Be cautious ordering generic blue wrap LFP cells

[shade]

Are they, though? 12 watt, 12 volt 2x4" silicone heater is $70.. and I thought Wolverine battery heaters were expensive... Imagine the made-to-order units are even less bang for your buck.

Maybe not. I quickly specified a size around that for around $35, iirc.

Otoh, if you're building an LFP battery of decent capacity and plan to use it in cold conditions much, it might be worth a premium to get the heating right the first time.

 

[shade]

I hear you on cells staring out cold, which they would with my plan to only warm the ammo can when I'm sensing that the day is starting and it's below charging temps. That's all to avoid needlessly heating the box... if it's 6pm and dark outside what's the point? Blowing hot air at ice cubes will result in a warm surface temperature and a frozen core...

If ambient temperature is expected to be consistently low enough for it to be an issue, I think you'll be better off leaving a heating system active 24/7 to keep the battery mass at an even temperature, which would avoid the hot-air-on-ice problem.

So, just saying, LFP is not good in the cold, automated systems are fragile, consider using personal attention and care if not instead, then in addition to sensors and heaters and microcontrollers.

Agreed. I'm also curious about the benefit of starting each morning with a high discharge event (microwave, coffee pot, hot plate) to dump some heat into the battery at the time it's likely to see its lowest internal temperature of the day, which happens to coincide with the beginning of a solar charging period. @dreadlocks has said that works well for him.

Manually engage the thermostatically controlled heating system installed on an insulated battery box, brew some coffee for breakfast, and don't worry about it too much.

 

[luthj]

Given how little heat the batteries themselves make, I don't think a discharge event is worth the trouble just to heat the batteries. The best approach is simply to insulate the box with some 1/2-1" rigid foam insulation. Then just leave the heater running all the time (with thermostat obviously). In zero degree ambients you would need something like 20-70W average to keep the pack above 55F. Obviously this is highly dependent on the box size and insulation.

My current design for a battery box uses an insulated box with a 120mm vent and 100W of heating pads (about 30W per square foot). The pads are attached directly to the battery pack. A thermostat drives the fans when the pack gets over ~90F, and the heaters when it drops below 60F.

 

[burleyman]

I have a permanently mounted, easily accessible diesel air heater. A piece of very flexible duct provides hair blow drying and clothes drying for my wife. Quick, temporary, focused heat source.

 

[john61ct]

Yes, an independently powered heat source is better than pulling the LFP bank power down to heat itself, but

to my mind, the thermal mass and gentler, more precise temp regulation of a hydronic solution would be better than forced air.

If left outside permanently, whether you maintain constant heat input or not

depends on just how cold it gets - at some point even isolated cells may get damaged,

and of course how frequently the rig gets used.

Thawing out gently should not require more than a 24-hour head start.

 

[luthj]

I wouldn't say hydronic is gentle, especially when the coolant loops can hit 160F. The electric heating pads have a nice even heat distribution. So a pad on both large vertical faces is about as even as you can get for a 4S pack. a 2p4s would benefit from a third lower output pad in the middle.

For the average user I don't think all the automatic stuff is needed. Just have a simpler overtemp thermoswitch, and a heating pad. On the off chance you need it, you can flip a switch.

 

[Rando]

Even within the LFP family, between say Winston cells and CALB cells, there are huge differences wrt specific number values.

Extrapolating between LI research and what the IRL reality for LFP might be, can only be done in the most general terms.

One sweeping statement seems clear - the relationship between (potential) lost cycles increases with either factor exponentially, not linear.

Professionally I have done a fair amount of research on using various battery chemistries at low temperatures (often much lower than we are discussing here) and the results are quite consistent across a range of lithium chemistries - at low charge rates the issue is minor at best. Even at higher charge rates (1C) the issue is not nearly as severe as folks seem to think:

Source. Note this is for LiFePO4 at 1C charge/discharge rates to 100% DOD. Even with this horrific use scenario at -10C you don't see a decrease in capacity until after 300 cycles.

If you look at lower charge rates, the picture is even better:

Source . In this case (again with LiFePO4) the capacity loss at 0.25C is actually much less than 1C at 25C. This is entirely consistent with the models for other lithium ion chemistries.

Anyway, does anyone actually have any data that shows that there is an issue at lower charge rates?

 

[john61ct]

My goal is near zero capacity loss through over 5000 cycles.

Each model battery will have its minimum "no charging anywhere near this temp or instant scrap" spec.

Look at Thundersky/Winston's data sheet as opposed to CALB's.

Again, getting exact numbers even with rigorous research

which has barely begun, is all oriented around EV usage where even 2C charging is considered "slow", and rarely includes **any** hard data for LFP

is hardly possible with all the greyscale variability among multiple factors.

Yes you usually can charge at a **very** low C-rate around 0°C

But in general, being conservative I'd start pre-warming below 10° if going over 0.1C

25° should be fine for 0.2-3C

Faster charging than this, anybody's guess, impossible to detect the factors leading to lost lifecycles without forensic autopsy, studies show different numbers for even slightly different chemistries.

So for 0.5C and above, I'd go to 30° first to maximize longevity.

Of course in many contexts people just accept the hit, which of course is fine, up to each owner to decide

most aren't even aware of the issue, think it's just a simple go/no-go binary.

 

[john61ct]

Another point, for a reasonably sized bank, say 600Ah, few owners will even have a charge source capable of going over 0.3-4C.

Fitting an alternator or genset that actually delivers just 200A to the bank continuously, is a pretty challenging and expensive bit of infrastructure.

So for most, this issue only pertains to a bank small enough to just keep inside the living space where we are usually maintaining around 20° anyway.

Of course, do not do this with most LI chemistries, but LFP is safe enough.

 

[john61ct]

LTO is really the way to go for such cold use cases, e.g. Toshiba Super Charged Ion Battery (SCiB)

Much better at low temps, can go to zero volts with no damage,

crazy high cycle life, 10,000+ is normal and

can be fast charged up to 10C with little stress, so full charging in 6-15 mins

and like LFP, are extremely stable thermally

Just a weird nominal 2.3V per cell, and a lower energy density, equivalent to NiCD

 

[Rando]

My question would be, which data are these recommendations based on?

I am not calling you out specifically, but one of the biggest issues with using forums as a source of information, is that they are an echo chamber for unsourced and often inaccurate information. A case in point would be the guy who added $300 of contactors to his battery to avoid voltage spikes which the evidence suggest don't exist. Or on this thread, folks suggesting glycol baths to keep batteries warm, this is insane. Or a well respected member who doesn't believe that ohms law applies when it comes to wires going to solar panels.

This echo chamber effect has also built the lore that you can't charge lithium batteries below freezing, which the data shows is just not true.

The criteria is to engineer a system for realistic operating conditions with realistic goals and for a realistic lifetime. For the vast majority of us, a few hundred cycles will last the lifetime (or the timeframe of our interest) of the camper and the design should reflect that. A few cold charges at 0.1C (or even 1C) each winter is not going to appreciably change the lifetime of your battery and is not something you should be overly concerned about.

With respect to the OP, he has made it clear that the challenge of designing these complex systems is part (or most) of the fun, which is great. But just to make it clear most of the complexity described here is not a necessity.

 

[luthj]

No one is considering a glycol bath. Its not unsual for larger rigs to use heated glycol/coolant lines to keep tanks and batteries warm in severe weather. With regards to john61ct's recommendations, he is quite extreme with his expectations. Trying to get more than 10+ years from a battery bank is noble, but most folks aren't building a $30k pack for an off grid cabin.

5,000 charge cycles with no capacity loss? Not possible. Less than 5%? Maybe, if the cycles are very shallow. 5,000 cycles with less than 10% loss is doable, but calendar aging has to be taking into account.

 

[Rando]

This guy appears to be be suggesting it:

Yeah I went through all the senarios. There is no liquid in Lithium so low and slow is the best option.

Glycol is the best way to heat the cells. This way each cell could have 40F-59F liquid surrounding it.

We'll head out when it is 20F which meant starting the heat (BUT NOT THE SOLAR CHARGING) 2 days beforehand. This and the possibilty of forgetting to disconnect the solar charge on after each winter trip is why I went with FLA.

 

[Alloy]

This guy appears to be be suggesting it:

Please tell me why you have issues with hydronic heat?

 

[luthj]

The confusion is that Rando thought you were suggesting a literal glycol bath for the batteries. At least that's how I interpreted the post.