This is related to this question as well, in which I inquired about the most appropriate float voltage for a battery that would be in very high ambient temperatures, as the float voltage should compensate for temperature changes to avoid over-charging.

A question I'd asked in the comments, and was recommended be a separate question here, is the following:

What compensation is appropriate for the cut-off voltage of a sealed lead-acid battery? If ambient temperature is 130-140 degrees [inside a vehicle], and I'm lowering the float voltage by .5Volts to compensate (0.003mV per cell per degree celsius over 25°C, 30°C difference to 55°C)

As an example, if my cut-off voltage would normally be 10.8Volts for a deep-cycle battery at 25°C, would that value be lower for a battery in an ambient temperature of 55°C? If so, how much / how would I calculate the compensation?

Tagged as battery charging because I wanted to tag as battery discharging and there wasn't one for it... feel free to edit if there are more appropriate tags.

  • \$\begingroup\$ Sealed lead acid is an ambiguous term. Do you mean AGM? \$\endgroup\$ – Glenn W9IQ Jun 26 '17 at 10:49
  • \$\begingroup\$ @GlennW9IQ I don't have a specific description of the battery type. From what I've seen, there's AGM, Gel cells, and 'flooded' as options (right?), I'm not sure which of these this is. A link to the one I purchased: amazon.com/dp/B00KQX5GSC \$\endgroup\$ – schizoid04 Jun 26 '17 at 14:48

Float voltage is lowered at higher temperature to prevent gassing. On discharge the open-circuit voltage is barely affected, but internal resistance is lower so voltage will be a bit higher at high current. At end of discharge the internal resistance increases and voltage drops rapidly, so a fixed cutoff voltage should be fine.

If ambient temperature is normally 55ºF then battery life will be severely reduced.

Deep Cycle Battery FAQ

"Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM..."

  • \$\begingroup\$ Thank you for your answer. Yes, ambient will unfortunately be 55C for periods of the year (Summer in Phoenix Arizona, battery installed in the back of a vehicle). Good reference with the note about battery life being cut in half per 15 degrees over 77. I'd like to find a way to correct this problem but I'm not aware of anything I could do in my region to help this; it's just that warm outside in Arizona. \$\endgroup\$ – schizoid04 Jun 26 '17 at 14:50
  • 1
    \$\begingroup\$ Shade and a fan could help, and white paint. The air temperature is probably not 55C very often, as that is right around the hottest air temperature ever recorded on Earth. \$\endgroup\$ – mkeith Jun 30 '17 at 5:01

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