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Old 08-14-2019, 08:23 AM   #1
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Best RV battery for solar system (2x100W)?

Dear forum members,

I hope all is good with you all. I am looking for some advice concerning the best battery given my regular needs in my RV. I currently reached a point of no return with my previous flooded lead-acid battery; it is in a bad state (~300). I think this was because I have been using under the following typical load for a day:
- 5 min water pump in the morning
- 24/7 refrigerator
- 10-15 min lights at night
- 5 min water pump at night
- charging phone during night

I have two 100W solar panels connected to the battery and currently a PWM controller. I live in a very sunny country with 3023 hours of sunlight per year (of a possible 4383), so usually bad weather is not an issue.


So my questions are:
1) Given the current normal expected load, and given that I can't use anything else other than solar light, how can I make this work sustainably? In other words, what would be the best battery to work with? AGM or Gel? I know already that a MPPT controller is by far the best solution to optimize solar energy, so that is a change I will certainly implement.

2) Shall I have 2 batteries rather than one to avoid overcharging, as this is a problem with AGM (and Gel batteries)?

3) Wouldn't the refrigerator constantly working influence/damage the battery in any way? (other than it would make it charge slower)

Your thoughts and ideas would be most welcome.
Thanks so much,
RAraujo
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Old 08-14-2019, 09:43 AM   #2
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Unfortunately you have not provided enough information for anyone to make a valid recommendation. I strongly suggest you install a battery monitor, like a Victron BMV-700. Until we have more data (from the monitor) we can not determine if your issue is insufficient battery storage or insufficient solar panels.

A couple of general comments.
  • MPPT controls ARE better than PWM
  • Flooded (liquid) lead acid batteries are still the most cost effective solutions
  • TWO 6V golf cart batteries is better than ONE 12V battery
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Old 08-14-2019, 10:22 AM   #3
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Quote:
Originally Posted by RAraujo View Post
......I currently reached a point of no return with my previous flooded lead-acid battery; it is in a bad state (~300).


What does that mean ~300 ? (300 what ?)
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Old 08-14-2019, 11:04 AM   #4
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I second the recommendation for starting with a good battery monitor. I have the Victron BVM-712 Smart (Bluetooth) which allows me to monitor from my phone app, my exact battery state of charge, current usage and remaining time (based on current usage) and can set alarms for when I need to charge. A good monitor will help you determine exactly how much battery capacity you need.

As for battery recommendation, a LiFePo battery like those made/sold by BattleBorn will allow you to use much more of any capacity compared to flooded, AGM or Gel without damage/life shortening and you can charge them as fast as you have the ability to (solar, genny, shore). They are expensive (~$900 per battery), but when you look at the total cost over the life (i.e you can use 80%+ of one 100AH Li- ~3,000 times verses only 20-50% of the other types only ~600 times), they are as good or better as all others, and with built in Battery Monitoring Systems (BMS) you don't have to worry about overuse, overcharge, maintenance or temps (the BMS will prevent damage from charging below 24F)

Between the monitor and LiFePo batteries, I know exactly how much capacity I have and how long it will last and won't be surprised to wake up to dead batteries.
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Old 08-14-2019, 11:55 AM   #5
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If money, location size, and temperatures aren't an issue, I'd add another 100W panel up top, and switch to 2 X 100 Ah Relion LiFePO4 RB100-LT batteries with an appropriate charger. I'm planning on doing just that when the factory installed FLAs are retired. I've already got the 3rd 100W panel installed, my ZAMP PWM ZS-30A solar charge controller has a Lithium charge algorithm, and my configuration probably wouldn't benefit from adding a MPPT controller, because it's too small capacity wise. I believe the 2 Relion RB100-LT batteries will just fit into the battery tray where the current jars are located. They're good to -4F charging with their self heating technology, and they all come with some sort of "idiot proofing" built in BMS these days.

Just waiting for the factory batteries to start to fail..... then pulling the trigger.
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Old 08-14-2019, 01:19 PM   #6
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Quote:
Originally Posted by RAraujo View Post
- 24/7 refrigerator
This is the elephant in the room. You need to feed this hog and that takes input, not just storage. Characterizing just how much power you're actually drawing daily will drive the system size you need, from there you can pick your battery chemistry.

Mark B.
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Old 08-14-2019, 04:26 PM   #7
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Knowing the year/make/model of RV, or of the refrigerator would help. If it's a typical 12VDC fridge, like the Norcold DC0061, it's only going to draw maybe 3-6 amps per hour, and only when the compressor cycles on. 2 100Ah Lithium jars can handle that no problem, and 300W of solar can replenish it pretty easily if the OP's statement about plenty of sun where he stays is correct.
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Old 08-14-2019, 04:52 PM   #8
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I second the battery monitor recommendation.

Take real readings...sample a few days. Make an informed decision. The battery monitor will continue to be your best friend in the Solar/Battery game.

There isn’t a perfect for everyone balance. I have an all-electric coach with a three door residential refrigerator. I didn’t design it...but after taking measurements...it included 8 x 6v deep cycle Interstate GC-2 batteries giving me 928aH of storage.

These can basically run the fridge and other static loads for about 40 hrs without going below 50% State of Charge.

I think that is a good target for storage. Once you know your storage...then you can better balance the solar.

Our batteries mfg. recommends a C/10 charge rate. So, a 928 aH bank divided by 10 is 92.8 A... at a typical 14.4 absorb voltage... = 1336 watts...

My goal is to charge them using solar like I would in shore power... I have 1440 watts, and the MPPT controller keeps the amperage below c/10 rate for me...

The PWM vs MPPT is an efficiency thing. Either version can still be not good for your batteries. Does your charge allow bulk, absorb, float, and equalization? Are the parameters fixed or can you custom program them? Does the unit compensate for temperature? Having the correct settings, and balancing plate life vs sulfation is important. Your usage is your usage. You will go thru batteries based on number of cycles...but what you do. It want is premature failure due to improper management. Not desulphating...deep discharging...improper maintenance as far as water and terminal connections remaining clean. Excessive temperatures.

Anyway...you’ll have to define your happy spot...and use real data to make the best decision for you.
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Old 08-14-2019, 07:47 PM   #9
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Quote:
Originally Posted by C.Martin View Post
I have an all-electric coach with a three door residential refrigerator. I didn’t design it...but after taking measurements...it included 8 x 6v deep cycle Interstate GC-2 batteries giving me 928aH of storage.

These can basically run the fridge and other static loads for about 40 hrs without going below 50% State of Charge.

I think that is a good target for storage. Once you know your storage...then you can better balance the solar.

Our batteries mfg. recommends a C/10 charge rate. So, a 928 aH bank divided by 10 is 92.8 A... at a typical 14.4 absorb voltage... = 1336 watts...

My goal is to charge them using solar like I would in shore power... I have 1440 watts, and the MPPT controller keeps the amperage below c/10 rate for me..
That does sound like an impressive, well thought out system.

Unfortunately, not everyone has the luxury of carrying/adding a battery bank that weighs just under 500 lbs. - 8 Trojan T105s will weigh that much. Throw in the weight of the 1440 watts of glass panels (assumption) and you'll crush just about anything shy of a DP.

The OP seems to have gone AWOL, so we may never know the exact environmental and structural parameters we're dealing with.
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Old 08-15-2019, 09:47 AM   #10
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More thoughts...

Hi all,

Thanks so much for all the replies. I do have a monitor; it gives me amperage and voltage. It is not as fancy or detailed as the Victron but it seems that I can derive the other measurements from a typical day, isn't it? I think that if I sample some days of typical use (and solar charge) I will have a good idea of what is going on. Would this work?

I agree that the elephant in the room is really the refrigerator. The one I have is a Waeco MD90 (45W and ~3A/h). From what I gathered from all the excellent replies (I can't thank you guys enough!) is:
1) Sure an amazing monitor is great, but I think that I can see more or less what is going on with the monitor I have... right?
2) Two batteries would be ideal (preferably LiFePo but that seems to be a bit out of my pocket range)
3) I got mixed answers concerning the MPPT controller (I thought it was the best in terms of efficiency and it modulates better the solar energy to be stored in the battery)... but I am not sure anymore...
4) I am not sure if modern AGM batteries have a BMS system today...do they? This sounds like a key component to lengthen the life of the battery...
5) If solar input is not enough, say if there is a sequence of cloudy days, I will certainly consider adding a third solar panel. However, I don't want to risk overcharging the battery in very sunny days (so the BMS sounds like a must). On the other hand, I don't want to overdimension the system with to much storage capacity (i.e., two 12v batteries) for supplying a 3A/h fridge only...


BTW, I meant 300EN (which is a measurement for the battery capacity. My lead-acid battery at max is 800EN, a flat battery is 200EN.

Well obviously money is a little bit my biggest concern here... as I checked the price of the LiFePo batteries I almost had a heart attack... However, I want to design a system that works for now but, still, I can upgrade it slowly in the near future. I.e., say, I add one battery now, and then later I can add another one... but the first battery I choose must be of the same type.

Thanks so much for your insights and more ideas/suggestions will certainly help me making an informed decision.


All the best,
RAraujo
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Old 08-15-2019, 10:17 AM   #11
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YOUR QUESTION WAS WHAT IS THE BEST BATTERY.

But you offered up solar charge load and panel information.

The best battery is the biggest you can buy with the most Amp hour ratings that will fit the space you have for it. Most people then consider the cost because the best is really expensive and not always the practical choice. The GC2 size is a popular choise and go with 2- 6 volt batteries in series instead of 12 volt batteries in parallel. AGM just means not having to deal with water levels but they cost more.

So for solar considerations you must find the balance point between watts used vs generated and consider variables such as cloudy days etc. The battery is not usually the limitation for a small system as probably the array can not generate the power used in one day anyway. Now that is for a no generator or plug in operations.

I have a 200 watt panel and a huge 750 amp battery bank and I will tell you the solar in inadequate, still must run the generator. But what it does do very nicely is keep my batteries charged when in storage.

A battery set should be able to power the coach overnight without solar. Your small 200 watt solar will extend that to some degree depending on use

The solar controller keeps the batteries topped of when the coach is not in use.
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Old 08-15-2019, 01:46 PM   #12
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Quote:
Originally Posted by RAraujo View Post
Sure an amazing monitor is great, but I think that I can see more or less what is going on with the monitor I have... right?
Sure, as long as you're willing to sit next to it and write the numbers down once a minute or so for 24 hours to get an idea of you daily power usage. You don't need a spendy battery monitor for that, the $20 chinese ones that count Ah work fine for basic source and load measurements.

https://www.amazon.com/Spartan-Power...dp/B07FN3VXH4/

Quote:
I got mixed answers concerning the MPPT controller (I thought it was the best in terms of efficiency
It can be but for small systems it's rearranging deck chairs on the titanic. It can make a small difference but a well engineered non MPPT system works well too. When every Ah is critical and you care, then engineer for that. But don't anguish over the minutia of a handful of additional watthours a day when you're burning through those running a refrigerator. In other words, I don't think it buys you enough margin to matter with this system size.

Quote:
I am not sure if modern AGM batteries have a BMS system today...do they?
Nope. The only reason I'd consider AGM is if I had enough charge capacity to take advantage of their absorption rate. I doubt it with this level of solar. You'd be spending twice as much on lead for no additional capacity or capability.

Quote:
This sounds like a key component to lengthen the life of the battery...
What is "life" in this application?

Quote:
If solar input is not enough, say if there is a sequence of cloudy days, I will certainly consider adding a third solar panel.
And that will make it sunny? Sure, you'll get 30% more from the array but you'll still be down 80-90% from full sun.

Quote:
I don't want to risk overcharging the battery in very sunny days
Even basic controllers manage that.

Quote:
I meant 300EN (which is a measurement for the battery capacity.
No idea what "EN" is. Storage batteries are rated in amp hours (Ah).

Quote:
I can upgrade it slowly in the near future. I.e., say, I add one battery now, and then later I can add another one...
You can do that with panels pretty easily but not so much with batteries. In order for them to perform optimally and not wear each other out, they should all be the same make, capacity and age as a set.

What I've found to be a useful exercise is to create the system in a spreadsheet. Using datasheets, insolation tables, and best guesses if necessary, you can calculate all of your input capacity and load demands both best and worst case on each side. From there you can play with panel sizes, controllers, battery capacity and load management to come up with a starting point of what it will take to realize this system. Can also tell you if what you have in mind is even possible before you go out buying equipment, only to find out you can't get there from here with it.

Mark B.
Albuquerque, NM
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Old 08-16-2019, 06:27 PM   #13
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Batteries and Solar

You have two questions really. One is whether your solar panels will produce enough on average to power the refrigerator and charge the batteries. The other is whether the batteries will store enough power to run the refrigerator when the solar panels aren't producing.

Assuming the equivalent of 5 hours of full sun your two panels will produce 1000W/h. If your refrigerator pulls 3 amps at 12 volts it will use 864 watt/hours in 24 hours. So your panels would support that load, assuming that five hours is realistic. A third panel would leave you with a much better margin of error for cloudy weather.

Assuming the longest you go without significant solar power is 67 hours (essentially two cloudy days) then you will need batteries that can give you 3*67 = 201ah. Assuming you want to limit your planned depth of discharge to 50% you will need a 12 volt battery bank that gives you about 400ah at a steady discharge of 3 amps.

Most batteries' AH is rated over 20 hours. So a 400AH rating assumes you draw down at a steady rate of 20 amps. At 3 amps it will produce a lot more than 400AH. How much more depends on the battery and good manufacturers will provide an AH at a variety of levels. Or find a knowledgeable battery dealer who can help you.

This is conservative. Your refrigerator probably isn't going to run 24 hours per day, you will get power even on cloudy days, and you can safely draw the battery down to 80% occasionally.

I would get the AGM batteries just for the maintenance issues and longer life. You don't need a fancier battery monitor. Your system should work without any intervention and monitoring the battery's voltage will alert you if there is a problem.
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Old 12-02-2019, 05:20 AM   #14
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Mission accomplished

Dear all,

I have come to a solution that after some initial testing seems to be working really well:

1) change the controller for an MPPT;
2) have not the initial 2, but 4 solar panels (100W each)
3) have two lead-acid batteries, instead of only one as it was initially installed.

This solution is working nicely so far, at least for the winter (with less solar energy available, yet less heat to control the temp for the fridge). And, we shall wait and see, but I believe it will work for the summer too (more solar energy available, yet more heat to control the temp of the fridge).

One might argue that the solar system is over-dimensioned (fridge consumption is 10Ah whereas each solar panel can maximally produce 8Ah, so multiply that by four), but honestly, it's better to be safe than sorry. My type of usage, i.e., long-term staying in a single place without electric connection, implies complete reliance on the sun's temperament... I might get a whole week of rainy days... and by then a single battery will be gone for good...

Anyway, I would like to thank you all for your input which was extremely useful for me before making my final decision.

All the best,
Ricardo
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