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Old 11-19-2019, 08:04 AM   #1
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The 50% Rule

This is a quick 2-part question...

First, I know that you should not discharge your battery below 50%. But how do you know when you have reached that 50% mark?

Secondly, are there RVs out there that run 12V televisions? I wonder if there would be some advantage to having a 12V television for efficient power consumption . After all, AFAIK, most RVs use 120V televisions, so when boondocking, they are using the inverter's ability to convert 12V to 120V. To me, this seems inefficient. I think that the TV is one appliance that most people would use more of when boondocking, so doesn't it make sense that they would be 12 volts?
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Old 11-19-2019, 08:18 AM   #2
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There are State of Charge charts that equate battery voltage with percentage you can Google. Hooking up a voltmeter, or using a multimeter to read the voltage off the battery posts (or anywhere, even a 12v outlet in the camper) will work.

Yes, you are correct, operating stuff directly off 12v without using the inverter is the most efficient and makes the most sense. But this won't work with the monstrous RV's I see out there with TV's bigger than the one I have at home!
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Old 11-19-2019, 08:25 AM   #3
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There are all kinds of battery monitors out there, you can spend as much, or as little $$$ as you like on these. Some folks are really passionate about their energy management, and others are not. I'm in the latter group, so I only use the 4 light battery indicator that came with the RV to give me a general idea, and a multimeter when I want more accuracy. We usually boondock, so we have a good idea of how to manage our battery as far as usage and recharge. We frequently take our cheap batteries below 50% and they still last about 5 years, so we don't worry much about that rule. Works for us.
As far as 12v television, sure, that's an excellent idea. I think they aren't standard due to the same reason manufacturers are going to residential fridges. Cost and features. I just did a quick search for 12v tvs and the largest was 40" and had limited features. Price was $400, which wasn't bad. Most I found were much smaller, but were reasonably cheap, so replacing your TV with a 12v is very doable.
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Old 11-19-2019, 08:39 AM   #4
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A rough rule of thumb is 12v reading with a reasonably accurate digital meter is approx 50% charge.
12.8 volts with a very light load such as a single light on indicates full charge.

FYI you can get all kinds of little LED digital voltage indicators on Ebay- here is just one example- https://tinyurl.com/vylthmt

I mounted one similar to that in my trailer on the wall next to my slide-awning-water heater-water pump switches. I also put a push on push off button next to it so I can turn it off at night as it is fairly bright.

As far as TVs go there are regular TVs out there that have external 12v power supplies so many of them could be used in an RV and connected to the 12v battery supply as long as the wire was large enough to handle the load.
Example I have a 55" Sony TV in my family room that has an external 12v supply that plugs into the rear of the TV. Keep in mind a TV that large is gonna use a lot of watts even if it is direct wired to your 12v system.
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Old 11-19-2019, 08:50 AM   #5
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Some good answers, but now it has prompted some followup questions...

Are the "battery indicator lights" really all that accurate? I see that you all are using different methods to measure your percentage of power. But I'm not sure that low voltage is the equivalent to 50% capacity. Does the voltage decline in a linear curve that would equal percentage of battery capacity? For example, you might have a larger battery with more amp capacity, but does a 12 volt reading on a large capacity battery equate to a 12 volt reading on a smaller capacity battery? Don't know!

And someone wrote about a 12 volt adapter for a television. In my opinion, this would be LESS efficient than just plugging in a 120 volt TV and letting your inverter handle the conversion. I guess I am wondering why RV manufacturers don't install some kind of standard 12 volt plug throughout their rigs. In mine, I have only one round 12 volt plugin in my front bedroom behind the TV. It would be nice to have several throughout the RV.
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Old 11-19-2019, 08:51 AM   #6
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Just a thought, your not boondocking if your watching tv. Your rving, at that point. Lol
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Old 11-19-2019, 09:04 AM   #7
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Quote:
Originally Posted by MOODMAN View Post
And someone wrote about a 12 volt adapter for a television..
If you're talking about my post what I was saying is my Sony TV in my house is a 12v TV out of the box. It has an external adapter that changes my house 120v ac to 12v dc. That adapter then is plugged into the rear of the TV. I believe I could cut the cord and hook it directly to a 12v battery and it would work.

I'm satisfied with the accuracy of my cheap 12v digital display. It agrees close enough to my $$Fluke digital VOAM. (volt ohm amp meter).
If I have a small load on in the trailer and if the battery switch is on there is ALWAYS some load- I figure it's close enough for me. I try not to let my 2 golf cart batteries get below 12v.
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Old 11-19-2019, 09:08 AM   #8
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Quote:
I guess I am wondering why RV manufacturers don't install some kind of standard 12 volt plug throughout their rigs. In mine, I have only one round 12 volt plugin in my front bedroom behind the TV. It would be nice to have several throughout the RV.
The ONLY 12 v outlet in my small trailer is behind the TV that the stock 12v tv plugs into. I had to add one by my bed to plug my CPAP into.
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Old 11-19-2019, 09:14 AM   #9
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Originally Posted by MOODMAN View Post
This is a quick 2-part question...

First, I know that you should not discharge your battery below 50%. But how do you know when you have reached that 50% mark?

Secondly, are there RVs out there that run 12V televisions? I wonder if there would be some advantage to having a 12V television for efficient power consumption . After all, AFAIK, most RVs use 120V televisions, so when boondocking, they are using the inverter's ability to convert 12V to 120V. To me, this seems inefficient. I think that the TV is one appliance that most people would use more of when boondocking, so doesn't it make sense that they would be 12 volts?
A resting voltage (no load) of 12.05-12.10 volts is generally considered to indicate 50% state of charge. My bedroom TV is 12 volt so we can generally watch a Carson rerun if we are boondocking and can get a mytv signal! I do have a pure sine wave inverter for the DW cpap and I can jumper it to power one half of the camper if we desire.
I usually carry at least one good multimeter and almost always have my clamp on amp meter to check individual loads, if needed. I did install a small and inexpensive voltage indicator at my switch panel for a convenient look. See my attached picture.
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Old 11-19-2019, 09:17 AM   #10
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Inherently idiot lights do not work for me.

I purchased a Bluetooth BM2 voltmeter from Amazon. $30-$40

Transmits to my phone.

It has alarms which I like. It records voltage and produces a graph. Important, yes.

By studying the graph one gets to understand the batteries better. Like the three stage converter charging. You can see big draws like the leveling jacks. You can see how little the tv charges. You do need the percentage graph, however I know 50% is about 12 volts.

The fancier meters show amps available or being used. Good info. But, pricey.

From my boating experience with a cpap machine batteries do not do well with large discharges. Needed a battery every 2-3 years.

The problem with 12 volt tvís is cost. I assume due to volume they are much more expensive and hard to find.

Bought the DW a 36Ē Roku tv for a convalescence after knee surgery. About $130. 110 volt.
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Old 11-19-2019, 09:23 AM   #11
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A lot of TV's these days are 12v, they have a brick built into the cord that converts AC to DC. One would just have to confirm the brick is outputting 12v, then cut off the cord going into the brick and attach it to a 12v plug. Some of those bricks are putting out a lot higher than 12v depending on the tv.

**edit: apparently people online have gone to stores and looked at the A/C to Dc converter bricks, and they are mostly 19v dc. You may find a smaller one that is 12v, I haven't looked. The Furrion that came with my trailer is a dedicated 12v tv.
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Old 11-19-2019, 09:59 AM   #12
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Off-topic a little bit...

But (truck stops) depending on where you stop...have large quantities of 12 volt 24 volt electronics and appliances for anybody wanting to purchase, in stead of online purchasing.
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Old 11-19-2019, 11:55 AM   #13
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Off-topic a little bit...

But (truck stops) depending on where you stop...have large quantities of 12 volt 24 volt electronics and appliances for anybody wanting to purchase, in stead of online purchasing.
This is a good tip. I had forgotten this fact, but you are correct. Thanks.
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Old 11-19-2019, 12:31 PM   #14
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Are the "battery indicator lights" really all that accurate?.
Not even on their best day.


Might be close if you only "push the button" after the battery has not been charging or used for a couple hours and is just "resting".

If you have to rely on your batteries for any length of time (boondocking) a good battery monitor will almost be required. One that measures current in and current out so you get real time readings on your battery's state of charge.


To the topic of this thread, the 50% rule, that's when you get the best balance of battery use and battery lifetime. If one goes lower and can recharge immediately the battery won't loose as much "lifetime" as it would if discharged deeply and allowed to sit in that state for any length of time.

On that note, if all you have is the LED "Light Tree", and you push the button while the furnace (for example) is running, it could show an almost dead battery when the actual state of charge is far higher. Reverse is true when charging. May indicate "full" but until the chemical reactions in the battery have equalized, battery may be far less than 100% SOC.

GOOD battery monitors overcome these errors and false information.
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Old 11-19-2019, 01:01 PM   #15
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Just a thought, your not boondocking if your watching tv. Your rving, at that point. Lol
Please don't gatekeep what "boondocking" is. Lots of us don't "camp" even if we are off-grid. My rig is a traveling hotel room. We'll watch TV, I'll work, and we'll also explore the outdoors. Just because my boondocking looks different than yours, doesn't mean mine is wrong (nor does it mean yours is). Variety is the spice of life, so they say.
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Old 11-19-2019, 01:03 PM   #16
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GOOD battery monitors overcome these errors and false information.
Any recommendations on a "good" monitor?
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Old 11-19-2019, 01:04 PM   #17
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For MOODMAN, I personally like having more information than less. The Victron BMV is expensive at about $200, but I like the detailed information that I get from it. I can see how much battery power different things use and can manage our usage to it. I can tell my kids if we have enough power to run the toaster or if my mother-in-law needs to not use her curling iron for 15 minutes.

This is what I have and like:
https://learntorv.com/victron-battery-monitor/

I've never mounted the display. I just use the Bluetooth/app interface.

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Old 11-19-2019, 01:08 PM   #18
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inquiring minds need to know .... thanks
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Old 11-19-2019, 01:22 PM   #19
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Here is a video that may be helpful for you.

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Old 11-19-2019, 01:31 PM   #20
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Any recommendations on a "good" monitor?
I use a Victron and am very pleased with it. Since I am using a pair of Battleborn LiFePo batteries a monitor like this is pretty much a necessity as the Lithium batteries have a much flatter discharge voltage curve. A battery that's almost "empty" could have enough voltage to light the top light on an LED tree.

For Lead Acid batteries a monitor like this is also a huge benefit as it takes into consideration the different loads and charge rates when calculating SOC. (Peukert Factor).

Yes, this monitor does cost more but it also does a lot more. Very useful in monitoring batteries and it even tracks "cycles" so when you need to replace the batteries you have a benchmark to see how the replacement's performed. Or let's you know when you're getting close to the end of an average life cycle for the battery type you're using.
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