Tuesday, Jan 26, 2010 at 19:42
Hi Dennis,
Speaking about specs:
Pull up their web site and start looking for battery internal resistance data.
You'll find, they don't publish the actual figure which may or may not raise some suspicion.
What I did find though, in the 'Technical Manual for Chairman', there are some good graphs (among other truly mind bogling stuff...)
Look at the
orange coloured peukert plot for their 100Ah AGM.
Note the 1 hour rate of discharge corresponds to 65A, and the 20 hour rate corresponds to 5A.
Now scroll up to the discharge curves and locate the 1hr and 20 hrs ones.
To work out the battery resistance, locate the 'spread' of the two graphs during the first few percent of DOD, which happens to be around 0.6V.
Because this voltage difference is associated with a current difference of 60A (peukert plot), you can then calculate the internal resistance which works out to be around 10 milliohm (which is nothing out of this world really).
Note that this figure becomes somewhat higher as the 'spread' increases with DOD, meaning only a fully charged battery has the lowest possible internal resistance.
Up to now it was pretty boring, but now comes the hilarious bit:
On page 20, they write about 'in-rush' currents of 5C or 500A for their 100Ah product.
Yes, 500 Amperes.
Quickly multiply this current with 10 milliohm, and you'll get 5V, or around 7.5V if the battery was discharged.
Take the OCV of a totally discharged AGM @ 11.5V, and add 7.5V equals 19V.
I don't know if this means anything to you, but at this voltage/current you most certainly induce heavy gassing inside the battery, and quick heating - a potentially dangerous mix, and certainly damaging to the batteries health.
Now you might say, yes but my alternator is fixed to 14V, it never can get this high.
True, but not without problems if the current isn't limited to down to earth levels:
Take your discharged battery and apply 14V to it, which would result in an initial current of around 160A.
The Joule heating would be limited by your alternator output in this case (150A?), resulting in around 350W.
The losses of the internal oxygen cycle go on top of this, and you could be easily heating up your battery with 4 to 500W. Due to lead having a specific heat 33 times lower than water, it'll only take a minute or two for your battery temperature to increase by 10 degrees. Add high external summer temperature to this, and chances are you're exceeding the maximum permissible boost charge voltage for this temperature.
In fact, looking up the 'charge volt. versus temp' graph, it becomes apparent that at 110/43 degree F/C, your boost charge voltage is around 13.9V (border line), and float charge at around 13V.
What could make the situation worse:
modern alternators boosting the output to 14.4V for the first few minutes: makes your battery temperature rise higher yet, due to the longer duration of the current peak until the charging runs into the (higher) voltage limit.
On the other hand, fixed volt. alternators will damage this battery just as quickly as it'll overcharge not only during the bulk charging, it'll do so on float as
well - quite a bit actually.
Last but not least, battery life is reduced by 50% for every ten degrees over 25 - you choose.
To add to your recommendation of '
check their specifications' - I agree totally, but may I add, that you also should take some dubious claims with a grain of salt.
Best regards, batterymeister
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