Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 26-06-2014, 07:17   #1
Eternal Member
 
monte's Avatar

Cruisers Forum Supporter

Join Date: Jul 2011
Location: Australia
Boat: Lagoon 400
Posts: 3,650
Images: 1
A Quick Question Re: Fast Battery Discharge Rate With Solar.

Hi
A quick question.
We have 1120W solar with max output of 85A through a victron MPPT charge controller.
I would like to plug my hot water boiler (1200W) in to the socket which is powered through the 1600W inverter
My concern is discharging the batteries too fast at around 100A
We have 4 / 140Ah flooded batteries.
It takes around 25 mins to heat the water.
So my question is, if the solar is putting in 50A, and I am taking out 100A, is that just a 50- 70A draw as far as the batteries are concerned and thus not a problem?


Thanks
Monte
__________________

__________________
monte is offline   Reply With Quote
Old 26-06-2014, 14:19   #2
Don't ask if you can't handle it
 
sailorboy1's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: On the boat somewhere
Boat: Hunter 410
Posts: 12,317
Re: A quick question re: fast battery discharge rate with solar.

I don't think it is a problem as far as the batteries are concerned. The same thing happens when I use my Mr Coffee maker. The voltage will probably drop a lot while you are doing it, but come back up once the big load stops.

The bigger problem may be whether the inverter likes it during with the lower voltage.
__________________

__________________
jobless, houseless, clueless, living on a boat and cruising around somewhere
sailorboy1 is offline   Reply With Quote
Old 26-06-2014, 14:27   #3
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,056
Re: A quick question re: fast battery discharge rate with solar.

No offense meant, but that is just so wrong.
Converting solar energy to 12V DC to convert to 120V AC to heat water is a pretty darn inefficient way of doing things
Using solar to heat water may be a better way to accomplish what your wanting to do, it's not really a boiler is it?
__________________
a64pilot is online now   Reply With Quote
Old 26-06-2014, 14:31   #4
Don't ask if you can't handle it
 
sailorboy1's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: On the boat somewhere
Boat: Hunter 410
Posts: 12,317
Re: A quick question re: fast battery discharge rate with solar.

Quote:
Originally Posted by a64pilot View Post
No offense meant, but that is just so wrong.
Converting solar energy to 12V DC to convert to 120V AC to heat water is a pretty darn inefficient way of doing things
Using solar to heat water may be a better way to accomplish what your wanting to do, it's not really a boiler is it?
Efficiency doesn't matter. Once you have the solar the only thing inefficient would be not using all it can produce.
__________________
jobless, houseless, clueless, living on a boat and cruising around somewhere
sailorboy1 is offline   Reply With Quote
Old 26-06-2014, 14:50   #5
Senior Cruiser
 
Sailmonkey's Avatar

Cruisers Forum Supporter

Join Date: Sep 2009
Location: Houston
Boat: '76 Allied Seawind II, 32'
Posts: 5,780
Re: A quick question re: fast battery discharge rate with solar.

I don't believe you have a problem at all with the load on the batteries, that is a hell of a lot of solar!! You may have a problem with the inverter though!
__________________
Sailmonkey is offline   Reply With Quote
Old 26-06-2014, 15:58   #6
Registered User
 
sparrowhawk1's Avatar

Join Date: Jan 2014
Location: Miami Beach Fl
Boat: Colombia Cc 11.8
Posts: 1,252
I have found that appliances often do not draw their rated wattage. 75w tv at the most draws 60w.(including inverter inefficiency) I run a 1000w hot plate off a 1000w inverter. And you are correct that if you have 50a input and 100a draw your batteries olny provide 50a. And I know cooking w electricty is not efficient but it's 100% efficient if it would have been wasted ie my charge controller cutting output. Which just so happens to happen around lunch time: )
__________________
sparrowhawk1 is offline   Reply With Quote
Old 26-06-2014, 16:32   #7
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,056
Re: A quick question re: fast battery discharge rate with solar.

my water heater draws 10 amps at 120V which is 1200W
__________________
a64pilot is online now   Reply With Quote
Old 26-06-2014, 16:32   #8
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,056
Re: A quick question re: fast battery discharge rate with solar.

How many panels is 1120 W?
__________________
a64pilot is online now   Reply With Quote
Old 26-06-2014, 19:52   #9
Registered User
 
sparrowhawk1's Avatar

Join Date: Jan 2014
Location: Miami Beach Fl
Boat: Colombia Cc 11.8
Posts: 1,252
Quote:
Originally Posted by a64pilot View Post
my water heater draws 10 amps at 120V which is 1200W
I don't mean to be snarky at all but did you test the voltage when you tested the amprage? I've been in marinas that olny had 90 something volts! Edit: it got me wondering what my inverter puts out. Just checked it. its 130 volts with no load and drops to 110 with a 1000 watt hot plate on. Just tried it for a few seconds usually only use hot plate when the sun is shining
__________________
sparrowhawk1 is offline   Reply With Quote
Old 27-06-2014, 02:42   #10
Eternal Member
 
monte's Avatar

Cruisers Forum Supporter

Join Date: Jul 2011
Location: Australia
Boat: Lagoon 400
Posts: 3,650
Images: 1
Re: A quick question re: fast battery discharge rate with solar.

Thanks for the feedback. 1180 W is 4 x 280W panels 1.6m x 1m each across the rear of a cat. I know its not the most efficient way to heat water, but assuming my batteries are full at mid day, and Id like to have some water for dishes and a shower tonight etc, why not make hot water? The water will be hot by 12.30, batteries down to 90% and back up to 100% by 13.30.
PS. I was reading Nigel Calders crusiing guide last night and noticed he does the same occasionally, although I dont think he has as much solar and his battery bank is 100Ah bigger. He did mention to divide the watts by 10 though when calculating AC current through an inverter, I assume for loss of efficiency, so we are likeley to be drawing closer to 120A. As far as batteries, I see they generally are rated 140Ah for 20 hrs, but I think the 5hr rating would be closer to 120Ah, meaning it can draw 24A for 5 hrs. So 4 batteries could supply 96A for 5 hrs, till dead, or 2.5hrs to 50% approx. 30 mins at 100A would reduce the battery charge by about 10%.
Im just typing my thoughts on the matter, so feel free to shoot holes in my calculations or possible downsides so I can consider them. I would like to add 2 more house batteries to lighten the loads and extend the between charge times, especially for on passage ( couple of cloudy days might have the batteries running low)
Cheeers
Monte
__________________
monte is offline   Reply With Quote
Old 27-06-2014, 03:00   #11
Registered User
 
sparrowhawk1's Avatar

Join Date: Jan 2014
Location: Miami Beach Fl
Boat: Colombia Cc 11.8
Posts: 1,252
I suggest getting an ahr meter if you don't already have one. It will allow you to use every last drop and keep you from deeply discharging your battery
__________________
sparrowhawk1 is offline   Reply With Quote
Old 27-06-2014, 05:49   #12
Registered User

Join Date: Dec 2011
Location: Boston Area
Boat: Beneteau 423
Posts: 223
Re: A quick question re: fast battery discharge rate with solar.

Quote:
Originally Posted by monte View Post
The water will be hot by 12.30, batteries down to 90% and back up to 100% by 13.30.
(Sorry if a double post but I got an error when I just tried to post)

Flooded batteries won't be to 100% in an hour. It will be more like 6-10 hours due to the dramatic drop in charge acceptance rate as FLA batteries go above 85% or so of capacity.

Also get a clamp-on ammeter or battery monitor to see the cycle in action a few times.
__________________
hlev00 is offline   Reply With Quote
Old 27-06-2014, 05:58   #13
Registered User
 
sparrowhawk1's Avatar

Join Date: Jan 2014
Location: Miami Beach Fl
Boat: Colombia Cc 11.8
Posts: 1,252
[QUOTE="hlev00;1573090"]

dramatic drop in charge acceptance rate as FLA batteries go above 85% or so of capacity.


I came back to the thread to post that but you beat me to it
__________________
sparrowhawk1 is offline   Reply With Quote
Old 27-06-2014, 06:15   #14
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,056
Re: A quick question re: fast battery discharge rate with solar.

Quote:
Originally Posted by a64pilot View Post
my water heater draws 10 amps at 120V which is 1200W
My Marina surprisingly is very close to 120V + or - 2v on my fluke and even 60Hz, again with little variation.
I only have a 300W Ebay invertor, but it's square wave and under light load it's 120V. I've not checked it under a higher load, just didn't think to try that, but would suspect a voltage sag as if for no other reason it's supply is from a 12V cigarette lighter plug.
I'm 30 amp shore power and now that i have put a 5K BTU AC in the forward cabin, I can't run both AC's and the water heater is why I know the amperage of "stuff".
Of course voltage is directly proportional to amperage, even a small drop in voltage requires a corresponding increase in amperage to keep current the same.

I wished we talked watts here, then the playing field is the same
__________________
a64pilot is online now   Reply With Quote
Old 27-06-2014, 07:02   #15
mrm
Registered User

Join Date: Feb 2011
Location: Poland, EU
Boat: crew on Bavaria 38 Cruiser
Posts: 651
Re: A quick question re: fast battery discharge rate with solar

Quote:
Originally Posted by monte View Post
Hi
We have 1120W solar with max output of 85A through a victron MPPT charge controller.
My concern is discharging the batteries too fast at around 100A
We have 4 / 140Ah flooded batteries.
Monte, how about approaching it from a slightly different angle?

First let the batteries finish the bulk phase as soon as possible. You have 560Ah combined battery capacity, so with your _theoretical_ 85A of charge current I would not worry at all. You will be at 0.15C _at_most_ and likely less, because your panels probably don't face the Sun at a right angle so must be de-rated.

When batteries reach acceptance phase (aka topping charge phase) you begin to have surplus power. How much? Not easy to determine as charge current will be decreasing with time and more power will be available for other uses (see a graph here Charging Information For Lead Acid Batteries €“ Battery University ). Now I would switch water heater, but would make sure that it uses LESS power than total available from solar panels, so that acceptance phase may continue. So maybe a 500W heating element?

This approach will maximise time spent in acceptance, which (hopefully) will let batteries reach 100% SOC and extend their useful life, while still heating water.

What do you think?
__________________

__________________
mrm is offline   Reply With Quote
Reply

Tags
battery, solar

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
battery discharge monitor question artist Electrical: Batteries, Generators & Solar 16 03-02-2014 01:25
Quick tour of an Atlantic 42 performance catamaran while sailing fast in Fiji Atlantic42 Multihull Sailboats 8 12-09-2013 05:45
Quick - I Need 10 Quick Easy ... and Meatless ... Recipes ! windtraveler Provisioning: Food & Drink 51 16-03-2012 14:42
Quick Discharge - Also How Bad?? ssullivan Electrical: Batteries, Generators & Solar 8 19-06-2008 12:44



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 21:11.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.