coulomb wrote: [With say gel lead acid, figures like the maximum charge voltage of say 14.4 V for a 6-cell battery are supposed to be derated my so many millivolts per degree Celsius. So an uncompensated charger is often overcharging a battery, since it forces the battery to 14.4 even when it is hot, and it should be limiting to 14.3 V or less. In very cold climates, a non temperature compensated charger could undercharge the battery.
It was my understanding that the temperature compensation was needed due to the secondary reactions inherent in the lead acid chemistry.
from my table of standard reduction half equation potentials:
O2(g) + 2H+ +2e- --> H2O2 +0.68v
H2O2 + 2H+ + 2e- --> 2H2(g) +1.77v
combine the two and you get hydrogen at 2.45v
given that at under charge, the voltage is usually limited to 14.4v (2.4v cell average), it is possible some of the cells in a block are already gassing (by a small amount if the pack is well balanced).
the amount of current this process shunts (steals) from the charging process varies with temperature, and how far above 2.45v the cell is.
The Arrhenius reaction-temperature relationship shows the reaction rate doubes (approx) with every 10 deg C increase in temperature.
Therefore, when operating at above rated temperature, to keep the shunted current below a certain level (so the current applied to the battery actually charges it), the voltage has to be lowered.
operating above rated temperature without temperature compensation especially has to be avoided with Gel and AGM batteries, as the recombiners give off heat when the hydrogen is recombined with oxygen (otherwise you get thermal runaway).
when operating at below rated temperature, the battery internal resistance will be much higher, so charge rate is reduced.
at lower temperature, the voltage can be raised, as the gassing reaction is more subdued anyway.
charging at lower voltage than specified will still allow a lead acid battery to fully charge, it will just take a long time (13.6v at 25 deg C takes 24Hrs from memory).
you only run into trouble if you don't allow the battery long enough to reach full charge.
my chemistry is a tad rusty (it was from all the way back in yr12), so if ive stuffed up, do point it out
back to the original comment, im of the opinion optimum charge voltage only changes for chemistries that have secondary reactions in the course of normal operation. LiFePO4 doesn't have secondary reactions (that i know of).
In case someone reads this through and gets the wrong idea, i made a mistake.
The voltage at which gassing occurs *does* change with varying conditions.
the voltage above which a reaction occurs is modelled by the Nernst equation:
basically the variable that have greatest effect on cell voltage are concentration of reactants.
The concentration of reactants depends a bit on cell construction. lead acid does have a bit of a wide variation in reactants concentration.
im lead to believe the concentrations vary markedly less in lithium cells.
this is something that is learnt in 1st or 2nd year university if you are studying chemistry (which im not, but i do have friends who are who set me straight).