Heating a Lithium Battery - One Data Point

[@Michael]

I'm lucky enough to have access to a deep-freezer cold enough to test battery heaters, and big enough to hold my campervan (I.E. my driveway....).

I have one Battle Born battery wrapped in the OEM 15 watt battery heater. The heater is essentially an Ultraheat tank heater sized to wrap three sides of the battery, with two 7 watt heating areas. Space is very tight, so the thickest insulation I can fit is 1/2" of XPS on three sides, 1/4" of EPS + a reflective felt pad on the other three sides. I didn't spend too much time fitting the XPS and EPS, so it has gaps.

With the relatively poor insulation and 15 watts of heat, I measured the following:

Ambient temp in the camper: -15F
Temp inside electrical cabinet: -8F
Least-insulated corner of the battery: +22F
Top of battery, under felt pad: +38F

Where the insulation is poor I see +30F temperature difference. Where the insulation is not-so-poor I see at least +45F above ambient.

My conclusion is that with better insulation (I.E. 1" of well-fitting XPS) the 15 watts of heat is more than adequate to keep the battery above freezing under most circumstances and above the low limit of -4F under nearly any condition.

Even with as little as 1/2" of well-fitting XPS I believe that the 15 watts of heat would keep the battery 40-50F warmer than ambient, adequate to prevent damage under discharge pretty much anywhere in North America, and above freezing in most of the continent. I'll likely re-fit the XPS/EPS insulation and use Thinsulate instead of felt next time I pull the battery out.

Power consumed will be 15W continuous as temps drop well below freezing. I have a temperature switch, so the pad cycles on and off when temps are just below freezing.

I do not see a reason to have much more than 15W of heat per battery if the battery is somewhat insulated. Battle Born advises against high wattage heating pads, as they have been known to warp the cases.

Unfortunately my drive-in freezer is only cold enough to test at -15F a few days per year, so my ability to test is limited. Might have to wait a year for more results.

 

[markopolo-ClassB]

It's not clear if your plan is to continually maintain the battery with thermostatically controlled heat or if it is to warm a cold soaked battery for use.

If warming a cold soaked battery for use then some caution is needed so that you don't trick the internal temperature sensor into permitting charging prematurely. That sensor appears to be very near the exterior surface of the battery. The heat wrap would warm the shell of the battery faster than the actual lifepo4 cells. See: https://youtu.be/G5E30u-66VI?t=140

 

[Davydd]

My 800ah Elite Power Solutions battery bank is made up of four modules (three more that is pictured in this photo.) So There are two electric resistant 6-1/2” x 14", 5 amp heating pads sandwich between module 1 and 2, 3 and 4 like in the photo for a total of 10 amps. The batteries are encased in a fiberglas basically uninsulated box outside under the van. The heating pads feed directly off the batteries so in 24 hours could consume a maximum of 240 amps from the battery. In real terms since the internal cores of 16, 3.4v, 200ah batteries are monitored the heating pads only come on to maintain a 41ºF. temperature. I can continuously monitor when the heat is being applied and in 0ºF. temperatures is seldom on and I think maybe consumes less than 100 amps in a 24 hour period. In these kind of temperatures you can bet I will be driving every day and with the second alternator it can replace those amps in about 20 minutes.

My lithium ion battery bank will be inside the van in our new van and, of course, on the road the cabin will be maintained well above freezing. I'm not sure if ARV is doing with batteries inside the van for external heat otherwise. Most all their vans are built with batteries inside the cabin now. I haven't asked since I store my Class B when not in use in a heated garage.

 

[GaryBIS]

Hi,
I know zip about Li batteries, so this is likely a dumb question.

Is there any concern with an insulated Li battery pack that under heavy load the batteries will heat up and be damaged because the insulation prevents them from cooling?

Gary

 

[@Michael]

Is there any concern with an insulated Li battery pack that under heavy load the batteries will heat up and be damaged because the insulation prevents them from cooling?

Gary

Yep.

Lithium batteries generate heat, and have upper temperature limits. Mine are rated to 135F.

 

[@Michael]

It's not clear if your plan is to continually maintain the battery with thermostatically controlled heat or if it is to warm a cold soaked battery for use.

If warming a cold soaked battery for use then some caution is needed so that you don't trick the internal temperature sensor into permitting charging prematurely. That sensor appears to be very near the exterior surface of the battery. The heat wrap would warm the shell of the battery faster than the actual lifepo4 cells. See: https://youtu.be/G5E30u-66VI?t=140

I'm planning on maintaining the battery at >32F all winter & have an on/off switch for the heat. I'll switch the heat off and remove the insulation from top and two sides in spring.

If I have a cold soaked battery, I'll run the furnace & warm up the whole coach. My guess is that it would take hours to transfer heat from the coach to the battery - but the battery happens to be right on top of a Truma heat duct, so the battery compartment will warm up much faster than the coach.

For reference, we got down to -19F night last night. The inside of the battery compartment got down to -10F, the coldest corner of the battery case was +19F, and the top of the battery was +35F. For next winter, I'll figure out better insulation.

--Mike

 

[avanti]

I'll switch the heat off and remove the insulation from top and two sides in spring.

It wouldn't be hard to add a little muffin fan controlled by a thermo PWM controller. They are readily available, cheap, and work well. That way you could just leave the insulation in place. That's how I keep the back of our fridge compartment cool.

 

[markopolo-ClassB]

Advanced RV's solution of maintaining the batteries above 41F is science-based. The batteries would be in an acceptable ready-to-use state.

The idea that full current charging is somehow permissible at 33F but not at 32F has unfortunately taken hold. There's no water in a lifepo4 battery.

 

[booster]

Advanced RV's solution of maintaining the batteries above 41F is science-based. The batteries would be in an acceptable ready-to-use state.

The idea that full current charging is somehow permissible at 33F but not at 32F has unfortunately taken hold. There's no water in a lifepo4 battery.

I agree with Marko, and it was recently discussed a bit on another discussion.


Most of the temperature/charge rate/storage time, etc type stuff appears to be of a sliding scale type thing, not a sharp cutoff. They just need to put a number on it because that is what people might actually use. You won't see people wanting to go to and use a graph of temps vs charge or plug numbers into a formula. Just like the 50% rule in AGMs, which we have manufacturer charts showing it is not accurate, but we still see folks that think, or were told, that going to 49% is going to instantly kill your batteries. 50% was a good place to pick as it sells more batteries and by cherry picking the points on the sliding scale of life, they can make it look much more accurate than it is.



Unfortunately for consistency, different companies choose a different point within that given range on the scale, so you get lots of different points that are said to be the definitive "point", but of course none of them are.


As was stated ARV chose a point where the batteries are fully functional without use limits at 41*. Others choose 32* because it is on the lowest end that you get by with even with limitations and it looks like they are better than ARV and others on the sales literature, most likely. Not great, IMO, to choose sales over science but it is done all the time in most industries.


Of course it would probably be best to design a system that took care of it all for you by automatically controlling charge and discharge rates based on temperature, but that would be a quite complex job. It is much easier to just heat the batteries to the safe level of your choosing, even with the problems heaters can cause with energy use and source. You will need them anyway once you get colder than the lowest choice of temp.

 

[avanti]

I have no idea of the physical basis for the "don't charge at low temperatures" rule. However, it does not necessarily follow that the cutpoint is fuzzy. There are many phase transitions in the physical world that are not approximate, so there is no a priori reason to assume that this number is or isn't a cutpoint. It depends entirely on the physical basis for the rule. (I do admit that it is suspicious that the recommended number is exactly the freezing point of water. But, coincidences do occur).

N.B.: I am NOT saying that I believe the number isn't fuzzy--I have no idea. But, any such discussion really needs to reference the underlying physics.

 

[booster]

I have no idea of the physical basis for the "don't charge at low temperatures" rule. However, it does not necessarily follow that the cutpoint is fuzzy. There are many phase transitions in the physical world that are not approximate, so there is no a priori reason to assume that this numbers or isn't a cutpoint. It depends entirely on the physical basis for the rule. (I do admit that it is suspicious that the recommended number is exactly the freezing point of water. But, coincidences do occur).

N.B.: I am NOT saying that I believe the number isn't fuzzy--I have no idea. But, any such discussion really needs to reference the underlying physics.

Absolutely correct about materials.



There is relatively little about the physics/chemistry behind it that I have seen, although it probably is out there somewhere. The main basis of it being a sliding scale is because a number of spec sheets actually list the acceptable charge rate over the range in question, showing the increasing safe charging level with temperature. It behaves as if it is an electron migration speed vs temp thing, but that is only a guess.

 

[@Michael]

Absolutely correct about materials.
There is relatively little about the physics/chemistry behind it that I have seen, although it probably is out there somewhere. The main basis of it being a sliding scale is because a number of spec sheets actually list the acceptable charge rate over the range in question, showing the increasing safe charging level with temperature. It behaves as if it is an electron migration speed vs temp thing, but that is only a guess.

There is enough research - but it seems to be locked up behind paywalls.

Here's one paper that explains quite a bit.

https://www.sciencedirect.com/science/article/pii/S1002007118307536

Summary: The exact chemistry will influence cold temp limits. Viscosity increases, chemical reactions slow down, and the possibility of permanent battery damage increases.

To know exactly how cold one could safely go, one either has to assume that the battery manufacturer knows the chemistry of the cells they are sourcing & has published accurate cold temp recommendations, or one would have to know the exact chemistry of the cells in your camper and make your own call.

--Mike

 

[Davydd]

I think it is about optimizing battery life. Charging below 32ºF doesn't destroy your lithium ion battery but they say it will incrementally shorten the life of them. Also, lithium ion manufacturers say optimum charging is 41ºF and above and I think ARV now sets it at 42ºF. That is automatically controlled with heat. Since I store and keep my garage at 50ºF my battery heat never turns on in storage. I have observed that the batteries shut down from charging at 36ºF set by ARV's controls. You can turn off the external heat manually as I did accidentally and learned all of this. Now, it is more fail safe in a blue flame light indicator comes on when heat is applied and there is also now an indicator that the heat is active and ready to use. That was an upgrade I got done under warranty.

The manufacturer of my batteries, Elite Power Solutions, say they will be permanently damaged at -4ºF but other sources say depending on the duration. I guess it is a better safe than sorry situation. Keeping batteries always above 41ºF is the very safe solution.

ARV also sets the lithium ion batteries to shut down completely at 20% state of charge which is a comfortable margin from 0 or 10% from other references. There is no auxiliary AGM battery to restart them as the old Roadtrek solution. All you have to do is start your chassis engine, hold a button down for one minute and they come back online with second alternator autostart and idling or just driving. I can set the percentage of SOC at any percent I want above 20% SOC. I've tested it at 90% just to see if the autostart kicks in. You have the option of turning autostart off in case you are storing in a garage. So the 20% is a belt and suspenders and setting it higher means you don't have to hold the button for one minute to get them online again.

 

[markopolo-ClassB]

I have no idea of the physical basis for the "don't charge at low temperatures" rule. However, it does not necessarily follow that the cutpoint is fuzzy. There are many phase transitions in the physical world that are not approximate, so there is no a priori reason to assume that this number is or isn't a cutpoint. It depends entirely on the physical basis for the rule. (I do admit that it is suspicious that the recommended number is exactly the freezing point of water. But, coincidences do occur).

N.B.: I am NOT saying that I believe the number isn't fuzzy--I have no idea. But, any such discussion really needs to reference the underlying physics.

I mentioned full current charging and by that I meant that the rate of charge is not being considered along with temperature. It's my opinion that you have to look at charge voltage, charge current & temperature together.

Several chip manufacturers implement JEITA guidelines in their charger control chip solutions. JEITA's charge voltage profile allows for full voltage (3.6vpc) only until 40°C then it drops. JEITA's charge current does not allow charging below 0°C, then it's reduced current charging from 0°C to 10°C. Full current charging is permitted from 10°C to 45°C. Then reduced current charging from 45°C to 60°C and no charging from 60°C and up.

Folks are being pretty selective if adhering to no charging at 0°C but ignoring the reduced current guideline at 1°C for example.

 

[avanti]

Folks are being pretty selective if adhering to no charging at 0°C but ignoring the reduced current guideline at 1°C for example.

Agree, and it is a good point.

However, there is still the issue of whether these specs are absolutes in that one risks a hard failure by ignoring them, or whether (as some have suggested) that these numbers are mere rules-of-thumb and that the only risk is reduced lifetime. I see now that this is a slightly different issue from the one you intended to raise, but it is an important and objective question that must have an objective answer.

 

[markopolo-ClassB]

Probably not absolutes. They're likely in the middle and meant to cover a variety of subtle lifepo4 chemistry differences that can accommodated by the same charger.

Bringing this back to Michael's first post on this topic. It's an indicator that 32°F isn't risk free if about to start loosely controlled charging. It's would be better to maintain the battery at least at 41°F like Advanced RV does IMO. Alternatively, it could be brought up to 41°F a day or more in advance of being used.

 

[booster]

Probably not absolutes. They're likely in the middle and meant to cover a variety of subtle lifepo4 chemistry differences that can accommodated by the same charger.

Bringing this back to Michael's first post on this topic. It's an indicator that 32°F isn't risk free if about to start loosely controlled charging. It's would be better to maintain the battery at least at 41°F like Advanced RV does IMO. Alternatively, it could be brought up to 41°F a day or more in advance of being used.

X2, in my opinion

 

[Davydd]

You have to know your conditions as well. Lithium Ion batteries connected and under load will have an internal temperature higher than ambient temperature and if inside the cabin the cabin may very well be higher than ambient temperature.