A Quick Question Re: Fast Battery Discharge Rate With Solar.

[monte]

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

[sailorboy1]

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.

[a64pilot]

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?

[sailorboy1]

Originally Posted by a64pilot

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.

[Sailmonkey]

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!

[sparrowhawk1]

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: )

[a64pilot]

my water heater draws 10 amps at 120V which is 1200W

[a64pilot]

How many panels is 1120 W?

[sparrowhawk1]

Originally Posted by a64pilot

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

[monte]

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

[sparrowhawk1]

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

[hlev00]

Originally Posted by monte

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.

[sparrowhawk1]

[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

[a64pilot]

Originally Posted by a64pilot

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

[mrm]

Originally Posted by monte

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?