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 > Sizing a battery bank. Ah VS wh?

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valhalla360

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Posted: 01/02/22 12:22am Link  |  Quote  |  Print  |  Notify Moderator

theoldwizard1 wrote:

For example, you want bacon and eggs with toast and coffee for breakfast. Easiest thing to use is an electric frying pan. Is you inverter up to the task of running the frying pan, toaster and coffee pot at the same time ? Throw in a hair dryer and few inverters could handle that.


Your average 30amp trailer on shore power isn't up to that...so it's a bit unrealistic. Even with a 50amp rig, you are going to have to be careful to keep those on separate circuits or you will be popping the breakers.

As far as amp-hr vs watt-hr...watt-hr is the more correct way to do it but as long as you keep the voltage the same, you can shortcut to using amp-hr (ie: you are generating, storing and using 12v all the way through, amp-hr works fine).

The problem with amp-hr is once you add an inverter, the voltage is changing (also DC vs AC). By going to watt-hr, it really doesn't matter (at least not enough to worry about).

Then it's a good idea to oversize by 10-30% because the system isn't going to be 100% efficient. Also, technically, 12v is not not exactly 12v. Depending on battery type and state of charge, it will vary a little from 12v but if you oversize a bit, that shouldn't be an issue.


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Lwiddis

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Posted: 01/02/22 07:58am Link  |  Quote  |  Print  |  Notify Moderator

“That seems like such a little amount. I thought it would be more about the solar's charging rates. Like "25amps per hour" or 25ah x 4.5hrs of sunlight = 100 amps charged in 'one day.' I havnt quite gotten to that research yet...”

Rarely do 300 watt solar panels produce 300 watts. The sun must be directly overhead, the temperature must be moderate and very importantly the batteries must be willing to accept 300 watts. During bulk charging they may but during absorption they won’t. The majority of the time recharging wet batteries are in the absorption phase. Lithium is different.


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valhalla360

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Posted: 01/02/22 07:01pm Link  |  Quote  |  Print  |  Notify Moderator

Lwiddis wrote:

“That seems like such a little amount. I thought it would be more about the solar's charging rates. Like "25amps per hour" or 25ah x 4.5hrs of sunlight = 100 amps charged in 'one day.' I havnt quite gotten to that research yet...”

Rarely do 300 watt solar panels produce 300 watts. The sun must be directly overhead, the temperature must be moderate and very importantly the batteries must be willing to accept 300 watts. During bulk charging they may but during absorption they won’t. The majority of the time recharging wet batteries are in the absorption phase. Lithium is different.


True they don't put out an instantaneous output of 300w often.

But I believe the point was if you take the nominal rating...it will generate total watt-hr per day of about 4 to 5 times the rating (ie: 300w of panels generate 1200-1500w-hr per day). They will actually output for far longer but at lower wattage for much of the day.

Solar also largely solves the issue of acceptance by default. Sometime after 2pm, as the angle of the sun gradually drops, so does the panel output as it gradually tails off for the next 3-4hrs (depends a bit on latitude, time of year and shading). Come 4-5pm, those 300w panels may only be throwing off 20-50w, so battery acceptance is less of an issue.

StirCrazy

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Posted: 01/03/22 04:26pm Link  |  Quote  |  Print  |  Notify Moderator

Lwiddis wrote:

“That seems like such a little amount. I thought it would be more about the solar's charging rates. Like "25amps per hour" or 25ah x 4.5hrs of sunlight = 100 amps charged in 'one day.' I havnt quite gotten to that research yet...”



if you looking at it as 25AH then no, they will never put out full power. a 12V panel on a PWM controler will put out 17ish v all day long which is 17v to the controler at 17.6 amps. since a PWM doesnt convert it only used a percentage of that and is faily inefficent so lets say 14.4 at about 15 amps.

this should be fairly close as 480 watts of 12V panels on a PWM in my 5th wheel will only give me about 23 amps.

this changes when you go to something like MPPT controler and say what I have on my camper which is a 72 cell 24V panel. this puts out aproximatly 32 to 36v depending on the panel so with a MPPT controler it is actualy a built in dc to dc charger so it takes that input and converts it directly to the output voltage and you only lose the effeciency of the controler. mine is 98.6% so lets pick 34V at 300 watts that is 8.8 amps but since the battery needs 14.4 it converts it to that output - the effeciency loss so you end up with 295.8 watts of 14.4v and 20.54 amps.

so the same output panel in different configurations with different controlers in ideal conditions can varry widely with out having to get into increased preformance in low light of a split cell 24V panel.

I realy started looking into this when I found I got more charging out of a 325 watt system than my 480 watt system and wondered why.

Steve


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BFL13

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Posted: 01/04/22 11:04am Link  |  Quote  |  Print  |  Notify Moderator

Surprisingly, Steve's above post has many errors wrt PWM and MPPT. It would be easier to delete it than try to correct it all. I am not feeling like trying to.


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2oldman

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Posted: 01/04/22 11:52am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

Steve's above post has many errors...
He could use a good spell checker too.

PerryB67

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Posted: 01/05/22 07:33am Link  |  Quote  |  Print  |  Notify Moderator

I'm dyslexic and have enough problems reading and writing.

So, why would I take seriously a person who: won't capitalize the first word in a sentence, put a period at the end of many sentences, writes a run-on sentence with 85 words and no commas, and/or doesn't bother to spell check. And then I'm supposed to rely on that person for accuracy/knowledge?

I will correct that person if he's quoting me, otherwise, like others here, just ignore his posts.

Perry

* This post was edited 01/05/22 07:42am by PerryB67 *


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StirCrazy

Kamloops, BC, Canada

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Posted: 01/05/22 08:08am Link  |  Quote  |  Print  |  Notify Moderator

2oldman wrote:

BFL13 wrote:

Steve's above post has many errors...
He could use a good spell checker too.


I could

StirCrazy

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Posted: 01/05/22 08:09am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

Surprisingly, Steve's above post has many errors wrt PWM and MPPT. It would be easier to delete it than try to correct it all. I am not feeling like trying to.


good you think so but it matches how my two systems work perfectly , in low light there is even more of a difference.

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