Michael Ross wrote:
EBikes and such often (always?) have a pack assembled with an
interface-less BMS. The low warning is it shuts off.
I have an Ebike with a lithium pack, and that's how it works. Its "fuel
gauge" isn't very accurate; but at least it shuts off the pack when it
reaches "empty". I don't really know what its criteria is for this, or
whether the pack really is empty or not. But that's OK. I can always
pedal to get where I'm going. :-)
Bottom balancing does obviate the need to worry about this somewhat, though
you still want to find your lowest capacity cell and stop charging soon
enough to pamper that one.
With the BMS built into the pack, you don't really know (and aren't
allowed to find out) whether the cells are bottom balanced, or top
balanced. You can't really do anything about it, anyway; it's a sealed
unit, with no access to individual cells. If it works right, the
internal BMS prevents overcharging, and over-discharging.
The drawback is that as soon as one cell fails, the whole pack fails.
I've cut open laptop packs that quit like this, only to find that
exactly one cell had failed, rendering the whole pack "bad". If I cut it
open and replace the bad cell, in works again. Maybe $10 for a cell
instead of $100 to replace the whole pack. :-)
These internal BMS often aren't very good. They do the minimum necessary
to prevent fires and other catastrophic failures, but don't do much to
extend the life of the pack.
Lee, I really like your website which I had not seen before. Thanks for
doing that. I hope you get payback from it.
Thanks! But it's mostly a labor of love. No flash and no glamor means
only a few sales a month...
See, I have a problem. I favor simple, cheap solutions. So simple and
cheap in fact, that no one will buy them! They either don't believe it
will work and so buy something expensive and complicated that has
extravagant advertising claims. Or, they see that it works, and so build
it themselves! Either way, I get nothing...
How much use would a Batt-Bridge that is adjustable be? As in set the
allowable difference between pack halves that send the warning? I don't
instantly see where any hysteresis comes from. Is it the difference in
drop between the green and red LEDs? Are there any disadvantages to the
Batt Bridge?
It's an analog device; not digital. The more the imbalance, the brighter
the LED lights. The two divider resistors determine how bright it can
get. With modern superbright LEDs, even 10ma is enough to see in sunlight.
The threshold at which it first begins to glow is set by the difference
in forward voltage drop between the green and red LEDs. You could make
it adjustable by using something like a TLV431 "adjustable zener" with a
trimpot in place of the green LED.
On Thu, Apr 17, 2014 at 7:50 PM, Lee Hart<[email protected]> wrote:
Peakfoto Digital Photo Still n Video wrote:
I have a an ev scooter, with 22 lfp cells. I'm using min bms board
and evpower cell moduals and an aq Cycle anlyist.
BUT it'd be nice to have a some kind of an setable audble alarm ,
while driving if a/any cells get below a set point. so I can know
"the end getting close " not " you better stop NOW!" .
I kinda know about CELL logs , but sound s a alttle messy. I don't
need a screen on the dash. any reasobale ideas?
If you already have a BMS, doesn't it have an undervoltage alarm?
You don't *have* to monitor every cell to know if one is undervoltage. If
you want something very cheap and simple, try my Batt-Bridge. It splits the
pack into two halves, and compares the voltage of each half. If they are
different, some cell has just went dead, and is at a lower voltage.
The Batt-Bridge normally lights an LED. But you can use the LED of an
optocoupler, and have the output of the optocoupler turn on a buzzer or
other more noticeable indicator.
The Batt-Bridge is described more fully at:
http://www.sunrise-ev.com/LeesEVs.htm#battbridge
--
If you're not stubborn, you'll give up on experiments too soon.
And if you're not flexible, you'll pound your head against the wall
and you won't see a different solution to a problem you're trying
to solve. -- Jeff Bezos
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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