a cost benefit comparison of lifepo4 and lipo for newbs

[neptronix]

The lipo chemistry I used is LG lipo used in cell phones. I'm not sure of the raiting, but across the board lipo and li-ion show pretty significant capacity fade at 800 cycles.

Those LG lipos did fantastic at 800 cycles. i CANNOT WAIT for a Chevy Volt new Chevy eAssist car like the Hyundai Lacrosse, or other various new GM/Hyundai hybrid to find itself in a crash and have the battery pack sellin' on eBay.

Those cells are drool worthy if they really last over 2000 cycles. HK Lipos will be kicked to the curb if we can get our hands on those.

 

[neptronix]

If all the cells stayed perfectly in balance it would work. That's a big if though. As cells age discrepancies develop between even well matched cells. Even if they appear balanced at 3.4v some of the cells might reach 4.2v before the others and continue to climb until they damage themselves. I think it's a bit silly that he wouldn't use the balancing tap if you had the option.

You could do things like hook up the balance tap to a cell-monitor and set an over voltage alarm. Then you'd have to be near it while it charges to disconnect it when the first cell hits 4.2v

It's possible to do these type of things, but it's a pain and can be risky if your not absolutely sure of what you're doing.

Here is my pack after 25 cycles of non-balanced charging, at it's storage voltage:

Here is the same pack, fully charged to 4.15v per cell, and left to rest for a few minutes:

Keep in mind this is a worse case scenario. Discharge has been at 4C-8C, getting the batteries warm. I've taken them down to 3.5v near the cliff more than once. They get warm when i hill climb. They are not in a parallel pack so if one cell gets out of balance, it is not corrected by other cells charging it.

I'm betting i can run for another 25-75 cycles without needing to balance these. I check the balance with the cell meter from time to time just in case.

I don't know where you get this idea that LIPO is a nightmare to live with. If you start out with packs that are known to be good, and check up on it from time to time, it's pretty easy to live with.

 

[auraslip]

Those LG lipos did fantastic at 800 cycles. i CANNOT WAIT for a Chevy Volt new Chevy eAssist car like the Hyundai Lacrosse, or other various new GM/Hyundai hybrid to find itself in a crash and have the battery pack sellin' on eBay.

Those cells are drool worthy if they really last over 2000 cycles. HK Lipos will be kicked to the curb if we can get our hands on those.

As been said here many times, the BMS in the volt will be limited to only use %65 of the batteries capacity

The Volt's 375 lb (170 kg), 220-cell lithium-ion battery (Li-ion) pack is anticipated to store 16 kW·h of energy,[1][69] but will be restricted (in software) to use only 10.4 kW·h of this capacity to maximize the life of the pack. It will only be allowed to charge to 90% of full capacity and to discharge only to approximately 25% SoC before the engine cuts in and maintains the charge near the lower level. When the vehicle is plugged into a charger the battery SoC is restored to 90%.

As far as the lg losing %20 of it's capacity in 800 cycles...that is far from fantastic when you compare to a123 or even ping.

I don't know where you get this idea that LIPO is a nightmare to live with. If you start out with packs that are known to be good, and check up on it from time to time, it's pretty easy to live with.

From the people that use the cells daily on high powered bikes and discharge them down low. There is a reason everyone talks about how great the Kokams stay balanced even when discharged down to %100 DOD.
You otoh, use them on a 36v bike. I imagine you don't ride daily, and when you do I'm sure you don't often ride them to %80 DOD. Take your batteries down to %80 DOD a few cycles, and then we'll see how well they stay balanced.

 

[neptronix]

As been said here many times, the BMS in the volt will be limited to only use %65 of the batteries capacity

The Volt's 375 lb (170 kg), 220-cell lithium-ion battery (Li-ion) pack is anticipated to store 16 kW·h of energy,[1][69] but will be restricted (in software) to use only 10.4 kW·h of this capacity to maximize the life of the pack. It will only be allowed to charge to 90% of full capacity and to discharge only to approximately 25% SoC before the engine cuts in and maintains the charge near the lower level. When the vehicle is plugged into a charger the battery SoC is restored to 90%.

That BMS would be thrown in the trash pretty quick. I think you noted that at 80% discharge, cycle life looked like it would head into the multi thousands. I'd use the LG polymer then, for sure.

I don't know where you get this idea that LIPO is a nightmare to live with. If you start out with packs that are known to be good, and check up on it from time to time, it's pretty easy to live with.

From the people that use the cells daily on high powered bikes and discharge them down low. There is a reason everyone talks about how great the Kokams stay balanced even when discharged down to %100 DOD.
You otoh, use them on a 36v bike. I imagine you don't ride daily, and when you do I'm sure you don't often ride them to %80 DOD. Take your batteries down to %80 DOD a few cycles, and then we'll see how well they stay balanced.

Yeah, kokams are basically high quality lithium polymers. I believe that konions are also
They will drain to whatever they consider 100% no problem as their cells are much better matched in mAH than turnigy/zippy, and the manufacturing process is probably much better too.

This pack i'm talking about has been down to 0-10% remaining a few times by now with no balance issues.
As quick as the voltage is drained from a cell, it comes back in at the same rate. Look at some discharge & charge curves if you don't believe me; better yet, use your cellog 8's monitoring function via USB if you want to discover this on your own.

Disbalance is what occurs as the cells start aging at different rates, or if a balancing charger / BMS is screwing up the balance. It is not really dependent on how they are charged or discharged, unless you are somehow massively overcharging or massively discharging one cell in specific.

Lipos don't really get disbalanced, and discharging under 3.6v is a bad idea anyway due to the variances in mAH that we get from the HK batteries.

So what you do with lipo is assume that there will be some variance and add a safety margin around it.

So the problems you see from lipo are really just due to the quality of HK stuff, and people not working around that margin of error. It is a good value in my eyes even considering the occasional dud.

No i don't ride daily. I will be very soon, though as the weather is vastly improving. My 20AH pack is ready to roll.

 

[vanilla ice]

I will contest this thought because it is the most misleading, here is my charging procedure:

Dang three steps? My $5 chargers have only one step. plug in JSTs!

BTW I use LiFe LiPo and SLA.. and I like all three of them. I, same as wine, don't see why it has to be an either or thing. Use both. Use everything.. use each for what they good for.

 

[auraslip]

BTW I use LiFe LiPo and SLA.. and I like all three of them. I, same as wine, don't see why it has to be an either or thing. Use both. Use everything.. use each for what they good for.

Sorry if it came off as either or --- I was writing this with beginners in mind.

That BMS would be thrown in the trash pretty quick. I think you noted that at 80% discharge, cycle life looked like it would head into the multi thousands. I'd use the LG polymer then, for sure.

What? What I actually said was that lipo discharged below %80 has drastically reduced lifespan. Seriously, google scholar. It's not that hard!

 

[liveforphysics]

1) Check the balance status of the brick with a balance charger, and now let's assume I see all six cells are at about 3.4 V
2) seeing no real balance issue, I hook the brick up to charge on a current-limited meanwell power supply set to 25.2 V and maxes out at 20 A
3) leave the thing on, since once the cells reach 25.2 V total (4.2 V/cell), charging current should drop to zero
4) take away the charger, check balance, and go

I could deal with this situation, since I'm considering a setup of trying to charge nine 6S packs in parallel, and I don't feel like getting three of those 4 x 6S smart chargers, or having to use one smart charger three separate times. I could just keep an eye out for imbalance before charging, and if I see it, I use my one and only smart charger where needed.

How's that sound? Am I a dead man?

Yes. This is exactly how Methods, Hyena, Gary, myself, and many others charge. We call it bulk charging.

I haven't used an RC charger with LiPo for about year (unless charging a little RC pack for a helicopter, or balancing or measuring capacity or something).

I just have a single 2 wire connector (a deans ultra). I get home, I plug it in, then flip on the power strip that the power supplies are plugged into. That's it. The whole charging process. Plug in 1 connector, turn on the power supplies. I've got a few hundred perfect cycles this way, Methods must be coming up on a thousand by now.


Also, as far as life-cycle for LiPo goes, the LG presentation I sat through yesterday showed 50,000 cycles at 40% DOD, 40,000 cycles at 50% DOD, then a big drop to 20,000cycles for 60% DOD, and something like 7-8,000 at 70%.


For myself, everytime I want to make a new ebike (or electric anything for that matter), I just spend a few hundred on LiPo, spend an hour or two assembling, and I'm done with the battery, and I have the best performance, power, weight, volume of anything.

Cheap. Fast. Simple. Powerful. Light. And if you don't know what you're doing, you can burn your house down or get KFD.

 

[auraslip]

Also, as far as life-cycle for LiPo goes, the LG presentation I sat through yesterday showed 50,000 cycles at 40% DOD, 40,000 cycles at 50% DOD, then a big drop to 20,000cycles for 60% DOD, and something like 7-8,000 at 70%.

Was that charging to %100 SOC? What about life cycles at %80 DOD or lower? Do they have a website I can look at the powerpoint? At that shallow of DOD almost all lithium chemistries would react the same way in ideal conditions. I'm assuming this is some new formulation not even on the market that's designed for long life cycles in full sized EVs and not for hobby packs in R/C toys. A hundred cool points for getting to sit through a presentation by LG, but minus ten million for being misleading to make a point.

Cheap. Fast. Simple. Powerful. Light. And if you don't know what you're doing, you can burn your house down or get KFD

You make it seem so easy until the last line

 

[JCG]

BTW I use LiFe LiPo and SLA.. and I like all three of them. I, same as wine, don't see why it has to be an either or thing.

Yes! All things in moderation - very good.

Thanks for the advice you guys. Very useful video, neptronix! Now I can see the way the paralleling is done very clearly (the balance paralleling is what I needed to see). Perfect.

 

[neptronix]

What? What I actually said was that lipo discharged below %80 has drastically reduced lifespan. Seriously, google scholar. It's not that hard!

Any tests of what happens if you discharge below that %? also voltage is considered to be 100% drained? 3.0v?
I don't think 5 or 10% is going to make a huge difference.

Check these out:

http://www.dowkokam.com/tech-cells.htm

High cycle life (2,000 cycles at 80% discharge)

The LG Chem lipo is weird; it they state it has a 2C discharge rating ( ??? ) Their specs for the large cells are from 2004, that doesn't seem right. So i don't know what the lifetime of those is.

http://lgchem.com/

I think what they're listing is their old 1st gen lipo :/

 

[Partycat]

I understand from reading here that most of this seems to have become geared towards a debate between hobbiest regulars, on the forums or otherwise. I've been reading all over the place in the last few weeks and have randomly decided to throw a bunch of money into not only repairing my bicycle, but, electrifying it at the same time. I'm absolutely what you'd throw under "n00b".

For some people you're going to be looking at cost comparison and penny pinching. When you're talking about a couple of hundred for hundreds, if not thousands, of cycles, I don't think you'd have any good reason to pick the lower cycle power source unless you really just don't have the money. Myself, I'm looking at it where I'm going to get a ROI on fun , and not driving my huge 8 cylinder car.

I'm not sure what the audience is that you're going for here. If you've got $2k to drop on an e-Bike, then you're going to choose the best parts for the application, and penny pinching isn't going to help. Either you're out of power, or, supposedly, you're on fire.

Looking at liveforphysics' video on youtube, these cells *shouldn't* burn your house down unless you overcharge them, yet, I see reports of people saying that the batteries just burst into flames with no provocation. That's rare, admittedly, but, if it burns down my apartment even occasionally, that's far too often.

I've been looking at things like the 48v20ah Ping battery, thinking I'll get along just fine at 30A peak, and I probably will. But, what if I want that power in reserve? What if I don't want to strap 15+ lbs of battery brick to my bike somewhere? I have a stupid Haro frame that has almost no workable 'triangle' area, so I'm out of luck.

I'm battery shopping and these graphs and such don't really help as much as all the difference in time and effort does. LiFePO4 is being cast as a "set it and forget it" sort of implement, and LiPo (Cobalt, primarily, from what I'm reading) is the performance and tweaker's cell. If I'm commuting to work, 15 miles each way, I don't want to plug something in, in my office, that might just perhaps burst into flames and burn it down, if I don't stare at it and have a fire extinguisher under my desk. That's how LiPo comes off to me. So the time and preparation elements are well worth throwing a few hundred at Ping instead of trying to plan my daily commute around some undefined time of staring at a pile of chargers and battery packs to make sure I 'm not dead or lose my job.

I really think you want to talk about more in your cost/benefit other than cell value and life. As already mentioned elsewhere, information is so scattered, and, for whatever reason, images just don't show up half the time, so for someone new to the hobby it just does not make sense to throw even a minimum investment into LiPo to see if it will work out, without someone outlining that "this is what you will have to do for daily usage assuming you'll have to charge somewhere on the road along the way (work etc) or just flat out saying, it's not for you. I've been in a number of weird hobbies and when I'm seeing that you're buying battery packs for R/C cars in quantities being described in terms like 'piles' or 'mountains' , bulk charging with random eBay chargers, regularly ordering nebulously described parts from China, I can tell you that the average layperson isn't going to do it, and the information here just isn't coherant enough to attract what I would consider to be middle-of-the-road persons who don't have a career in this field.

My 2c, and, note, I'm definately getting into this. Be it a ping and a run-of-the-mill travel bike, or something that, to paraphrase Methods, can zip through an intersection when you need it to, I'm going to end up paying my way through the learning experience, but, I can just tell you from trying to read and get the information I need, without just flat-out asking every question, it just isn't a clear picture.

 

[auraslip]

I really think you want to talk about more in your cost/benefit other than cell value and life. As already mentioned elsewhere, information is so scattered, and, for whatever reason, images just don't show up half the time, so for someone new to the hobby it just does not make sense to throw even a minimum investment into LiPo to see if it will work out, without someone outlining that "this is what you will have to do for daily usage assuming you'll have to charge somewhere on the road along the way (work etc) or just flat out saying, it's not for you. I've been in a number of weird hobbies and when I'm seeing that you're buying battery packs for R/C cars in quantities being described in terms like 'piles' or 'mountains' , bulk charging with random eBay chargers, regularly ordering nebulously described parts from China, I can tell you that the average layperson isn't going to do it, and the information here just isn't coherant enough to attract what I would consider to be middle-of-the-road persons who don't have a career in this field.

The pics are broken since a few weeks ago when the server crashed. We really do need some videos and pic threads of lipo charging setups, and I nominate NEP since he seems like the most likely to actually do it.

That's rare, admittedly, but, if it burns down my apartment even occasionally, that's far too often.

:lol: :lol: :lol:
Funny man. welcome to the server. the way I hear it is that you really need to try to make them catch fire now days.

My 2c, and, note, I'm definately getting into this. Be it a ping and a run-of-the-mill travel bike, or something that, to paraphrase Methods, can zip through an intersection when you need it to, I'm going to end up paying my way through the learning experience, but, I can just tell you from trying to read and get the information I need, without just flat-out asking every question, it just isn't a clear picture.

If you have the money, and your commute is only 15 miles than an a123 pack from cell_man would be a much better value than ping. Provided you could charge at work that is... I'd really go for a 20ah pack just for head room.

 

[SamTexas]

My thanks to you, auraslip. Very detailed and balanced comparison.

 

[Partycat]

That's rare, admittedly, but, if it burns down my apartment even occasionally, that's far too often.

:lol: :lol: :lol:
Funny man. welcome to the server. the way I hear it is that you really need to try to make them catch fire now days.

I'm going off the uneducated's view of laptop fires and the claims that if you damage the cells, you're in for it. I'm not particularly worried about roasted nuts, but moreso the unattended charging and an issue. From what I gather overall, if the cells are balanced, a bulk charge won't cause death. Would I still feel comfortable commuting on these things? Probably not, at least, initially.

If you have the money, and your commute is only 15 miles than an a123 pack from cell_man would be a much better value than ping. Provided you could charge at work that is... I'd really go for a 20ah pack just for head room.

Okay, I guess, I'll check it out. I have $ but not to buy 3 or 4 batteries. Thundersky/B-whatever are large. LiPo is small but has additional considerations. LiFePO4 is either requiring construction of a cell (in various difficulties depending on Headway or loose cells) or could be pre-assembled, but at a low C for discharge. There's not real happy medium here. I'm going to go with the 20Ah just because I never really will know where I am going, and getting boned with a heavy or hard-to-pedal bike with miles to go is no fun. Ping ? Well, again, depending on what you read, he's a good value for up to 2C. Some say 2C is peak and that means not on a regular basis. LiPo is huge as far as current but, can I plug in and go do something else? Probably. But even if I put it in a metal cabinet and walk away, and smoke comes out and sets off some thing, well, I'll have some explaining to do, about why there was smoke, and why I was "stealing" electricity. I was hoping a Ping , while not powerful, would let me get away with it, with the best odds. Then, I would read up on 'booster packs' and other such things, to play around with LiPo and a RC charger, to see how it was to manage. You can always swap out the battery for something else, and having a backup, given that you have the funds, why not?

 

[neptronix]

The pics are broken since a few weeks ago when the server crashed. We really do need some videos and pic threads of lipo charging setups, and I nominate NEP since he seems like the most likely to actually do it.

Actually i plan on doing this. Arguing with you has been a great motivator to take pics and explain my setup, thanks auraslip :lol: 8)

That's rare, admittedly, but, if it burns down my apartment even occasionally, that's far too often.

:lol: :lol: :lol:
Funny man. welcome to the server. the way I hear it is that you really need to try to make them catch fire now days.

If you have the money, and your commute is only 15 miles than an a123 pack from cell_man would be a much better value than ping. Provided you could charge at work that is... I'd really go for a 20ah pack just for head room.

Yeah if you are traveling 15 miles and can charge at work, and don't have any mega hills on the way, and don't mind pedaling, 10AH would actually work quite well. It would be under 10lbs and could fit on a rear rack without breaking it.
Add to the fact that cost is not an issue, and i would say that's probably the perfect battery for you.

Welcome to the forums also.

 

[x88x]

LiFePO4 is being cast as a "set it and forget it" sort of implement, and LiPo (Cobalt, primarily, from what I'm reading) is the performance and tweaker's cell.

I think that's a big bit of it. If you don't want to worry about it, and cost is not an issue (and, well, let's be honest...if you're already dropping $500+ on the battery pack alone, cost is not a huge contributing factor), A123 is the way I would go. The only current reliable source I know of that's anything approaching a feasible price is cell_man; PM him on the forums or check out his website. If those are too expensive, Ping seems to be the way to go. If you want maximum power in minimum weight and volume, LiPo is where it's at. LiPo also makes it easier to re-configure your pack if you want to, and if you're building a really big pack (like the one I'm planning for my electric motorcycle conversion), it does offer a big price difference (~$4,140 for cellman A123 vs ~$2,852 for HK LiPo, with my current estimate of a 5.18kWh pack with the LiPo hitting 5.18kWh @ 80% DOD). With the size pack that you deal with for ebikes, though, the price difference is much less pronounced, especially if it's your first purchase.

The danger of LiPo is, I think, greatly exaggerated by some. From my limited experience (hardly worth mentioning at this point...haven't really done anything with my LiPo yet except charge it), but more so from the writings of much more experienced members (liveforphysics is one of our resident LiPo experts...well, battery in general, really), as long as you treat it right, there's not really anything to fear from LiPo at the discharge rates we deal with.

 

[liveforphysics]

My thanks to you, auraslip. Very detailed and balanced comparison.

LOL

(assuming you're being sarcastic)

 

[wesnewell]

My 2c, and, note, I'm definately getting into this. Be it a ping and a run-of-the-mill travel bike, or something that, to paraphrase Methods, can zip through an intersection when you need it to, I'm going to end up paying my way through the learning experience, but, I can just tell you from trying to read and get the information I need, without just flat-out asking every question, it just isn't a clear picture.

People want different things from their batteries. A battery pack that might take one person 5 miles may take another 20 miles. If you want to go 20 miles without pedaling, then get at least a 20ah pack. If you want to average 20mph then you might even need a 30ah pack. Like an ICE, it depends on how heavy you are on the throttle as to the mileage you get. That's why there aren't any real clear answers. each persons wants/needs can be different. I've only got a 5ah pack and I rode 20 miles just using half of it. And then again I rode 9 miles using 40% of it because of wind, hills, going faster, etc. Once you get to the 20ah battery, C rate shouldn't mater for most people, so take your choice.

 

[sangesf]

How about this...
I need two 36v 15AH battery packs..
Give me a cost analysis and charging routine for as cheap as possible (while still having "decent" equipment) using both or either chemistries..
My costs needs to be at about $700usd...
My current lifepo4 runs typically at 39v for 85% of the usage and I NEED the ability to charge "on the road" and almost never at home..
I run two v-power 36v 15ah batts/bms/chargers that have been in use for 6 months with no problems whatsoever... At a cost of $600 (yes.. eBay auctions CAN rock)
My controllers have regen and never use more than 25a and typically run at 8-10a.

Is there a lipo setup that would fulfill ALL my needs?

What do you guys think?

 

[dogman dan]

Sounds like you have your need satisfied. Great that you got lucky on ebay.

 

[BikeFanatic]

I use both PING Lifepo and LIPO, Can't say it any better than Dogman. Note to Noobs, Dogman has ALOT of experience and is very wise.

LIPO can be a good intro pack for noobs -Especially someone who only needs 2 X 6S 5AH to get started and play in the neighborhood. Some one afraid to invest 700 in a battery (new to them technology ).

The other problem with EBAY, Alibaba and LIFEPO from an unknown vendor is reliability. If you do not get your lifepo from Ping or Cellman, and it is not well put togther or cells die, you are out 500-700$ .
I know this from experience.

 

[neptronix]

How about this...
I need two 36v 15AH battery packs..
Give me a cost analysis and charging routine for as cheap as possible (while still having "decent" equipment) using both or either chemistries..
My costs needs to be at about $700usd...
My current lifepo4 runs typically at 39v for 85% of the usage and I NEED the ability to charge "on the road" and almost never at home..
I run two v-power 36v 15ah batts/bms/chargers that have been in use for 6 months with no problems whatsoever... At a cost of $600 (yes.. eBay auctions CAN rock)
My controllers have regen and never use more than 25a and typically run at 8-10a.

Is there a lipo setup that would fulfill ALL my needs?
What do you guys think?

30ah of 10S will come close to that $700 but exceed it

12x $42 5ah 5S 20c zippy lipo shipped via USA warehouse: ~$516.01
Charging stuff & various doodads = $200-$250

$300 is mind blowingly cheap for 36v 15ah lifepo4. You should have bought a carton and resold them for a profit!!!

Occasionally hobbyking will have sales on their lipo though. I have seen up to 28% off the packs i used.

 

[wineboyrider]

I use both PING Lifepo and LIPO, Can't say it any better than Dogman. Note to Noobs, Dogman has ALOT of experience and is very wise.

Yep. Listen to the Dogman most of the time, especially if you are a newb. Dogman tells it like he sees it and does it. I have learned a lot of things the hard way on this forum (but I have had a hell of time doing it ). The "gurus" know their stuff, but it takes many people like me without any electrical training a lot longer to grasp some of the concepts most especially batteries. Some people want to commute to work and some people want a bicycle/motorcycle that goes 80mph! I started out, because I lost my license and wanted to commute to work and now I have 5 ebikes and tons of batteries.

 

[wesnewell]

How about this...
I need two 36v 15AH battery packs..
Give me a cost analysis and charging routine for as cheap as possible (while still having "decent" equipment) using both or either chemistries..
My costs needs to be at about $700usd...
My current lifepo4 runs typically at 39v for 85% of the usage and I NEED the ability to charge "on the road" and almost never at home..
I run two v-power 36v 15ah batts/bms/chargers that have been in use for 6 months with no problems whatsoever... At a cost of $600 (yes.. eBay auctions CAN rock)
My controllers have regen and never use more than 25a and typically run at 8-10a.

Is there a lipo setup that would fulfill ALL my needs?/quote]

Yes, it could be done for under $700. 12 5000mah 5s 15C lipos would be $500. Configure as 2 10s/3p packs for 15ah 42V each. Charging on the road isn't a problem. There's several ways to do it. Cheapest would probably be an AC/DC lipo charger and charge each pack in parallel with all 6 batteries at once. Don't know how long it would take to charge like this, but probably several hours. Those chargers can be had for ~$40. Might want a couple of them. I've actually charged my 14s lipo pack with the SLA smart charger that came with my motor kit and it worked fine, but also isn't real fast There may be better/easier/faster ways, but I haven't looked for them since i don't need to charge on the road. If I did, I wouldn't have a problem using my SLA charger in a pinch, but wouldn't go over an 80-90% charge to be on the safe side.

 

[liveforphysics]

I charge on the road with power supplies.

They come in many flavors, but I always just charge at the limit a standard 15amp wall outlet breaker can sustain, which is around 11-12amps, which is roughly 1000-1200w charging.

Tiny. Compact. Light. Cheap. And unlike little toy chargers that come with an off-the-shelf pack, these actually charge at a power level that let's you add meaningful watt-hours back into the pack in a little 30min break for a snack or drink.