How to ruin an RV battery

[Bdinla]

Thanks WindyJim for another really helpful post. Don't mind you're being verbose at all.

[lewster]

Mind if I chime in??

Originally Posted by WindyJim

General comments: Using your batteries to heat water can really suck your batteries down. If you use a 1000 watt microwave running off an inverter, you are discharging your 12 volt batteries at the rate of 1.4 AH (ampere-hours) per minute, or 84 AH per hour. Battery discharge is measured in amp/hours (A/H). If you wish to obtain amp/minutes, divided A/H by 60. You then have amp/minutes.There is no measurement for battery discharge called amp/hours per minute I believe the fans draw about 3 amps each when running, so the two of them add another 72 AH discharge, per 12 hours (if they are running constantly and not cycling on and off via thermostats) (142 AH per 24hours). I believe the batteries you referenced have a capacity of approximately 84 AH each (when new) (total 168 AH) and I think the guidelines are to not let your batteries go below 40% of capacity (if so that means you only have 100 AH per day available for use, excluding solar generation).The generally accepted rule of battery discharge is a 50% depth of discharge, which for Interstate Group 24 liquid cells would be 80 amp/hours of usable capacity.

You state the solar was putting out 10 amps at 12 volts. That may be the max, with less in the morning and tapering off in the evening, going to nothing at night. Lets say that effectively you produce 10 amps for 10 hours per day, or 100 AH per day from your solar (more in summer, less in winter). No solar charging system puts out a consistent, continual amount of power during a sunny day. There is minimal charging from dawn that increases with the sun's azimuth, and hence the solar radiance that the solar array receives and increases to 'solar noon' which is generally 2 hours before and after peak sun angle. If one assumes a 10 amp maximum solar output during solar noon, then the projected output of this system would be approximately 70-80 amps including 10 amps/hour during solar noon and a tapering of this amount for the remainder of the day. Let's call it 80 amp/hours of solar charge per day. This is assuming a summer sun pattern.

If you have incandescent lights, they draw about 5 times the power of LED lights, so depending on how much lighting you are using they can suck the batteries down too. To heat a quart of water from 60 degrees up to boiling (without boiling) takes 7.4 AH (continuing to heat it with 1000 watt inverter would be adding another1.4 AH per minute). Don't know where you got these figures from, but there are few electric kettles that will run from a 1000 watt inverter, as many have 1200 or 1500 watt heating elements. And remember, there is an increase in the load by a factor of 10 going from AC to DC, so a 1000 watt heating element appliance (8.3 amps) would draw 83 amps plus 15% for inverter inefficiency or a total of almost 100 amps from the batteries for each hour of use, That translates to 1.7 amps for each minute of use that will be coming out of the battery bank.

All that said and done, I’m a believer in using the propane stove to heat water, and run the fans and lighting judiciously. A heavy overcast day can mess up your anticipated solar generation. Don’t let the batteries get too far discharged. Make sure they have the plates in the batteries complete covered (by adding distilled water if necessary). You can play around with the numbers, but still see that without additional power being supplied you still have to be conscious of your rate of consumption. This is good advice for any system.

Sorry for being so verbose…

[RAH]

I have also made the mistake of over-estimating the solar output. For me it was just running the fans. We were in Taos, NW with no-hookups. I ran both fans on high for 6 hours. The figures I remember is each fan uses 1.5 amps on low, 2 on medium, and 2.5 on high. so I used 30 amps just to run the fans, which was my total solar input for the day. So I got exactly zero charge put back into the batteries that day. Actually less than zero. But I learned that I needed to think in terms of "the total amount you take out of the batteries has to equal the total you're putting back in" or else you finish each day with less power than you started. What a revelation.

I also learned that running 2 fans on high is kind of pointless with the factory setup. They both blow out. A couple of days later on the same trip, and we were courtesy parking at my in-laws on a street with lots of traffic. Could not open any of the windows, and it was August and hot. We opened the fan above the bed, but turned it off. We set the front fan on low, and had nice circulation of air throughout the AS as air was sucked into the opening above the bed, and discharged out through the front fan. Two revelations on the same trip!

[Bdinla]

@lewster
Thanks for the clarification on WindyJim's post. I'm getting the itch to go out again just so I can put all this new found knowledge to work!

@RAH
I appreciate those refinements on amp usage for the various fan settings. We also discovered how to circulate the air more effectively by using just one fan.

[idroba]

Like most threads, this one has drifted, but if it helps people understand batteries and their use, that is OK.

One thing that many RVers have little concept of is just how little energy they have stored in their batteries. Let me try to put that into perspective.

Most Airstreams come with two group 24 batteries, with a storage capacity of about 75 amp hours each. An amp hour is what it sounds like. These batteries will supply a one amp load for 75 hours. They would supply a 5 amp load for 15 hours. Not to try to confuse things, but the more load you put on them, the lower the amp hour capacity would be. So, loading them hard, like running a 1000 watt inverter with a coffee pot would reduce their capacity considerably. But most RV loads are relatively small these days with LED lights and so on, so lets say that the 75 amp hour capacity is a good number.

With two batteries, you then have a total capacity of 150 amp hours, in theory, assuming they are fully charged and you can run them down to no charge.

But, that is a very bad idea, running down to no charge. With the charge varying between 40 to 100% you might get 300 to 400 cycles if promptly recharged to 100%. If you left them sitting at 40 to 50 % charge, never bringing them back to 100%, they soon will have a permanent capacity of 40 to 50%. In other words, damaged.



If you discharged them to 0% you might get 10 to 20 cycles before failure. If you ran them down to 0% and left them un recharged for a few days to a week or two, I doubt you would get 5 cycles from them. Flooded cell batteries hate to be left discharged. AGM batteries are somewhat better, but not much. This is battery abuse for any lead acid battery.

So, the common recommendation is to not discharge a lead acid battery to less than 40 to 50% of it’s capacity, and then promptly recharge it to 100%. Prompt is a day or two, less than a week.

So, you have two 75 amp hour batteries in your Airstream. You don’t discharge them lower than 40%. (you use 60%) Thus you have a capacity of 75 x 2 x .6 = 90 amp hours.

Lets put this into household perspective. We all get electric bills. The energy you pay for is listed in kWh (kilowatt hours, 1000 watt hours). Most households use 400 to 1400 kWh a month.

Your RV batteries were rated in amp hours, so how to convert? A watt is a volt x an amp. We had 90 amp hours of capacity and a 12 volt system. So the battery capacity of our two RV batteries, not used lower than 40% is 90 amp hours x 12 volts = 1080 watt hours, or a bit more than 1 kWh.

So, in your home you probably use 400 to 1400 times that amount of energy in a month. Think of 400 to 1400 pairs of 2 batteries sitting around your yard.

RV batteries have VERY limited capacity. That is one reason they are so often abused and fail regularly, and are replaced often. But if not abused, they can hold up quite well. You simply must not ask more from them than they can deliver, and replace it quickly. (recharge).

I hope that helps in your understanding of RV battery capacity.

[Bdinla]

@idroba
Like most threads, this one has drifted, but if it helps people understand batteries and their use, that is OK.

Sorry to have been the one to pull the thread in a different direction. (I was afraid from the outset that I might violate proper Forum protocol.) That said, this has been extremely informative to me, and I've appreciated the contributions of all who have take the time to post to my question on your thread. I've learned a lot. Your latest post provides great perspective on how to think about the limited power we have available to us in our coach, and the need to conserve all the energy we can.

If I may, I have one final question pertaining to this topic. We have LED lights throughout the coach. Can you/anyone quantify how much power they tend to pull from the batteries? Should they be the least of our concerns?

Thanks in advance, and Happy Easter/Passover to all.

[idroba]

You have not violated anything, rest easy. Threads drift. When they go too far off, they either self limit, or there is a slight possibility that a moderator will say something.

I am posting a chart I have posted in the past, but don't know just where it is now. It may help answer your question about LED lighting and some other loads. My 2014 20' FC has only LED lighting.

Amp draw for things in a 2014 20' Flying Cloud "Bambi"

Measured with a Tri-Metric meter and 100 amp shunt.

Water heater, when flame on: .74 amp

Refrigerator, flame on: .32

Refrigerator fan only: .55

Bed lights, reading, each: .17

Bed lights, overhead, 3: dimmest .1
brightest .52

Main overhead lights, 6 dimmest .23
brightest 1.18

Step light: .06
Scare light (floodlight) .13

Table light: High .62
Low .31

Kitchen lights (either, there are 2) High .38
low .19

Bath lights (2, each with high and low) All .72
one high .38
one low .19
Every LED light inside on at the same time: 3.9 amps


Fantastic fan, each: High 1.55
Medium 1.22
Low .93

Bath fan (little, 4" round) 1.5

Kitchen range fan: 2.4

Kitchen range fan lights (halogen) 3.5

Furnace, 18,000 btuh, direct vent burner on: 3.65
fan only: 2.9

Radio, all off, no lights showing: no measure
Radio, disco lights on, no volume: .7
Radio, lights on, medium volume: .9 to 1

Propane detector: .06
(note: mis measured and posted an error recently on the forum, I was off by a factor of 10)

Pump: (variable speed) 2-4

1000 watt sine wave inverter, on, no load 1.76
with TV and DVD plugged in, but not on: 1.76

TV on and DVD playing: 3.6

TV on DVD playing, + radio on for sound: 4.9-5.3

Very small microwave, supplied by inverter: 89

Toaster, supplied by inverter: 75

Apple Mac Book Pro, charged running from
inverter: 3.1 to 3.3

[gandttimes]

Not that it makes a whole lot of difference but the group 24 Interstates in the trailers are aprox 84ah each, not 80 and 75 as has been quoted in this thread.
I know ,picky, picky!


George

[Bdinla]

@idroba
I am posting a chart I have posted in the past, but don't know just where it is now. It may help answer your question about LED lighting and some other loads. My 2014 20' FC has only LED lighting.

Awesome list. Thanks a heap, idroba. Your measurements can't be too far off our 2012 19' Flying Cloud. Much appreciated.

@gandttimes
Not that it makes a whole lot of difference but the group 24 Interstates in the trailers are aprox 84ah each, not 80 and 75 as has been quoted in this thread.
I know ,picky, picky!

Thanks, George. I appreciate the precision. It's very apparent from this thread that knowing what we're working with will help us stay out of trouble. I think I'll assume 80ah each, just to be on the safe side, and give us some cushion in the event we push the limits of the batteries again.

[idroba]

I expect that there is a variation in the AH capacity of group 24 batteries, from brand to brand, and then battery to battery. I like 75 as a conservative number which will catch most of the variations. When it comes to batteries + or - 10% is probably about as close as you will ever get. The important thing to remember is that there is not a lot of energy stored in any group 24 battery, but with care you can live with it.

[mhilley]

Dropped wrench across battery terminals once. it turned bright red. Knocked it off with a hammer. Let it cool on the floor and when cooled it was sort of a blue color. it was the hardest wrench I ever owned! Drop it on the floor and it would ring a high pitch. Yes, the battery was fried.

[dznf0g]

A friend and coworker of mine once got his ring caught between a truck fender and a wrench connected to the + terminal. His ring was literally fused to his finger and took surgery to remove it. A nasty reminder to lose the jewelry before working on cars.

[Bdinla]

@idroba, et al,
Hoping not to belabor the topic, this is what I think I’ve learned, applying the various posts, especially idroba’s chart, and comments on WindyJim’s post. We started with two fully charged Group 24 Interstates (conservatively 75 a/h each, or 150 a/h total). Our first afternoon/evening in Death Valley we used:

2 fantastic fans (on low 15 hours): (2 x .93 x 15) = 27.90 a/h
Refer w/fan (15 hours): ((.32 + .55) x 15) = 13.05 “
Bath fan (15 mins.): (.25 x 1.5) = 0.38 “
Lights (various, throughout the evening): = 5.25 “
Propane detector (15 hours): (.06 x 15) = 0.90 “
Pump (15 mins): (.25 x 3) = 0.75 “
Water heater (one hour) = 0.74 “
TOTAL POWER OUT OF BATTERIES = 48.97 a/h
TOTAL INPUT INTO BATTERIES = 0.00 a/h

The next day we left the two fantastic fans running for 24 hours (bad idea); the refer with fan for 24 hours (necessary in DV, hottest spot in the nation for four of the six days we were there); we ran an electric kettle (1050 watts) off the inverter for 15 mins (25.50 a/h!!); and increased usage of the bath fan, lights, propane detector, pump, and water heater. My guesstimate is that we drew the batteries down another 83 - 85 a/h, or more than the equivalent of one battery. While it wasn’t summer, the sun was intense, so I’m guessing that we put back (a max of) 70 a/h that day.
So, in our first two days in DV, we drew down roughly 135 a/h, and recharged 70 a/h, for a total discharge of 65 a/h, or 43.33% of our battery capacity. Each day (and night) thereafter we used progressively less power, especially the fantastic fans, which we turned off during the day, then only ran one in the last three evenings. We could not cut back on powering the refrigerator, so it was the primary draw. We used a blender to make smoothies one day, but it only draws 25% of the power of the electric kettle (maybe 7.00 a/h total). I think the real damage was done the first two days in DV; after that, we never recovered our power.

[timhortons]

I find it's easier to work/think in watts watt/hours.

Especially when there are 5V,12V and 120VAC via Inverter devices around.

Your 84AH group 24 battery has nominally ~1KW/hr to go on and less in practice.

so if you have 2 batterues you have 2KW/hrs on the best conditions of the year and each 100W of nominal solar will probably only top up 0.4 to 0.5KW/HR of solar a day under good conditions adjusting for angles, etc. Or you can top it up same in 4HR of driving. (.4-.5KWH)

LED lights use very little, radio and such not too much, charging phones and devices, then fans, and then pumps (but pumps are on demand so not much unless you take an extended shower which you don't have power for anyways)

Your furnace fan is 4AH or about 50W when running. saving enough for that if cold and the fans and fridge fans if hot should be your main concern and not a problem if you DON"T RUN INDUCTIVE LOADS (Heaters/Incandescent lights of any kind, bug killers, whatever)

Inductive Heaters and Cookers will kill your battery. That's what gas is for.

The the AC inverter is also a gateway to using up your battery, it burns power just being on. If your main computing device(s) is USB charging(phone tablet) you'll be much better off than if you have a laptop,charging plugged into an AC, running off an inverter, drawing from your 12V !

[Bdinla]

@ timhortons
I find it's easier to work/think in watts watt/hours.

Thanks, Tim,
That's a helpful way to look at it, and certainly one I'm more familiar with. Appreciate your input.

[Plan-B]

Originally Posted by idroba

Like most threads, this one has drifted, but if it helps people understand batteries and their use, that is OK.

One thing that many RVers have little concept of is just how little energy they have stored in their batteries. Let me try to put that into perspective.

Most Airstreams come with two group 24 batteries, with a storage capacity of about 75 amp hours each. An amp hour is what it sounds like. These batteries will supply a one amp load for 75 hours. They would supply a 5 amp load for 15 hours. Not to try to confuse things, but the more load you put on them, the lower the amp hour capacity would be. So, loading them hard, like running a 1000 watt inverter with a coffee pot would reduce their capacity considerably. But most RV loads are relatively small these days with LED lights and so on, so lets say that the 75 amp hour capacity is a good number.

With two batteries, you then have a total capacity of 150 amp hours, in theory, assuming they are fully charged and you can run them down to no charge.

But, that is a very bad idea, running down to no charge. With the charge varying between 40 to 100% you might get 300 to 400 cycles if promptly recharged to 100%. If you left them sitting at 40 to 50 % charge, never bringing them back to 100%, they soon will have a permanent capacity of 40 to 50%. In other words, damaged.



If you discharged them to 0% you might get 10 to 20 cycles before failure. If you ran them down to 0% and left them un recharged for a few days to a week or two, I doubt you would get 5 cycles from them. Flooded cell batteries hate to be left discharged. AGM batteries are somewhat better, but not much. This is battery abuse for any lead acid battery.

So, the common recommendation is to not discharge a lead acid battery to less than 40 to 50% of it’s capacity, and then promptly recharge it to 100%. Prompt is a day or two, less than a week.

So, you have two 75 amp hour batteries in your Airstream. You don’t discharge them lower than 40%. (you use 60%) Thus you have a capacity of 75 x 2 x .6 = 90 amp hours.

Lets put this into household perspective. We all get electric bills. The energy you pay for is listed in kWh (kilowatt hours, 1000 watt hours). Most households use 400 to 1400 kWh a month.

Your RV batteries were rated in amp hours, so how to convert? A watt is a volt x an amp. We had 90 amp hours of capacity and a 12 volt system. So the battery capacity of our two RV batteries, not used lower than 40% is 90 amp hours x 12 volts = 1080 watt hours, or a bit more than 1 kWh.

So, in your home you probably use 400 to 1400 times that amount of energy in a month. Think of 400 to 1400 pairs of 2 batteries sitting around your yard.

RV batteries have VERY limited capacity. That is one reason they are so often abused and fail regularly, and are replaced often. But if not abused, they can hold up quite well. You simply must not ask more from them than they can deliver, and replace it quickly. (recharge).

I hope that helps in your understanding of RV battery capacity.

Thank you! Using your formula I have been able to determine my maximum wattage of my panels.