accurate Amp Hours

Shadowcatcher

Have an aging AGM and it is no longer what it once was. I am bumping up against my aspergers (subset of autism) in trying to get some handle on what remains. I am mind blind to math and similar kinds of organizational things. 
I would think that a device could be cobbled together that would get an accurate Amp Hour reading. Current thinking, I am looking at some of 150 W dischargers along with a digital voltage cutoff and a digital timer. All of the items seem precise and doing a short term test, a fraction of the 50% discharge would/should be representative?

Photowhit

The problem with testing capacity, is the discharge rate is part of the capacity number. We commonly leave out the total information. If you have what is commonly called a 12 volt 100amp hour battery, What you have 'usually' is a 12 volt nominal, 100 amp hour battery at a discharge rate of 1/20th of it's capacity.

Since a quicker discharge effectively reduces the size of the battery bank it's hard to apply a load and test.

In addition, the process is electro chemical, so there are losses in heal at high charge/discharge rates.

All this makes for a non linear profile.

Here's some info that might help;
https://en.wikipedia.org/wiki/Peukert's_law

Estragon

As well as the Peukert factor, also keep in mind that the voltage indication of state of charge will be resting values. You'll want to start the test with a voltage around 12.8 with no loads or charging sources for several hours, apply a known load for a period of time, then let the battery sit for several hours before taking a voltage reading to establish a terminal state of charge.

Under a heavy load, the battery voltage can sag and give a misleading result unless left to recover to "true" voltage.

BB.

For smaller systems/loads, you can get a Doc Watson (or equivalent--there are a lot of similar devices out there these days):

http://www.rc-electronics-usa.com/

The real question is does the system meet your needs? Say you want two days of storage with your loads (loads, radio, cell phone charging)--Running the two day test and Estragon says, measure the resting voltage of the battery to estimate how much stored energy it has left.

When the battery no longer supports your needs, then it is time to replace it.

Also, AGM batteries tend to fail by not recharging correctly... Basically, there is a small catalyst pellet in each cell that converts the Hydrogen+Oxygen gases (from charging) back into water. Over time, the catalysts "wear out". When this happens, the gases are not recombined and instead, the cells "over pressure" and vent gases (and a bit of electrolyte) and the battery eventually "dries out". And needs replacing (sealed battery, cannot replace water).

Note that failing AGM (and any lead acid battery) need to be replaced. It is possible for them to explode/catch fire if failing batteries are still being recharged. Does not happen often, but you do not want to risk the danger.

-Bill

Shadowcatcher

OK I left out a key factor. I have a Victron battery monitor and need to calibrate it to read accurately, the main reason to get an accurate amp hour rating.