>>> Comments on your advice <<<
All good advice with a couple of minor oversights:
1) Manzanita chargers have a substantial output capacitance. (I use one
to charge our race vehicle(s) and I have a pre-charge circuit to
gracefully charge the capacitors on the battery side.)
2) The Manzanita chargers simply don't tolerate open output when
operating. Thus, whatever pre-charge circuit or battery disconnect
circuit needs to carefully safeguard against either opening the output
when the charger is energized, or energizing the charger when the output
is open/disconnected. I have personally blown up a charger because of a
momentary loose battery connection. :-(
>>> Question about why one would want to routinely disconnect the
charger <<<<
I am wondering what the motivation is for routinely disconnecting
the charger from the battery pack. I don't know of anyone that installed
a charger in their EV that did this. I am puzzled why one would
want/need to do this.
The charger doesn't have any parasitic current draw on the pack
when the charger is not running (to my knowledge.) There is no hazard or
disadvantage that I can think of that might occur if you simply leave
the pack connected to the charger. It seems quite difficult to devise
some circuit that would allow routine disconnection without introducing
additional hazards/complications as opposed simply leaving it connected.
Just my two amps. :-)
Bill D.
PS
I have damaged a BMS by connecting a charger to a battery without
pre-charging the capacitors in the charger. This is one of the reasons
that I built a pre-charge circuit for my racing charger set up, and why
I don't disconnect a charger routinely if I don't have a strong reason
to do so.
How, you might ask, did I damage a BMS by simply connecting a
capacitive load to my battery pack? The pack was unevenly charged, with
more than one module near zero SOC. When I connected the semi-infinite
load of the capacitive battery charger to the pack, the large number of
"full" modules briefly reversed the voltage of the modules that were
very low SOC (and were thus high impedance). This briefly reversed the
polarity on the unfortunate BMS units that were connected to those
modules. This let a little bit of the "magic smoke" out of those BMS
units, and left them partially brain damaged. Sad story. :-(
On 8/16/2024 3:25 AM, Lee Hart via EV wrote:
Precharging isn't as simple as it first appears. Here are a few things off the
top of my head:
Precharge circuits *must* used DC-rated parts with an appropriate voltage rating. An
AC-rated switch, relay, or fuse will fail *on*! If the switch or relay fails
"on", the precharge resistor can overheat, or simply run down the pack.
Your precharge circuit should include some way to sense the pack voltage, and
abort if it fails to precharge in a reasonable time. Some load on the pack
(like a DC/DC converter) can prevent it from precharging.
A simple precharge resistor works, but will get extremely hot and fail (or even start a
fire) if some fault condition leaves it on for too long. If you use a resistor, it should
be a flameproof type that is guaranteed to fail "open" in case of a fault.
It's hard to specify the wattage for a precharge resistor. It depends heavily
on how much current and how long it will be connected. Resistors have peak
current specifications that can be hard to find. The usual approach is to use a
much higher wattage wire-wound resistor than you might expect, just in case
something goes wrong.
You can use a tungsten light bulb as your precharge resistor. Their "cold" resistance is very low
(about 1/10th) of their "hot" resistance), so you get a high peak precharge current. Then the bulb
lights, and you get a much lower "holding" current that can stay on without damaging it. Obviously,
use enough light bulbs in series to handle your pack voltage (one for 120v or less, two for 240v, etc.)
There are also special PTC (positive temperature coefficient) resistors, whose
cold resistance is low, but go to a much higher resistance when hot.
If I recall correctly, the Manzanita chargers have little or no output capacitance. They
depend on the battery as their output "capacitor". You must be sure the battery
is connected to the output before AC power is applied. Otherwise, the output will go
overvoltage and fail (bang)! Check with Rich Rudman at Manzanita Micro for details and
advice on this.
Lee Hart
--
Excellence does not require perfection. -- Henry James
But it *does* require attention to detail! -- Lee Hart
--
Lee A. Hart https://www.sunrise-ev.com
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