Deep cycle battery question

[Bedelliott]

My new battery has 960 wH and I have a 400w inverter hanging off it to run my wife’s CPAP at night, charge phones, etc. Last weekend the CPAP stopped running halfway through the second night, but when I got home after a 2.5 hr drive home the battery was about 1/3 full (judging by how long it took to fully recharge with my 1a charger). I don’t think it would have charged 1/3 of the way after a 2.5 hr drive, would it? But my real question is, should the CPAP machine run fine all the way until the battery is completely empty, or does a battery weaken as it discharges and can’t really support something drawing 30-40w?

 

[Henry J Fate]

It should definitely absorb 1/3 charge from the chassis engine charging system and I would expect much more. ( I am assuming you are driving a motorhome )

The 400 watt inverter should shut down and discontinue power inversion at an internally set battery voltage minimum. This should protect any devices connected to the inverter that are sensitive to low voltage. ( mostly motors and pumps )

You should be able to find the minimum voltage shut off set point for the inverter either by looking at the specification sheet or by calling the manufacturer

 

[Bedelliott]

My apologies, I left off crucial information, I have a bumper pull camper and pull it with an F250. Good thought on the inverter low-voltage shut down, I will look into that. In theory should a new 12V battery put out 12V all the way through complete discharge?

 

[Kirk]

A 12V battery isn't really 12V but a variable voltage, depending on the type of battery and it's state of charge. If you have the typical, deep cycle, flooded cell battery, the actual voltage when fully charged would be about 12.6V, then slowly drop to about 11.5V which is as low as the battery should normally be used. Most RV appliances are designed to work properly between 14V and 10.5V. if is battery is continued to be drained that voltage will start to fall much more rapidly and drop eventually to 0V. No battery maintains the same voltage from fully charged to completely discharged. The lithium batteries of today do come far closer to doing that than do typical RV batteries. The answer to your question about voltage depends on what battery you have.

The rate that a battery in a towable RV is recharged by the alternator of the tow vehicle is usually limited by the wire size & length that supplies the voltage to it and so takes much longer to recharge than does the tow vehicle's battery. Just as a small diameter water hose supplies less water than a larger one with the same pressure, that is true for a wire size at the same voltage. If your wires between the RV and the tow vehicle as the same as most standard cords, I would consider 1/3 recharge in 2.5 hours to be pretty good but you also need to understand that as the battery is recharged the rate of recharge at the same voltage supply will decline.

 

[Henry J Fate]

Lead Acid Battery Voltage Chart

Just like any other type of battery, lead acid batteries have a different voltage at different stages of charge. In this article we talked about everything you need to know about lead acid battery voltage chart.

naturesgenerator.com

This should offer the info you need.

 

[Bedelliott]

A 12V battery isn't really 12V but a variable voltage, depending on the type of battery and it's state of charge. If you have the typical, deep cycle, flooded cell battery, the actual voltage when fully charged would be about 12.6V, then slowly drop to about 11.5V which is as low as the battery should normally be used. Most RV appliances are designed to work properly between 14V and 10.5V. if is battery is continued to be drained that voltage will start to fall much more rapidly and drop eventually to 0V. No battery maintains the same voltage from fully charged to completely discharged. The lithium batteries of today do come far closer to doing that than do typical RV batteries. The answer to your question about voltage depends on what battery you have.

The rate that a battery in a towable RV is recharged by the alternator of the tow vehicle is usually limited by the wire size & length that supplies the voltage to it and so takes much longer to recharge than does the tow vehicle's battery. Just as a small diameter water hose supplies less water than a larger one with the same pressure, that is true for a wire size at the same voltage. If your wires between the RV and the tow vehicle as the same as most standard cords, I would consider 1/3 recharge in 2.5 hours to be pretty good but you also need to understand that as the battery is recharged the rate of recharge at the same voltage supply will decline.

I do have the typical deep cycle, flooded cell - I have the O'Reilly's Super Start Marine Group 27M, 27DCMJ. My inverter says the low voltage shut-down is 10-10.5v. Will a typical $25 multimeter accurately test the battery's voltage, or maybe only after it's sat for a bit (meaning probably not accurate immediately after pulling the camper charging)? If I could trust this, I could test throughout my next camping trip to see how the battery is acting (it's a new battery). You mentioned that the battery shouldn't be used below 11.5v, but does that mean that most all of the aH have been consumed by then? I can chk the wire going to my 7-pin connector to see what size it is. Basically, I had (probably erroneously) assumed that a 960 wH battery would support a 25w CPAP for 38 hrs (doing the simple math of 960 wH / 25w = 38.4 hrs), but maybe batteries don't work exactly like that?

 

[Kirk]

The multimeters in the $25 to $35 range are reasonably accurate, at least when new and will be fine for most amateur uses even later. For battery state you really need at least two decimal places and as long as it measures consistently, then keep records of the voltage when fully charged to compare to and you would be fine. Better records mean better knowledge.

Group 27 batteries will range between 85 amp hour (Ah) and 100Ah and bigger lasts longer. To know more about how long it should last you need to know the amount of power used by the C-pap machine. If the inverter shuts down by 10.5V it is pushing the battery some but not enough to be serious if it only happens occasionally. At 11.5V the battery experiences no ill effects beyond normal aging but as you discharge it below that the rate of battery aging will increase. The 11.5V figure comes from battery manufacturing companies but there is really no exact way to define the wear on a battery. Its like when you drive a car harder it doesn't last as long but no two are alike. If you always keep it above the 11.5V level you should expect it to serve well for 3 to 5 years but if pulled down to 10V frequently it will probably be more like 2 to 4 years, possible less. If your battery is not a sealed one you will also need to check the electrolyte levels more frequently if it is discharged further. I think that yours is a sealed battery.

The umbilical cord wire size is typically 12 - 16 AGW. For best recharge rates most people find that it is good to run a separate wire from the tow vehicle to the RV battery and many use welding cable for a large wire size. With most standard wire sizes there is about a 2V loss between the alternator and the RV battery. The higher the charge rate the greater the voltage loss for the wire size. There are a couple of things you didn't consider in your battery life calculations. The inverter has a significant power consumption in converting and what that loss is varies. High quality sine wave inverters are rated at 90-95% efficiency. Lower quality modified sine wave inverters are less efficient - 75-85%.

 

[Bedelliott]

Regarding the aH, I had read online 85-100aH as well for group 27 but my battery specs say 175 mins @ 25a (seems to equate to 73aH or 876 wH @ 12v) but also says 20 HR CAP 80 (meaning I can draw 4a for 20 hrs, but this equates to 80aH, or 960 wH @ 12v). So it appears to be somewhere in the 73-80aH range but not overly close to the 85-100 range and I guess the 2 different ratings for the battery imply that I can get more out of it at a slower draw (drawing 4a vs 25a)...? But anyway, my manual says the inverter's energy conversion is >80%, I wonder if that's the efficiency like you mentioned. I had the CPAP connected to a meter and it draws 25-35w, it fluctuates a bit. I'll see if I can read the AWG on the wire from my truck, but I know it's the typical small wire, it looks to be 14-16, 12 would be the max. 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 say that because a multimeter will only tell me the voltage, but that doesn't overly indicate how charged it is or not. My apologies for the rookie questions, I could sure stand to take some electrical classes. My need is pretty simple though, I would have thought that my 900-ish wH would be plenty for a 2-night campout running this CPAP for 8 hrs each night (so that draw might be 240wH per night, or 480wH for 2 nights, meaning about 1/2 my battery) and the only other draw on the battery would be a very few LED lights, a phone charge or 2, the occasional igniter on my fridge (running on gas) and a little water pump here & there.

 

[ferfer]

In a perfect system you would have 960wH or 80ah going to your inverter and your inverter would not need power to run. But in the real world the power system is not 100% efficient and you can expect something less than 960wH or 80ah. You are asking a lot of 80ah battery over two day without charging.
Everything device plus parasitic is using 12V power thus reducing the amp hours.

 

[Henry J Fate]

Voltage readings are a very good and simple way to estimate state of charge for a lead acid battery. Voltage readings are used by full time battery level monitors.

 

[Bedelliott]

Sounds like I should stop operating in my theoretical world, hoping for the ideal and just throw a 200w solar panel on top w/ 30a charge controller (or a 2nd battery). But along those lines, if I ever got all sun and generated 200w, that's charging my battery at a little over 16a - isn't that way too fast of a charge rate to get a good, deep charge? If I went with a 100w panel it'd probably be a better charge rate but then again, how often will the panels ever catch 100% of their capacity, and if I ever wanted to add a 2nd battery and up my inverter to 2000w (to make coffee or whatever else) I'd sure wish I had 200w panels then. What's the prevailing approach to a simple/small solar solution?

 

[Bedelliott]

Voltage readings are a very good and simple way to estimate state of charge for a lead acid battery. Voltage readings are used by full time battery level monitors.

When I got home from my trip my battery read 12.5v, but so far my battery has taken 48a and it's still going (on a 1a charger), and based on my above msg it seems my group 27 battery holds somewhere in the 73-80aH range. What percentage full would 12.5v roughly represent?

 

[John From Detroit]

Full charge 10-12 hours of CPAP only. but oh wait there's more.
You have the Explosive gas detector (Propane) possibly a CO and/or SMOKE detector (some are self contained battery only some are "line" powered.

Control power for Fridge (even if on GAS) and Water heater and possibly other things.
Any lights. the water pump. all draw power. And that is assuming Full charge.

Upgrade to pair of GC2 or 100-200 amp hours of LiFePO4 for better run time
NOTE. this will lengthen the amount of drive time before it's re-filled.

I just dropped a bunch of LiFePO4 on my apartment Kitchen Fridge/microwave.. The Microwave won't be used during power fiil (The clock is easy to reset but....Why_) Figure 40 hours of run time and another 18 hours of safe time.

 

[Bedelliott]

Full charge 10-12 hours of CPAP only. but oh wait there's more.
You have the Explosive gas detector (Propane) possibly a CO and/or SMOKE detector (some are self contained battery only some are "line" powered.

Control power for Fridge (even if on GAS) and Water heater and possibly other things.
Any lights. the water pump. all draw power. And that is assuming Full charge.

Upgrade to pair of GC2 or 100-200 amp hours of LiFePO4 for better run time
NOTE. this will lengthen the amount of drive time before it's re-filled.

I just dropped a bunch of LiFePO4 on my apartment Kitchen Fridge/microwave.. The Microwave won't be used during power fiil (The clock is easy to reset but....Why_) Figure 40 hours of run time and another 18 hours of safe time.

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.

 

[Henry J Fate]

Might be time to start thinking about a lithium battery upgrade. Lithium charge profiles are much friendlier with the type of usage you are trying to meet. Available power is also much better. You can increase your lead acid capacity by adding an additional battery but you would definitely see better results with the same capacity you have now, in the form of a lithium battery. Double your current capacity in the form of lithium and you will have a good amount of wiggle room day to day. You will still require external charging by either a generator, plugging into shore power with an adequate charger or from a chassis engine alternator.

Rule of thumb is to work with what your system will provide, which in most cases is never enough.

 

[ferfer]

If your 12.5V battery reading was shortly after charging it means nothing. The battery must rest many hours for accurate SOC (state of charge). Check post #5 above. Henry J Fate post chart of SOC for different battery types.

You did not get the full calculated CPAP hours because the inverter turned off when SOC was ~ 11.5V. Also @ 11.5V the SOC for flooded = 0 % sealed 30%.

Depending on the type of boondocking you plan you might consider upgrading your battery (200ah neighborhood or lithium) or a portable generator that plugs into your 30amp socket. I do not think running the truck as a charger is viable as routine charger.
Jennifer

 

[Bedelliott]

Might be time to start thinking about a lithium battery upgrade. Lithium charge profiles are much friendlier with the type of usage you are trying to meet. Available power is also much better. You can increase your lead acid capacity by adding an additional battery but you would definitely see better results with the same capacity you have now, in the form of a lithium battery. Double your current capacity in the form of lithium and you will have a good amount of wiggle room day to day. You will still require external charging by either a generator, plugging into shore power with an adequate charger or from a chassis engine alternator.

Rule of thumb is to work with what your system will provide, which in most cases is never enough.

Thanks, but 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.

 

[Bedelliott]

If your 12.5V battery reading was shortly after charging it means nothing. The battery must rest many hours for accurate SOC (state of charge). Check post #5 above. Henry J Fate post chart of SOC for different battery types.

You did not get the full calculated CPAP hours because the inverter turned off when SOC was ~ 11.5V. Also @ 11.5V the SOC for flooded = 0 % sealed 30%.

Depending on the type of boondocking you plan you might consider upgrading your battery (200ah neighborhood or lithium) or a portable generator that plugs into your 30amp socket. I do not think running the truck as a charger is viable as routine charger.
Jennifer

My 12.5 reading was the next day after driving home, it hadn’t been touched in 24 hrs. I’m definitely not planning on the truck charging it when I pull, I’ll either put a solar panel on top or bring the battery inside to stick on the charger each time.

 

[Henry J Fate]

Thanks, but 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.

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.

 

[CharlesinGA]

Either get a CPAP that will run directly on 12v or if it has a cord with a power brick in it, find a cord with a power socket plug (ie lighter socket plug) and turn the inverter off, its eating extra current.

Charles