Lipo with its clothes off

[knoxie]

Hello

Well I pried a point-1 pack open in order to pop the battery pack and BMS in to a nice case with a switch and a fuse and a charging socket all built in, here you can see the pack split open, the space at the end is to allow the cells to expand (its polystyrene foam ), will post a picture of the box later.

The main bundle of cells (37V 15AH) are packed in beeswax for some reason? anyway we shall see how it goes, you can see the BMS cell monitoring wires coming from each one, the bms will be mounted on top of the cell behind an aluminium plate.

If we can get these packs sorted nicely Team Hybrid will be offering them as standard, I am not sure there is anyone else offering a high capacity lipo pack in a small frame mounted box with switch, fuse and charging socket? yet! ha ha.

Cheers

Paul

 

[BrendaEM]

The wax potting immobilizes the batteries, and binds them in one thermal mass, and perhaps to the sides of the case. The good thing about wax is that it can be removed more easily than epoxy.

Perhaps they thought they might have to repair a pack at some point, they could just melt it out. Wax is probably cheaper than epoxy anyway. I wouldn't be surprised if the batteries operate warm enough to soften the wax.

I probably don't have to remind you to be careful in there, but sometimes the obvious eludes the genius. If you short a cell, it will have a way of letting you know.

 

[knoxie]

Hi Brenda

Yes Being very careful here, there are some more photos of the case, It may have to go in the next size up, there is room for cell expansion in this case, it would be nicer if there was more, these cells run very cool under load and have a low resistance, I have never felt the packs get warm.

Some more shots here

Cheers

Knoxie

 

[Ypedal]

Very cool !!!..

Any markings on the cells to be seen in there ? I beleive Point1 is a distributor.. i'm curious to know who's cells they use..

 

[knoxie]

Hi

Not sure of the cells, I will pick them apart and have a look for you! I will show you all the detailed close up picks when i get time.

Cheers

Knoxie

 

[xyster]

sometimes the obvious eludes the genius

:lol: Great quote -- that's one for the "Best Of ES" thread, if ever there is one.

 

[BrendaEM]

sometimes the obvious eludes the genius

:lol: Great quote -- that's one for the "Best Of ES" thread, if ever there is one.

I got it from a co-worker of mine, named Ted.

 

[fechter]

That wax is interesting. I've never seen a battery packed in wax before.

If the battery got hot enough, the wax would melt. That could be messy.
It looks messy even when it's not melted.

If one of the cells ever got unhappy and "vented with flame", it would make a really big candle. Let's hope that never happens.

I'd like to see the BMS. Most of them look suspiciously similar.

 

[knoxie]

Hi Richard

Yes it was a little odd, I think I have allowed enough expansion not sure how much the pack will expand? not much i dont think, yep the BMS may have to go on the side, I will post pictures but you are right it looks the same as the other one posted on here! something fishy going on?

Knoxie

 

[Reid Welch]

Thoughts:

=I love that quote, Brenda.

=The wax idea is a good one. In the event of conflagration, believe the RC guys: the fire is oxyacetylene-hot, and it is self-oxygenated too. It's the lithium. Candle wax or other added fuel will count for nothing.

=The wax immobilizes -everything- that could otherwise possibly rub, chafe or fret to short. Our applications are high vibration and high shock--
similar, I think, to RC flight guys' lipoly abuses, but worse in a way because we expect hundreds more cycles from our packs---and each of our 'flights' are much longer than theirs.

=The expansion foam---another great idea. However, consider how much expansion room is really wanted to allow for---a single cell if shorted or in thermal runaway will swell as round as a sausage. One bad cell going fat can infect its neighbor. And so it seems to me that any case is going to pop eventually.

=If the case is hard and strong, it will fail with a bang of some sort.
If the case is bolted with nylon screws instead of metal screws, and if thes nylon screws are half-sliced through at assembly time, in order to make them rupture easily--- or how about tape of some sort? Point: I think the case should be made to hinge open when/if a cell puffs.

Another advantage of the low temperature wax, is that if a pack is abused unintentionally, you may have a warning of trouble---paraffin wax would seep from the seams.

These are just my rough and speculative thoughts.

Kokam has some lovely 30A lipo cells nowadays. 40s too. Yumm


bang the drum slowly

 

[Reid Welch]

Another point of interest, which I do believe is vital to get the most for the longest from a big lipo pack, is to monitor each cell during discharge.

The mimimalist way, and the most practical way, I think is to employ the
LVD-1T lipo monitor. This is a unique product; there is no clone or equivalent on the market. It is not a big seller.
The RC guys use multi-cell LED pack monitors instead.
screenshot, not active


I've ordered ten of these
http://www.bphobbies.com/view.asp?id=V936613&pid=EF-LVD-1T
by which make a cluster monitor pod which will go on the bike's stem or bar in my field of view.

This way, when I run hard, when the weakest cell dips to near the danger point, bingo: a red light flashes. And I will know each time I use the pack,
that all cells are OK. For, if one cell were to go bad suddenly, in short, the current which the other cells are putting through the bad cell, would exacerbate the heating, and explode the cell in short order.

With single cell monitors when anything happens to any cell, but before it goes flat or into reversal, I will know it and I will close the throttle.

Plus, in normal usage, I may be so bold as to ignore the usual "stop at 37V" (for my 37V pack). I may well run it down a bit lower, knowing that the LVd-1T monitors will at least tell just when I must pack it in and go home.

 

[xyster]

I may well run it down a bit lower, knowing that the LVT-1 monitors will at least tell just when I must pack it in and go home.

You're supposing the drop between 3.7 and 2.9 will take some time! In my experience, the voltage at some point just below 3.7v drops like a rock -- that exact, rock-dropping point likely varies from cell-to-cell and from pack-to-pack. Look away from your lights for five seconds at 37v, look back and some cells will be at 2 volts or less, permanently toasted or seriously compromised. After all, the lights are not a per-cell LVC; and numerous posters at the RC site complain that a 3v LVC is much too low. Don't say you weren't warned!

 

[Reid Welch]

Unlike you with your 240 or so, I have only ten cells in all.
And they are big capacity cells of 8AH.

Now, when/if a cell peak-loads and drops to that critical point, a point I will not be trying to reach,


the very idea is that the monitor pod will be mounted in such a way as to catch my eye with a laser flash.

This will be especially easy to see at night when I do most all of my biking.

Dude, the concept is sound.
There are two scenarios: Unexpected cell failure, and then cells aging, losing capacity in dissimilar ways;

I'll learn early on when a particular cell washes out as a weak sister.

I can then opt to replace the weak sister before she turns her siblings into flaming silver blisters, Xyster.

:wink:

I know, furrrrr.

 

[xyster]

Unlike you, I have only ten cells in all.
And they are big capacity cells of 8AH.

A good point. But I still don't know why you'd take this big risk for a miniscule payout of perhaps 1% greater capacity, while for this low-reward/high-risk venture, paying the high price of significantly decreased pack longevity.

Dude, the concept is sound.

Perhaps, but the reasoning behind it isn't.

For, in time, the pack's cells will lose capacity, and at different rates. I'll learn early on when a particular cell shows that it is the weak sister.

So you plan to accelerate capacity loss, and the big risk of puffing a lipoly thereby crushing her buddies, by doing this regularly? Silly!

I can then opt to replace the weak sister before she turns her siblings into a blister, Xyster.

The poor, weakest lipoly sister wouldn't need replacing (nor nun of her other 10-sister convent you risk popping like a blister and blowing to kingdom come) if you didn't first mistreat her. Your plan is dumb.

-Xyster

 

[Reid Welch]

Conversation with a rock
or with a safe
is apt to lead
to hardly....


_______


I don't plan to overstrain the pack by habitual heavy discharges.
I do know that will shorten the cycle life in proportion to the depth of discharge.

Am hoping for a 300 cycle useful life. Perhaps the BS specs are inflated (likely).

By then, a year or more from now, better cells of larger size and lower cost will be available.

But if that happens, a an overdischarge, and cells aren't tracking because they do not have perfectly equal capacities (and they won't),

by seeing that the one cell has gotten way to low to be good for it,
I can save it from genuine damage. Genuine, instant, heavy damage does not occur at the 2.9 figure.
And adjudging the pack whilst under load is the only true measure of its remaining Moxie;
you just go ask that of Knoxie.




:wink:

 

[Matt Gruber]

i like the LED idea, but fear it is too low at 2.9v.

 

[xyster]

But if that happens, a an overdischarge, and cells aren't tracking because they do not have perfectly equal capacities (and they won't),

by seeing that the one cell has gotten way to low to be good for it,
I can save it from genuine damage.

So long as you promise to wear full-bodied, fire-resistant garb and a helmet, and so long as your folly doesn't increase my taxes or insurance rates, I support your right as a semi-competent adult to learn from your mistakes. :wink:

 

[fechter]

I like the idea of little "building block" pieces that allow you to make any configuration. At $9 ea, though, you could proabably get a multicell BMS for less.

You could get a bunch of Matt's super-cheap DVM's and monitor each cell's voltage. I think such a display would look good on Xyster's bike (held on with duct tape). Imagine all the wires.
Keeping an eye on 10 little voltmeters might be challenging though.

What would be really cool is to have an analog switch chip that multiplexes the cells into a single voltmeter. The analog switch would measure each cell for a short period of time, then switch to the next cell. All the cells could be scanned several times per second. A single voltage comparator could detect when any cell went below the desired threshold (adjustable) and trigger an LED. It would be fairly easy to have multiple thresholds and LEDs.

With one of those spiffy Atmel programmable things like Swordman has, you could have a digital display of the critical parameters.
You could dispaly the voltage of the lowest cell (and which cell it is) and trigger a LED or something in the display when you hit a certain threshold.

 

[xyster]

You could get a bunch of Matt's super-cheap DVM's and monitor each cell's voltage. I think such a display would look good on Xyster's bike (held on with duct tape). Imagine all the wires.

It'd certainly be in keeping with the general aesthetic theme...
I've noticed on discharge my pack is quite good about tending towards balance instead of away from it. My cheap chargers seem to charge at widely varying rates. So sometimes, if I don't feel like ending the charge at the same voltage for each subpack, which requires ending the charge at different times, I'll unplug with the subpacks as much as 0.1 V apart. Like 4.18V, 4.08V, 4.12V......

As the pack moves into 50% SoC, the voltages converge:
3.84V, 3.82V, 3.83V......

Neat! I don't understand though how it does this as these subpacks in series I thought should all be discharging at the same rate (as opposed to the cells in parallel).

Anyway, I've read reports of other lithium packs tending towards balance on discharge too. The tendency makes multi-cell surveillance less necessary. And in Reid's case, I fear he will have too little time to stop the bike from the time the first light goes on at 2.9v. It may literally be one second or less before the whole pack is fatally damaged, the cells down to less than 1V, or in reversal.

 

[Matt Gruber]

LOL good topic
u know what i'd do?(not reid)
i'd tap 3 cells at a time and watch for a dip below ? (to be determined).
only 1 to 4 DVM's needed(depending on paranoia level LOL)
if i see a low pack, i switch to read each cell in low 3 pack during gentle ride home. 3 DVM's should do it.
Hey,that's me today. next week who knows?

 

[Nimbuzz]

Uncle Knox, I'm getting ready to take apart a Point One to place it in another container also.
1) How did you open the aluminum?
1.5) Why are those yellow wires disconnected?
2) Did you remove or retain the bees wax/ parafin(which?)
3) How is your new box working out? Does it mount on bottle mounts?

Thanks,

 

[texaspyro]

The wax potting immobilizes the batteries, and binds them in one thermal mass, and perhaps to the sides of the case. The good thing about wax is that it can be removed more easily than epoxy.

Perhaps they thought they might have to repair a pack at some point, they could just melt it out. Wax is probably cheaper than epoxy anyway. I wouldn't be surprised if the batteries operate warm enough to soften the wax.

I probably don't have to remind you to be careful in there, but sometimes the obvious eludes the genius. If you short a cell, it will have a way of letting you know.

Also the wax adds to fuel to the excitement when a cell spews its firey breath...

 

[Spacey]

Holy blast from the past.....ressurect the dead

 

[Lock]

Hehe... texaspyro, pretty sure this 2007 thread got "bumped" by a spammer, and spam msg just deleted, but I have to ask... about the bees wax... In another more current thread here, folks are debating some aspects of a pack assembled by these folks:
http://www.allcelltech.com/

...who tout a Li pack assembled as:

Like an ice cube cools a drink, AllCell's patented phase change material (PCM) surrounds each Li-ion cell to absorb and conduct heat away from the battery. This maintains a constant temperature throughout the battery pack, which effectively doubles the pack's life and prevents catastrophic thermal runaway (i.e. a fire or explosion).

and:

AllCell's PCM uses paraffin wax microencapsulated in a custom designed graphite matrix to provide superior cooling. As the battery discharges, the graphite matrix conducts the heat evenly throughout the pack while the PCM absorbs the heat as it melts to control the temperature. During resting or charging, the PCM re-solidifies and becomes ready for re-use.

When ya look at a table listing the Specific heat capacities of different materials:
http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html

Two materials just about top the long list... Paraffin wax at 2.9 (kJ/kg K) and bees wax at 3.4...

So, these folks used bees wax just as "glue", without regard for it's other properties? Or is the specific heat capacity of bees wax/paraffin wax immaterial w/out the fancy graphite matrix?

Just curious... as I tend to go w/the point, better to start w/cells that don't need "thermal management" in the first place.
LocK

 

[spinningmagnets]

How to cool a battery that gets warm inside a locked enclosure? theres got to be some kind of waxy goo to create a thermal paste. I don't think LiPo that is properly sized will have any discharge/charging heat issues, but when presented with several form-factors, it would seem long and thin would present more heat dissipation surface area than a cube.

Then perhaps place a flattish aluminum tube with internal fins between the packs with some type of thermal paste (wax?). There must be some type of waxy substance that softens when hot, conducts heat away well, and doesn't catch on fire when very hot? Here's some extruded Al tube to show what I mean for placement between packs.

Of course, exposed packs don't need this, it would only be for packs enclosed inside a housing, and air could be directed to flow through the tube that passes down the middle.