Deep cycle battery question

[Bedelliott]

Understood. Good lithiums are still expensive, though it would help you meet your power needs.

To get the most out of your current power set up, you should be bulk charging the battery early morning every morning. You can do that several ways. Solar can provide additional charging throughout the day if your daytime load is less than your solar generation. Maximizing the system you currently have should be first on your list. Once you are able to maximize your storage and battery health, you then can manage the power provided.

Bulk charging can be done each morning with a portable generator, shore power and a bulk charger, or from your vehicle chassis engine charging system. This is an important cycle if you are depleting the battery through the night. Solar can be helpful following bulk charging but it is not a dependable source to meet your power needs.

Regarding daytime use, it’s hardly anything, all it is is fridge support, occasional water pump and parasitic draw. I would think a 200 W solar system would be more than adequate because with full sun it could add 16 A per hour, so if a sunny day gave me 3-4 hrs of good sun that could add 50-60a which isn’t far off of a full charge, given mine seems to be 73-80a at full capacity. Or am I missing something? Granted full sun isn’t dependable, so maybe a second battery would provide good back up support. 2 of these batteries ($220) and a 200w solar system ($250) is still less than 1 lithium battery so I’m not seeing the attraction of the lithium.

 

[RV_Lee]

Before upgrading to 460AH lithium I had two 110AH life lines and a 100 watt panel. It was only reliable for approx 24 hours, by day two it would drop to 11.6~V and not even have enough juice to start the generator. The inverter just being on uses battery power (along with everything else posted earlier), I would only have that on to use the plug to charge phone and laptop. That and a couple lights was all I used. Lead acid / AGM is just not enough for dry RVing unless its for a quick overnight.

Also keep in mind you can run lithiums down to 0, you cant do that with lead acids. A cheap 100AH lithium would be double what you currently have, but I'd shop for a 200 or 300 if running a cpap all night. Amazon had deals on same size 300AH last winter. That will last for days with no solar.

Lead acids may have worked 30 years ago when all RVs used was a couple lights, but not in our power hungry world today.

 

[ferfer]

When you boondocking amp hours become important. The amp hours become even more important when you do not have a generator or reliable method to recharge the batteries. Sealed or flooded lead acid battery are dead at 11.6V - as you discovered. 50% SOC is 12.2V sealed or 12.4V flooded. Running battery below 50% SOC frequently reduces the life of the battery. At 11.6V dead battery it will dies quicker.

I do not have lithium batteries but some advantages of a lithium battery is you can almost use all amp hours available. Fast charging but I understand the speed and full charge depends on your converter. The more knowledge will respond. Several Forum members use a cheaper lithium with great success. If you decide to evaluate lithium start a new post and I am sure you will lots better advise then me.

The bottom line is your wife uses a CPAP. Only she can tell you consequences when not available. You know your rig & the type of off grid camping you both enjoy. If you are thinking of spending ~$500 you could find lithium or true deep cycle 12V 200+ ah batteries. Google is your friend and this forum has lots of knowledge.

Jennifer

 

[Henry J Fate]

Regarding daytime use, it’s hardly anything, all it is is fridge support, occasional water pump and parasitic draw. I would think a 200 W solar system would be more than adequate because with full sun it could add 16 A per hour, so if a sunny day gave me 3-4 hrs of good sun that could add 50-60a which isn’t far off of a full charge, given mine seems to be 73-80a at full capacity. Or am I missing something? Granted full sun isn’t dependable, so maybe a second battery would provide good back up support. 2 of these batteries ($220) and a 200w solar system ($250) is still less than 1 lithium battery so I’m not seeing the attraction of the lithium.

This will not provide you with what you need and your batteries will quickly fall victim to long days and long nights in an unhealthy condition while never enjoying being fully charged. Have a good generator ready each day or provide a good power feed from your vehicle engine. Plan to run one of those daily.

 

[John From Detroit]

How could you only get 10-12 hrs of CPAP that draws 40w max? 40w x 10 hrs = 400wH, which is less than half my capacity.

You said Deep Cycle. Not LiFePO4. LiFePO4 you buy 100 AH you get around 99 AH usable power.
Lead acid True Deep cycle derate 50% MARINE/deep cycle derate 75% after that the voltage drops to where the inverter says "By By"

 

[Kirk]

I would assume from this that you usually do not have 120V power available when you are out in the RV? Have you considered carrying a small, portable generator?

I can get more out of it at a slower draw (drawing 4a vs 25a)

Yes, that is true of any battery and especially so of flooded cell, lead-acid batteries.

my manual says the inverter's energy conversion is >80%, I wonder if that's the efficiency like you mentioned.

That is exactly what it means. If the inverter is to supply 8a to the system it will be using 10a from the battery.

I had the CPAP connected to a meter and it draws 25-35w, it fluctuates a bit.

Were you using something like the Kill-A-Watt to measure it?

It seems there's no way to know how full (how many aH/wH are left) other than sticking a charger on it and seeing how long it takes to charge (multiplied by the charge rate), true?

I have never heard of a device that can measure how full a battery is, accurately. The best that you can do is to measure how much goes out and how much goes back in as that equipment is available but not inexpensive.

this battery is only a month old so I’m not ready to trash it, I’m not sure I would want to pay four or five times the price for lithium though.

With a battery which is that new, why not just add a second one that is just like it, connected in parallel? That will double your capacity and thus run-time for the C-pap and it wont cost a lot and will be easy for you to do.

 

[Gary RV_Wizard]

In theory should a new 12V battery put out 12V all the way through complete discharge?

NO. A fully charged 12v lead-acid battery yields 12.6v but the voltage drops as it discharges. The standard battery rating system (20 hour performance) considers it "dead" at 10.5v. The voltage drops faster and further as the amp load increases, but a CPAP isn't a huge amp draw. But if running it via inverter rather than on 12vdc, the inverter has a 15%-20% overhead cost that increases the amp load quite a bit. If you can find a 12v adapter for her CPAP, the battery will last longer.

The easiest improvement, tho, is to put a second identical battery in parallel with the first and double the capacity. Voila!

 

[Bedelliott]

I would assume from this that you usually do not have 120V power available when you are out in the RV? Have you considered carrying a small, portable generator?

Yes, that is true of any battery and especially so of flooded cell, lead-acid batteries.

That is exactly what it means. If the inverter is to supply 8a to the system it will be using 10a from the battery.

Were you using something like the Kill-A-Watt to measure it?

I have never heard of a device that can measure how full a battery is, accurately. The best that you can do is to measure how much goes out and how much goes back in as that equipment is available but not inexpensive.

With a battery which is that new, why not just add a second one that is just like it, connected in parallel? That will double your capacity and thus run-time for the C-pap and it wont cost a lot and will be easy for you to do.

True about your 1st statement - my RV is in storage until I use it so no access to plug it in overnight, all I have is charging when I pull it (typically 1-2 hrs) or bringing the battery inside each time & sticking it on a charger. I have a pair of those Honda 2000s but figured I didn't really need them and could come up with a better solution, but would one of these give the battery a good deep charge in a short amount of time? I thought you needed to charge at a very low rate to do that? Yes, I was using a kill-a-watt to measure it. I thought of adding a 2nd battery in parallel but still have the charging issue - the more I learn from y'all (which I've really appreciated all the input!) the more I think I need to move to a LiFePO4 battery, then I'd get nearly all my aH/wH, but if I had one or 2 of those would a 200w solar on top be a good charging solution (assuming there's sun)? If those would add 15+ amps/hr (for a couple hrs each day) that'd probably be more than enough given this limited nighttime use & not much daytime use...? I probably need to start a different thread about a lithium solution, but I've seen those 1600wH solutions at Costco for $1700 - if I get my own Litium battery & appropriate inverter I'd think I'm replicating those expensive solutions, so I'm curious why they're so expensive, or maybe those batteries (like Lion Energy, Jackery, Bluetti) are just better batteries, or maybe just overpriced for the convenience?

 

[Ernie n Tara]

You're not listening well:
a. The load as you measure it is 40 W per hour for the CPAP.
b. The parasitic loads as described above aould easily be one Amp plus or another 12 W all 24 hours.
c. The other loads you describe will easily amount to 20 or more Watts. Example: One 12 V light is one Amp ~ 12W, etc.
Above is 400 + 288 + 150 ~= 938 WHours per day
d. The Inverter is about 90% efficient and will be on most of the time unless you turn it off certainly for long enough to provide to charge to the above 938 WH while consuming another 94 WH. you state the battery is 80 A hours or 800 W hours!

You simply don't have enough battery to rely on it for a critical application.

Add a second battery and then ensure adequate charging is provided.

Ernie

 

[RV_Lee]

I only manage to run my 460AH down to 90% over night with the max air fan running and stuff plugged in and my 300 watts of solar has it back to 100% by noon (with everything still being run off the battery at the same time). They say you need to double the solar watts (in my case it would be 800+ watts) but my little 300 watts of solar works (100 on roof, 200 folding plugged)

Its overkill but I like it that way, I was stuck at a truck stop for 3 days this past spring and it still never hit 50% SOC

 

[ferfer]

I have a pair of those Honda 2000s but figured I didn't really need them and could come up with a better solution

If you already have the generators (zero cost solution) why not use one to recharge the battery during the day. Especially since you now know the 80ah battery must be charged to complete your stay. Even better if you can connect generator to 30A power socket to support house loads while charging.

Even if you go to solar or lithium or more am hour battery, having a generator available is wise backup plan.

Jennifer

 

[CharlesinGA]

You plug the RV into the generator(s) via the shore power cord. The battery is charged by your trailer's power converter which is probably 45 or 55 amps output. The generators have a 12v power output but it is minimal, much better to power the trailer via the shore cord and let the converter do its thing.

The inverter is consuming more current than it produces, by far, so you are throwing away precious battery power using it. This is why I suggested using a 12v CPAP. Those will run on 120v AC via a cord with a power supply "brick" in it, like a laptop computer does, or will run directly on 12v DC using a different cord that plugs into a 12v power socket. Most likely they will be designed to be more energy efficient also.

Camping with CPAP: Solutions and helpful tips

Charles

 

[Bedelliott]

I only manage to run my 460AH down to 90% over night with the max air fan running and stuff plugged in and my 300 watts of solar has it back to 100% by noon (with everything still being run off the battery at the same time). They say you need to double the solar watts (in my case it would be 800+ watts) but my little 300 watts of solar works (100 on roof, 200 folding plugged)

Its overkill but I like it that way, I was stuck at a truck stop for 3 days this past spring and it still never hit 50% SOC

Does your charge controller accurately tell you how much solar production you’re getting as well as how many aH your battery has, or maybe they all do that now?

 

[RV_Lee]

Does your charge controller accurately tell you how much solar production you’re getting as well as how many aH your battery has, or maybe they all do that now?

The Epoch battery is bluetooth and it tells me everything on the Epoch app, the charge controller and dc to dc charger is Victron and that app tells me whats going on with those.

 

[Mark_K5LXP]

Lots of arm waving and gum flapping, with no hard numbers to make an accurate assessment. Wonder and guess or measure and know. We know the OP has an 80Ah marine battery, and 25W@120V CPAP. The rest - starting state of charge, inverter efficiency, inverter cutoff point, other loads are all TBD. Back of the envelope math says this battery can easily run the CPAP for more than one night but the other variables cast this in doubt. A basic $25 battery monitor would eliminate the guessing. Understand that lithium doesn't create capacity. If your lead acid isn't enough, then lithium doesn't fix that. Quantify the goal of the system and design to it, rather than throw parts at it.

Mark B.
Albuquerque, NM