In article ,
says...

"Glenn Ashmore" wrote

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com


I like the idea of diverting to the water heater, but most of us do not have
the option to add a second 12V element to our existing heaters.

One thought, would be to add a second heater in an external loop around the
water heater - something like the add-on heaters that are sold for cars in
lieu of a block heater. This site has the elements:

http://www.realgoods.com/renew/shop/...204/ts/1025078

But, if the solar panels put out, say 150 watts for 8 hours, what happens
when the water becomes too hot? With a small boat type water heater say 6 or
10 gal, this could happen quite quickly. Maybe then we throw the towel?

According to this handy calculator:

http://chuck-wright.com/calculators/watts.html

Heating 6 gallons of water by 100F will require about 240 Watts
for 6 hours. You're not likely to boil the water in in
the heater with that amount of input. With losses, 6 hours
at 150Watts is probably just enough for a hot shower. OTOH,
on a day sunny enough to give you 150 Watts for 6 hours,
you probably don't need a HOT shower. ;-)

Mark Borgerson

Matt,
6hrs at 240watts or 8 hrs at 150 watts - either one would heat water by
about 100F. But what was the water temperature when you started?

If the engine had not been run, it might be say 80F, so we get 180F - not
enough to boil, but enough to scald anyone opening a faucet.

If the engine had been run, the batteries could very well be fully charged
and the hot water tank hot - lets say 150F. Now we anchor and put 240 watts
into the tank for the next 6 hrs. Now the water IS boiling!

Whether or not this is a real world problem, depends on how the boat is
used. For live-aboards or those on extended cruises, it is likely not an
issue. But, for weekend cruisers it could be.

Suppose we return from a long weekend cruise and the batteries are low. We
don't have shore power and rely on the solar panels to recharge the
batteries unattended during the week. We get 8hrs/day for 5 days of bright
sunshine on our 2x75watt array - about 430 Amp hours and we have a 200AH
bank plus starting battery that need a total of about 150AH to recharge.

In a case like the above will the charge controller safely cut off current
to the batteries? Or should we cover the array?

GBM

"Mark Borgerson" mborgerson.at.comcast.net
wrote
GBM wrote:

But, if the solar panels put out, say 150 watts for 8 hours, what
happens
when the water becomes too hot? With a small boat type water heater say
6 or
10 gal, this could happen quite quickly. Maybe then we throw the towel?

According to this handy calculator:

http://chuck-wright.com/calculators/watts.html

Heating 6 gallons of water by 100F will require about 240 Watts
for 6 hours. You're not likely to boil the water in in
the heater with that amount of input. With losses, 6 hours
at 150Watts is probably just enough for a hot shower. OTOH,
on a day sunny enough to give you 150 Watts for 6 hours,
you probably don't need a HOT shower. ;-)

Mark Borgerson

First of all 150 watts of panels is not going to produce 430 AH. You would
be very lucky to get 300 or so. If you can produce 56 AH/day extra just
leave the refrigerator running and have the cold beer ready and waiting for
next weekend. A half way decent refrigeration system should keep cold
without being opened on 50 AH a day.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"GBM" wrote in message
...
Matt,
6hrs at 240watts or 8 hrs at 150 watts - either one would heat water by
about 100F. But what was the water temperature when you started?

If the engine had not been run, it might be say 80F, so we get 180F - not
enough to boil, but enough to scald anyone opening a faucet.

If the engine had been run, the batteries could very well be fully charged
and the hot water tank hot - lets say 150F. Now we anchor and put 240
watts
into the tank for the next 6 hrs. Now the water IS boiling!

Whether or not this is a real world problem, depends on how the boat is
used. For live-aboards or those on extended cruises, it is likely not an
issue. But, for weekend cruisers it could be.

Suppose we return from a long weekend cruise and the batteries are low. We
don't have shore power and rely on the solar panels to recharge the
batteries unattended during the week. We get 8hrs/day for 5 days of bright
sunshine on our 2x75watt array - about 430 Amp hours and we have a 200AH
bank plus starting battery that need a total of about 150AH to recharge.

In a case like the above will the charge controller safely cut off current
to the batteries? Or should we cover the array?

GBM

"Mark Borgerson" mborgerson.at.comcast.net
wrote
GBM wrote:

But, if the solar panels put out, say 150 watts for 8 hours, what
happens
when the water becomes too hot? With a small boat type water heater say
6 or
10 gal, this could happen quite quickly. Maybe then we throw the towel?

According to this handy calculator:

http://chuck-wright.com/calculators/watts.html

Heating 6 gallons of water by 100F will require about 240 Watts
for 6 hours. You're not likely to boil the water in in
the heater with that amount of input. With losses, 6 hours
at 150Watts is probably just enough for a hot shower. OTOH,
on a day sunny enough to give you 150 Watts for 6 hours,
you probably don't need a HOT shower. ;-)

Mark Borgerson

"Glenn Ashmore" wrote
First of all 150 watts of panels is not going to produce 430 AH. You
would
be very lucky to get 300 or so. If you can produce 56 AH/day extra just
leave the refrigerator running and have the cold beer ready and waiting
for
next weekend. A half way decent refrigeration system should keep cold
without being opened on 50 AH a day.

You are right Glenn - 150 watt panels will probably not produce 430 AH. But,
according to this link it should have been about 405 AH:

http://www.qsl.net/ve3lgs/solarpnl.htm

My neighbour at the marina has the problem I described and has recommended
we either install just one 75 watt unit or more batteries. I don't like the
idea of running the refrigeration just to consume power . It gets everything
quite cold in about 2 hrs anyway, once we get to the boat.

Can't the controller simply switch off the current from the solar panels
when the batteries reach full charge?

GBM

My common knowledge, experience from other cruisers and other research
differs from the below:

GBM wrote:
"Glenn Ashmore" wrote
First of all 150 watts of panels is not going to produce 430 AH. You
would
be very lucky to get 300 or so. If you can produce 56 AH/day extra just
leave the refrigerator running and have the cold beer ready and waiting
for
next weekend. A half way decent refrigeration system should keep cold
without being opened on 50 AH a day.

You are right Glenn - 150 watt panels will probably not produce 430 AH. But,
according to this link it should have been about 405 AH:

http://www.qsl.net/ve3lgs/solarpnl.htm


Everything I've heard suggest that you'll get 1/4 typical minimum, 1/3
usually, and about 1/2 AH/watt..

This was confirmed recently by someone who came to check out my wifi,
whose 250W through an MPPT controller reliably gives an average of
80AH/day in St. Petersburg FL, for the last 8 months' experience.

I expect my 360W bank to provide us minimum 90, and probably more like
180 given that we're in the tropics (well, will be when we get there),
and on the order of 120 during our Bahamas winter time. During the day
here in St. Pete, with my batteries typically full because it easily
keeps up with our profligate use of electricity, but no electronics to
speak of, I routinely get 15-20A mid-day, and from 7 on get 7 or more
until the sun's up, or it's going down. If I have a 12 hour sunlight
day (typical in the tropics) and the top is 20-25A in peak times, I see
it as reasonable to get the above.

However, I'd love to be proven wrong and get a full 300AH from our
bank!!

L8R

Skip and Lydia, on the boat, over wifi while Lydia talks to her mother
in England on Vonage, free with the unlimited package we buy...

"GBM" wrote in news:BeTzg.53656$Uy1.27051
@read1.cgocable.net:

Matt,
6hrs at 240watts or 8 hrs at 150 watts - either one would heat water by
about 100F. But what was the water temperature when you started?

If the engine had not been run, it might be say 80F, so we get 180F - not
enough to boil, but enough to scald anyone opening a faucet.

If the engine had been run, the batteries could very well be fully charged
and the hot water tank hot - lets say 150F. Now we anchor and put 240 watts
into the tank for the next 6 hrs. Now the water IS boiling!

Whether or not this is a real world problem, depends on how the boat is
used. For live-aboards or those on extended cruises, it is likely not an
issue. But, for weekend cruisers it could be.

Suppose we return from a long weekend cruise and the batteries are low. We
don't have shore power and rely on the solar panels to recharge the
batteries unattended during the week. We get 8hrs/day for 5 days of bright
sunshine on our 2x75watt array - about 430 Amp hours and we have a 200AH
bank plus starting battery that need a total of about 150AH to recharge.

In a case like the above will the charge controller safely cut off current
to the batteries? Or should we cover the array?

GBM

Once again, use the controller in PWM mode. Solar panels don't need a load,
only devices like wind generators do.

-- Geoff

"Geoff Schultz" wrote

Once again, use the controller in PWM mode. Solar panels don't need a
load,
only devices like wind generators do.


Not all controllers have PWM.

I am considering a marine kit made by Soltek in British Columbia. It comes
with their own controller called a GPR-22 - also sold for RV use under Go
Power name.

The company packages this controller with their kits and presumably it is
satisfactory for basic systems and presumably cuts out panels once batteries
reach full charge and cuts back in at some lower voltage.

GBM