@krby Do you use a BMS? If so, which one? This may open some additional options. Also, you may not need to break the precharge circuit at all before making the main circuit, because as soon as you connect the main circuit, the resistance of the main circuit is nearly zero, so nearly all current will pass through the main circuit and nearly none through the precharge circuit. This principle is used by e.g.
Jack Rickard as shown in
the user manual p23 of the " ESP32 BMS Controller Assembly". Theoretically, without Jack's hardware, you could achieve similar behavior as Jack' s system using two contactors (kind of high-current relay) in series, of which the second one has a precharge resistor R in parallel with itself as shown in Figure 1. Works like this: Contactor 1 closes first. This allows current to flow trough itself (duh) and R, but not yet through contactor 2. This allows the precharge. Next, after a time delay of your choosing, contactor 2 is closed (effectively fully bypassing R, but R is
not disconnected). The time delay can be easily set using
a cheap compact simple programmable timer (This link is key to this solution! So no design & soldering of capacitors, diodes etc., like
here with a soft starter). Assuming that the contactors are controlled by the BMS, now you only need to (dis)connect the (low) power connection from the battery to the BMS to turn off/on the whole system without
sparks, IF the BMS allows such. This avoids the (perhaps negligible) risk that
someone in a hurry quickly moves the switch/button
past the precharge contact, possibly damaging the inverter. Why neglegible: the precharge spike may last only
in the order of milliseconds.
Figure 1: The manual disconnect in rightmost purple box can be closed, powering the buck, in turn powering the BMS (or maybe it is already on through power on pin 4 when a charger is present), in turn performing checks, and if all OK, closing both contactors with a pause between them. Later on, the manual disconnect can cut the buck power, in turn opening the contactors and cutting power to the BMS so it does not drain the battery. (download PDF version to read small text).
Figure 1 system's disadvantage: BMS gets power through port 2
or 4. So even after a manual disconnect cutting power to port 2, the BMS can theoretically still drain the battery while powered by mains, if present, through port 4. This can be avoided by a better (?) design, shown in Figure 2.
Figure 2: New version. The manual disconnect in bottommost purple box can be closed, powering the BMS, in turn performing checks, and if all OK, closing both contactors with a pause between them. Later on, the manual disconnect can cut the BMS power, in turn opening all contactors and relay (download PDF version to read small text).
?
Update:
Forget about the previous two designs, someone else has already invented the wheel:
Tiny BMS has an integrated timer
and a programmable precharge duration
and the option to use the on-board FET to replace one (expensive!?) external contactor
. For less than half the price of an Orion.
Alternatively, see also:
[1]
http://liionbms.com/php/precharge.php
In addition to the two contactors used by Jack, this approach has relay "K1" [1] to disconnect the precharge resistor "when the system if off" [1] (I do not understand this - doesn't K3 already fulfill this function!?). Maybe in the rare case that K3 is arc welded closed, K1 should prevent a closed circuit as a back-up.