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I've been searching for an answer to the following question for a while and haven't found one. What's the lowest temperature that a battery with sla/agm chemistry can tolerate? Will it withstand below freezing temperatures or not? The link is an example
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All temp effects assuming a healthy charged battery is capacity and charge voltages. As temps go down, capacity goes down and charge volts should go up. How much capacity loss depends on the temperature. You would have to look at a battery manufacture spec sheet to see what the loss is vs temps. As a general rule down to 30% rated capacity at -40 degree F. Makes no difference if it is flooded or sealed.
To answer your question you can take a AGM easily down to -40 degrees again assuming it is fully charged. The reason lead acid batteries can work at such low temps is the electrolyte is basically anti-freeze with a catch. It only works if the battery is kept fully charged and not allowed to be discharged and exposed. As the battery state of charge decreases, freeze temp goes up. So a fully charged battery can tolerate -40 F just fine, By the time it is fully discharged freezing point is basically the same as water of 32 degrees F.
AGM can take lower temps than flooded because the are sealed and pressurized. The higher the pressure is, the freeze point temp goes down, and boiling point goes up. So that gives a AGM a slight advantage is cold and is one reason you can justify their use. But we are talking -40 degree cold for extended period. Secondly AGM batteries have strong jars to deal with the pressure, so if the battery does freeze, the case will not split open. Once thawed is just fine with no damage. FLA jars are not made to handle the pressure of freezing electrolyte, and thus the jars can split, so when the battery thaws you have a nasty acid spill to deal with.
Bottom line is unless you are talking -30/40's degree F, you have nothing to worry about. Just keep the battery fully charged.
FWIW if you had searched battery capacity vs temperature you would thousands of hits with charts like this one.
Last edited by Sunking; 11-17-2016, 12:51 AM.MSEE, PE
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