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RV Battery size question.

HomeTownHockeyFan

HomeTownHockeyFan

Solar Rookie
Joined
Dec 23, 2019
Messages
61
Location
Cary, NC
I'm building a solar system for my RV travel trailer...and need some help with battery sizing.

Configuration so far:
Six 100 watt HQST Poly panels configured 2S3P. (Planning for shade in campgrounds)
Victron SmartSolar MPPT 150/35 Solar Controller
WildKat 55 amp charger converter
Renogy 2000 watt inverter
Go Power Transfer switch
All running 12 volts
Lots of 1/0 wiring. :)

My energy usage calculations are 100-162 ah per day.

My question is what size batteries should I go with...my plan is to have 2 Lifepo4 batteries wired in parallel for a 12 volt system.

My choices are 152 ah, 200 ah, or 280 ah batteries (times 2)

I am leaning toward the 280ah ones...my question is...is there such a thing as too much battery?

My concern is with my configuration can I charge them back fast enough...or am I wasting my time and should go with smaller batteries?

Thanks in advance.
 
Last edited:
In good solar conditions your array will give you about 3kw/hr of production a day. This is about 250 a/hr. Based on that I would say a pair of 200a/hr would provide about 5 kw/hr and allow you to operate using about 60% capacity and the ability to coast through a couple of days with lower solar conditions. I myself have a similar set up with 200a/hr at 24v but with 1000 w of solar and this is enough to use 240v appliances such as toasters and kettles, microwaves, hairdryers etc without concern. I very rarely go below 60%, and most days have recharged from solar before midday. Hope this helps
 
But lets do some math (using 10 hour full sun to make the math easy):
100w x 6 x 10 hours = 6000WH x .75 efficiency best case = 4500WH produced daily
(Noting: Wellydex estimates 3000WH daily as i write this... could be!)

Assuming your system is 12v (stating AH without voltage is insuffieient)

162 AH x 12v = 1994WH used daily.
(162 AH x 24v = 3888WH used daily. ruh roh!)

Your question REALLY revolves around storage which means how much you need after the sun goes down. How much of your daily use needs to come from the batteries at night (or cloudy days, parked in the shade...)?
 
MisterSandals said:
But lets do some math (using 10 hour full sun to make the math easy):
100w x 6 x 10 hours = 6000WH x .75 efficiency best case = 4500WH produced daily
(Noting: Wellydex estimates 3000WH daily as i write this... could be!)

Assuming your system is 12v (stating AH without voltage is insuffieient)

162 AH x 12v = 1994WH used daily.
(162 AH x 24v = 3888WH used daily. ruh roh!)

Your question REALLY revolves around storage which means how much you need after the sun goes down. How much of your daily use needs to come from the batteries at night (or cloudy days, parked in the shade...)?
Click to expand...
Mister sandals, a couple of explanatory notes. I live in New Zealand and travel around the 40 s latitudes. My experience of 5x solar capacity per day in w/hr is based on flat mounted panel and that in NZ you very rarely get a day without some cloud. I would agree that in many places closer to the equator you will get more.
 
Wellydex said:
a couple of explanatory notes.
Click to expand...
No need to explain to me! The idea was that the OP would substitute his/her real numbers in the formula. I'm pretty darn good at multiplying (and dividing) by 10!

Love, love, love Wellington. We did a 4 month cycle tour and went thru there between islands, twice. Looking forward to the next tour there.
 
Thanks for the feedback so far. I edited my original post to indicate 12 volts.

My easy math concluded IF I used up 560 ah in one day, it would take me more then a day to recover that...but, since I would never use that much in one day...it did not matter that I could not recharge that in one day...

More of my usage would be at night. Furnace fan, limited microwave, TV, etc...
 
HomeTownHockeyFan said:
I'm building a solar system for my RV travel trailer...and need some help with battery sizing.

Configuration so far:
Six 100 watt HQST Poly panels configured 2S3P. (Planning for shade in campgrounds)
Victron SmartSolar MPPT 150/35 Solar Controller
WildKat 55 amp charger converter
Renogy 2000 watt inverter
Go Power Transfer switch
All running 12 volts
Lots of 1/0 wiring. :)

My energy usage calculations are 100-162 ah per day.

My question is what size batteries should I go with...my plan is to have 2 Lifepo4 batteries wired in parallel for a 12 volt system.

My choices are 152 ah, 200 ah, or 280 ah batteries (times 2)

I am leaning toward the 280ah ones...my question is...is there such a thing as too much battery?

My concern is with my configuration can I charge them back fast enough...or am I wasting my time and should go with smaller batteries?

Thanks in advance.
Click to expand...

I believe there is a risk of memory effect from incomplete charging (and not immediately discharging). If your battery bank is over-sized this could be something to consider.
 
Last edited:
Carly said:
I believe there is a risk of memory effect from incomplete charging (and not immediately discharging). If your battery bank is over-sized this could be something to consider.
Click to expand...

Is that really a thing for lifepo4?
 
smoothJoey said:
Is that really a thing for lifepo4?
Click to expand...

There was a thread on it back in December, I think. And this article was recently linked here (snippet below):

Memory effects in LiFePO4 cells were discovered and studied by Sasaki et al. [6] and the results published in Nature Materials in 2013. The authors illustrated that, under specific circumstances, the prior cycling history of a cell alters the voltage curve during charging by causing the voltage to increase faster and earlier than expected.

For a memory effect to appear, an incomplete charge cycle followed by a rest period and a discharge must have taken place earlier (memory-writing cycle). A partial charge followed by an immediate discharge is not sufficient to record a memory of the incomplete cycle [7]; this is important because the practical consequence is that a charge-and-hold strategy is particularly harmful when full charge was not achieved. It is not uncommon for DIY lithium battery systems to implement deficient charging strategies which in fact result in this scenario taking place and it is detrimental to the long-term performance of the battery bank.

Marine lithium batteries in operation | Nordkyn Design

This article details the relations between voltage, state of charge, current and temperature for lithium iron phosphate (LFP or LiFePO4) battery banks.
nordkyndesign.com
 
diysolarforum.com

Most useful article I've ever found on LFP battery charging

Came across this as a reference in a thread, it's possibly the most useful article I've ever came across about LFP charging regimes. Evidently there actually is a well documented memory effect on LFP cells and that always holding charge voltages low to avoid overcharging the cells is...
diysolarforum.com diysolarforum.com

That's the link thread that I mentioned, above.
 
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