AGM Battery under the hood

Hi Rockape,

interesting you mention spiral wound AGM batteries.

Do you give them occasional boost charging at a higher voltage, or is alternator charging all they ever get to see?

How deeply and how often do these get discharged?

For anyone interested in the maths:

In order to find out the relationship between life expectancy and battery temperature, you can use this simplified formula:

x=y*(1-((z/168)*(1-0.55^a)))

x...life expectancy in years
y...design life in years which is based on 25°C
z...average number of hours driven per week
a...exponent (increases by 1 for every 10°C step, i.e. under bonnet temperatures: 25°C a=0; 35°C a=1; 45°C a=2; 55°C a=3; and so on)

I've based the formula above on the Arrhenius factor of 45% for every 10°C temperature increase.

e.g:

y = 8 years
z = 30 hours
a = 3


If your under bonnet temperature is 55°C, then you can plug your 30 weekly-hours-behind-the-wheel into the formula,
and the result is: x = 8*(1-((30/168)*(1-0.55^3))) = 6.8 years

If the battery isn't going to last this long, then it's because the charging voltages aren't adjusted according to the battery temperature, and/or the charging current exceeds the max specified value.


cheers, Peter

Peter,
this is the history of the batteries.

The yellow tops I have are ex mine winder batteries, they spend 2 years on and off the skips, one week powering the radios and one week on the chargers, being a critical part of the system they are replaced every 2 years and I am lucky enough to get them for nothing. Up until 3 yrs ago they were charged with Smartcharge BC-012-15A chargers, these have now been replaced with 15a ctek units, again only because they are safety critical. The charging stations sit in full sun with no airflow and summer temps are normally 40 to 45 degrees C so the batteries would be around 60 degrees C during the day. The early batteries were discharged down to around 11 volts, the newer ones don't get below 12 volts as the skip radios now don't report as often.

I got a total of 9 yrs out of one and 8 yrs out of the other. I don't discharge them below 12 volts. My main starter is a red top 5 yrs old which I killed when the charger was accidentally turned to power supply mode and left on. I use the red and yellow batteries in parallel for starting most times because of the 6.5 chevys size and compression.

I now have a time clock on the charger and it runs approximately .5 of an hour in the cool of the morning 5 am.

My alternator is set at 14.2 volts and the smartcharger at 13.8 volts. The engine bay temps are high but nowhere near as high as the old 2H motor.

Have a good one

Rockape,

the secret to the batteries' longevity in your setup is the shallow discharge of the batteries.

And because the spiral wound AGMs have the best chemistry of all of them (pure lead/tin), they're pretty forgiving in terms of high temperatures.

That's because there's nothing much prone to corrosion inside them (compare this with calcium and associated alloying metals like aluminium which get eaten away in no time under high temps and then leave the positive grids brittle and thin).

If you'd discharge them deeper, you wouldn't get away with the 14.2/13.8V charging setpoints though.
In such a case you'd let the terminal voltage rise to 14.8V, and after a few hours, the batteries would be fully charged.

But congratulations, you've found a pretty good charging routine for your app!

cheers, Peter

Peter,
thanks for the info and Guy may also benefit from it.

Have a good one and may all the rocks you kick turn to diamonds.

Why do you say metal?
The factory shrouds are some sort of composite plastic for safety (short circuit) reasons.