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Stupid question: how to know when to run the generator to charge batteries?


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Hi all, I just purchased my first RV.  After a brief bookdocking tryout, I realized that I have no idea how to guage when my house battaries need to be charged.  All I have is a display that shows the voltage - which started out at a little above 12v and slowly climbed down into the 11's.  Is that the number I am supposed to be using to guage the amount of energy left in the batteries?  If so, what level would indicate that I need to turn on the generator to charge them, and for how long?  Thanks!

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An important thing to remember is that the chart provided by Hemsteadc is based on the resting voltage of the battery.  If the battery has any load on it, these voltages will not correlate well to State of Charge. So if you are trying to judge State of Charge using voltage, read the voltage with as little 12V load on the system as possible. 

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In case your display stops working or becomes inaccurate there is a cheap voltage read out tester that I bought at the auto parts store.  Has one prong for negative and the other for positive that you touch to the battery post.  The readout in green shows 12.5 and yellow as low when below 12.5 and then red for very low. 

When generator is running it will also show status of generator and if it is charging the battery.  Very small tester that fits inside a small sandwich zip lock bag.  Can use as backup only and can use I would think on tongue mounted RV battery/s as well as truck battery. My display in the TT became inaccurate and unreliable.

This little tester unit has saved me from battery failures out in the boonies several times.  Cheers and Happy Trails.

Retired Acct & SEC CFP. Former legislative aide and pilot to two Texas Governors, Adjutant General, State Senators, etc. 

Four Honorable Discharges from Active Army, Guard and Reserves.

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Edited by NamMedevac 70
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As stated above, trying to determine state of charge by voltage alone is difficult to do.  There are conversion charts to reference like the one posted, but they relate to a battery "at rest" and an RV battery is pretty much never "at rest".  There is always some type of load on the system.  The only accurate way to know what state of charge your batteries are at is to install some type of battery monitor.  A battery monitor will use a shunt on the negative line between the battery and all loads.  The shunt measures current going in and coming out of the batteries and uses an algorithm to determine state of charge.  They are very accurate and very useful for maintaining a healthy battery bank in an RV.  The two most common stand alone battery monitors you will see in the RV world are the Bogart Engineering Trimetric 2030 and the Victron BMV or Smart Shunt.  There are others as well, but these have the largest market share for stand alone usage in the RV world.  The Trimetric is older technology, but still works well.  The Victron has a lot more bells and whistles and allows for Bluetooth connectivity to an app on your wireless device for monitoring as well as physical display options.

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As a career electric service tech, I have learned that a cheap volt meter is of very little value when testing a working battery and even a resting battery needs the use of a quality, mid-priced, digital meter to know much at all. If you are planning to spend a lot of time camping where you depend on the battery, I think that you should follow Chad's suggestions. You are looking at $150 - $200 for one that would give you what you need and it can make life much better. You may also want to check exactly what converter or inverter/charger you have as the charge rate of it will plan a major role in the amount of time needed to recharge the batteries. 

We used to live without a good measure of battery by just using the onboard generator for things like the coffee maker, TV, microwave, and such, but that only served well because we rarely ever spent more than a single night where we were depending on it. A great deal of what is actually the best will depend upon the way that you plan to use your RV. 

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Thanks everyone!  Great info.  The chart at least gives me something to go on.  I have a multimeter, so would using that to test the batteries with the main power shut off give me the most accurate reading (without buying extra equipment)?

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Electricians are expected to have expensive latest greatest tools of the trade however not all RVers have a need for those expensive sophisticated gadgets sport depending on type of RV used.  No cheers for

My mission is complete and successful!!!!!!!!!!!!

Edited by NamMedevac 70
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7 minutes ago, John Reich said:

I have a multimeter, so would using that to test the batteries with the main power shut off give me the most accurate reading (without buying extra equipment)?

Two be accurate you need to let the batteries site with an open circuit for at least 2 hours and Battery University site says 4 hours. It isn't a quick thing to do. Just shutting off the power for a few minutes would tell you more than the little meter on the RV indication panel inside but again it depends on how you use the RV to be sure just how accurate of information you need. Do you plan to be a serious dry camper or just an occasional overnighter like we are? 

Edited by Kirk W
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13 minutes ago, Kirk W said:

Two be accurate you need to let the batteries site with an open circuit for at least 2 hours and Battery University site says 4 hours. It isn't a quick thing to do. Just shutting off the power for a few minutes would tell you more than the little meter on the RV indication panel inside but again it depends on how you use the RV to be sure just how accurate of information you need. Do you plan to be a serious dry camper or just an occasional overnighter like we are? 

Just occasionally.  I'm going to need to work so I need to be where reliable internet is.  At least during the week

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You may want to consider a solar setup and Chad is a good source of advice on that as well. While I once had the educational background for those systems, I never did enough dry camping to justify the expense. My wife told me that after she spent years camping with me in a popup towed by a 4WD, tenting while traveling on horseback, and even some on foot, she was not about to move into an expensive motorhome and live in it in some remote area. For that reason we never spent more than an occasional weekend where we didn't have hookups. For internet we did have satellite internet for a number of years but there are better choices today. 

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18 hours ago, John Reich said:

Hi all, I just purchased my first RV.   All I have is a display that shows the voltage - which started out at a little above 12v and slowly climbed down into the 11's.  Is that the number I am supposed to be using to guage

John your statement you just purchased your first RV and all you have to monitor is voltage leaves a lot of questions going forward. You stated you have a generator and a method to charge your batteries.  Is the generator built into the RV or a separate unit. Like wise is the battery charger a separate unit or built in.

Most individual battery chargers have a amp meter indicating the amount of charge going into the batteries and it tapers down as the batteries charge. If you have a built-in invertor/convertor it may have available a remote that could be used to provide more battery condition.

15 hours ago, sandsys said:

The only stupid question is the one you don't ask even when you need to know.

Linda

You came to one of many places for answers. Tell us what you need and give up feed back.

Happy Camping,  Clay

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On 4/29/2021 at 5:46 PM, John Reich said:

 The chart at least gives me something to go on.  I have a multimeter, so would using that to test the batteries with the main power shut off give me the most accurate reading (without buying extra equipment)?

Your voltage display, if accurate, should be fine. I used voltage for years with no problems.  

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Lead acid batteries expected life diminishes fast if you go below 50% charge. Very few built in charger/inverters are good chargers. An automobile charger plugged directly into your generator will do a much beter job in most cases. 

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22 minutes ago, Lance A Lott said:

Lead acid batteries expected life diminishes fast if you go below 50% charge. Very few built in charger/inverters are good chargers. An automobile charger plugged directly into your generator will do a much beter job in most cases. 

Chargers in (many) RV's have gotten much better over the past decade. Many RV's now come with 3 or 4 stage chargers with charge rates of 50 or 60 amps, or even higher. While I immediately replaced the converters in my first two RV's, the last two came with high-quality chargers that did an excellent job. 

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Check the manual for your batteries.  Mine came with a chart that show approximate depth of charge based on voltage AND specified loads.  I can get a fairly accurate estimate with lights and small loads on the battery bank.

BTW, my alert point is 12.2v with a light load.  Keeping above that greatly prolongs the life of the batteries.  Next you will need to learn about the proper way to charge your batteries.  It is a long process to get to completely charged which is also important for battery life.  You should be able to find a manual for your battery online.  Quality batteries can be quite expensive.  You will want to learn how to take care of them.

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The easiest answer to the original question is, when you get up in the middle of the night to use the restroom, and the light won't turn on and the water pump doesn't work to flush the toilet.

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1 minute ago, 57becky said:

The easiest answer to the original question is, when you get up in the middle of the night to use the restroom, and the light won't turn on and the water pump doesn't work to flush the toilet.

If you let it get to that point you've damaged the battery and significantly shortened its life.  It is wise to adopt some type of monitoring to prevent this from occurring.

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11 hours ago, mptjelgin said:

If you let it get to that point you've damaged the battery and significantly shortened its life.  It is wise to adopt some type of monitoring to prevent this from occurring.

Absolutely.  Look at the chart above.  If your battery voltage has dropped into the yellow zone, you have way overdone it and shortened the battery life.  If you dropped the voltage into the red zone, there is no fix.  The batteries will have a greatly shortened lifespan and a reduced load rating.  It is time to replace them and not try to rely on them for RV use.

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21 hours ago, mptjelgin said:

Chargers in (many) RV's have gotten much better over the past decade. Many RV's now come with 3 or 4 stage chargers with charge rates of 50 or 60 amps, or even higher. While I immediately replaced the converters in my first two RV's, the last two came with high-quality chargers that did an excellent job. 

That would be a good thing. I was watching, well listeneing, to a guy trying to get enough charge into his batteries using, a construction generator, for 3 days so I finally went over to talk. He couldn't make them last the night, it was a rental, I told him to use his jumper cables and his car. He was sceptical but tried it.  he came over a couple of, fairly quite, hours later to tell me that he had much more charge than he had been getting with the generator. This was a few years ago. My living quarters horse trailer is 10 now and it has a cheap one in it, I put a decent stand alone charger from Backwoods Solar in. But even that is outdated now.

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On 5/1/2021 at 8:01 AM, mptjelgin said:

Chargers in (many) RV's have gotten much better over the past decade. Many RV's now come with 3 or 4 stage chargers with charge rates of 50 or 60 amps, or even higher. While I immediately replaced the converters in my first two RV's, the last two came with high-quality chargers that did an excellent job. 

Interesting. The RV I just bought is 15 years old.  What upgrades would you recommend and what would be the benefits?

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3 hours ago, John Reich said:

Interesting. The RV I just bought is 15 years old.  What upgrades would you recommend and what would be the benefits?

My older RV has a single stage charger fixed at 13.4 volts.  Whether I used shore power or a generator, that was way less than recommended.  It shortened the life of my batteries, but even worse it took hours upon hours to charge the batteries.  I also had a solar system with a fixed 13.6 volts of charging.  Again this was too low for optimum battery life and prolonged with length of time needed to charge.

Replacing the solar controller was quick and easy with just simple electrical connections.  Finally I realized I needed to replace the built in 110 v charger.  That was built into my electrical system and seemed to be a bigger job than I wanted.  Instead I just added an additional charger capable of maintaining 14.4 volts.  I use it for my generator instead of the built in charger.

As I remember each of the charger upgrades was about $200 or less.  

Again, I recommend you download and review the manual for your batteries.  You will be able to verify 2 key points.  First you do not want to routine draw your batteries down to less than 50% of capacity.  Routine operation in the 50-70% range will greatly extend battery life.  Second it is important to properly charge your batteries and to do so at roughly 14.3 volts for an extended period of time until the rate of charging (amperage) drops to a level equivalent to 0.5% of the battery bank rated amp hours.

Once you have made the upgrades, I would recommend upgrading your batteries.   Consider deep cycle AGM batteries that will need no maintenance and are likely to last at least 7-10 years if properly maintained.

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4 hours ago, John Reich said:

Interesting. The RV I just bought is 15 years old.  What upgrades would you recommend and what would be the benefits?

This first thing to do is to find your converter. It may be incorporated into a "Power Center" that also contains 12V fuses and 120 volt circuit breakers. At 15 years old you may already have a decent, multi-stage converter so you should find the make and model number, and either report it back here or look it up online to see what the specs are. 

If it turns out to be a simple, single-stage converter you will likely be able to find a converter from a different manufacturer, (Progressive Dynamics is a common choice), that will be a "drop-in" replacement for the converter in your RV. 

But first determine what you have...

 

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