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· Registered
157 Posts
Here's Jackbauer's post on EVTech about headways blowing up when overcharged:

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.

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.

· Registered
157 Posts
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.
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