Battery bank beginner - Stumped.

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