Battery voltage to % charge relationship

newcamp

I recently purchased a Renogy 12V AGM battery.

I measured the voltage out of the box to 13.00V

During our first trip we went as low as 12.58V

After recharging (and allowing overnight settling without charging) it is back to 13.00V

Now, I have found lists like these to interpret the voltage in terms of % remaining charge:


I have seen similar lists posted on this forum.

According to the list, I would be at 90% when seeing 12.62 on the meter. But that seems inaccurate, considering my 100% is 13.00.

Some adjustments should be made. I'm just speculating, but how about something like this:

100% - 13.00
90% - 12.88
80% - 12.76
70% - 12.63
60% - 12.50
50% - 12.38

Adjustments??? Thoughts???

Did anyone else run in to this "issue"? Is Renogy at fault for increasing their voltage (to make it seem like it has more charge than it does)?

Basically, I am trying to get a better idea, so that I don't assume I have more charge than I really do and drain my battery and run it down prematurely. I would at least like to know what my "bottom" is. I guess if I stay above 12.38 I'll be ok, but maybe that is overly cautious? 

Thanks,

VictoriaP

A quick Google for Renogy’s state of charge (SOC) chart for their specific batteries didn’t produce much…shame on them, good manufacturers make that information easy to find. They do have one, it’s just buried in a blog post:

https://renogy.com/blog/expectations-vs-reality-making-sense-of-battery-voltage-percentages/

AGMs are more resilient to deep discharges than traditional lead acid. Many folks run them down to 20% or 30% remaining rather than stopping at 50%. One member here did the calculations and determined the overall lifespan of the battery was shortened by very little doing that.

If you’re using a Victron shunt to track your usage, you can set the 100% voltage to match that of the battery so your readings are more accurate. If you’re not using a shunt, it’s one of those things I’m still kicking myself over for waiting so long to buy one…one of the best upgrades you can make for boondocking.

pthomas745

Everything about "battery" is a guess, an estimate.  A battery is a box full of chemicals and lead plates or mats.  State of charge is dependent on many variables.  It is best to take all of these numbers with a grain of salt.  There are many many State Of Charge charts on the web, and they all will be a bit different.  (I have no idea why.)  So, look at anything "around" 12.6 as 100 percent and go from there.

On all the battery charts I've used for my old lead acid battery, 12.6 is 100 percent.

This battery FAQ is a bit dated, but it does cover a lot of things about batteries.  Find the section on battery voltages.

https://www.solar-electric.com/learning-center/deep-cycle-battery-faq.html#Battery Voltages

Grumpy_G

I dug into this a while back: https://tab-rv.vanillacommunity.com/discussion/14588/battery-soc-vs-internet-myth

Basically voltage is not a good indicator of SOC because it depends on the load on the battery. I have seen "shot" batteries that had over 12V open circuit voltage but drop below 11V with any useful load. 

newcamp

Victoria, that article was excellent! That is actually my exact battery pictured on the right. (Feeling silly for not doing that Google search myself  , thanks for digging that up).

I took a screenshot of the chart, to keep it for myself:

So it, seems like the spread of the chart is greater for the AGM. Dropping around 0.25 initially per 10% drop, as opposed to 0.12 in my guesswork. 

That's great to know that I was being overly cautious and it seems I can go down to 12.05 with less worry!


Grumpy, yes, I have noticed that the voltage reading drops, when connected to stuff that is pulling from the battery. I guess the best way to get a good read is to shut of the battery switch and then read it. 

My 12.58 on the trip was taken when connected, so switched off it may have been 12.68 or something, so about 85%. 


pthomas, yes a grain of salt is probably good to use here!

Sharon_is_SAM

Here is a Renogy article on voltage
https://renogy.com/blog/expectations-vs-reality-making-sense-of-battery-voltage-percentages/

PNWtabber

Thanks for this thread.  I am trying to learn!

My 2018 320S has an Interstate Batteries Marine/RV lead acid battery (model HD24-DP).  According to the manufacturer's website, it is rated at 64 Amp Hours.  It also says the battery has a "reserve capacity" of 100 minutes (???).

I am trying to figure out what the 50% state of charge level is, both for my own information and because my son is going to borrow the trailer so I want to make sure he keeps an eye on it while boondocking.

After reading this thread, and the related "Battery SOC vs Internet myth" thread, linked above by @Grumpy_G, I am very confused.

 

Sharon_is_SAM

@PNWtabber - in the above chart, your flooded lead acid battery would follow the first column.  These are rested readings as a reading underload will be lower.  You should aim to not dip to 50% if possible, so keep it at 12.2 or higher.  Too many visits to 50% will limit your battery life.  The SOC is impacted by many factors which is why a shunted battery monitor is invaluable.  Voltage alone is not the best way to monitor.

PNWtabber

Sharon_is_SAM said:

@PNWtabber - in the above chart, your flooded lead acid battery would follow the first column.  These are rested readings as a reading underload will be lower.  You should aim to not dip to 50% if possible, so keep it at 12.2 or higher.  Too many visits to 50% will limit your battery life.  The SOC is impacted by many factors which is why a shunted battery monitor is invaluable.  Voltage alone is not the best way to monitor.

Thanks.  What is a "shunted" battery monitor?  I have the Victron Energy BMV-700 battery monitor (I assume it is original to the trailer), is that a shunted monitor?

Sharon_is_SAM

Yes, that is shunted.  If set up correctly you can use the VictronConnect to monitor the %SOC which it is designed to formulate, taking in multiple factors besides voltage.  

PNWtabber

@Sharon_is_SAM thanks.  I will look to see if I have a manual for that!

Grumpy_G

@PNWtabber

According to the manufacturer's website, it is rated at 64 Amp Hours.  It also says the battery has a "reserve capacity" of 100 minutes (???)

That's actually a manifestation of the chart in my old post. 100 minutes equals  1.66 hours x 25 A = 41.6 Ah. The 64 Ah number is derived from a 20 hour discharge, or at 3.2 A. It shows the capacity is dependent on the discharge current. I don't know if the Victron accounts for the varying discharge currents when calculating state of charge. 

Personally I use the reserve capacity and calculate Wh from it which is a pretty decent approximation of how much energy can be drawn from the battery before the SOC gets dangerously low. 

Sharon_is_SAM

@PNWtabber - you need to download VictronConnect app to your phone.

PNWtabber

Sharon_is_SAM said:

@PNWtabber - you need to download VictronConnect app to your phone.

I assume that would require my Victron monitor to be blue-tooth compatible.  How do I know if that is the case?  (Purchased my 2018 trailer second-hand)  

Denny16

The 2018 TaB320 has a Victron 75/10 BT which is blue tooth compatible.  You will probably need to unmount the blue Victron controller box, and get its ID number off the back to get it to sync with the App.  You do need a smart phone, IPhone or Android type phone to run the app., which is free from Victron.
 Cheers 

Sharon_is_SAM

@PNWtabber - you said you have the BMV 700.  To use Victron Connect with BT, you would need a dongle.  You should have a monitor screen with the 700.  It gives the same info.  

newcamp

Denny16 said:

The 2018 TaB320 has a Victron 75/10 BT which is blue tooth compatible.  You will probably need to unmount the blue Victron controller box, and get its ID number off the back to get it to sync with the App.  You do need a smart phone, IPhone or Android type phone to run the app., which is free from Victron.
 Cheers 

I'm confused (but pleasantly surprised)... the T@B comes with a Victron already installed???

Where is it located?

Thanks

PNWtabber

@newcamp -- in my 2018 320S, it is on the wall between the sink and the wet bath.  Can't speak to other models/years.

VictoriaP

newcamp said:

Denny16 said:

The 2018 TaB320 has a Victron 75/10 BT which is blue tooth compatible.  You will probably need to unmount the blue Victron controller box, and get its ID number off the back to get it to sync with the App.  You do need a smart phone, IPhone or Android type phone to run the app., which is free from Victron.
 Cheers 

I'm confused (but pleasantly surprised)... the T@B comes with a Victron already installed???

Where is it located?

Thanks

@newcamp No Victron shunt came standard in 2019 or 2020. Pretty sure it also wasn’t standard in 2018, @PNWtabber bought their trailer used, it was likely installed by a previous owner.

Controllers were only installed if you had onboard solar, which was an option by 2018 if I recall, but didn’t come standard on any trim level of the 320 until the refresh in 2021. Zamp panels come with their own (as do most suitcases), so the Zamp port wouldn’t have one connected.

PNWtabber

@VictoriaP thanks for the clarification.  Yes, I bought my trailer second hand from the original owners.  So I am not sure what is "standard" and what they may have had added.

Denny16

If you have solar panels, which I thought was part of the 2018 Boondock package (and assumed your’s did), you should have the Victron Solar controller.  The Victron Shunt power monitor is an optional item, I talked after the trailer is built.  Most of the Shunt/meter or Smar Shunt monitors are user installed, some dealer installed and a few installed by nüCamp service department.
 Cheers 

VictoriaP

@Denny16 The 320 Boondock package at that time did not include solar as standard. In fact, when I was shopping for new 2018 & 2019 320 models, it was very rare to find one on the lot with solar, at least in my region. You pretty much had to special order it, and solar was a separate line item on the order sheet. Additionally, I’d swear I’ve seen several models here on the forum or on FB from that period with factory solar that did not use Victron controllers, but Renogy or another brand, but I don’t have time right now to chase down where I saw those.

The 320 Boondock package with solar as standard did not happen until the 2021 model refresh.

PNWtabber

I do have "suitcase" solar panels that the previous owners included.  But I haven't had the opportunity to use them yet.  I don't tend to sit around camp much, usually out birding or hiking or something, so don't want to leave the panels out as I've heard they are subject to theft.  

The case says Zamp Solar on it.  There is a connection in the propane/battery box on the front of the trailer for hooking up the solar panels to the battery.  If it was all optional, I am guessing the previous owners added it when they bought the trailer.  Maybe that's why I have a Victron monitor???

Horigan

Having a suitcase solar panel and a battery monitor are unrelated.  The trailer probably came with the Zamp port in the tub and the original owners bought the suitcase panel, as we did when we bought our 2016 T@b.  If there is a battery monitor, they likely installed that after purchase.  If it's a 700, then there should be a circular display mounted somewhere.  If it's a bluetooth 712, then the display may be under the bed and the original owners just used bluetooth, as we do with ours.  I would download the Victronconnect app, fire it up, and see if it sees any Victron devices when near your powered (12V) trailer.  Passcode is typically 000000.