I've read that the float voltage for lead-acid batteries needs to be adjusted based on the ambient temperature that the batteries are charged in; that lower temperatures merit slightly higher voltages, and higher temperatures merit lower voltages on the battery for float charging.

I have had trouble finding a reference that breaks this out based on the ambient temperature, in a chart or table for example.

I live in the Phoenix, Arizona region, where the temperatures in the summer are easily above 115-120°F.

I have a 12V, 50Ah sealed lead-acid battery in the back of my vehicle, that is currently being used to power some accessories that are in the back of the vehicle, separate from the car's battery and electrical setup.

This battery is charged from a flexible solar panel that's mounted to the top of the vehicle.

The charge controller for this allows me to specify a float voltage, and the default value is 13.7V.

I'm sure, since it's frequently 120°F+ outside, and likely 130-140°F inside the vehicle while it sits in the sun, I need to adjust the floating voltage for this, but I don't know what level would be appropriate for this environment.

To summarize, What would be an appropriate float voltage for a sealed lead-acid 12V battery, in ambient temperatures of 120-140°F?

Update: Asked a separate question regarding the cut-off voltage when discharging and whether similar compensation is needed / how it should be calculated here

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    \$\begingroup\$ The lower the better. At that temperature, battery life will probably be reduced. Maybe 2.2V per cell as a guess. So that is around 13.2V. I might be tempted to go down to 13 even. \$\endgroup\$ – mkeith Jun 25 '17 at 23:27
  • \$\begingroup\$ Thank you @mkeith . Do you have any idea what I should use as the cut-off voltage for the load, so when I'm discharging this I don't go too far? It is a deep-cycle battery but of course depth of discharge will affect its service life, so I'm trying to figure out at what point I should cut off a load as well. \$\endgroup\$ – schizoid04 Jun 25 '17 at 23:30
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    \$\begingroup\$ @schizoid04 -- that's worthy of a full-blown question (it's a darn good question, too) \$\endgroup\$ – ThreePhaseEel Jun 26 '17 at 4:00
  • \$\begingroup\$ Not really an expert on the matter. I do know that the higher the load, the lower the discharge voltage can be. If you have a really light load, you might want to stop at 12V even! But most inverters assume you are going to be discharging at a very high rate, and they cut off at around 10V which is probably too low for most applications. \$\endgroup\$ – mkeith Jun 26 '17 at 5:30

According to Battery University:

The recommended compensation is a 3mV drop per cell for every degree Celsius rise in temperature. If the float voltage is set to 2.30V/cell at 25°C (77°F), the voltage should read 2.27V/cell at 35°C (95°F). Going colder, the voltage should be 2.33V/cell at 15°C (59°F). These 10°C adjustments represent 30mV change.

So, for a 30°C difference in temp from 25°C (55°C is 131°F), I'd have 90mV per cell of compensation, multiplied by 6 cells to reach a temperature compensation of 0.54V for the battery.

So, instead of 13.8V float, I should be floating at 13.26V or less.

As a result, I've modified my float voltage to a safe 13.1V just to leave some additional room / buffer.

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    \$\begingroup\$ You will find that several battery manufacturers recommend suspending the charging cycle when the core battery temperature exceeds 50° C. Monitoring the temperature of a thermally insulated battery post can serve as a proxy for core temperature. \$\endgroup\$ – Glenn W9IQ Jun 27 '17 at 12:10
  • \$\begingroup\$ @GlennW9IQ So if you have a battery that's in an environment that's over 50C for an extended period of time, are you just not supposed to charge the battery? \$\endgroup\$ – schizoid04 Jun 29 '17 at 21:44
  • \$\begingroup\$ Yes, if that is what the battery manufacturer recommends. You may find another brand that does not have this restriction. \$\endgroup\$ – Glenn W9IQ Jun 29 '17 at 22:07

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