications where life and property may depend on it,
is to calculate the worst-case load and double it.
Thus (for the middle case cited):
600A * 30 sec * 10 cycles * 12V * 2X safety factor = 4320k W-Sec
In other words, the charging system would need to be designed to restore
up to 1.2kWh - a much more realistic figure.
> Consider a 10W solar panel:
>
> 1) Assume solar panel efficiency = 50%.
Poor assumption #3: Solar panel output is highly dependent on the amount
of insolation available. This varies greatly, depending on location,
time of year, weather, etc. These days and in these latitudes, my ~250W
bank is giving me about 6.5A during the peak 2 or 3 hours of the day -
and that's only if the weather is good. For the remaining 6 or 7 hours,
it's doing something around 1-3A most of the time.
> 2) Assume available charging time = 8 hours/24 hour day.
>
> Thus 10W solar panel conservatively generates:
>
> 10W x 50% x 8Hrs = 40 Watt Hours/24 hour day.
If we carry across the ratio that I'm actually getting with real -
rather than imaginary or calculated - solar panels, it would be more
like 10Wh/day. This would improve somewhat in the summer or lower
latitudes, of course.
> Thus a 10W solar panel will on average, recharge a starting battery
> within a half day after starting engine.
Only if the world worked the way we want it to. :) In reality, no.
> Probably have to add a means to dump excess power generated by the 10W
> panel.
Sure - if you were charging, say, a flashlight battery. In the real
world (back to *that* annoying concept again), if the incoming current
is under 15% of battery capacity - at least for lead-acid batteries -
it's considered to be self-regulating.
--
OKOPNIK CONSULTING
Custom Computing Solutions For Your Business
Expert-led Training | Dynamic, vital websites | Custom programming
443-250-7895 http://okopnik.com http://twitter.com/okopnik
_______________________________________________
Liveaboard mailing list
[email protected]
To adjust your membership settings over the web
http://liveaboardonline.com/mailman/listinfo/liveaboard
To subscribe send an email to [email protected]
To unsubscribe send an email to [email protected]
The archives are at http://www.liveaboardonline.com/pipermail/liveaboard/
To search the archives http://www.mail-archive.com/[email protected]
The Mailman Users Guide can be found here
http://www.gnu.org/software/mailman/mailman-member/index.html
