Proposed: Precharge bet taken away from the controller or BMS master and acts as a stand alone component.
Precharge only needs to happen when the contactor is open. When the contactor is closed the BMS slave stack knows the stack voltage, there for Precharge need only report while contactor open
Precharge has the purpose of diffusing a potential, therefore it can *inefficiently* draw from that potential. While precharging it can report on voltage differential - which is all that is really needed.
Precharge typically involves a big resistor which is heat sunk to the chassis.
Precharge is typically located next to the contactor - where first there is potential between the posts and then there is potential across the coil
Precharge can measure a voltage range of 3 orders of magnitude by using a voltage divider which is hard-clamped with a Zener.
Say we need to range down to 5V... where anything over that will create magic smoke.
We clamp hard at 4.2V
Any reading over our target reading (which is driven by the divider stack) reads as OL
We keep bleeding until the timer runs out or the voltage drops to the point where it is not clamped
This allows us to read anything from 12V to 1200V through a single channel ... since we dont need to know the absolute value but only the threshold of minimum value.
It could even be a simple comparitor... or stack of comparitors...
The only place a BMS master needs to touch Battery ground is:
* Measuring precharge voltage (tho not controlling it if using a relay which we will)
* Measuring (+) leakage to chassis and (-) leakage to chassis, and that happens only every now and again
Thats about it. Everything else is galvanically isolated. This means a BMS master can rest on the 12V AUX ground and isolate only at the times/places where Batt +, Batt -. and Load + need to be viewed.
Its arguable whether we need to measure V on the batt side of the contactor.
If you dont trust your slaves you should... but we will trust our slaves... as we will program an expectation into the master of # slaves + range + worst case delta + history comparison etc.
Assuming we dont need to then we definitely dont need to read Load + (other side of contactor... we care only about contactor delta)
We may be able to read the *current* running through the precharge resistor....
Got it... WE DONT MEASURE THE VOLTAGE... WE MEASURE THE PRECHARGE CURRENT.
Mmmm... nope... wont work...
Cant predict the load... so cant use current to determine when sufficient precharge has occurred.
Good idea tho.
So generally speaking we want to wait to close the contactor until the voltage is like... uh... 10V say.
Thats pretty reasonable.
Whether its a 12V system or a 1200V system... 100%, 10%, or 1% (it should not be a ratio of voltage... it should be an absolute value... IMHO... but I am not an expert)
Lets say we target an absolute value
We set up a comparitor
It takes TINY power... microwatts
It uses a linear regulator or a power scavenger to milk its tiny bit of juice out from between the terminals.
It clamps hard with at least 2 diodes the voltage coming it... so we can basically just run through a couple of very large inline resistors (10M... 100M...) strap at our trigger voltage... and blip when safe.
So really... there is no need for an analog value to be reported that represents the precharge voltage. It is a binary state that needs to be reported.
This is much easier, much cheaper, much more straight forward.
Comparitors... I have seen a lot of those used in highly isolated systems due to the fact that they draw next to nothing and can easily work with an opto.
Now to power it...
We want Linear Technology to produce a scavenging device that accepts 10V to 1000V and turns it into a very low but stable voltage. Lets go see... I know they micro scavengers...
http://www.linear.com/parametric/energy ... 1033,1!gtd
Nope... thats not the path.
How about a simple buck converter?
Ah... our dream buck again
4V to 150V
So then the question could be... how do you get 12V to 1200V to look like 4V to 150V?
We could divide by 4 to get 3V to 300V... nah
We could... bring it down with an inline resistor and a zener diode (yep... so long as you dont take much current.
AH HA... a use for Linear Regulators!!
Linear regulators dominate where current draw is super low.
Here is a batch that starts at 300nA (0.3uA.... yea...)
So... assuming we have to drop a round thousand volts off at the pool...
1000V * 1uA = 1mW
Uh... can you find a linear regulator that can shrug off a mW?
So we can easily harvest power from between the posts of a contactor up until the time it converges
The circuit would have to default to true... for the case where delta is close to equal
The hitch would be powering the opto coupler or isolator... those are not 1uA!
now wait... times have changed... ANalog Devices claims they can provide digital isolation for as little as 1uW
http://www.analog.com/en/about-adi/news ... power.html
Thats food for thought.
Now off to my nap.
I am an old man with a separated right shoulder, sprained right thumb, and my right elbow is all ballooned out and swollen like an egg.
I need my beauty rest so I wont eat people.
Increasing battery voltage and controller current limit will result in a non linear experience