building a large lipo pack safely

Starting to build up what will be by the fall a 1.4kwh lipo pack out of 6s5ah packs @ 18s 20ah.
The pack will be permanently mounted inside an enclosed housing. It will be bulk charged with meanwells and balanced with battery medics. LVC will be via 2x the capacity I'll need and with cell-logs as a fail safe.
The only safety issues I'm foreseeing is the unexpected: crashes, vandals, tornados, equipment failure, and QC issues with the cells themselves.

How do I do this safely?

First is fuses


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This seems like the easiest way to parallel packs and give them some protection. He uses 200 amp ANL fuses. Should I use one for each parallel group - positive and negative? Should I use 200 amp or lower?

Packs will be connected in parallel with 12 awg and 4mm bullets and in series with 8 awg(possibly double runs?) and 5.5mm or 8mm bullets.

Someone mention a fire resistant tape to wrap the packs in to protect against fire and physical damage. Another option is foam to protect against physical damage and cold weather.
My worst nightmare is passing out at my girlfriends with the bike charging, a packing going thermal, and burning down her apartment.

This is the bike:

This is the battery pack layout:

The on board charger will be from BMS battery. It will be slower than my at home charger, but a since I'm @ school for 3-4 hours it should be perfect. I'm contemplating removing it from it's metal shell. This guy has done something similarand cut 1.2lbs off the weight of the charger.

The box will have 3 cell logs - which will also be where the battery medics plug in.
A temp sensor for the motor
Possibly a watt meter to keep track of the charging process
and all the switches.
And a 5v phone charger

test all packs with something to discharge them and give them multiple balances before OKing them to be in the large pack.
You can use a celllog 8m/8s with a USB port + logview to do this.

You will also need a balance charger that does not suck. May want to invest in something better than the half-working imax.

Good point -
I guess I should of mentioned I'll be using these packs with my electric lawn care equipment before they go on the bike, so they'll have a cycles on them before they go on the bike.

BTW I found a good way to break in lipo with the imax.
1: set to li-ion.
2: it charges to 4.1v
3: set capacity charge/discharge limit to 4ah
4: it should discharge to 3.7~

It takes a long time, but wtvr. I can wait. I've been using the usb port on the imax to see the capacity of the cells.

I just ordered a 10 pack of 150a ANL fuses.
$10....
Anyone have an idea of how much resistance these add and what voltage range they're meant for?

3.7v is not low enough to see a bad cell, you need to go down to about 3.2-3.6, and stop immediately when 1 cell hits 3.0v.

It is important to do this to identify a runt that will become problematic in the future.

That's cool that the imax has USB capability.

The weedwhacker will find the runts I bet.

We wrap our lipo's in adhesive backed fiberglass tape. Might not prevent a battery from combusting, but it will mitigate the impact, as well as prevent it from damaging nearby batteries.

http://www.amazon.com/CS-Hyde-Temperature-Fiberglass-Silicone/dp/B000REJN4S/ref=sr_1_4?s=industrial&ie=UTF8&qid=1304170040&sr=1-4

Not exactly the same tape we use, as it comes in 4 foot rolls for us and uses a different kind of fiberglass, but we have performed some light testing on it and found it would hold it's integrity for ~15-30 seconds with a propane bottle torch (3,600 Degrees F) before it fails, and up to a few seconds resistance with a oxy-acetylene torch (~6,300 Degrees F).

Click to expand...

After Keplers post I'm dotting my dotting all my Is and crossing all my Ts!

So I have two options for containment of a potential fire.

1) try to keep the bad pack isolated from the others and save the pack from going up too. This doesn't seem feasible. How long does a pack burn for? Surely long enough to set the others off even if insulated.

2) Try to keep the fire inside the battery box. Yeah it'll toast the bike, but not the house. The question is, how can I fireproof the wood? Some sort of coating? Or maybe that tape would help...


To isolate the batteries from physical damage like impacts and the bumps that come with everyday riding I read about a foam used to wrap injured horses legs. Apparently it sticks to itself and nothing else. Could be a good way non-permanent non-messy way to deal with it.

I suppose one possible safety measure would be to add pressure sensors to the packs, between cells or between packs or both, so that if pressure goes above some limit (which would need to be determined experimentally), power is cut to the charger, and an alarm goes off.

If the pressure does not continue to rise, things will be ok even without intervention, as charging will have ceased.

If there is a discharge-type balancer on the pack it will proceed to drain the high cells, theoretically helping to reduce swelling caused by overcharging, if any, but it will be likely very slow and not help in any extreme case.

If pressure continues to rise, then hopefully the alarm will have continued to go off and intervention can be made on the swelling pack.

auraslip said:

2) Try to keep the fire inside the battery box. Yeah it'll toast the bike, but not the house. The question is, how can I fireproof the wood? Some sort of coating? Or maybe that tape would help...

Click to expand...

There are things you can soak wood with that would help (sorry i don't remember any names), but I don't know if any of them actually fire*proof* it, though they may well retard the flames long enough to prevent additional combustion in it.


I expect any kind of metal internal liner, backed by the stuff from those lipo bags (between the metal and wood), would do a good job of preventing any fire from igniting the wood itself, as long as the pack wiring does not also ignite and then spread the fire outside the box to start with.

To be honest.. the flames are going to go somewhere.. and they are going to damage something.. that is just a plain fact.. no getting around it. That energy has to go somewhere. You've seen lipo fire videos. That fire goes on for a damn long time.. around a minute.

The only hope is separating them so that you only have one 5AH pack blowing up at a time rather than 4-12 packs.. but that is not easy to do.

The trick is to treat them well, stick them in a place where you can periodically inspect the cells for puffing, and test them thoroughly before putting them in a big pack.
You should never have a fire, or have to worry about a fire that way.



I keep mine in a squishy little bag with double padding ( the bag has it's own hard padding too ) that allows for easy inspection for puffing / easy dis-assembly for diagnosing bad cells etc.

Found this foam. It changes electrically when compressed. http://www.inventables.com/technologies/conductive-foam-sensor

Might be able to make a pressure sensor to detect puffers with it.

AW - that's my hope - to make it so the wood won't combust. The smoke will be pouring out of the case, but not the flames! It just needs to withstand high temperatures for... how long?

Asbestos lining maybe, haha.

The thing that spookes me, is not amount of good pack design can help against a failure like JohninCR had, where a pack just sitting on his desk that had all normal voltages when it arrived just vented fire randomly.

I honestly don't know what the truly safe option would be for an RC lipo pack.

I think the only thing you can do is try to thoroughly test each pack with a handful of deep cycles before assembling.

I think I'm overreacting?

The issue here is that these packs can be dangerous, but the type of danger you can't predict can be discovered by thorough testing?

my guess is that there are still going to be some defects that cannot be caught in the few cycles of testing most people are going to be able (or willing) to do, and that at some point the (possibly several) conditions required to cause failure will all occur at the same time, and the pack will fail dramatically.

When I worked at Honeywell, we had some connectors filled with diodes, and the incandescent-filament displays they held, in a cockpit annunciator unit. These failed quite a lot due to manufacturing problems after the manufacturers changed locations/processes/people, and they'd fail randomly at all sorts of handling conditions.

We fixed their major process problems, but still got occasional failures, still due to the same causes, but these would not show up just in regular usage at first (only after dozens to hundreds of "cycles"). The only things that really brought out the failures were doing our "burn in" testing (thermal cycling and vibration testing) while actually power cycling the displays, segment by segment and all at once, etc. That wasn't our normal test, because if you power cycle a filament while under vibration, especially in extreme thermal conditions, it's pretty likely to damage it. But it was the only way we could weed out all the problems they were creating, before they ended up in airplanes and prevented a pilot from gettign the correct info needed to fly.

We couldn't get hte manufacturers to do this testing anymore, because it wasnt' in our contract--it was simply how they had always done the testing before, and we of course expected them to continue their QC at the same level. When they began cutting costs, naturally QC was one of the first cuts, and labor costs for assembly were next, which without the QC would never catch the now-myriad assembly problems, and then costs for raw materials whcih again, without quality assembly processes/people and no or little QC, will pass these problems on to the end user.

So it might require us, as the end user, to create our own testing regimen that includes all the conditions that could bring out the latent defects in these packs, in order to catch them before they go on our vehicles. That may well include thermal cycling while in use at varying levels and states of charge, as well as simply "storage" conditions. And even with all the testing we can do, there will still be packs that escape.

And the costs of that testing will be high, since each individual will need logging equipment to track all the variables, and then labor hours to go thru all that data, or labor hours for someone to write a program that will do that automatically (which will be a MUCH lower cost overall, by several factors of 10, most likely, even in just one year, assuming that everyone that builds a vehicle pack uses the program instead of doing their data analysis on their own). And test equipment to do all this in an automated way (or else the labor hours go way up).

Another cost is the cost of wasted material--what do you do with the packs that don't work out? Return them? It probably costs more in postage to send back and forth than the pack is actually worth, especially if you get more than one dud pack in a series (send a bad one back, get a different bad one in exchange, send that back, etc.).

At a factory, all those costs are much much less than at the end-user level, but they are not zero cost, and there's also no way for us to get the factory to do this, even if they raised prices to compensate. Other factories that don't test would then be under-selling them, and get all the business. And the factory would still have a bunch of bad cells that they'd have to deal with selling to someone else (since I'm pretty sure they wouldnt' recycle them for raw materials if there was any chance of getting a quick buck out of them, more than the materials cost).

Next bet would be at a distributor, but again, that's not going to happen at least with HK/HC, without raising prices enough to get people to buy most of their stuff from places that don't do the testing, and thus don't cost as much. So a distributor isn't going to do it, either.


This is most likely the biggest reason why all these problems happen in the first place--because most people are not willing to pay the extra costs for the QC required to deal with the problems before they reach the market. They'd rather pay a lot less for a product that might or might not work, and just toss it and get a new one instead when or if it does fail, and gripe about it being crappy.


I suppose it would be possible for everyone that cares to, to send their packs to a testing facility setup just to do this, which would greatly decrease the total equipment costs and labor, but it would still cost money for hte testing itself, and for postage back and forth. Plus with the increasingly-stringent shipping requirements for batteries, at some point it will become impractical to do even this, as you'd wind up sending each individual pack as it's own shipment, and risk of damage to them increases with every trip thru the shipping system, too.


So it still comes down to the end-user most likely having to do the testing. Thus testing will be inconsistent at best, and nonexistent at worst. Most likely the very simple tests proposed so far of a few deep cycles, voltage and capacity testing, etc., will be all that happens to most packs that even get any testing. They'll be sufficient to catch many bad packs, but some are still going to sneak thru that work fine under that kind of testing....


I still think that failures like John in CR's and Kepler's are probably caused by either:

--pinholes in the casing from mishandling or materials defects, and moisture ends up entering the cell, eventually contaminating the lithium compounds,
--moisture contaminated the compounds at the factory, prior to sealing into the packs,

and then as soon as conditions are right, it flames out.

Almost no testing we as end users can do is going to show those defects, if they are microscopic amounts, if they don't show up during normal pack cycling.

auraslip, if you've been following the 'nasty lipo fire' thread, then that seems to be the case.

Of my 13 packs, i knew which ones were bad within the first 2 cycles.
One was a potential flamer and never got charged.

Both instances of non-abuse related lipo fires here happened within the first cycle.

Amazing write up amberwolf. It really puts the the price of our batteries into perspective. I still don't think lipo is any cheaper than a pre-made life pack... and know everyone is realizing we have to spend hours doing our own QC? Yeah... really attractive! All though... I don't mind doing it now... I'm sure it'll get tedious later.

Dogman mentioned how some of his started to puff up after 15 cycles, so maybe it's not just defects present after the first few cycles? It certainly makes building a pack all that much harder because you have to visually inspect it... That involves taking off the sides of my bikes.... maybe lipo isn't the best choice for this project?


Another thing I'm worried about is physical damage.... how should I lay the batteries? With the cells horizontal so all the pressures is on the bottom cell, or vertical so all the cells get share the weight? I'll be using a very thin layer of foam between the wood and the packs as well as some method of firmly securing them so they don't bounce around.

You can get packs to puff easily if you overdischarge them, overcharge them, or run them at too high a C rate.
Doing your own QC sucks but i spent about an hour per pack personally.. not a big deal to me.

Look at my pic again. I can visually inspect all my lipo easily, and the lipo will never see forces hard enough to bump them against the frame with my design. The pedals, cranks, derailleur, handlebars, and axle would prevent a side impact to the lipo from happening.

This is why it has taken me over a year to find the perfect mounting system. You can't just stick lipos in a hard box and forget about them forever.