what is a safe limit for battery temperature during use?

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veloman   1 GW

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what is a safe limit for battery temperature during use?

Post by veloman » Jun 29 2013 11:12pm

I have some 18650 low c rate cells that got up to 115 f. They were 105 before starting the ride due to being parked in central TX weather. I know the lifespan is reduced by the higher temps, but what sort of limits are best?
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Re: what is a safe limit for battery temperature during use?

Post by neptronix » Jun 30 2013 12:30am

115f measured on the outside is pretty hot. I read somewhere that 140-160f is where some internal chemicals start to decompose.

Parallel some more ah onto those bad boys, stat. A real ideal situation for a battery is for it to never rise above ambient.. especially at >100f!
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Re: what is a safe limit for battery temperature during use?

Post by dogman dan » Jun 30 2013 6:37am

I'm fairly sure hot weather is part of why I never quite got 4 years from a ping. 120F in my garage all summer.

But what are you going to do about it out on the road? A light colored box beats a black one, but other than that what can you do if you have to park in the sun?

I do see heating up a lot past 100F as a problem, requiring less load or more ah. So yeah, getting above 120F in use would be a good way to accelerate wear. On the other hand, when it's above 100, you really feel the batteries perk up. I wouldn't want to chill the batteries below 80f, that's for sure.

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Re: what is a safe limit for battery temperature during use?

Post by friendly1uk » Jun 30 2013 8:20am

dogman wrote: what can you do if you have to park in the sun?
Aluminium foil is a good heat shield. It's around 95% reflective. Often thin polystyrene sheet is laminated with it to go behind radiators. Sometimes on car windows while parked. NASA developed mylar sheeting which is better still, and often used for emergency poncho's or them shiny things runners get at the finish line. It's main use seems to be indoor growing though, So it is best not to wave it around, unless you want following home.
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Re: what is a safe limit for battery temperature during use?

Post by dogman dan » Jun 30 2013 8:49am

Yeah, beats a black bag. But ambient was kinda cool the other day in my town. Only 107 in the shade. :roll:

Anything in the actual sun can very quickly reach 140, 160F. I dog dare you to touch aluminum that's been in the sun barehanded. Hell, it's been almost 120 in the shade here in the west lately. Once you roll, a sun soaked surface cools down to just ambient, only 105 or so for me, at higher altitude. There's a really good reason few of my bikes have a black seat.

That's what I meant by, "what can you do?" You can start out with a cool battery, room temp of 80F or so. That helps. But if you park and can't do that in shade, it's going to get real hot in no time. Once the center of your battery equalizes with ambient at 105, it won't cool for hours with all that mass.

This is the climate where Allcells system might work really good.

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Re: what is a safe limit for battery temperature during use?

Post by melodious » Jun 30 2013 9:17am

The mylar I have experience with. Not on bikes, but on (ahem) other stuff I used to do in the past. I couldn't tell you about it's heat shedding properties, but it is very reflective. Reflective enough akin to a glass mirror. I wouldn't put it on a bike, as it would shine directly into peoples' eyes.
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Re: what is a safe limit for battery temperature during use?

Post by wb9k » Jun 30 2013 12:10pm

For A123 cells, 65C (149F) is considered max safe operating temp. Electrolyte can start to break down above that, and 85C is the highest temp a cell should ever see. Like Dogman says, batteries perform better at these high temps (because cell impedance is at a minimum), but this comes at the expense of calendar life. Note that electrolyte breakdown is not the big culprit in accelerated capacity loss/impedance growth with high temps--it's the loss of cyclable Li into the SEI layer. This is a very slow-motion chemical reaction that accelerates as temperature rises. I can't speak to what is "typical" for Li cells, but for A123 cells, this reaction is completely arrested below about 10C (50F). In other words, you can totally prevent capacity loss in storage of A123 cells by storing them at 50 degrees F or cooler. Cold is not a problem (meaning damage of risk to the cell) until you get to about -20C (-4F) and you try to charge too fast--this plates Li.

So, in hot climates like yours, batteries give better day to day performance, but have a shorter overall lifespan due to accelerated capacity loss and impedance growth. Lifespan penalty should not exceed 20% or so in a worst-scenario provided you stay withing the absolute ratings.

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veloman   1 GW

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Re: what is a safe limit for battery temperature during use?

Post by veloman » Jun 30 2013 4:15pm

I will clarify.
I have a temp sensor inside the 18650 pack. They are wedged in the frame of my gf's ebike with just a vinyl flap to protect from light rain. The bike parking is probably 60% shaded through tree cover. I could add some more reflective material to help keep the sun off it. The issue is that the 105 degrees is what the cells will start at for the ride home.

Ambient temperature is 105 degrees F. It's central TX. The bike is parked outside all day while she is at work. It would be possible to reconfigure the battery to be removable but it still will take some work each time it's taken on/off.

Aside from using high quality cells like my a123, there isn't much you can do about keeping them from generating heat. They are cheap 18650s, run at 20amps peak or ~1.3c rate. One pack is 24v - the currie 10ah lithium, the other is laptop cells off ebay 11.1v 16ah.

In these hot climates, it almost seems better to build a battery with insulation, if the ride distance is short.

It sounds like I shouldn't worry at 115 F. Lifespan will be reduced a little, but she doesn't ride more than 100 days a year. I just didn't want the cells to be a fire risk. Yes, I've read the laptop cell thread where some members say they are more dangerous than lipo. But that's why I put the temp sensor in for her to watch. If they go over 60 C, then it might time to rethink using them and keep the bike outside.
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Re: what is a safe limit for battery temperature during use?

Post by wb9k » Jun 30 2013 5:21pm

Your clarification is very helpful....and sorry if I'm being obtuse, I am new around here.

There are a bunch of options really, and they would probably be worth doing given how much of your cells' lives will be spent in high heat if you do nothing.

I would definately pursue the idea of making the pack detachable and taking it indoors whenever possible in hot weather. This is probably the biggest bang-for-buck thing you can do, and the effectiveness of most other things you might want to try will be greatly amplified if the pack is detachable and kept in an air-conditioned environment.

Other things you might want to try (most of which have already been suggested):

--Add heat sinking and/or a DC muffin fan for enhanced air cooling
--Add more capacity to the pack
--More esoteric cooling methods like cold packs or solid state DC heat exchangers

Don't know if I'm being helpful, but there's my 2 cents.

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Re: what is a safe limit for battery temperature during use?

Post by neptronix » Jun 30 2013 7:05pm

dogman wrote:This is the climate where Allcells system might work really good.
What the thermal mass in the all cell system does is buy you time. And you need time with a battery that will shed about 5-7% of it's power as heat.

If you left an allcell battery outside at 100f, the waxy stuff would be at 100f and would not be nearly as useful. in a way, it is sort of a heatsink..
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Re: what is a safe limit for battery temperature during use?

Post by jag » Jul 01 2013 4:42pm

Hi dh,

Thanks for your good advice. I would like to ask what you consider "charge too fast" at low temp to be?
wb9k wrote: Cold is not a problem (meaning damage of risk to the cell) until you get to about -20C (-4F) and you try to charge too fast--this plates Li.
dh
To be more specific, I have a 16s pack of 20Ah A123 on my eBike. For overnight charging I use a small 1.8A charger in the garage. Is charging with c/10 all right at -20C? How about -25C or -30C? (If it gets too cold I could bring the bike in, but it is a bit of a hassle to get up the stairs and my wife doesn't like it sitting inside)

Martin (Canada)

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Re: what is a safe limit for battery temperature during use?

Post by dogman dan » Jul 01 2013 5:07pm

True, starting at 105, you are kind of pooched from the get go, if you want to keep the pack below 100.

But if you could bring the pack inside to keep it at 80F, then the allcell heatsink would help as you rode.

But back to the problem at hand, one of the best things you could do would be to limit amps even more in the hottest weather. Lower the c rate, and they won't get warm discharging as much.

But I wouldn't fret so much till it got really hot. 115 F wouldn't scare me. 140 might though.

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Re: what is a safe limit for battery temperature during use?

Post by wb9k » Jul 01 2013 8:25pm

jag wrote:Hi dh,

Thanks for your good advice. I would like to ask what you consider "charge too fast" at low temp to be?
wb9k wrote: Cold is not a problem (meaning damage of risk to the cell) until you get to about -20C (-4F) and you try to charge too fast--this plates Li.
dh
To be more specific, I have a 16s pack of 20Ah A123 on my eBike. For overnight charging I use a small 1.8A charger in the garage. Is charging with c/10 all right at -20C? How about -25C or -30C? (If it gets too cold I could bring the bike in, but it is a bit of a hassle to get up the stairs and my wife doesn't like it sitting inside)

Martin (Canada)
Hi Martin,
up to c/2 is fine at -20. c/10 is OK at -30, but only up to 60% SOC, above which you should not charge with constant current at all. I would keep the cells indoors at night with temps like that, if for no other reason than they will perform better if you don't let them get that cold.

dh
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2008 Prius with Hymotion L5 PCM (passed on to daughter)
2011 Rose Xing Fu 48V 500 Watt commuter scooter (now departed)
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Under construction: 2008 Miles ZX40ST truck with altered DC drivetrain
Departed: 2002 VW Jetta TDI which was run for a time partly on WVO.
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Re: what is a safe limit for battery temperature during use?

Post by friendly1uk » Jul 02 2013 6:23am

The oz890 based smart bms has a temp probe. The oz890 chip actually supports two. I don't know how it works, but I suspect currant limitation would work for discharge not just charging. It's not the answer alone, but could be helpful.
It's so easy to forget peoples weather is different to mine. With 105 start temps one could get really silly with both insulation and peltier heat pumps. You need a mini fridge :)
bmsbattery sent me broken and incorrect stuff, and won't even talk to me about it.

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Re: what is a safe limit for battery temperature during use?

Post by wb9k » Jul 02 2013 7:05am

friendly1uk wrote:The oz890 based smart bms has a temp probe. The oz890 chip actually supports two. I don't know how it works, but I suspect currant limitation would work for discharge not just charging. It's not the answer alone, but could be helpful.
It's so easy to forget peoples weather is different to mine. With 105 start temps one could get really silly with both insulation and peltier heat pumps. You need a mini fridge :)
Any really decent BMS system will monitor both cell temps (at least 1 cell in a small pack) and air temps inside any enclosure.

The kinds of features you call "silly" are daily reality in the automotive sector with this stuff. Liquid cooling has rather quickly become the standard for a variety of reasons, though methodology has varied considerably. New materials are making other schemes possible. There was an article on a phase-changing material (a gel at RT) in the previous issue of Charged magazine. I can see that kind of thing being ideal for these bike packs, but you'd still want to bring the thing indoors when not in use and outdoor temps are extreme.

Anyone here reading Charged? (Edit:slightly tweaked for clarity)

dh
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2013 Chevy Volt
2009 Prius with Hymotion L5 PCM
2008 Prius with Hymotion L5 PCM (passed on to daughter)
2011 Rose Xing Fu 48V 500 Watt commuter scooter (now departed)
1984 Honda Interceptor 48V electric conversion motorcycle
Under construction: 2008 Miles ZX40ST truck with altered DC drivetrain
Departed: 2002 VW Jetta TDI which was run for a time partly on WVO.
Amateur radio station run entirely on solar/battery power.
"You can tell I'm a troublemaker, because I plug in my car."

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