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Old 10-15-2020, 05:09 PM   #1
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Battery sizing

It has been several years since I had to size a house battery that I need a refresher course. I think this is right but not sure so thought I would ask in a very simplified fashion. Let me know if my thinking is correct.

A new AGM battery should only be discharged to 50%. So a 100 amp hour battery only has 50 amps available.

Assuming no diversity and a continuously running fridge drawing 5 amps a 100 amp battery will last 10 hours, 50 amps available/5 amp draw=10 hours before the battery reaches 50% discharge. With a 50% diversity on the fridge the run time increases to 20.8 hours before reaching the 50% drawdown limit on the battery.
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Old 10-15-2020, 08:02 PM   #2
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Everyone's approach will differ in some way. Been on the short end too many times to make hopeful assumptions, So I never assume what could be...I approach this sort of challenge with the mindset that includes the worst I could expect.
So I'll answer your question with another: If you get 20mpg and your itinerary requires you to accomplish 100 miles today, would you be comfortable carrying 5 gallons in your tank??? Theoretically it works, but offers little room for the unexpected.
If you have a decent capacity alternator, you could run your engine for a while to drop some amps in the coffer, but if you're in a campground and have a diesel you may end out being "that guy".
Lot of the solution you seek has to do with expectations. Do you only expect to get 3/4 of a day with the fridge running? Are you on the move each day for several hours so you can re-charge your battery through the engine running? Solar - assuming you will get benefit of some sun?
I keep a Yeti style cooler in the van with all the beverages in it, just so I don't have to open the fridge 3 times more than necessary.
Personally, I have a 220amh Lifeline and at some point will double that (when I decide what gets eliminated to make the room), just so I don't have to go through the occasional frustration associated with capacity,
Probably could have (or should have) stated my recommendation to simply say "put in as much as you can" larrie.
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Old 10-15-2020, 09:36 PM   #3
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At the moment am just trying to understand the basic theory. Figure if I get that right then I can add up the rest of the loads in the van add in the safety factors and the solar to come up with the battery bank size.
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Old 10-16-2020, 07:22 AM   #4
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You have the theory bang-on, other than saying "[...] has 50 amps available." rather than " [...] has 50 amp-hours available." (yes, I am retentive - I am an electrical engineer).
You have the right plan - work out what the loads will be, decide what the target run time you want, multiply by a safety factor, and that will tell you how much battery you need. You may also want to consider choices other than lead-acid - Lithium Manganese Iron Phosphate batteries are lighter, can be drawn down to 20% charge remaining (80% usable), and depending upon how much capacity you need can be reasonably priced. If you need less than 100Ah@12V, an all-in-one battery like a BattleBorn can do very well, and if you decide you need the capacity, a solution based around discrete batteries and an external battery management system can be reasonable. I have 300Ah@24V in my van, total cost for the system was $4k ($600 per battery * 6 batteries + BMS + contactors).
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Old 10-16-2020, 07:30 AM   #5
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Larry: Your assumptions above are correct.


The engineers around here use the term "duty cycle" instead of "diversity" but we've been known at times to not be politically correct.


Your assumptions above are valid for lead-acid chemistry...Flooded, AGM, Gel.....Lithium chemistry batteries can be discharged around 80% and still have long lives.


You can discharge lead acid batteries more than 50% but it will shorten their life.
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Old 10-16-2020, 07:49 AM   #6
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Applying the 80% "rule" isn't a bad idea. For example, if your calculated usage is 200 amp hours resulting in a need for total of 400 amp hours of flooded or AGM battery capacity, I suggest increasing it to 400/.8 = 500 amp hours to be safe.

Not only does this provide for some wiggle room in your calculations, it gives you a safety factor relative to battery aging, lower than expected opportunity for recharging, etc.
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Old 10-16-2020, 08:29 AM   #7
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Quote:
Originally Posted by Wowbagger View Post
You have the theory bang-on, other than saying "[...] has 50 amps available." rather than " [...] has 50 amp-hours available." (yes, I am retentive - I am an electrical engineer).
You have the right plan - work out what the loads will be, decide what the target run time you want, multiply by a safety factor, and that will tell you how much battery you need. You may also want to consider choices other than lead-acid - Lithium Manganese Iron Phosphate batteries are lighter, can be drawn down to 20% charge remaining (80% usable), and depending upon how much capacity you need can be reasonably priced. If you need less than 100Ah@12V, an all-in-one battery like a BattleBorn can do very well, and if you decide you need the capacity, a solution based around discrete batteries and an external battery management system can be reasonable. I have 300Ah@24V in my van, total cost for the system was $4k ($600 per battery * 6 batteries + BMS + contactors).
Good info, which leads me to a question. I've been watching for the cheaper 100ah LifePo's on Amazon to come down. The other day there was this one for only $411. Would you trust it? Edit to add that price has gone up some. Now with 10% discount it's around $450, still a great deal IMO for a 100ah lithium.

https://www.amazon.com/dp/B08KY3TXR4...v_ov_lig_dp_it
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Old 10-16-2020, 03:34 PM   #8
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That wouldn't be too bad a choice. The only thing I have against the integrated BMS batteries like that is that they tend to only support a 1C discharge rate (a 100Ah battery can supply at most 100A). My batteries are rated 3C sustained discharge (100Ah battery can supply up to 300A), and the BMS and contactors I used are able to support that. But, if you are planning on pulling more than 100A from a battery like that you probably want 2 anyway, just to have the run time.
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Old 10-16-2020, 08:44 PM   #9
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Now that the subject of lithium has come up, I have see and read mixed comments about charging them off a vehicles alternator while driving. Some people say yes others no. Anyone have a definitive answer? Would like to go lithium and need to resolve the charging issue first.

Thanks
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Old 10-16-2020, 08:58 PM   #10
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You might want to check out Firefly batteries. They are a Carbon-foam AGM battery and have become very popular in the boating world. I just replaced my 4D battery with two G31 Firefly’s and now have a 220 Ah capacity. There are some of good threads on this website and lots of good articles in the boating media.
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