I ran across a label shot of the Xantrex "Trucharge 40" smart charger that indicated that its voltage cutoffs varied considerably depending on temperature. At the Xantrex site at:
I found that the setting for AGM batteries were:
The manual points out:
Selecting the correct temperature setting selects the appropriate charging voltage to compensate for battery temperature.
*Effect of temperature
*Cold battery temperature* If you choose the Cold setting,
Truecharge + delivers, for example, 14.8 volts in absorption mode and 13.9 volts in float for a Flooded battery.
*Hot battery temperature* If you choose the Hot setting for the same battery, the charger delivers 14.0 volts in absorption mode and 13.1 volts in float, nearly one volt of difference from the cold battery charge.
*Consequence of wrong selection* If you choose a cold temperature
setting for a hot battery, you risk overcharging your battery. If you are in doubt about which to choose, or if you are leaving the charger connected for an extended time, select Hot to reduce the risk of overcharging. The same is true for Gel and AGM type batteries."
Since the Soneil bricks have a charge cutoff of 14.6 and a float voltage of 1307, they conform to Xantrex's "Cold" settings. (Perhaps this is because Soneils are designed for Canada?) My use of these bricks through a Maryland summer would have done exactly what Xantrex predicted. The more so because 18AH AGM batteries have a limited overcharge tolerance.
With temperatures around 18F, I guess it would again be safe to use the Soneil's.
Too bad they no longer answer their e-mail.