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valhalla360

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Posted: 01/18/21 07:19am Link  |  Quote  |  Print  |  Notify Moderator

naturist wrote:

Not enough information to hazard a guess, IMHO.


Yep, people stating as fact you need X watts solar and Y amps battery have no clue what you really have, so they could be completely off.

Some items to look up:
- How many days do you want to operate without alternative sources of power (shore power, generator, connected to the truck, etc)?
- What is the running wattage of the fridge? Running is different from starting. 12v DC vs 120v AC doesn't matter as much as the wattage. If it's 12v DC, there is a small advantage that you can skip the inverter.
- What is the duty cycle expected? The compressor doesn't run 24/7 but it cycles on and off. When it's off, it's not drawing power. So if it's a 50% duty cycle, there is roughly have the watt-hours used per day compared to 100% duty cycle. So if it's 50w while running and 50% duty cycle, 50w * 24hr * 50% = 600 w-hr used per day (this matches the rating on our 12v fridge but a true residential is likely significantly higher).
- Solar panels don't generate their rated wattage 24/7. A good estimate is to take the wattage rating and multiply by 4 to get watt-hours generated per day. So a 100w panel would generate around 400 w-hr per day.
- Batteries (assuming lead-acid) are rated in amp-hr but if they are 12v multiply the amp-hr by 12 to get watt-hour. There is a catch, you can only use about 50% without causing damage. So a 200amp-hr battery holds about 2400w-hr but you only have about 1200w-hr that is usable (lithium are a whole different ballgame).

Now if you want to run for multiple days, 1200w-hr of usable battery bank will keep 600w going for a couple days. If you want to stay out longer, you either need a bigger battery bank or break out a generator (or other source) to recharge the batteries.

A few additional thoughts:
- 20-30% oversizing is a good idea. You may have missed something in the calculations. Also battery banks tend to lose capacity over time.
- Are there any other 12v draws? If you can't turn them off or you want to run them, you need to increase the size to account for them.
- Inverters aren't 100% efficient, so you need to add a little more storage to account for that plus most will use a little power just in standby not powering anything...again that adds a little power demand.


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ajriding

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Posted: 01/18/21 05:08pm Link  |  Quote  |  Print  |  Notify Moderator

At some point you have to do math and see how much all the batteries and solar panels will cost,
and how much a propane fridge that needs little to no electricity to run cost.
Cost in propane to run a propane fridge: negligible .
Cost of batteries if you run resi fridge all the time and wear out batteries quickly?
Replacement cost of batteries that you wore out?
How often to replace the batteries that are quickly wearing out?

Don't forget about the Danfoss 24/12 volt or 120 household voltage (which converts to 24 for fridge) compressor refrigerators (typically in chest configurations).

wintersun

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Posted: 01/19/21 05:03pm Link  |  Quote  |  Print  |  Notify Moderator

An electric only fridge is a poor choice for dry camping. It will take 300AH of battery capacity and 300W of solar panel production at a minimum. With lead acid batteries the 300AH will provide only 150AH for the fridge and everything else in the RV as going below 50% SOC will greatly decrease their useful life and increase the time it takes to recharge them.

For an electric only fridge the best solution is to invest in lithium phosphate batteries that can take a 80% discharge without damage and that can be recharged at 4 times the rate of lead acid batteries so the generator run time will be greatly reduced and solar charging will be more effective during the limited hours of daylight.

n0arp

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Posted: 01/19/21 05:21pm Link  |  Quote  |  Print  |  Notify Moderator

pianotuna wrote:

Start with an energy audit. Decide how much reserve capacity you want. From that design the battery bank. Build a solar panel system that will recharge the battery bank.


This is the proper way to go about it. If you refuse to do an audit or don't have the ability to do so for some reason, most 18-22cf residential refrigerators use around 2kWh/day, which will need to be added to your assumed base load.

* This post was edited 01/19/21 05:30pm by an administrator/moderator *


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time2roll

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Posted: 01/19/21 05:28pm Link  |  Quote  |  Print  |  Notify Moderator

jshupe wrote:

pianotuna wrote:

Start with an energy audit. Decide how much reserve capacity you want. From that design the battery bank. Build a solar panel system that will recharge the battery bank.
This is the proper way to go about it.
I prefer to max out the solar and deal with the consequences.
Solar is so inexpensive today there is no reason to aim for the minimum.


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Posted: 01/19/21 05:31pm Link  |  Quote  |  Print  |  Notify Moderator

time2roll wrote:

jshupe wrote:

pianotuna wrote:

Start with an energy audit. Decide how much reserve capacity you want. From that design the battery bank. Build a solar panel system that will recharge the battery bank.
This is the proper way to go about it.
I prefer to max out the solar and deal with the consequences.
Solar is so inexpensive today there is no reason to aim for the minimum.


See my sig - I will counter though, that batteries are heavy, expensive, or both when in large quantities.

Dusty R

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Posted: 01/19/21 05:38pm Link  |  Quote  |  Print  |  Notify Moderator

You also need to consider where you spend most of your time with your RV. For example, Michigan is 47th of the 48 continental states.

BFL13

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Posted: 01/19/21 06:53pm Link  |  Quote  |  Print  |  Notify Moderator

wintersun wrote:

An electric only fridge is a poor choice for dry camping. It will take 300AH of battery capacity and 300W of solar panel production at a minimum. With lead acid batteries the 300AH will provide only 150AH for the fridge and everything else in the RV as going below 50% SOC will greatly decrease their useful life and increase the time it takes to recharge them.

For an electric only fridge the best solution is to invest in lithium phosphate batteries that can take a 80% discharge without damage and that can be recharged at 4 times the rate of lead acid batteries so the generator run time will be greatly reduced and solar charging will be more effective during the limited hours of daylight.


I don't think you have done the calculations for that.

You keep seeing this claim about faster charging with LFP, but nobody explains how to do that. EG this "4 times the rate" claim that will reduce gen time.

To me that must mean restoring four times the AH in the same time that it would take with FLAs . Or same AH in 1/4 the time.

If you now have four 6s at 460AH and a 75 amp charger powered by a 2200w gen (like my set-up), that means you can do a "50-90" (184AH) in about 3 hours.

To do that in 1/4 the time- 45 minutes-that means charging at 245 amps. (184 x 60/45) So first you need a 245 amp charger or combination of chargers, and a generator to run that.

I doubt that is what was meant! So how about some realistic numbers with a realistic RV set-up for chargers and gen sizes and the LFP bank vs the four 6s, a 75 amper, and a 2200w gen?

EDIT--ok, say I had the same set-up only with LFP so no amps tapering. It all gets done at 75 amps. 184/75 is 2 hrs , 27 min. so I would save 33 minutes (from 3 hours) of gen time. Not quite the same as "four times faster" is it?

How about I do a 50-80 with my four 6s? Now it all gets done at 75 amps where amps start to taper at 80% SOC. 150AH in two hours. Swap to LFP and--oops--still 150AH in two hours.

* This post was last edited 01/19/21 07:27pm by BFL13 *   View edit history


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Posted: 01/19/21 07:47pm Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

...

EDIT--ok, say I had the same set-up only with LFP so no amps tapering. It all gets done at 75 amps. 184/75 is 2 hrs , 27 min. so I would save 33 minutes (from 3 hours) of gen time. Not quite the same as "four times faster" is it?

How about I do a 50-80 with my four 6s? Now it all gets done at 75 amps where amps start to taper at 80% SOC. 150AH in two hours. Swap to LFP and--oops--still 150AH in two hours.


The "4x faster" component comes from the C rates the batteries are rated for. Most GC2 batteries are rated for C/3 charging while LiFePO4 1C or above. That's three times the speed - not accounting for the "aboves". They also don't have a slow absorption stage at the end (as you alluded to in your edit). If you have the chargers/generators to support the charge rates, they do charge that much faster. If you want the most life out of your lead acid batteries, most manufacturers say not to charge at more than C/10. That means for optimal life, you should be charging your bank of four GC15s at only 46A.

In practical application, you can buy larger chargers (or parallel them), larger generators, and charge off your solar array at the same time - so you can get closer to those theoretical limits. I've seen as much 6.3kW going into the LiFePO4 bank in my fifth wheel from multiple sources - that's a charge rate of over 500A in 12V terms. If I had a bigger genset, I could add more chargers or swap out my current ones to charge faster. A lot of people who upgrade to Lithium also upgrade their chargers at the same time, to take at least partial advantage of this.

BFL13

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Posted: 01/19/21 10:57pm Link  |  Quote  |  Print  |  Notify Moderator

Yes, that is correct. In my case, the max size gen we can carry is the 2200 and it runs max at 1700VA which is what the 75 amper wants, so 75 amps is our max amps no way around it. (In the woods, no solar to add more amps)

There is not a single advantage to get LFP in our situation. No weight issue, got enough AH , Gen time is ok. What we have works, no problem for our situation.

There can be situations where an RVer can take advantage of LFP for weight, space, usable AH, etc, but "faster charging" is too easy to be misleading. An RVer must use some actual numbers for his comparison with what "speed" of charging he would get anyway using ordinary batteries, so he is not fooled by the advertising.

He may find that like us, he is gen size limited to a charger size that means no "faster charging" by enough time that makes his life better.

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