Battery bank beginner - Stumped.

sarahps33sarahps33 Registered Users Posts: 5 ✭✭
Hi there, 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. 
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  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    The first pass answer is you can do the calculations in either Amp*Hours or Watt*Hours. Just need to know the working votlage:
    • 2,510 WH / 12 volts = 209 AH @ 12 volts
    • 2,510 WH / 120 Volts = 20.9 AH @ 120 volts
    In the "olden days" when everything was "one voltage" -- I.e., everything in the the RV ran off of 12 VDC -- You could simply stay in Amps and Amp*Hours without getting confused because everything was 12 VDC.

    Today's systems which are both 12/24/48 VDC (DC battery bank) and 120/240 VAC (with AC inverters), you can still do the same calculations and just make sure you "convert" between DC AH and AC AH as needed (power=Voltage*current; current=power/Voltage, etc.).

    There is a more complex second pass answer too... In the 12 VDC lights and appliance days, The current tended to be a bit more "stable" with varying battery voltage... Near 10.5 volts (dead battery) and 15 volts (charging battery)--The current was not much different being drawn by appliances.

    Today, with modern electronics, LED lighting, AC inverter, etc., these are "constant power" devices. Meaning that your 13 Watt LED, 1,200 Watt microwave, 120 VAC loads on AC inverter draw pretty much the same "power" as battery voltage varies.

    For example, say you have an 800 Watt microwave or induction cook top in your RV (these are getting popular--Can avoid having propane in the RV--Use diesel or gasoline space heaters from vehicle fuel tank--No propane tanks, fills, etc.). Just to give an example--That same 10.5 to 15 VDC battery bus voltage range:
    • 800 Watts * 1/0.85 AC inverter efficiency * 1/15.0 volts battery charging = 62.7 Amps @ 12 VDC battery bus current (charging)
    • 800 Watts * 1/0.85 AC inverter efficiency * 1/10.5 volts battery near dead = 89.6 Amps @ 12 VDC battery bus current (near dead)
    • 800 Watts * 1/0.85 AC inverter efficiency * 1/12.0 volts battery near dead = 78.4 Amps @ 12 vDC battery bus current (nominal)
    Or almost 50% more current into the AC inverter from battery bus between battery charging and near dead...

    We could go through the "modeling" of each and every load--But frankly that is too much work and gets us "lost in the numbers". Using "nominal assumptions" (like 12 VDC battery bus, and not running below ~11.5 VDC on the battery--Roughly 50% SoC for FLA battery bank)--We get numbers that are "close enough for solar" design for cloudy days--And on sunny days--Have more than enough energy to run "everything" (within reason).

    Nothing about solar is "cheap"... It all cost money.

    Flooded Cell Lead Acid Batteries (FLA) are the cheapest and most forgiving of batteries (still not hard to "murder" FLA battery). But they are heavy and take up a lot of space.

    AGM (a form of lead acid battery) are somewhat more expensive, and have better "surge current" support. But they are still large and heavy banks. And tend not last as long as properly maintained FLA batteries (at the same quality/price point).

    Li Ion (usually LiFePO4 or lithium iron phosphate for RVs and Homes) are close to the ideal battery. No maintenance, much better charging and discharging characteristics, smaller foot print, lighter weight. But they are $$$ expensive. And generally need a BMS (battery management system) to help ensure that the LiFePO4 batteries are not over charged or over discharged as this can pretty much ruin a battery bank the first time that the specifications are exceeded.

    With RVs, you also run into issues with roof space... If you have heavy electrical loads, you need a bunch of solar panels on the roof to harvest the solar energy. And the farther away from the equator, the more "tilting" roof array can help with your harvest (especially towards the winter months).

    Before we get into designing your solar power system--I highly suggest looking at your loads first. You want all of your loads to be highly energy efficient--Doubly true for RV appliances. LED lighting, smaller/efficient DC refrigerator--None of those 3 Watt absorption fridges than run on propane/AC/DC power--They are fine for propane, but on electricity they are huge energy wasters. If you use computers, as small and efficient laptop as you can get away with saves lots of energy too (full size desktop may take 300 Watts. A laptop can be down towards 10-30 Watts).

    It is almost always cheaper to conserve energy than to generate it. Refrigerator and Computers (used for work/editing/etc.) are usually the largest users of off grid energy--So doing the research and spending money here will allow you to have a smaller (and less expensive) solar power system.

    Energy usage is highly personal... The solar power system needs to support your needs. You can see what various folks have done on "van life" videos and websites--All very interesting and lots to learn there... But your energy needs are critical here.

    After you have measured (estimated the best you can) your daily loads, then look at the space for batteries. How large of space (WxLxH) and how much weight can you manage.

    Also need to look at your roof space. How many Watts of solar power can you put on the roof. And solar panels need direct sunlight. Any shading from roof vents, A/C units, etc. will kill the solar electric output. And the farther north you go, the possible advantages of a tilting solar array can help. The down side with solar--No sun (far north, cloudy winters, etc.), there is no solar harvest (or very little).

    We have rules of thumbs which help us to quickly and easily do paper designs for sizing the solar power system. And you will do multiple designs with different loads, battery types/sizes, solar array designs, etc. to see what will be "optimum for you". Size, weight, costs, physical constraints, backup genset usage/driving and charging from vehicle alternator (or not), etc. will all give you "different" answers--And you will need to sift through those to figure out what will work for you.

    Don't get too much into specific hardware/battery configurations and such... They will matter--But not yet with the "paper" sizing of your system.

    For example... FLA batteries. You can by 2x 12 volt @ 100 AH batteries and connect them in parallel for 12 volts @ 200 AH. Or you can buy 2x 6 volt @ 200 AH and connect them in series for 12 volts @ 200 AH. They will be the same size, weight, and (roughly) costs. However, I personally, try to avoid paralleling lots of smaller batteries together for a large battery bank. For various reasons, I would tend towards the 2x 6 volt @ 200 AH batteries in series over 2x 12 volt @ 100 AH in parallel. But that is for the "battery bank design post".

    More or less--Your FLA battery bank (say 6 volt @ 200 AH "golf cart" battery bank) would need to be almost 2x larger AH capacity (at 12 VDC) vs a Li Ion battery bank @ 12 VDC. This is because Li Ion batteries can output more current and charge "more efficiently" than an FLA battery bank. Down side--Where a FLA battery bank will give you more "no sun" energy (we tend to design FLA for 2-3 Days "without sun"--Both because of limited current output and because "less efficient solar charging") vs a "1/2 size" Li Ion bank which will hold around 1-2 days of energy before recharging needed.

    Just to give you some random numbers... Say your daily load is 2,510 WH. And typical rule off thumb for off grid cabin design (RVs generally have smaller system because of size/weight restrictions). An FLA bank might look like this:
    • 2,510 WH * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge (longer battery life) * 1/24 volts = 492 AH @ 12 VDC
    An FLA battery bank out of "golf cart" FLA batteries could look like:
    • 4x 6 volt batteries in series = 24 volts
    • 2x parallel strings of 6 volt batteries @ 220 AH = 440 AH @ 24 volt battery bank
    • 4 x 2 = 8x 6 volt @ 220 AH "golf cart batteries
    • 8 batteries * $120 each = $960 battery bank (Costco/Walmart batteries)
    • 8 x 64 lbs = 512 lbs for bank
    • Life ~ 3-5 years typical
    Doing a "minimum" design for an RV with Li Ion batteries:
    • 2,510 WH * 1/0.85 AC inverter eff * 1 days storage * 1/0.70 max discharge (longer battery life) * 1/24 volts =176 AH @ 24 volt Li Ion bank
    • 176 AH * 24 VDC battery bank = 2,224 Watt*Hour @ 24 volt battery bank (same energy spec. as 176 AH @ 24 volt battery bank)
    Looking at our host's website for Li Ion batteries:

    https://www.solar-electric.com/residential/batteries-battery-storage/deep-cycle-batteries.html?nav_battery_type=537

    Just picking a random (good quality) LiFePO4 battery. 2.8 kWH @ 24 volts:

    https://www.solar-electric.com/discover-energy-aes-2-8kwhr-24vdc-lithium-ion-battery.html

    $3,385 battery (bank) @ 88 lbs @ 10+ year life(?)

    The FLA bank will run for 2-3 days without sun. The Li Ion bank will run 1 day without sun.

    Just a quick example of what you may be looking at just for the battery bank. How much does cost vs size matter to you?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    edited January 2 #3
    I should add... Li Ion batteries are "better" for high temperatures (>85F). And will not/should not charge if the bank is below ~40F (BMS shutdown or ruin batteries). Need to "keep from freezing"--Insulated box, heater, etc..

    FLA batteries are good down to -20 to -40F (discharge/charge--Less useful capacity when cold/sub freezing). Over >~85F, high operating temperatures have a shorter useful life (faster aging when hot--FLA is less efficient and run "hotter" too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Wheelman55Wheelman55 Registered Users Posts: 178 ✭✭✭
    edited January 1 #4
    You are closer than you think in understanding this. 

    Watt hours = amp hours*volts

    100 ah at 12 volts produces 1200 wh
    100 ah at 24 volts produces 2400 wh
    100 ah at 48 volts produces 4800 wh

    You’ll most likely end up with either a 12 volt or 24 volt system. 

    Some questions to ask:
    - will a 12 volt system power all of the  appliances you want to use?  If yes go 12 volt, If no go 24 volt. 
    - are you camping in the same spot or are you moving from place to place?
    - solar panels need full sun, so camping in the shade would necessitate remote/moveable panels
    - how many of your watt hours do you want to store in batteries vs powering by solar or a small generator. 
    - a small Honda inverter-generator could also supply power to run a microwave for a few minutes a day, allowing a 12 volt system to power the rest. 

    My wife and I lived for years on two different 12v systems. 

    The first was two 6 volt 220 ah Trojan flooded lead acid batteries which we charged a couple of times a week with the Honda. This battery was 130 pounds or so. Our loads were two ARB fridges, laptop, tablet, rechargeable headlamps, fan and phones. We cooked on propane. Also charged tool batteries with the Honda. 

    Two things changed our lives to the positive. 

    We got one 12 volt 100 ah Battle Born LiFePO4 battery. This battery weighs 35 pounds:). Loads stayed the same.  Much quicker and easier to charge. 

    The big plus was adding a 180 watt folding solar panel with PWM charge controller. No more noise:)) and we could move the panels to follow the sun. 

    FYI our location was just one spot, where we eventually built a house and put in a robust 48 volt system, once again with lithium batteries. 


    Building Off-Grid in Terlingua, TX
    14 CS 370 watt modules. HZLA horizontal tracker. Schneider: XW6048, Mini PDP, MPPT 80-600, SCP. 3 - Discover AES 42-48-6650 48 volt 130ah LiFePO4 batteries
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