[T3] Electrical questions

[Jim Adney]

On 25 Aug 2015 at 9:52, Keith Park wrote:

> flooded lead acid cells are 80% charged when they start to boil.

Keith, you're killing me here.  ;-)

Now you've forced me to go back to my Bosch handbooks and double- 
check everything.

Okay, gassing (commonly, but incorrectly called "boiling" starts at 
~2.4 V per cell. That's 7.2 V for a 6 V system or 14.4 V for a 12 V 
system. Gassing refers to electrolysis of the water in the acid. We 
can charge higher than that, but it's hard on the cells and reduces 
their lifetime. Gassing is undesirable because it can cause breakdown 
of the sintered lead and lead oxide cell plates.

At 2.4 V per cell, Bosch claims we're at ~90% of "full" charge, but 
going higher than that, while possible, is hard on the battery, so we 
try to avoid it.

It's possible to continue charging above 2.4 V per cell, but never at 
a rate higher than 10% of the A-Hr rating (and more like 1% if the 
battery is sulfated after long discharged storage.) Charging like 
this will increase the cell voltage rapidly to 2.7 V, which is 100% 
charged.

The numbers above are for batteries sold with acid diluted for use in 
temperate climates. Batteries sold for arctic or tropic use, use 
weaker or stronger acid and follow different curves. US and European 
batteries all use the temperate climate dilution.  

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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