12 volt fridges

While technically true, there's really no reason to worry about the 80% thing. The spec that all quality manufacturers (those that use grade A cells) state is that you can discharge their battery 100% and then recharge fully on a daily basis and still have 80% of the overall capacity left after 10+ years. So with this kind of service life, who cares about trying to keep them between 20~80%? Most of us wont use them that hard and can probably expect something more like 20 to 30 years of life.
I think that's a lifetime of camping for most of us and after that much time something better will have come along anyway.

valhalla360 wrote:

- How many "usable" amp-hours does your battery bank have (lead-acid batteries should only use 50% of the rated amp-hr and lithium should only use 80% of the rated amp-hr...using more can prematurely age the batteries). Convert to watt-hours by multiplying by 12.

Where is the reference about “only use Lithium to 80% rated amp hour” stated clearly by a battery maker? I’ve never seen it referenced except on this forum. I’ve read where a LiFePO4 retains 80% capacity after its expected recharge cycle life is exceeded. I’ve also read where you can fully discharge them and then fully recharge them with no damage.

Most controllers have an screen that shows the voltage. It should be north of 13v if charging. That or just check the battery voltage on a sunny day and it should be north of 13v.

If its just the fridge and you run almost nothing else 12v, it's likely OK for a night or two of boondocking with 100w. A second battery would be more useful than more solar. This assumes a 12v compressor fridge...an absorption fridge (also runs on propane) will easily use 10 times as much when running on electricity. It also assumes it's not killer hot out and you leave the door open a lot.

If you want to run more 12v stuff (lights, fans, furnace, etc...) and boondock for multiple days, an energy audit is the way to go. Simplified a bit:
- How long will each 12v device be run per day, then mulitply the wattage by the hours it is on to get total amp-hours @ 12v.
- How many "usable" amp-hours does your battery bank have (lead-acid batteries should only use 50% of the rated amp-hr and lithium should only use 80% of the rated amp-hr...using more can prematurely age the batteries). Convert to watt-hours by multiplying by 12.
- Divide usable watt-hours by how many watt-hours you estimate you will use. This will give you an estimate of how long the batteries will last in days. Discount this by 20-30% to cover items you missed or aging of batteries eventually reducing usable amp-hr.
- Solar is really about longer term boondocking. If you are stopping for a night or two, solar is mostly a bonus. If boondocking longer term, you want enough solar to cover your daily consumption. Take the rating of the solar panels and multiply by 4 to get an estimate of how many watt-hours it will generate (100w panel would be 400watt-hour generated each day). In reality it will vary based on time of year and weather (clouds are a big impact) but 4 times the rating is a good starting point.

Does your TT have a battery disconnect?
Most solar charging bypass the battery disconnect to keep the battery charged in storage.

Size of solar depends upon your usage. For storage only and battery maintenance then 100W might be OK. Otherwise do an energy audit to determine what you need for solar. Info/calcuators available on line.

The better question: "Is the solar large enough for my needs?"

Being critical: Salesman "Comes compete with solar" ie Total nonsense until you know it's capability and YOUR needs.

Measure the battery voltage, turn the solar controller on and the voltage should start for rise. Assumes battery a little discharged. Even better if you have a battery monitor that measures voltage, amps and battery charge state.