Thanks to everyone who has responded so far. This is definitely a
challenge !
Summary first, question at bottom.
I've refined my modeling and determined that about 85Wh is sufficient
and 6.6kW max constant discharge.
If I did my calcs correctly, that means
240VDC total. With
3.7V per cell, requires
65 total cells
They would have
28A max current and need at least
350mAh capacity.
Choosing a higher capacity cell, say
1500mAh brings down the minimum C-rating to
18C, and means using only about
25% of the cells capacity, keeping it nicely in the middle charge state
area
As an example, I've looked at the LG HB6 (which is a discontinued cell,
but cheaper :)
3.6V
1500mAh
30A max continuous current.
QUESTIONS:
If I use these cells as I describe above (keeping in the middle 25%
charge state), will they probably last a long time, meaning 1000s of
cycles ? Even at near max continuous current ? (Bear in mind the
discharge time will be short and there will be ample cooling off between
uses.)
Peri
------ Original Message ------
From: "Cor van de Water" <cor.vandewa...@gmail.com>
To: "Peri Hartman" <pe...@kotatko.com>; "Electric Vehicle Discussion
List" <ev@lists.evdl.org>
Sent: 13-Apr-20 10:59:58 PM
Subject: Re: [EVDL] custom battery
Peri,
That is 50C discharge, not impossible but also not trivial.
The good old A123 cells could do this.
The cylindrical cells are almost 10Wh and can discharge at metal
melting speeds.
I have a bunch of them here, in case you are interested.
Cor.
On Mon, Apr 13, 2020, 9:33 PM Peri Hartman via EV <ev@lists.evdl.org>
wrote:
Hi,
I need a battery with about 150Wh and a discharge rate of 8kW for up
to
a minute or so. I've spent a few hours searching and there doesn't
seem
to be anything on the market that fits this. There are, of course,
higher capacity batteries that can deliver 8kW, but that's not what
I'm
after.
This isn't for an EV, so I suppose this is somewhat off topic,
although
BMSs are a relevant topic to EVs. It's for a home appliance that I'm
prototyping.
I'm pretty sure I can find affordable cells that I can put together to
do this, but then I'm stuck with a custom BMS. I suspect that's the
route I'm going to have to take. So, let's talk about that.
Each cell (or group of parallel cells) needs a monitoring circuit (is
there a name for this?). And theses monitors need to communicate with
a
BMS. I'm less concerned about designing the BMS as I think I can use
an
arduino and do it in software. The arduino can control the charger,
too.
I can revisit this later. But for the monitors, I need to get it
right.
Since the battery will be in the house, it cannot cause a fire. Ever.
Of
course, I will have over temperature sensors which shut off the
charger
and shut off the load.
Here's what I think each monitor's role is, based on what I've read on
EVDL over the years and some common sense :)
- cell over voltage
- cell under voltage
- cell over current
- cell temperature
In all of the above the circuit should shunt the cell.
In addition, the monitor should constantly report the cell voltage,
current, and temp to the BMS.
If there's something reliable I can buy, at a reasonable price, that's
my first choice. Otherwise, I guess it's getting someone to help
design
and fabricate these.
I would be grateful for your suggestions.
Peri
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