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Jennifer McM

A question on Solar charging batteries

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Wondering as we're upgrading our system to 3 x 175 panels - MPPT 150/35 could this cause a battery problem for us? Our batteries are 4 x 100A AGMs?

 

Thanks... :) 

Edited by Jennifer McM

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4 minutes ago, Jennifer McM said:

Wondering as we're upgrading our system to 3 x 175 panels - MPPT 150/35 could this cause a battery problem for us? Our batteries are 4 x 100A AGMs?

 

Thanks... :) 

No you will just have more power for longer in the year

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Shouldn't be any problem at all, now you need to find a way to use the surplus power available for 6 months a year!!!!

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Just now, matty40s said:

Shouldn't be any problem at all, now you need to find a way to use the surplus power available for 6 months a year!!!!

Now if only the power could be used to heat the water when we're not on the move! Maybe that would be just too much to ask the batteries to heat water with our newly installed 2KW immersion heater? Inverter is 3KW.

 

This might sound like an obvious question to some, but for me (sadly) it's not :) 

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12 minutes ago, Jennifer McM said:

This might sound like an obvious question to some, but for me (sadly) it's not :) 

You mean its not an obvious question for you ?

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7 minutes ago, Alan de Enfield said:

You mean its not an obvious question for you ?

Perhaps I should rephrase it, would the immersion be too heavy for the batteries? The washing machine, heating the water from cold, is only switched on when the engine is running.

 

Obviously don't want to run the engine to run the immersion heater ??

 

 

Edited by Jennifer McM

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14 minutes ago, Jennifer McM said:

Now if only the power could be used to heat the water when we're not on the move! Maybe that would be just too much to ask the batteries to heat water with our newly installed 2KW immersion heater? Inverter is 3KW.

 

This might sound like an obvious question to some, but for me (sadly) it's not :) 

It's not a lot more than simple arithmetic to explain why:-

 

A 2Kw device would draw about 200 amps from your battery per hour. That would take an hour to completely flatters / destroy your 400Ah battery bank.

The solar will make an  useful contribution - but that's dependant on (ideal) weather conditions.

 

The eaqsiest answer that fits all is to run the engine while washing -

  • Waste heat from the engine heats the water (or it should in a properly designed system
  • The engine charging resolves the shortfall in power requirement

 

Seemples 

 

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1 minute ago, Jennifer McM said:

Perhaps I should rephrase it, would the immersion be too heavy for the batteries? The washing machine, heating the water from cold, is always switched on when the engine running.

 

Obviously don't want to run the engine to run the immersion heater ??

 

 

I would say that it is certainly possible to re-route some of the 'surplus' solar production into water heating, but not with a 2kw immersion heater (that'll be drawing 200 amps), you can get away with a 1kw immersion heater (drawing 100 amps) but would either need to put in some fancy electrics/electronics, or, VERY CLOSELY monitor your voltages and SoC.

 

Your panels will probably be producing around 30 amps (in reality) so with your 2kw heater you'd be (in simple terms) using the 30 amps produced by the panels and taking 170 amps from the batteries. If it takes 1 hour to heat the water you will have taken your batteries down to 50% (ish) state of charge.

Fine if you know that the sun will still be shining once the water is hot and the solar can then recharge to batteries.

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Thank you for your explanations! :) 

 

Pretty certain we bought a 1Kw immersion from Midland Chandlers, looking at it now (it's not been installed yet), the 'thing's' 2Kw. ?

Edited by Jennifer McM

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I was wondering a similar question to the o/p. Could the load connection on the mppt controller not be set up to power a low-ish rated heating element for the water? Most times my batteries are only taking a handful of amps when there is full sun and I know lots of potential power is not being made the most of. 

Can one not get say a 250w / 12v element that could be used? Over a few hours when the batteries are drawing not a lot?

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12 minutes ago, Mike the Boilerman said:

 

Oh dear. And I thought you were beginning to understand what an Amp is.

 

 

I could have the explanation more technically correct :-

An immersion heater rated at 2Kw at 240 volts would require 200 amps at 12V (ignoring losses) to power it.

The OP has a battery bank of 400 Ah nominal

Running that sort of load would goose her batteries in less than an hour = don't try it...

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53 minutes ago, Johny London said:

I was wondering a similar question to the o/p. Could the load connection on the mppt controller not be set up to power a low-ish rated heating element for the water? Most times my batteries are only taking a handful of amps when there is full sun and I know lots of potential power is not being made the most of. 

Can one not get say a 250w / 12v element that could be used? Over a few hours when the batteries are drawing not a lot?

Yes you can, and you could run it from the load outlet on the solar controller if you can program it to kick in when the batteries go on float.

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6 minutes ago, matty40s said:

Yes you can, and you could run it from the load outlet on the solar controller if you can program it to kick in when the batteries go on float.

My drive solar 2500 watts is currently heating water direct as I dont need it to charge the drive batteries or move me, so its a perfik use of excess solar

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1 hour ago, OldGoat said:

I could have the explanation more technically correct :-

I think the point was 200Ah for 1 hour rather than 200 amps per hour.

 

(It MtBs hobby-horse - he is correct,  although we all know what you meant)

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56 minutes ago, Alan de Enfield said:

I think the point was 200Ah for 1 hour rather than 200 amps per hour.

 

(It MtBs hobby-horse - he is correct,  although we all know what you meant)

 

Or put simply and correctly, 200 Amps. 

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3 hours ago, OldGoat said:

A 2Kw device would draw about 200 amps from your battery per hour. 

It would also draw 200A every second, of every minute, of every decade... how many Amphours it would take would of course depend on how long it was drawing 200A for...

15 minutes ago, Mike the Boilerman said:

Or put simply and correctly, 200 Amps. 

Indeed. 

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Everyone is forgetting that a 100Ah battery is only rated 100Ah at what they call a C20 Discharge Rate. In other words the power held by the battery when it discharges over 20 hours or at 5 amps. When you discharge a battery quickly or at any rate over the C20 rating the battery loses power much quicker and the amp/hour rate drops dramatically.. 200 amps from 4 batteries means 50 amps per battery which  would probably only last 10 minutes.

 

https://batteryuniversity.com/learn/

Edited by rogeriko
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7 minutes ago, rogeriko said:

When you discharge a battery quickly or at any rate over the C20 rating the battery loses power much quicker and the amp/hour rate drops dramatically.

 

I get your general point which is a good one, but is this a valid use of the term "amp/hour rate"? I think not....

 

"Capacity", perhaps? 

 

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Yes, because the battery is only 100Ah at a discharge rate of 5 amps. At a discharge rate of 50 amps it becomes about a 40Ah battery and going the other way at a discharge rate of 1 amp it will be come a 150Ah battery.

 

https://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html

 

If you enter 50 amps for 1 hour you will see you need a 236Ah battery. Tick the box if your battery is more than six months old.

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6 minutes ago, rogeriko said:

Yes, because the battery is only 100Ah at a discharge rate of 5 amps. At a discharge rate of 50 amps it becomes about a 40Ah battery and going the other way at a discharge rate of 1 amp it will be come a 150Ah battery.

 

https://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html

 

If you enter 50 amps for 1 hour you will see you need a 236Ah battery. Tick the box if your battery is more than six months old.

 

You didn't address my question, I don't think. You just repeated your original point that battery capacity depends on the rate of discharge.

 

In what way is the unit of measurement "amp/hour" you used, valid?

 

 

 

 

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I see what you mean. I meant the amp/hour rating of the battery is not fixed, it can fluctuate from the manufacturers specification.

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5 hours ago, rogeriko said:

I see what you mean. I meant the amp/hour rating of the battery is not fixed, it can fluctuate from the manufacturers specification.

I think mike is questioning your typing. Amp/hour is not the same as Amp hour. Amp/hour could be taken to mean amps per hour or amps divided by hours.

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