A handy passive to active battery cooling blog post?

[Logic11]

This looks like it may be interesting/useful to battery case designers and builders?
I've often wondered about the sealed battery boxes, with almost no mention of cooling, in most posts here..?
Thermal Management System in Li-ion Battery using Air Cooling. (Blog by Aniket Bajare)

 

[neptronix]

I wouldn't bother with this personally. it's just more weight and space that isn't necessary.

Just spec a battery so that it doesn't produce heat now at your current application, and preferably, 5 years from now too ( accounting for IR drop ). it's not too hard to do. You'll never need battery cooling in that case.

You need air cooling only when you're stretching a cell past it's limits, which isn't a great idea because you will lose range to heat. The better thing to do is not make the heat all.

I used to run 20AH 20C lipos at 5C ( 100A ) inside a triangle bag. I could never detect any warmth from them even during multi mile hill climbs at 55mph.

Same goes for my 21700 Samsung 50S battery. 3C rated, but i run it at an average 1C. 3v of voltage drop under max load. No detectable warmth after totally discharging it. No need for cooling - we didn't make the heat in the first place.

 

[Logic11]

I wouldn't bother with this personally. it's just more weight and space that isn't necessary.

Just spec a battery so that it doesn't produce heat now at your current application, and preferably, 5 years from now too ( accounting for IR drop ). it's not too hard to do. You'll never need battery cooling in that case.

You need air cooling only when you're stretching a cell past it's limits, which isn't a great idea because you will lose range to heat. The better thing to do is not make the heat all.

I used to run 20AH 20C lipos at 5C ( 100A ) inside a triangle bag. I could never detect any warmth from them even during multi mile hill climbs at 55mph.

Same goes for my 21700 Samsung 50S battery. 3C rated, but i run it at an average 1C. 3v of voltage drop under max load. No detectable warmth after totally discharging it. No need for cooling - we didn't make the heat in the first place.

Ah... Thx neptronix

 

[Chalo]

My take is that hot cells aren't a problem to solve-- they're evidence of a problem that already happened, that caused the cells to heat up. Usually the cause is beating on the pack hard enough that not only does it heat up, but cycle life and capacity are adversely affected even if you cool it by other means.

 

[Logic11]

So EV cars and trucks (Like Edison Motors' Topsy that will use the aircon to cool the battery when necessary) are all banging on the batteries way harder than the E-Bike crowd is?

ie: You guys all run more than enough cells in parallel for heat=amps per cell not to be an issue,
whereas;
The CAR guys all opt for a higher per cell amp load for some reason?
Weight?? Weight's an issue everywhere, so??

 

[harrisonpatm]

So EV cars and trucks (Like Edison Motors' Topsy that will use the aircon to cool the battery when necessary) are all banging on the batteries way harder than the E-Bike crowd is?

ie: You guys all run more than enough cells in parallel for heat=amps per cell not to be an issue,
whereas;
The CAR guys all opt for a higher per cell amp load for some reason?
Weight?? Weight's an issue everywhere, so??

I can think of multiple reasons. Weight is one. Desire to advertise higher power ratings, another. Some people get off on having super fast acceleration. Tinfoil-hat-consumerism reason: if they give you a battery that gets worn harder than necessary, then it gets worn out faster and you have to buy another one sooner.

But likely one of the biggest reasons is just that it's a big heavy car, that requires more power to go. That doesn't necessarily require cells to be pushed at high discharge rates, but it can happen more readily than our lightweight bikes and motorcycles.

Also, we as DIYers can make the design choice to add more cells in parallel, or choose cells that are overspecced for our use case, because we want our batteries to last longer and not be as stressed. This choice might be more expensive in the short term (buy the bigger battery now so I don't have to buy a replacement battery in 2 years when the first battery wears out prematurely). Car manufacturers might make the cheaper short-term decision (spend less on the battery pack now in order to have an overall cheaper car, even if the battery won't last as long).

Tl;dr = variety of variables at play

 

[docw009]

EV's need the batteries to meet consumer expectations. probably at least a six year life in all kinds of weather. Environemental control of the battery is necessary,

I'm just riding a light conversion bike with an electric motor assist. It's as rugged as any appliance, but I have to know when it's too cold to recharge, and be smart enough to size the battery so I don't cook it,

Maybe the next "advance" in ebike packs will be BMS that prevent recharge when it's too cold,

 

[Diggs]

EVs have to respond to all challenges. It's not so much the current draw that requires cooling but everyone is trying to charge faster and faster. Fast charging generates quite a bit of heat. Don't forget, these cars are expected to be able to wander Death Valley in the middle of July (with 5 people keeping cool inside). Try that on an ebike......

 

[harrisonpatm]

EVs have to respond to all challenges. It's not so much the current draw that requires cooling but everyone is trying to charge faster and faster. Fast charging generates quite a bit of heat. Don't forget, these cars are expected to be able to wander Death Valley in the middle of July (with 5 people keeping cool inside). Try that on an ebike......

Good point, I forgot that manufacturers' biggest selling point on new EV tech is how fast the battery can charge. Pumping 50kw into a pack at a time is certainly gonna take some cooling

 

[bananu7]

Maybe the next "advance" in ebike packs will be BMS that prevent recharge when it's too cold,

ANT BMSes already have that feature. Mine has 4 temp sensors to spread around the battery, and you can set a low cutoff where it will close the charge FETs below that.

 

[BlueSwordM]

I'd argue most ebike packs are just built to a lower standard than leading edge phones and most electric cars, particularly when it comes to cooling and charging algos.

However, designs can still be pretty good: putting a good thermal pad below the cells inside of a metal casing can help a lot.

 

[harrisonpatm]

ANT BMSes already have that feature. Mine has 4 temp sensors to spread around the battery, and you can set a low cutoff where it will close the charge FETs below that.

Same on JKBMS

 

[neptronix]

Yeah, lithium batteries are the hottest in the center, external cooling is difficult. Tesla ended up using liquid cooling to make laptop cells work out @ ~50% more power than they were designed for.

That was so they could take most dense cell technology of the day ( imagine 2C maximum rated ) and advertise both a good range and good power.. pushing the cell to 3-4C.
But due to the power demands of cooling and weight, you're throwing away a small amount of efficiency all the time.

Otherwise with air cooled, you need to tradeoff range for power because you need a big safety buffer.

The electric car is extremely constrained.. picking a 6C rated cell to have stable temperatures means you've lost 20% of your range compared to your competition. And because you're keeping a safety buffer.. you have less power, too. The net gain is a less complex system.

We don't have to make these kinds of choices on small EVs though.

EV cars haven't caught on to things like tabless cells yet.. those kinds of cells don't have a capacity/power tradeoff... so you get this big temperature/IR safety buffer for free.. allowing you to get high power without added cooling... and also unlock some extra range. since you are not lugging around some 200lbs of extra metal, liquid, and pumps..

 

[classicalgas]

It's easy to forget that the portion of the retail cost of a cell phone that's in the battery is minuscule, and that ( because run time is a big marketing feature) cell phone batteries are worked hard.

With cars, on the other hand, the cost of the battery is a large fraction of the cost of the vehicle, so you want to be nice to that battery...while still charging and discharging fast. The very few electric cars of recent years that didn't use active cooling for the pack have a rep for killing their batteries quickly (before the warranty runs out)

The design decisions in production E-bikes seem to be more like those of a cell phone than a car.... but maybe they shouldn't be.

I read a paper a while back that compared basic phase change (wax) cooling of a lithium pack to un-cooled...the performance improvement with even basic cooling was on the order of 10-15%.

Mayne that's why the best yard tool packs use wax envelopes in the pack?

The heat transfer paths in a typical e-bike pack are atrociously bad, just adding more cells to reduce the heating doesn't do much good if the cells are essentially insulated with air gaps, in an non vented plastic enclosure

 

[neptronix]

Most prebuilt bikes are designed in such a way that they waste 7-11% of the battery's as heat when new. As the IR increases over calendar life and cycles, this percentage increases, and that's one of the ways you get kaboom. But before you get kaboom, you see reduced performance, especially in cold.

There's a lot of batteries on prebuilt bikes that will only last a few years due to this.

This is why i like to overspec the C rate of my batteries by around 4x. This is how you make a 10 year battery pack that never overheats and gives good performance and range for a long time.

No cooling solutions needed.

 

[classicalgas]

This is why i like to overspec the C rate of my batteries by around 4x. This is how you make a 10 year battery pack that never overheats and gives good performance and range for a long time.

Doesn't that mean you're carrying 2X the mass of batteries you really need? (for range)

With batteries improving as fast as they are, I don't see the point in overbuilding a pack in order to get ten years of use out of it. Five, sure....and then replace it with something better and cheaper.

If you can dump any heat (there will always be some heating on charge/discharge) effectively, the batteries will be happier. Why do you think that essentially all electric cars have active battery cooling?

 

[BlueSwordM]

Doesn't that mean you're carrying 2X the mass of batteries you really need? (for range)

With batteries improving as fast as they are, I don't see the point in overbuilding a pack in order to get ten years of use out of it. Five, sure....and then replace it with something better and cheaper.

If you can dump any heat (there will always be some heating on charge/discharge) effectively, the batteries will be happier. Why do you think that essentially all electric cars have active battery cooling?

The answer to the 1st question is that Neptronix either draws less power than rated, or they pick higher power density cells.

As for electric cars, almost all of the modern ones have any form of active cooling; most of them integrate liquid cooling to properly thermally manage their packs, which is particularly important for heating up the pack during fast charging and cooling at the end of the charging cycle and discharging. Does that mean all of them have top tier cooling?

No. In fact, some of them have abysmal performance despite the availability of cooling.

 

[neptronix]

Doesn't that mean you're carrying 2X the mass of batteries you really need? (for range)

Not necessarily. The high C rate penalty isn't that bad these days. In fact it can be nonexistant or close to, with tabless cells.

With batteries improving as fast as they are, I don't see the point in overbuilding a pack in order to get ten years of use out of it. Five, sure....and then replace it with something better and cheaper.

We gained 80whrs/kg in the last 12 years. Just enough to warrant a technological upgrade. Batteries that are available to the DIYer are 280whrs/kg today. Historically, it's a bad bet to assume that the rate of technological advancement would be high, because it never has been.

I think a 10 year horizon is reasonable based on that. I'd also like to create less e-waste.
Another thing you get when you overspec the C rate is resistance to cold, lower voltage drop.

If you can dump any heat (there will always be some heating on charge/discharge) effectively, the batteries will be happier. Why do you think that essentially all electric cars have active battery cooling?

All electric cars have active battery cooling because they follow a range maximizing design at the expense of added maintenance cost, lower efficiency, and higher complexity. I don't agree with this design because the useable lifespan of the battery would be much lower than it could be, due to aging affects on a high IR cell. And you've got other crap to fix down the line.

Ebikes are weight, complexity, and cost minimizing designs. All attempts at selling cooled battery packs have been a failure so far.

Another thing that an ebike doesn't have to deal with that a car does is extreme temperatures exacerbating the problems that come with super large battery packs - IE the cells in the center are the ones cooking - which reduces the thermal headroom your pack has a lot, therefore to get more thermal headroom, you add liquid cooling, just to cool those cells in the center and prevent them from going thermal. Otherwise you need more like an 8x or 10x multiplier on the C rate, which with the technology of the past, would reduce range.

We may see air cooled batteries in cars though when we get tabless technology into cars. It's feasible to make a pack with super low IR, use a fraction of it's output capabilities, and never see enough heat to warrant liquid cooling. I'd very much prefer such a battery.

 

[classicalgas]

If you're using a small enough fraction of the output of a pack that it never heats up, you're carrying around a bigger pack than you need ( Or, if you get the c-rate by specing higher rate cells, you paid more money )

Where does "normal operating temperature" slip into "overheating"?

Why do the big yard tool packs have phase change sleeves and pack cooling vents?

 

[neptronix]

I'm fine with carrying a bit larger pack than i need for longevity, low voltage drop, and resistance to the cold.
I like the absence of battery problems that come with that.

Where does "normal operating temperature" slip into "overheating"?

80C or 176F is where most cells go thermal. So if you're operating the vehicle in 120F, you don't have much headroom for a pack that creates a lot of heat. Your 'cell in the middle' problem gets worse the larger the pack you have.

Why do the big yard tool packs have phase change sleeves and pack cooling vents?

That's an aggressively power to weight ratio optimizing design. It also counters the cell in the middle thermal problem. Does the attached tool draw a crapton of power ( IE above say, a north american 120v outlet worth of power? )? that might be the reason.

 

[BatteryMooch]

80C or 176F is where most cells go thermal. So if you're operating the vehicle in 120F, you don't have much headroom for a pack that creates a lot of heat. Your 'cell in the middle' problem gets worse the larger the pack you have.

For “standard” chemistry li-ion cells 80°C is about where the electrolyte starts to break down and certain exothermic reactions start. But it’s a long way from the roughly 250°C (or even higher) needed to break down the cathode material and start the violent thermal runaway reaction. I’ve brought NMC and NCA cells to over 100°C hundreds of times and 120°C often, all without issues.

I‘m not sure though if you were referring to full runaway or just the start of exothermic reactions when you mentioned “go thermal”.

 

[neptronix]

^-- listen to this guy instead of me, all i know is 80C is entering the danger zone for certain cells.

 

[Pajda]

I have a similar experience as @CamLight

Just some information about cell safety certifications. UN38.3 prescribes heat up the cell to 72 °C and keeping this temperature for 6h and then at -40 °C the same time. UL1642 then prescibes to heat up the cell in the temperature chamber with forced air to 130 °C and keeping this temperature for 10 min.

 

[neptronix]

I am better educated, thanks for your insightful posts, both of you.

 

[classicalgas]

That's an aggressively power to weight ratio optimizing design. It also counters the cell in the middle thermal problem. Does the attached tool draw a crapton of power ( IE above say, a north american 120v outlet worth of power? )? that might be the reason.

See, from my point of view, a yard tool is a good example of an optimized power system, and "aggressive power to weight ratio" is exactly how I think an e-bike should be designed. Being able to easily carry your E-bike up a few stairs matters to a lot of potential users, so does "bicycle" agility and handling.

Cordless yard tool have to achieve a power output that's usefully close to an ICE powered tool to be marketable, they have to be dead reliable and safe under fairly extreme operating conditions, the battery is hot swap capable, easily portable, and available in different capacities ( so you aren't carrying more weight than necessary for a given trip) The batteries have a BMS in each one of them, capable of shutting down the pack under high draw, high temperature, or overload conditions (so in combination with most Ebike controllers you have a redundant battery protection ) and the charger is small and light enough to easily carry with you for mid trip charging. In some cases the BMS is smart enough to tell you what the problem is if it's shutting down.

I think you get a better built design than many E-bike batteries, but you do pay for that (in limited output (20-25A, depending) limited capacity (under ten amp/hours, for most of them ) and cost. You can work around the last issue with careful used purchasing.

From what I've read, it's keeping a battery hot (or fully charged for days) that does the most damage. Phase change cooling increases conduction paths, drops peak mid pack temperatures, and moderates temperature spikes.