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AGM batteries contain electrolytes that can freeze in cold temperatures. Electrolytes have much lower freezing points than water alone. The freezing point depends on the concentration of sulfuric acid:
30% acid concentration: -13°F (-25°C) freezing point
40% acid concentration: -31°F (-35°C) freezing point
50% acid concentration: -67°F (-55°C) freezing point
Most AGM batteries have around a 30% sulfuric acid concentration. This gives the electrolyte a freezing point of around -13°F. If an AGM battery drops below this temperature, the electrolyte can start to freeze and crystallize.
The crystallized electrolyte prevents the chemical reaction that produces electricity. It also expands in volume and can crack the battery case. Both these effects can severely damage the battery.
So AGM batteries are susceptible to permanent damage if they freeze in cold temperatures. Using the battery’s electrolyte concentration, you can determine the approximate freezing point to avoid.
When an AGM battery is exposed to freezing temperatures, the electrolyte inside can begin to freeze and expand. This causes severe damage to the internal lead plates and separators as the expanding ice puts pressure on the internal components.
Specifically, the electrolyte between the lead plates will freeze first. As it expands during the phase change from liquid to solid ice, it will push against the lead plates with significant force. This force can warp, bend or crack the lead plates, damaging their structural integrity.
The separators between the plates are also vulnerable to damage from the expanding ice. The thin sheets that isolate the positive and negative plates can easily tear or develop holes from the freezing pressure. If the separators are compromised, it can create shorts between the plates when the battery thaws.
In addition to physical damage, the freezing process alters the electrochemical properties of the electrolyte. This can reduce conductivity and battery performance even after thawing out. Essentially, the freezing causes both mechanical and chemical damage that degrades battery function.
Partial freezing may only cause minor issues, while a deep freeze can permanently destroy an AGM battery from expansion damage. Checking for leaks, bulges or cracks can help identify if freeze damage has occurred.
AGM batteries are rated to operate within certain temperature ranges, outside of which their performance and lifespan can be affected. Most AGM batteries have an optimal operating temperature range of -4°F to 140°F (-20°C to 60°C), though some are rated for lower temperatures.
AGM batteries can struggle in very cold temperatures below 0°F (-18°C) due to the increased resistance and sluggish chemical reactions within the battery at lower temps. Performance and capacity will be reduced. Cranking amps can fall to around 50% capacity at -4°F.
In extremely hot temperatures above 120°F (49°C), AGMs may experience accelerated self-discharge and drying out of the electrolyte. Heat also accelerates corrosion of the lead plates inside. Sustained high temperatures above 140°F can significantly shorten the lifespan of an AGM battery.
Proper charging and maintenance is required to maximize performance and longevity of AGMs in both temperature extremes. Using an AGM battery within its optimal and maximum/minimum ratings will provide the best results.
Absorbent Glass Mat (AGM) batteries are designed to withstand cold temperatures better than standard lead-acid batteries. However, additional care should be taken when using them in freezing conditions.
During the winter, the capacity and cranking amps of AGM batteries can drop. To maximize battery life and performance in cold weather:
Fully charge the battery before winter sets in.
Check that the battery is securely mounted in the vehicle to minimize vibration damage.
When it’s below freezing outside, insulate the battery with a heater or battery blanket.
Limit use of accessories like lights and heated seats to prevent draining the battery.
Drive regularly and avoid leaving the vehicle sitting for weeks unused.
Before you drive, let the engine run for a few minutes to let the battery get recharged.
To keep the vehicle’s charge stable while it’s not in use, think about using a trickle charger.
Check the charge level monthly and recharge as needed. Consider a battery tender.
Before winter arrives, have the battery checked up at a shop to make sure it’s still in good shape.
In extremely cold climates, consider switching to a AGM battery with a higher cold cranking amp (CCA) rating.
Taking steps to protect, maintain, and monitor AGM batteries during winter will ensure they continue to provide reliable performance despite the cold.
If your AGM battery has frozen, the first step is to safely thaw it out before attempting to recharge. Take the battery inside and let it get to room temperature to fully defrost. Avoid attempting to quicken the defrosting process.
Once thawed, you’ll need to recharge the battery to assess its condition. To restore the battery voltage to its maximum level, use a battery charger. Avoid quick charging, which could harm the battery, and instead charge at a regular pace. While the battery is charging, keep a tight eye on it.
If the battery accepts and holds a normal charge, then it may still be functional after having frozen. However, there could still be unseen damage that causes issues down the road. It’s best to have the battery tested to determine its Cold Cranking Amps and condition. A battery that has frozen is more vulnerable to freezing again.
Consider switching to a lithium battery if you experience frequent freezing with an AGM in frigid climates. Even after freezing up, an AGM battery can continue to operate dependably with the right maintenance.
Once an AGM battery thaws, it’s important to test it to see if it still holds a charge and functions properly. Here are some steps for testing a battery that has frozen:
Check the voltage – Use a voltmeter to test the voltage. A fully charged AGM battery should show about 12.8 volts. If the voltage reads significantly lower, like below 12 volts, that indicates the battery has been damaged.
Check for bulging or cracks – Carefully inspect the battery casing and terminals for any obvious signs of damage like bulging sides or cracked plastic. This can indicate the internal plates and connections have been damaged.
Charge and discharge – Fully charge the battery and then disconnect it and drain it using a load like lights or electronics. If the battery quickly loses its charge, that is a sign it has been compromised.
Check for electrical shorts – Use a multimeter to check for any abnormal drops in resistance that could indicate shorted cells. Electrical shorts can develop when the internal plates shift due to freezing.
If testing determines the battery has lost capacity or has internal damage, it will likely need to be replaced. It’s not recommended trying to recharge or repair a battery that has frozen and failed testing, as it may be unsafe.
Proper maintenance is key to ensuring your AGM battery lasts through cold winter months. Here are some tips:
Keep the battery fully charged – Never let it discharge below 50%. A fully charged battery withstands cold better.
Use a battery tender or trickle charger – Keep the battery plugged into a maintainer when not in use. This prevents discharge over time.
Store in a warm area – If possible, bring the battery indoors or keep it somewhere insulated when not in use.
Regularly clean terminals – Clean the battery posts and cable connectors to prevent corrosion buildup. Use a wire brush and battery cleaner.
Check water levels – For AGM batteries with removable caps, periodically check the electrolyte level. Top off with distilled water as needed.
Avoid deep discharges – Deep discharging in cold weather can more quickly damage the battery. Try to limit depth of discharge.
Use a battery blanket – Wrapping the battery in an insulated blanket helps retain warmth when stored in the cold.
Charge before storing for the winter – Give the battery a thorough charge before keeping it stored. Recharge the device one or two times in the winter.
Consider a higher CCA battery – Upgrading to an AGM battery with a higher cold cranking amp (CCA) rating improves cold weather performance.
Proper winter battery care ensures your AGM battery survives the cold weather and is ready for reliable starting when needed. Follow these tips to maximize battery life.
The freezing point of an AGM battery is approximately -4°F (-20°C), but allowing your battery to freeze can permanently damage it even if you are eventually able to revive it. Here are some signs that indicate a frozen AGM battery may need to be replaced:
The battery will not accept a charge even after thawing – This suggests the lead plates inside the battery were damaged by freezing. A frozen battery that will not take a charge likely has internal short circuits and permanent plate damage.
Reduced cranking power after thawing – If your battery has noticeably less starting power after being thawed out, it means the freezing compromised the battery’s ability to deliver high amperage current.
Faster discharge after freezing – An AGM battery that loses its charge much faster after being frozen has likely suffered internal damage that reduced its overall capacity.
Visible cracks or bulging of the battery case – The swelling and extreme expansion that occurs when an AGM battery freezes can actually crack or warp the outer plastic case. Any visible case damage is a sign of catastrophic failure.
Cloudy or milky battery electrolyte – Peering inside a translucent AGM battery case may reveal cloudiness or white streaks in the normally clear electrolyte. This suggests permanent separation of the electrolyte.
A sulfurous odor from the battery – The sulfuric acid electrolyte can decompose when frozen, creating a rotten egg smell around the battery. This is a clear red flag something is wrong internally.
If your AGM battery exhibits any of these symptoms after freezing, it’s time to retire and replace it. Allowing it to continue operating in a damaged state will lead to premature failure down the road. Investing in a new battery is recommended.