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Old 06-25-2022, 08:29 PM   #1
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How to keep AGM batteries warm/do I even have to?

I'm planning my battery setup for my upcoming travels. Right now the RV has 1 shot group 24 battery. I was going to install dual group 27 AMG batteries. The batteries are tongue mounted.

I've found a lot of discussion about keeping lithium batteries warm and the answer almost always is put them in the basement use ducts or heating blanket to keep them warm.

I can't figure out why I can't find DC battery warmers. You can have jackets and gloves with DC heaters. What am I missing for large 12v batteries.

I'm hoping to go boondocking in fairly cold weather.
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Old 06-25-2022, 10:16 PM   #2
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Are you getting AGM or lithium batteries?

Here is a very brief overview. Both will work when cold.

Lithium can not be safely charged below a certain temperature, and that is the battery temperature, not the ambient temperature. There are lithium batteries with built in heaters.

There are not AGM batteries with built in heaters because there is no need. How do you keep your batteries warm in your car in freezing weather?

There are tons of 12 heating pads for sale with a simple google search.
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Old 06-25-2022, 11:17 PM   #3
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Quote:
Originally Posted by Nwcid View Post
Are you getting AGM or lithium batteries?

Here is a very brief overview. Both will work when cold.

Lithium can not be safely charged below a certain temperature, and that is the battery temperature, not the ambient temperature. There are lithium batteries with built in heaters.

There are not AGM batteries with built in heaters because there is no need. How do you keep your batteries warm in your car in freezing weather?

There are tons of 12 heating pads for sale with a simple google search.


I wasn't typing in the right thing when searching on Google.

I had been reading that AMG performance was best at 40-90 deg.

I thought car batteries you had your initial cold cranking but then they were warmed up by the engine.

I'm guessing something like the attached is what I'd want if I wanted to heat the batteries?
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Old 06-26-2022, 04:39 AM   #4
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It is true you will lose some capacity of the battery in cold weather, but I don’t think you can fully make up for that by using the battery power to heat itself…. If that were true, electric cars would not show a mileage reduction in winter (which they do !)

Just drawing power from the battery heats it internally as it experiences internal resistance to the electron flow.

If you want to relocate the battery inside where it is heated by your propane furnace, it will work longer than at ambient temperature of 0F
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Old 06-26-2022, 05:36 AM   #5
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Agree that with AGM you will lose more AH heating than will be gained by warmer temps so it's a loss vs a gain.
If boondocking in coldcWx I would just insulate the batty box to retain any heat generated by charging/ discharging the battys. You might better utilize a small solar charger to add some charge and heat to the batty... even a portable unit hooked up when desired.
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Old 06-26-2022, 06:43 AM   #6
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AGM batteries should not be kept warm , unless you need all the available capacity from them . The hotter the batteries are, the shorter their life. For longest life , keep the batteries cool. The only time you may need to warm the batteries , is if you are in sub zero temps. The battery capacity will be reduced to about 30% at -30 , but will return to its full capacity once it warms up , if it is not used at -30 .
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Old 06-26-2022, 06:55 AM   #7
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To keep lithium batteries warm try searching for ”rv tank heater pad”
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Old 06-26-2022, 07:29 AM   #8
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There's a few elements to this scenario.

One, lead acid batteries are spec'd to 77F. At 32F, both their capacity and current delivery is diminished, capacity by about 25%. So keeping them warm is useful if you need that capacity. Depending on how much energy you actually need between charges you may not need to maintain 77F, maybe 50 or 60F will be enough.

Below freezing lead acid batteries are OK to store, but not necessarily use. The colder it gets, the less capacity they can deliver before they will freeze and be damaged. Car batteries don't typically need to be heated because the start cycle for a car does not use much of the battery's energy, so there is little risk of freezing.

Thermal mass is your friend. A couple of 50lb batteries can hold heat for a while. Insulate the box, and you will keep the heat longer. You will not generate much heat during cycling, so don't count on that. The typical scenario where a battery heater is used is to power it from 120V during charging, then letting thermal mass carry the heat through to the next charge cycle. If you're 100% solar you'll either need to budget for that heat energy, or provide it with some external heat source. Being AGM you can safely recirculate heated cabin air through the enclosure, or mount the batteries inside heated space.

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Old 06-26-2022, 07:40 AM   #9
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Battery capacity or energy storage is a chemical characteristic. It does not go away at cold temperatures.

Getting the energy out of the battery to do useful work is a chemical process. Most batteries are a corrosion process. Lead in a lead acid battery is corroded by the sulfuric acid electrolyte into lead sulfate.

Colder temperature slows the chemical action. Up to a point, cold reduces how fast the energy can be removed. The Cold Cranking Amps rating of lead acid batteries is designed to reflect how fast the charge can be removed. CCA does not address total amount of charge.

Total amount of charge is basically the quantity of lead contained in the battery. It does not change at colder temperatures.

Of course at low enough temperatures, various chemicals undergo phase change. Phase change such as freezing can stop discharge and / or break physical components.

Battery heaters have been in use for a hundred years. Commonly available heaters are usually designed to run on grid power or from generators. Typically they were used with older lead acid designs to be able to start cars in freezing weather.

Lithium battery heaters also typically run on grid power or generators. They are used to raise the lithium battery temperature above freezing in order to charge them. This is necessary to make Li charging chemistry work properly.

NASA uses heaters to keep space vehicle batteries working in extreme conditions. NASA also installs them in insulated and sealed spaces.

For RV use, it is more efficient to install lithium batteries in a heated space. So they are effectively kept above freezing using some of the RV furnace heat. Shore power heaters are often installed because the Li batteries are being installed in space formerly designed for flooded cell lead acid batteries.

AGM batteries can also be installed in heated cabin spaces because they are safely sealed. This allows them to run high current inverters powering things like microwaves better than externally installed flooded cell batteries.

Flooded cell batteries require ventilation while they are being charged. Installing in heated cabin spaces often leads to dangerous conditions.

Avoid installing battery heaters. RV use of lead acid rarely requires heaters.

Install AGM and LiPO4 in minimally heated cabin spaces like in a cargo space or under a bed where ever possible.
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Old 06-26-2022, 02:34 PM   #10
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Thank you everyone for your feedback.

Unfortunately I can't move the batteries anywhere inside the trailer. While I don't have a ton of weight I do have bulky items that utilize a lot of space. I.e. down sleeping back for hiking (shouldn't be compressed), various bags (back packs an luggage), full face helmet for mountain biking, etc.

And then the "regular stuff": generator, portable solar panel brief case, sat dish, dog crate, all of my clothes including bulky winter gear, scrubs, activity specific active wear, activity specific shoes, foldable electric bike, water filters, sewer hose attachments, Jack/stabilizer blocks, leveling chocks, etc.

Just out of room internally and externally.
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Old 06-26-2022, 02:38 PM   #11
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Unless does my cargo pass though that goes under the bedroom count as a minimally heated space?

I could invest in a tongue box to put things like the leveling gear, sewer hose attachments, etc in. And move the batteries into the pass through.
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Old 06-26-2022, 02:43 PM   #12
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I could potentially add a 120v heat pad like someone would use for achy muscles or a battery specific product, build a little box insulated with bubble wrap foil, and control the warmer with an external thermostat such as an inbird. If I'm running a dual heater control inbird (I have a brand new one laying around for fish tank applications).

I could run dual heating that could potentially reduce power cycling if one heating source would suffice.
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Old 06-26-2022, 04:57 PM   #13
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If you keep your batteries insulated, be sure to use a temperature compensated battery charger. Here is a little more general info from the batteries that were used in solar and UPS installations that I maintained.

Battery capacity is rated at room temperature about 77 F. At approximately -22 degrees F , battery Ah capacity drops to 50%. At 32 F capacity is reduced to 80%. Capacity is increased at higher temperatures – at 122 degrees F, battery capacity would be about 12% higher.

Battery charging voltage also changes with temperature. For a 12v battery ,it will vary from 16.4 volts at -40 F to 13.8 volts at 122 F. This is why you should have temperature compensation on your battery charger or charge control.

A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. This is why temperature sensors should be attached to one of the terminals, and wrapped with some insulation . The sensor will then read very close to the actual internal battery temperature.

Battery capacity at high temperatures is higher, but battery life is shortened. Even though battery capacity is reduced by 50% at -22 degrees F ,the battery LIFE increases by about 60%. The higher the temperature, the shorter the battery life . For every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of lead-acid battery, flooded, sealed, GEL or AGM.

The bottom line is the cooler you keep your batteries, the longer they will last.
For example , a 100 ah battery that has a life of 5 yrs at 77F (full Capacity 100 ah) w At 122F the capacity will increase to 112 ah but the battery will only last 2.5 yrs. . At -22F the capacity will be reduced to 50 ah , but the life will be increased to about 8 yrs.

Batteries that are on a continuous float charge will not last as long as batteries that sit for months without being charged.
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Old 06-26-2022, 06:52 PM   #14
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That is a whole lot of new info for me. Generally I thought the rule for battery storage was try to discharge to around 80% and then let it sit for a while.

Re: temp compensating charger. I was just going to let the onboard charger handle it, although I've considered a DC to DC charger from my tow vehicle but not sure if that's wholly worth it if I have a gen and moderate solar array (200 w).

Again re: the temp compensating charger. What do people usually do who don't even try to manage battery temps. I.e. those who just run tongue boxes? I was thinking the two stage temp regulator connected to heating pads would give me control over the activite heating and otherwise just let them swing with ambient temps like unheated batteries would do.
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