[rwaudio]

As I learn more and decide on the goals for my EV I keep changing my mind on batteries/motor/controller. This week I'm looking hard at headway cells because of the increased performance for the same price as a TS/CALB pack but with reduced range. My commute is only ~12miles each way and I can charge at work and even the headway pack should give me 50-80 miles at 80% which is probably more than enough, and it provides 2-3x the peak power of the TS/CALB pack. And for this project power is more important than range. The Headway pack is also lighter, so I could add more cells at a later time if I needed more range.

Since I will be ordering tons of cells and probably a few extra in case I have a bad cell or two, I thought about replacing the LA 12v battery with a pack of headway cells. Maybe a 4s3p type pack that would be "charged" by the dc/dc converter. Would this need a shunting bms, or simple voltage monitoring or nothing at all?

I will be building my own bms for the main pack that measures the voltage of each cell in real time and show me the high cell, low cell and average of all the cells. I'm sure it wouldn't be too hard to extend this idea to the "12v" battery but I'm wondering if it's required? The dc/dc converter would probably be set to about 14v which is 3.5v per cell (if they're equal of course)

Remember this pack would have minimal draw unless the dc/dc converter failed or can't keep up with the load. I won't have a power steering pump, I'm switching to a manual rack, so just the usual car stuff, vacuum pump, stereo will be on the 12V line. Having no 12v battery isn't an option so it's stock or something like a headway pack.

 

[alexcrouse]

For a pack that small, id go for it. Cheap is good. Frankly, i haven't found a correct answer, BMS seems purely optional on this site, with both camps very committed.

I would, because again, a small pack, only needing 4 units. I would use a shunting style because overcharge is the worst that can happen.

Low Voltage is bad, because the cells will fail, but over voltage is worse, as they can explode.

 

[frodus]

Explode? I'd like to see your source for that info.


ANYWAY....


There's a nice little 4-cell BMS here:
http://www.manzanitamicro.com/index...ategory_id=18&option=com_virtuemart&Itemid=64

 

[alexcrouse]

I can't find it in a quick search, but i saw pictures of headways blown up. One end cap blew off and hit the ceiling. Not what i want under my hood.

 

[frodus]

Please find the link. Its nothing but a rumor unless you've got some proof to back it.

I've been working with headway (buying, selling, testing) for almost 2 years now and haven't seen that once. The only time I saw one even catch on fire was ignited by a heatgun, overdischarged and shorted and it was done on purpose to show how non-volitile they are and what it actually takes to make them fail.

Lifepo4 doesn't explode, lipo does.

I seriously doubt that what you saw was a headway.

 

[MPaulHolmes]

Here's Jackbauer's post on EVTech about headways blowing up when overcharged:


Folks,
First off if this is old new i apologise but thought i'd mention the
results of some tests i performed today.

Just prior to the evcomponents meltdown I had purchased and received a
quantity of 10 headway 10ah s cells. I've done a few simple tests with
them over the summer and was quite pleased. Recently I purchased and
tested more samples direct from headway and just this week payed for a
big batch of 16ah cells for my ev. Anyway today I decided to do a few
overcharge tests to see what happens. this is especially important for
myself as i'll not be running a bms or shunt balancer so what would
happen if things got a bit out of hand like a failed charger etc. Not
wanting to waste good cells I decided to use 4 of my much battered
original stock from evcomponents. A 24v 50amp forklift charger provided
the power. At switch on and for several minutes the charger maxed out
and nothing much happened. Then the current fell away and the voltage
leveled out to about 29v. Still not a thing. About 5 minutes later i
decided to test the cell temp with a probe. Just them my phone rang and
i was about a minute on a call. Just as i hung up BAM! One of the cells
went off like a stick of dynamite. Five seconds later BAM! another let
go. Copper foil everywhere. Being of above average intelligence , i
quickly concluded this was bad.

The ends of the 2 cells had launched a good 30 feet away and one of the
cells had uncurled like a roll of toilet paper. This i found strange as
i had assumed (i know i know) that the cells had vents to relieve
internal pressure. Clearly this did not happen in this case. Next move
was to repeat the test. This time i used 4 cells from my newer headway
stock. Same setup. Power on. While i'm contemplating the purchase of a
bomb suit two very sedate pops ensued and current fell to zero. Tiny
whisp of smoke drifted upwards. I left it on for 30 minutes to be sure.
Nada,

First noticable difference was a code on the cell label. The
evcomponents shotgun shells read : JC23 while the newer relaxed versions
read : JG06. Means nothing in itself could just be a batch code or build
data. More significantly however was the postmortem of the positive
end. The JC23 cells have 4 grooves in the positive end but the JG06 have
12. Crucially , the JG06 have a burst disc inside the positive cap. This
had popped on the newer cells but seems to be absent from the older JC23.

Time to prove the theory. Took 2 more of the newer JG06 and wired them
in parallel with a pair of headway 2 cell busbars. on one cell I used
the supplied m6 screw. On the other i used a long m6 bolt and nut and
tightened it down on both positive and negative ends. My working theory
being that the bolt would prevent the burst disc from popping on that
cell. Placed them in a metal box , connected the forklift charger and
hit the power. One minute into the test the cell with the headway screws
pops gently and smokes. 2 Minutes later all hell breaks loose. BAM! ,
smoke and flames. Once things calmed down , i inspected the results.

The cell with the normal screws had popped its disc. The other had not
due to the bolt. Instead it had stretched about a half inch in length
and split its case down the middle , vented and flamed.

The most important lesson for me here is the bolt length when joining
cells. DO NOT impede the bust disc on the positive end or you got a
stick of dynamite.

 

[frodus]

HAHA, yeah I did see that.....

but that is a situation that will NEVER happen.... I mean, when are you EVER going to charge a single cell at 24V .... that's way over its rated 3.7V max charging voltage. The rated charge wattage is what, 3.7V x 60A (6C) = 222W. He was hooking to a charger that could go over 6x the voltage and 50A, and potentially charge it at 1200W.

And he Admitted he blocked the disks on one of them at least, which doesn't allow it to rupture correctly.


Lessons:
Don't overcharge a single cell at 6x its voltage.... duh
Don't block the pressure disk at the end with the screw/bolt.

Now, a cell, being used as it is designed (with proper clearance of bolts/screws) and never being allowed to overcharge in an extreme manner, will not explode.

I've overcharged these (accidentally) to 4.5V and yeah, it got hot, but never exploded. The cell didn't like it and never had the same capacity, but it did not get physically damaged in any way.

 

[kansas]

I'm using Headways (8ah) for my lithium upgrade for the same reasons you are. I built a 4S2P pack of Headways to replace the existing 19ah lead house battery. I'm using an IOTA DC/DC convertor, set at 14 volts. And I'm not using any BMS. Last year I built a 24 volt 4S4P pack for my ebike. After riding 1500 miles last year, it looks great. Last week the cell voltages on the bike pack were all sitting at 3.2 volts after resting for siz months. So I believe it is safe enough to go without any BMS.

BTW, I bought 460 8ah Headways from Manzanita Micro and they are the best! All 460 cells initally charged to 3.7 volts, no clinkers. I found 6 cells where the screw holes and threads are a tiny bit out of square, but not enough to warrant asking for replacements.

 

[MPaulHolmes]

But let's say you made your own buck charger, like jack. Let's say that the igbt fails on for some weird reason, which is their failure mode. You could end up with a situation that would dramatically overcharge. The constant current would fail. I think it's nice to know that if you use the wrong bolt or something, that the LiFePO4 cells that never blow up can actually blow up. Not following specs == dynamite is useful to know.

 

[jackbauer]

My main aim for that test was to determine a difference in manufacure between the date codes on the cells. As i think i said in that post , the newer cells will not explode or flame even when hooked to the fork charger. The burst disc just pops.

 

[ElectriCar]

But let's say you made your own buck charger, like jack. Let's say that the igbt fails on for some weird reason, which is their failure mode. You could end up with a situation that would dramatically overcharge. The constant current would fail. I think it's nice to know that if you use the wrong bolt or something, that the LiFePO4 cells that never blow up can actually blow up. Not following specs == dynamite is useful to know.

There's an app for that! Seriously there's devices called Voltage Relay's that monitor your pack and can be wired to interrupt the charger if it over volts. I installed one made by ABB. Click it then you can read the specs. It's adjustable up to 330V I believe. Mine is wired so that a contactor is energized until the voltage reaches a set point, whatever you want it to max at then it kills power to the contactor. Problem solved. The only drawback to this relay is the voltage is not very precise, you have to tweak it a few times to get it to cutoff where you want it. Mine shuts off now at just over 171V.

IMO, if you're going to invest the equivalent of the price of a small car in batteries, it's a no brainer to have a backup such as this IN CASE of charger failure to shut down and continues to ramp up in voltage. It's CHEAP insurance. I've been running it on mine for a month or so before I upgraded to lead. I wanted to get it working before I installed the new pack. Works like a champ.