Electrical concept question - SailNet Community
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Electrical concept question

So if I am seeing 10 amps from my solar panels and I decide to run my fridge, laptop etc during the peak power output would that in effect be more efficient then trying to charge my batteries? My understanding is batteries are very inefficient losing about 25 percent of the energy they see. So when my solar is putting out the most power would probably be the best time to run my systems - what ever is left over would be used to charge the batteries. I guess the heart of this question is will the power bypass the battery and go straight to the fridge etc as its a potential difference in the system or will the power still be lost running through the batteries?

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Mariner777 is offline

Old 07-04-2011
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If your panels are outputting 10 amps and you are using 3 amps for a fridge the batteries get 7 amps. It is the same as living at a dock with a 10 amp shorecharger and skimming amps off the top for powering appliances.

There is a loss in charging the batteries but not as high as 25%. And there is the added convenience of using the battery power at any time desired.

Brian
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Yes it will be more efficient to use power as you are suggesting. The gain will depend on the state of battery etc. The savings will not be high as 25%, but are still helpful.
Make sure you always use as much power as possible if your solar panels are regulating all the regulated power is wasted so you are getting this power for free
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Nope, simply not true. The batteries are a big STORAGE device. Put amps in take amps out. Pretty simple, doesn't matter when.

Stu Jackson, C34, 1986, M25 engine, Rocna 10 (22#)
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Sorry what is not true? Batteries are quite inefficient and wasteful in fact a better battery/ electrical storage device is holding back the entire world one of the greatest innovations will be an efficient way to store energy I think grid tied PV systems are more efficient then battery based ones.

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Originally Posted by Mariner777 View Post
Sorry what is not true? Batteries are quite inefficient and wasteful in fact a better battery/ electrical storage device is holding back the entire world one of the greatest innovations will be an efficient way to store energy I think grid tied PV systems are more efficient then battery based ones.
That's not quite correct either, because you're comparing apples to oranges per se. Stu has a point, and methinks you have some research to do. The way you connect electrical systems on a boat has less to do with "efficiency" and more to do with getting it working in the first place.

In your OP you mentioned fridges and laptops.. both these "devices" have extremely high in-rush current demand when first turned on. As Stu pointed out, batteries are storage devices: current in now, current out later at a set voltage. Solar panels are Base Load generators: they supply roughly constant current to a maximum value - once the load current increases past the maximum, the output voltage drops and they don't work so well.

In a boat's electrical system, a battery bank provides exactly the same function that the grid does for household electricity - it allows increased current draw when equipment is first turned on - well in excess of a solar panel could provide - without the voltage altering so much the equipment never gets a chance to get started.

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I understand the point of having a large battery bank but was just wondering how exactly the output of the panels would be used if I run the devices during charging - just if the output will go through the battery bank and its resistance etc before hitting the rest of my devices. Lets say I could only run my fridge for 5 hours a day - should I run it from 3am to 7am when the temps are often the coolest so it has the least work to do or should I run it from 11am to 4pm or so when the panels are producing the most current? If the batteries actually stored all the power then it would be a no brainer but I have heard/ read that they lose quite a bit of energy in the reactions used to store the power as heat/ entropy etc. So I could max out my fridge while my panels are at peak production as I would in effect be using more of the available power. Not sure if this makes any sense...

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A quick googling confirms my memory that lead acid batteries were something like 75 percent efficient at storing energy the rest being lost as heat and warming the battery Sealed Lead Acid Battery Applications - Transwiki

It also mentions the rest of the inefficiencies in the charging circuit - I suppose they will still plague energy consumed directly from the panels as well - but I still can't find the answer - since the batteries are all in parallel and the fridge is plugged in to the batteries is it possible that the energy will go to the fridge without going into charging the battery first?

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Simple - the panels are connected to the positive and negative post. The fridge is connected to the same posts. Example - if the fridge is on the 10 amp output of the panels will be reduced by the amount the fridge uses.

I think you are best to use the fridge whenever necessary to keep the contents below 38 degrees F so the food doesn't spoil.

That is where batteries shine - you can run the fridge at night or when the sun is obscured.

Brian
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Originally Posted by Mariner777 View Post
I understand the point of having a large battery bank but was just wondering how exactly the output of the panels would be used if I run the devices during charging - just if the output will go through the battery bank and its resistance etc before hitting the rest of my devices.
Given that solar panels and similar base-load charging devices (including battery chargers, alternators and the like) produce a maximum amount of current at set times (eg. solar panels at peak of the day; alternators whenever the engine is running) some sort of "accumulator", like a battery, is needed to provide the rest.

If the panels and battery are connected as mitiempo describes, whether the panel or the battery, or both, supplies the end devices will depend upon the conditions at the time: full sun, no sun, fully charged battery, empty battery, too much load, not enough load, ... so there is really no set answer to your question.

The usual way to monitor what is happening at any given time (to measure the "efficiency", if you like) is with a battery monitor like the Xantrex LinkPro or Victron BMV-600. I'd think that one of these gadgets will answer all your questions directly and would recommend you install one if you haven't already.

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Last edited by Classic30; 07-04-2011 at 10:18 PM.
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