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Sarahps33's avatar
Sarahps33
Explorer
Jan 01, 2022

Sizing a battery bank. Ah VS wh?

Hi again!

So, I'm trying to size out the battery bank I will need for my husband and I to live off grid in a camper. We don't have tons of time or money, so that's a bit of a factor. We plan on using an inverter for pretty much everything. And using either standard lead acid batts or agm. We plan on getting a few solar panels and possibly a small generator if needed.

I've read tons of forums and watched hundreds of youtube videos and it seems like everyone I look into tells me something vastly different, from calculations and formulas to bank size & voltage and so on.

I believe we need roughly 2510 watt hours.
I calculated this by getting the watts of each appliance and multiplying it by the number of hours we'd use it each day.

I tried to do the same thing with amps. I divided the watts by volts to get amps then multiplied it by how many hours we'd use it. I came up with 202 amp hours.

I've read that some folks use 6v batts and wire them both in series and parallel to equal 12v with higher amps, while others just use 12 in parallel, or wire in series for 24v or even 48v.

What really confuses me here is that from what I understand the concern should be amp hour capacity? Or should it be watts hour capacity?

I've seen people ramp up their watt hours by using 24v and 48v batteries while the amps stay quite low. While others ramp up the amp hours by using 6v golf cart batts and get tons of amp hours.

Which one should I be concerned about and why? Do they equal the same thing essentially?

And how in the world do I figure out a batt bank that will fit our needs with all this contradicting info. I'm stumped, lol.

Thanks for reading, looking forward to your responses.
  • “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.
  • 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.
  • 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
  • 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.
  • BFL13 wrote:
    Steve's above post has many errors...
    He could use a good spell checker too.
  • 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
  • 2oldman wrote:
    BFL13 wrote:
    Steve's above post has many errors...
    He could use a good spell checker too.


    I could
  • 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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