Determining health of AGM battery

bern64

2020 T@B400 Boondock Lite (Solar), purchased Sept 2023

Harris Battery, AGM, 2 x 6V batteries 

I just returned from a trip to Colorado and Utah where on 3 occasions I boondocked at National Parks for 1 night.  The only measure of battery strength I had, was the panel display(yes, I know it is not reliable). Within 1 hour of setting up camp, the display indicated battery was low.  Also, when I tried to use propane setting on the Alde for heat, a low battery warning alert displayed.  

Today, at home I turned off the battery switch and used a multimeter to test battery.  Everything was turned off in the camper.  The reading I got was 13.7, which I know is NOT accurate.  How do I get an accurate battery health reading?  Do I have to disconnect the converter/charger? If so, how?

Sharon_is_SAM

Use the Victron App to disable the solar charging, turn off the battery switch, make sure the inverter is off.  This should isolate the battery and allow a resting measurement.  

AnOldUR

Agree with @Sharon_is_SAM. Your high reading could be from solar. Off topic, but educate me on the 400. Is that battery compartment vented?

rfuss928

Measuring resting voltage will only tell the state of charge.  It is not an indicator of capacity.  An aging battery may still charge and discharge normally just with a lower capacity than when new.
To determine capacity the battery needs to be fully charged then a known load applied and timed while state of charge is monitored.  Generally best accomplished on an analyzer at a battery shop.
Depending on use and care, 4 to 6 years is common life expectancy of AGM batteries.  The OP batteries fall in this range.
AnOldUR  - AGM batteries do not require venting. 

Grumpy_G

@AnOldUR if you look to the right side of the batteries there is a hole in the bottom that vents to the outside. AGM batteries do not need venting in normal operation but they can gas off when something goes haywire with the charging voltage.

As @rfuss928 stated the only good way is to determine the health of batteries is a proper capacity test. You already have a voltmeter, if you have a way to measure current you can calculate capacity from current x time to discharge from full to 11.5 V with a 5-10A load.

bern64

Sharon_is_SAM said:

Use the Victron App to disable the solar charging, turn off the battery switch, make sure the inverter is off.  This should isolate the battery and allow a resting measurement.  

Thank you, Sharon_is_SAM.  I disabled solar charging with the Victron App, the battery switch is off, and ensured the AIMS power switch is OFF on the panel by the door.  I took a reading right after and got 13.6.  Then 1/2 hour later got 13.2, and 1 hour later got 12.8.  How long should I have to wait to get a reliable resting reading? 

Everyone - thanks for the feedback.  I have no problem replacing the batteries, if necessary.  What is the purpose of the chart below that I copied from a post on this forum?  I thought this chart showed battery health.  

AnOldUR

Grumpy_G said:

@AnOldUR if you look to the right side of the batteries there is a hole in the bottom that vents to the outside. AGM batteries do not need venting in normal operation but they can gas off when something goes haywire with the charging voltage.

I thought I had read somewhere about AGM's off gassing under severe conditions and requiring venting because of this, but at a first look at the picture I didn't notice the hole in the bottom. Was expecting to see something on the side.

Sharon_is_SAM

@bern64 - you are supposed to wait 24 hours before getting a rested battery voltage reading, but, for most, waiting a 1-3 hours will come close enough.   The chart provides the amount of available energy in the battery as a percentage of a rested battery.  But, voltage alone is not the best way to monitor the status of a battery as a load applied will drop the voltage temporarily and a charge will temporarily raise the voltage.   A shunt factors in multiple variables like voltage, age, efficiency, current etc to give a more reliable indication of state of charge.  

We too have an aging battery.  We have a Victron shunt and we adjusted (reduced) our efficiency and capacity to have the shunt reflect a more accurate state of charge.  When our battery was healthier, we could charge it fully and it would still be close to 100% after 6 months.  Now, we charge it fully and the charged voltage settles down to 12.6 to 12.7 and drops to 12.4 after a couple months.  The aging battery can’t hold a charge as long and discharges sooner.  BTW, there are a lot of charts out there that do not define charging vs rested voltage.  See below.

Voltage	Capacity
13.00V	100% (charging)
12.85V	100% (resting)
12.80V	99%
12.75V	90%
12.50V	80%
12.30V	70%
12.15V	60%
12.05V	50%
11.95V	40%
11.81V	30%
11.66V	20%

Grumpy_G

bern64 said:

Thank you, Sharon_is_SAM.  I disabled solar charging with the Victron App, the battery switch is off, and ensured the AIMS power switch is OFF on the panel by the door.  I took a reading right after and got 13.6.  Then 1/2 hour later got 13.2, and 1 hour later got 12.8.  How long should I have to wait to get a reliable resting reading? 

Everyone - thanks for the feedback.  I have no problem replacing the batteries, if necessary.  What is the purpose of the chart below that I copied from a post on this forum?  I thought this chart showed battery health.  

Lead acid batteries have a "surface charge" effect where they read higher than their nominal voltage right after the charger is disconnected. It depends on the battery but after 2hrs it should read fairly accurate.
The chart shows the state of charge (SOC) vs measured voltage, it does not indicate battery health. It's not a very useful chart in my opinion because different batteries behave differently. I have some old batteries sitting around that "recover" to over 12V after dropping below 10V under load. 

pthomas745

You can get a "rested" readout in just a few minutes that will help you considerably, especially if  you are out camping.  A few hours may be good sitting in your back yard, but out in the cold and dark, go with a short time period.  Not many of us have meters that will actually show 5 digits of voltage.

The "state of charge" charts that are very common around the web don't vary much.  That 13.0 number is a bit of an outlier.  Voltage for a lead acid battery is more of a "very good estimate" rather than something you can take to the bank.

bern64

I think I confirmed with many multimeter tests over the weekend that my battery is toast.  The rested readout is about 11.09.  Can you look over my chart and confirm?  

manyman297

Our AGMs went out this spring. The main symptom was a huge and fast drop in voltage and then once the solar panel got just the smallest amount of sun the voltage shot right back up. Extremely weird behavior. If you have a Victron smart shunt you can clearly see this behavior in the graph/history.

pthomas745

@bern64 I salute your great testing process and the spreadsheet!

And, I would agree with your assessment.  The battery is not holding a charge.  The only positive readings you are getting is from voltage going into the battery from either the solar or the shore power connection. The only time it seemed to hold a charge was on that first day.  After that, even after anywhere from 5 to more than 12 hours of charging, the battery kept returning to that very low voltage.

The Harris batteries just seem to be failing in a wave over the last year or so, after only 3-4 years of use.

There are many threads about replacing the dual 6V with a single 230 amp hour lithium battery that will increase your usable amp hours to around 200 compared to the dual 6V's 135 or so.

This thread discusses a lithium replacement, along with the possibility of replacing your current battery setup with equivalent 6V AGM's, which are still available if you want to stay with them.

https://tab-rv.vanillacommunity.com/discussion/17889/what-batteries-to-replace-old-ones-in-2021-tab-400

Trailpixie

A side note on AGMs--some have a standard resting voltage of 12.8 volts. This doesn't affect its capacity. Check the spec of your battery to see what its resting voltage should be.

Cherokee

The best way to prolong the life of your batteries is to keep them on a trickle charger that keeps them fully charged when the trailer is stored