Saturday, Nov 22, 2014 at 13:30
Lets talk about stewing batteries.
We frequently get this concern about stewing batteries with increased charge voltage......mostly this is completly unwaranted.
In times past when we used non sealed screw top batteries the charge voltage on alternators was in general kept to arround 13.8 volts.
This was to keep gassing and electrolite loss to a minimum.
with the advent of sealed maintenance free batteries...with calcium in their plates, modified electrolite and a preasurised container.
They will tolerate 14.5 volt charge.
The calcium in the plates and the modified electrolite actually require a slightly higher charge voltage.
The calcium in the plates and the modified electrolite are required to allow the battery to be sealed...it reduces gassing.
The preasurisation allows the recombination of what gasses are generated into liquid which is retured to the electrolite.
Glass mat has not a thing to do with this recombination cycle or the need for the calcium in the plates or the modified electrolite OR the charge voltage.
How does this play out in reality...I have a dual battery system in my 4wd.
It consists of two"Seamaster gold" N70 batteries (sealed maintenence free wett cell batteries) connected by a simple VSR.
The old alternator charged at 13.8V ( because the vehicle came from the factory with a screw top battery).
I replaced the alternator due to age concerns, the new one charges at 14.5 volts as would be expected for a pre ECU controlled alternator for a vehicle fitted with a sealed maintenance free battery.
This arrangement has been in place for over 3 years....for the last 18 months this vehicle was driven daily minimum of two 30 miniute trips and a number of 3 or 4 hour highway days.
All is good.
Now back to my hobby horse about AGM.
There are sealed maintenance free battreries with glass mat packed between the cells, that have all the technology and advantages of AGM ( absorbed glass mat) batteries, but they have not been starved of electrolite, ( there is more electrolite than can be absorbed by the glass mat) so they tolerate higher temperatures and higher chage rates better.
If you have a modern sealed maintenence free battery and an AGM battery...it is the AGM battery that will suffer from being stewewed.
AND not simply from the charge voltage, but because of the charge voltage combined with temperature.
AGM is being heavily over sold and there are people with AGM batteries being disapointed... because..... contrary to what they have been sold, AGM has no advantage to them and the AGM battery is suffeering due to poor heat tolerance..particularly when installed under bonnet.
cheers
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