Battery charge table for dual 6-volt batteries

[Valerie]

We've got the dual 6-volt batteries in our 2020 Escape and I'm using one of the simple battery monitors that plug in the 12-volt cigarette outlet to watch the charge. When we had our Casita with a 12-volt battery I used one of the tables that show what percentage the battery is down based on the voltage readout. I'm wondering if that same table is accurate for our dual 6-volt batteries. We boondocked in the National Forest outside of Brevard, NC last week and were thrilled that after two days we were still over 90% but now I'm wondering if my numbers weren't right. is this the right table to use? As always, thanks for your help!

[rubicon327]

Dual 6V batteries in series are the same as a single 12V battery for all intents and purposes. Accurate is not a word I would use when trying to use voltage to determine state of charge in either arrangement though. It is inaccurate while in use (charging/discharging). The battery would need to rest for 24 hours to give a decently accurate reading which is impractical. If you will be boondocking regularly consider a battery monitor with shunt like the Victron BMV-712. Much more accurate.

https://batteryuniversity.com/learn/...tate_of_charge
https://www.amazon.com/Victron-Energ.../dp/B075RTSTKS

[Hawkeye Ed]

Valerie, welcome to the club of all the folks who are looking for a simple, easy to understand way of gauging their RV house battery's state of charge (SOC) by measuring the battery voltage.

But as you're finding, there are so many variables (type and age of battery, whether under load or not, etc.) that it's difficult to do more than get a ballpark idea of SOC by measuring the battery voltage. That's why folks often recommend battery monitors that measure current in and current out to give the most accurate answer.

But don't give up hope if all you're trying to do is to get a general idea of SOC to gauge whether the battery can last another day without being so discharged that it's damaged.

For instance, I contacted Lifeline, the manufacturer of my great AGM batteries and asked your very question. Lifeline publishes all sorts of charts, graphs and articles about charging. But I got a very simple answer to my question: To avoid discharging the battery more than the recommended 50%, don't let the battery drop below 12.0 when under load or 12.2 volts when loads are turned off.

Yes, this is very general information. But it is helpful, and with experience over time will keep the battery safe and your lights and appliances running.

Now AGM batteries have different SOC characteristics than regular flooded lead acid batteries like you may have installed in your Escape (you don't say). Nevertheless, it appears to me that the chart you posted in Post #1 is for voltage readings taken while the battery is being discharged under load. There are lots of online resources that will help you with the details, but generally speaking, it seems that one should not let a battery under load drop below 12.0 volts if you want to keep the battery healthy and happy for several years.

So if you are running lights and appliances and the battery is below 12.0, that's not good. It needs to be charged right away. Sure, you can get away with lower voltage than that but your battery won't last very long.

So when you say you're impressed that your batteries are still over 90% after boondocking for a couple of days, I'm betting that your voltage was around 12.5 volts and you either didn't have any appliances on or, perhaps, just a light or two. I'd suggest you turn on the Maxx Fan, all your lights and the fan over the stove. Then see what your voltmeter is reading. If it's getting down toward 12.2 volts, you'll know you're more likely in the 70% or so range.

And remember, you only want to discharge to 50% before recharging to preserve the life of those expensive batteries. So, with special care, your new batteries should last 3, maybe 4 days before they need recharging, but watch them carefully.

Hope that helps.

[John in Santa Cruz]

to clarify the previous two responses...

your table is correct for a battery thats under rest, not being charged, and at 68F.

change any of those things, and its all wrong. if theres any more than a minimal current load on the battery, the voltage will read lower, depending on how many amps are being drawn. if the battery is being charged, it will be around 1-2 volts higher. if the battery just came off a charger, ti will still read a volt or more higher, it takes severla hours at rest to give a 'clean' state. and if its warmer or colder than 68F, those voltages are also wrong, but only by tenths...

if you really want to track your battery charge state, get a battery monitor like a Victron 700 series. this is a 2 inch round digital display face, that you connect to a 'shunt' that is put between the battery and the negative cable, the shunt also has a wire to the battery plus connector. If you're into smartphones, get the BMV-712 bluetooth model, and you can monitor your battery from your phone as well as the display screen. The basic BMV-700 model does the same stuff for about $20 cheaper without the bluetooth stuff.

https://www.amazon.com/Victron-Energ.../dp/B075RTSTKS

[rubicon327]

Originally Posted by HawkeyeEZ

But don't give up hope if all you're trying to do is to get a general idea of SOC to gauge whether the battery can last another day without being so discharged that it's damaged.

Good point. Even with the likely error in the reading you are still getting a general idea of charge level. Kind of like a wobbly needle on a gas gauge.

[Valerie]

Thanks for the good answers to my question. I'll look into the Victron and see if my brother is willing to install it when we camp together in a few weeks (he's an electrician). I'm not willing to go to our local RV repair place to do it - they are marginally competent on too many occasions.

With our Casita at Dome Rock outside Quartszite, AZ the battery was down to about 50% on the plug-in monitor in 24 hours so we wanted to give the Escape a test run (we did run the furnace in the morning to warm things up). On our Escape trip to the Smokey Mountains last week we had the refrigerator on gas and used the LED lights only a little bit and the water pump to flush the toilet and take one shower each. I charted the voltage reading every few hours and was pleased to see that it wasn't plunging. We are planning to convert our Honda 2200i to propane so that in the future when we're in the forest and want to stay longer we could charge the battery. The few nights boondocking were my favorite part of our 11-day trip so I'd like to be reasonably confident in monitoring the battery!

[Iowa Dave]

Hi Valerie
Have you looked into conversion kits and if so have you made a decision on a source? I was looking at Hutch Mountain and thought that unit looked pretty good. I did not realize that you can switch between gasoline and propane without modification once the conversion unit is in place. That has an attraction too.
Gasoline for incidental home use would be fine but the attraction on lower exhaust fumes,
Safe portability of the fuel (propane) is attractive to me.
I’d appreciate hearing your thoughts.
Iowa Dave

[Hawkeye Ed]

I just ordered the Hutch Mountain propane/natural gas conversion kit for my Honda EU2200i. Shipping on their product is delayed a few weeks but I'll report back when it arrives and I've installed it. Reports from others are promising.

[Valerie]

Originally Posted by Iowa Dave

Hi Valerie
Have you looked into conversion kits and if so have you made a decision on a source? I was looking at Hutch Mountain and thought that unit looked pretty good. I did not realize that you can switch between gasoline and propane without modification once the conversion unit is in place. That has an attraction too.
Gasoline for incidental home use would be fine but the attraction on lower exhaust fumes,
Safe portability of the fuel (propane) is attractive to me.
I’d appreciate hearing your thoughts.
Iowa Dave

Dave, Hutch Mountain is where I plan to order our conversion kit from. We've just (thankfully) had a series of camping trips planned with 3 weeks in between each and I didn't want to order and have the package sit on my porch while we were gone (and potentially be stolen). We tow with a Chevy Tahoe SUV and had the famously unavailable OMW over-the-propane-tank cabinet on the trailer tongue that we could put our generator and gas tank on for travel. With the Escape we really hadn't come up with a way to transport the generator and gasoline - so converting it to propane will mean that it can be transported in the SUV (that's the plan anyway!) Not having to deal with gasoline just sounds like such a plus.

[Iowa Dave]

That’s what I thought too. I have the dual 20 lb tanks and an 11lb tank in the front box. If I need more I’ve got a couple spare tanks I could put in a heavy duty plastic milk crate. Either in the Escape or back it the Highlander. I’d prefer in the trailer though. The generator looks more practical than an additional solar panel for the way we camp but honestly I rarely drop below about 12.8 volts. We are light electrical users. Thanks for the confirmation
Iowa Dave

[75thRanger]

Has anyone actually installed the Hatch kit yet? Looks good to me too. Lots to like about propane over gasoline. Never have liked dealing with gasoline.

I didn't see a manual that could be downloaded from their site. Could be there and I just didn't see it. Anyone know where the manual could be found? Like to read the install instructions.

Ordered it anyway. Going to be awhile before I get it.

[Iowa Dave]

I watched the video wheee the guy does basically the whole install including drilling a hole in the case for the propane fitting, removing and reinstalling gaskets on the carburetor hookup and even changing the sticker by the fuel shutoff. Doesn’t look too tough. I can always call my brother, he’s got unbelievable mechanical aptitude.

When I was in college and came home one Friday my brother and a friend had disassembled an Alfa Romeo transmission on the kitchen table. There were parts every where. It was about 3:30 in the afternoon. He said “Will you go in and work for me till about seven at the gas station I gotta have this together and out of here by the time Mom gets home at 5:30 and then we’re going to put it back in the car. They came driving into the station about 6:45 and I made my date with my girl, now my wife. That was about 52 years ago.

I’ll probably order one before long. I’ve still got several bucks left from the stimulus payment.
Iowa Dave

[75thRanger]

Originally Posted by HawkeyeEZ

I just ordered the Hutch Mountain propane/natural gas conversion kit for my Honda EU2200i. Shipping on their product is delayed a few weeks but I'll report back when it arrives and I've installed it. Reports from others are promising.

I ordered the Hatch conversion also. I contacted them to ask about an installation manual and they sent me the manual in a pdf file. Doesn't seem that difficult to do and I am by no means a mechanic.

He said I would have plenty of time to study it as they were 6-8 weeks behind. If I remember correctly the order form on their site stated about 3 weeks for delivery. That was Saturday past.

[Dcboyd]

I believe when the trailer has dual solar and a transfer switch, the monitor for that has a voltmeter. I could be wrong, but the orientation video shows the battery voltage in a digital screen.

[Vermilye]

Originally Posted by Dcboyd

I believe when the trailer has dual solar and a transfer switch, the monitor for that has a voltmeter. I could be wrong, but the orientation video shows the battery voltage in a digital screen.

Unless they have changed things since 2017, the battery monitor that comes with dual solar, a pair of 6V batteries, inverter & transfer switch is still the multi function LED readout column that is used for tank levels.

There is a voltage measurement available on the solar controller that can be used to measure voltage, current from the solar controller, etc but the voltage measurement, like the LEDS, is only accurate after dark when there is no solar input.

If you want to accurately track the state of charge of your batteries, an amp hour meter is by far a more accurate. It uses a shunt to track the amount of amps going in & out of your batteries, as well as current & voltage. With a bit of internal math, it determines the amp hours stored in the batteries. The voltage measurement, like that of the solar controller, a plug in volt meter or the LEDs, is only accurate when there is no active charging source, no load, and after a 1-2 hour rest after charging has stopped.

For those not needing to depend on accurate state of charge battery condition, a simple volt meter is fine, but if you are stretching things dry camping, accurate state of charge measurement can prevent you from needing to leave early.

[Dcboyd]

Originally Posted by Vermilye

Unless they have changed things since 2017, the battery monitor that comes with dual solar, a pair of 6V batteries, inverter & transfer switch is still the multi function LED readout column that is used for tank levels.

There is a voltage measurement available on the solar controller that can be used to measure voltage, current from the solar controller, etc but the voltage measurement, like the LEDS, is only accurate after dark when there is no solar input.

If you want to accurately track the state of charge of your batteries, an amp hour meter is by far a more accurate. It uses a shunt to track the amount of amps going in & out of your batteries, as well as current & voltage. With a bit of internal math, it determines the amp hours stored in the batteries. The voltage measurement, like that of the solar controller, a plug in volt meter or the LEDs, is only accurate when there is no active charging source, no load, and after a 1-2 hour rest after charging has stopped.

For those not needing to depend on accurate state of charge battery condition, a simple volt meter is fine, but if you are stretching things dry camping, accurate state of charge measurement can prevent you from needing to leave early.

Thanks for the info. I appreciated it.

[Valerie]

OK, my brother was not interested in installing the Victron for me-- but he suggested this Accutire monitor. It's only about $36 on Amazon and can "supposedly" be monitored via my iphone and bluetooth. It sure doesn't have many reviews, as in a handful, and they are mixed. Anyone seen something this simple? Think it would be better than my cigarette plug-in style volt "estimator" that I've got? Thanks for your thoughts and advice.
https://www.amazon.com/Accutire-MS-5...ct_top?ie=UTF8

[John in Santa Cruz]

Originally Posted by Valerie

OK, my brother was not interested in installing the Victron for me-- but he suggested this Accutire monitor. It's only about $36 on Amazon and can "supposedly" be monitored via my iphone and bluetooth. It sure doesn't have many reviews, as in a handful, and they are mixed. Anyone seen something this simple? Think it would be better than my cigarette plug-in style volt "estimator" that I've got? Thanks for your thoughts and advice.
https://www.amazon.com/Accutire-MS-5...ct_top?ie=UTF8

thats just a volt meter, it can't possibly measure current (amps) as it doesn't have a shunt, and you need voltage+current to track charge rate and load.

was it the cutting the 2" hole for the Victron he didn't want to do? or the wiring? the wiring is really quite simple, the shunt connects to the negative battery terminal, the existing negative/ground wire connects to the other side of the shunt, the skinny red wire connects to the positive battery terminal, and a simple RJ12 ''telephone" style cord plugs into the shunt and the display. you'd need a short negative cable with suitable ends for that first connection.

[Valerie]

Originally Posted by John in Santa Cruz

thats just a volt meter, it can't possibly measure current (amps) as it doesn't have a shunt, and you need voltage+current to track charge rate and load.

So if my goal is to dry camp and not ruin my batteries by running it down too low -- this won't do the trick? I thought I could just make sure the volts don't drop below about 12.20. My apologies for apparently not "getting it."

[alanmalk]

Originally Posted by Valerie

So if my goal is to dry camp and not ruin my batteries by running it down too low -- this won't do the trick?

If you don't run the batteries below 12.2 on a regular basis, and you recharge as soon as possible and you preform required maintenance, your batteries should have a long and healthy life. Deep cycle batteries can take the occasional abuse with a minimum of damage compared to car batteries. Yes, perhaps you will shorten the life by a year or so, but if dry camping is your goal than just put aside an extra $50 per year for the new batteries when the time comes. Meanwhile enjoy your camping. There is more to camping than counting amp-hours. But do remember, this advice comes from someone who rarely even looks at the volt meter.
--
Alan