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TI BQ769X0 Based Monitors - Project thread
#21
Feeling pretty stupid right now.

So this whole time I was expecting that we'd want a higher balance current than what most boards offer (normal is 50mA or less, with some offering up to ~100mA). So I wanted to have an external board for running much higher currents that would be fan cooled. However everything I'm seeing now seems to be pointing to ~100mA being sufficient for even the large packs we are running. The idea being that if even larger currents are needed to balance a pack, there's something wrong and it should be addressed.

In that case I'll take inspiration from the Libresolar design - at this point I should have just basically used his whole darn board. He uses external FETS to pass the balancing current through a 33 ohm resistor that determines the balancing current. 4v thru a 33ohm resistor comes out to ~120mA (which is sufficient) at around 1/2W power dissipation. I'm not exactly sure how much that will heat up the board or resistor but it does concern me. So what I'm going to do is to get the largest surface mount resistor and use that on the bottom side of the board with copper under the resistor so it spreads the heat out to the ground plane. That way, if cooling is needed, it'd be easier to do on the bottom via a heatsink. Since I can just go with a standard size, anyone can choose to use a lower/higher value resistor to increase or decrease balancing current.

Considering CNY, I need to make the changes and order within the next couple days. Anyone got any suggestions for additional expansions or improvements in the next couple days?
Also Korishan, I looked it up and the suggested schematic does have a resistor between V5x and the voltage input.
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#22
By supporting something like a bp-100 (they have larger as well) you can switch massive current without running it through the board. That would be nice for me. To have the board to keep track of cell voltages and keep it from over charging/discharging. Other than than my portable setup doesn't have too much else going on. Some DC stuff to usb and lights. The main current drain is the victron phoenix 24/1200 inverter which could be pulling 60ish amps.

I'm running the Victron Venus GX software on a rpi with a small touchscreen and usb ve.direct cables to each victron device. I can see how much solar or how much ac/dc power is going out pretty clearly. I can also see total battery voltage but I can't see the individual cells or protect the battery as a whole. So that is where a BMS would come in. Ideally it would run in the background with programmable voltage cut offs and then I can see my usage on the screen.
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#23
I was thinking of using something this: https://www.ebay.com/itm/2Pc-50A-100A-75...2788295709

I haven't used one of those yet, so I don't know how accurate they are. Obviously they would have to be tested for accuracy on the Ohms and possibly at different voltage/amp ratings.

If the BQ couldn't read directly from it, then I was thinking of possibly using an ATTiny to do the remote monitoring, then report back the proper voltage back to the BQ current sensor. After all, it's only reading a voltage, so the ATTiny would just change the voltage on the return.
Again, I haven't done this, so I don't know how accurate this would be, or if it'd even work. I was thinking of going this route so the current shunt could be away from the monitoring station if need be, and still get good results. That way the heavy power leads wouldn't have to be ran in an awkward position. Just drop the shunt right at the battery, or right at the busbar connection before charger/inverter connections.
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#24
I understand the need for something to disconnect for OV/UV protection - what I was wondering was why you chose the VIctron product over any of the other available technologies. EG: DC Breaker, latching relay, SSR etc.

Well, the BQ chip supports a +-200mV signal for a shunt - the board I already have is setup for it, you just gotta plug it in.
For 200A that's a .001ohm shunt, half that for 100A etc. It has a 16bit ADC for current measurement. My problem with using a shunt is the amount of power lost to just using a shunt in general. If you open this other thread:
https://secondlifestorage.com/t-quick-ar...bms?page=4
You'll see the issues with using a shunt for larger currents - basically you are wasting energy as heat whenever you use any current at all - it's always on. By using hall effect measurement, you get less accuracy and probably less robustness - but you lose no energy. The amount of heat generated at 200A is huge - 40watts - that's basically a soldering iron worth of heat.
For now, I'm going to move forward with a shunt since it's already there and supported. I was interested in using the hall effect sensor mentioned in that other thread, but that has a full scale voltage range that is much higher than what is supported by the BQ chip - and more of a nice to have than a need ATM.
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#25
How often are you running 200A?

Wouldnt it be better to just remove one 40w bulb in the house or just make sure your wiring is 1 size bigger? Smile Doing that and you save more than you loose on the shunt and still can keep the shunt and its higher resolution. Many people often get stuck in obvious losses... But whats more obvious is losses everywhere else like in the inverter or in wires between stuff Big Grin
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#26
For me, I'd be using around 40A max, and that's with every light, fan, device in the house turned on. The only reason I'd need a 100A sensor is for those start up times of the compressors to know how high the spikes were. And even then, that surge is only for about 2 seconds, tops.

It's not like we're all running electric heaters to bake paint on cars with these power walls Wink


If you'd like a good hall effect sensor, check this out by ROHM: https://www.allaboutcircuits.com/news/a-...conductor/
It outperforms even a standard shielded sensor

Altho, it's not quite available for prime time yet. Buuuut, might be able to do something comparable on a slightly larger footprint.
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#27
I like the BP for portable installations because I can "power off" the system. I also have a breaker for the system as well but this is a big relay that can take some load.

For the system to power on the BMS would have to agree and the user would need to push the ON Switch. To power off the user breaks the circuit with the switch or the BMS senses a problem and shuts it down.

There are good BMS systems for large scale power wall but not in the 2-4kwh portable range. I'm not going to try and stuff a batrium system into a pelican case that is already heavy. This seems like a good portable alternative.
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#28
(01-25-2019, 01:02 AM)gpn Wrote: I'm not going to try and stuff a batrium system into a pelican case

Common now, it'll fit. You just need a bigger hammer Wink
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#29
Well, my issue is that I am currently running a 12v (soon to be 15v) system. With my 3kw inverter and running a microwave to heat up my depressing bachelor dinners, and let's say the fridge kicks on... It's not so much that it will be often, but it will need that peak power capacity. Eventually, I am going to move to a 24V system for obvious reasons - but until then I don't want to invest in a new inverter, water pump, buck converters for all of my lights, etc.

I actually only use the inverter very occasionally - I've converted most of my electronics to DC - Only my fridge, microwave, and power tools run on the inverter.

I'm working on updating the schematic now - my goal is to finish today and put in an order tomorrow to get parts and boards before CNY. When I finish updating the board I'll send out an update to this thread if anyone wants to pick it apart.

What I really want to do is have the ability to output 24v to trip a breaker, and an on-board relay to trip sensors like the one on the BP. I'm going to try and get that into my next board order. Anyone else got ideas of what else I should add?
For now I'm sticking with the shunt just for ease of use reasons.
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#30
Well it only took 5 hours, but I finished adding:
~200mA of balancing (adjustable, I put in FETS that can do up to 600mA)
5v 3A buck converter to power the ESP8266 (it'll need its onboard voltage reg) as well as to power accessories such as 5v relays and other goodies.
2 MCU pin ports for controlling accessories (not sure what for yet Smile)
Nets so everything should be easier to understand

https://easyeda.com/mikethezipper/10s-ba...-balancing

Now that I finished with the schematic I'm working on the board I'm hoping to have this board ordered tomorrow, so if anyone wants to take a gander and pick at it, now is the time Smile
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