35V-60V DC voltage detection

Thread Starter

deepak4you

Joined May 1, 2010
51
Hi,
I'm working on a project for a hybrid with Lithium batteries and I wish to incorporate automation to detect battery voltages as below:

1. Cut-off battery pack completely when voltage reaches 47.6V (14 cells x 3.4V/cell)
2. Cut-off battery charging when voltage reaches 57.4V (14 cells x 4.1V/cell)
3. Some other extreme and intermediate cases as well which I will incorporate as and when I develop them

I am inclined towards using a uP for voltage detection since it gives me flexibility and ability to add complex optimisation algorithms for battery management, whereas a simple analog cut-off circuit is ok as well for now.

I could use a simple resistive divider and an ADC/Comparator to achieve the above by dropping the voltage down proportionately and measuring the same at the uP/Comprator directly, but the variation of the output of the divider with temperature will be far too huge to be applied to extremely sensitive Lithium batterries.

Is there any other method I could use? I wonder what is used for digital voltmeters that measure such wide and high voltage ranges?

Appreciate your help with this one.

Regards,
Deepak
 

danadak

Joined Mar 10, 2018
4,057
Is there any other method I could use? I wonder what is used for digital voltmeters that measure such wide and high voltage ranges?
Basically resistive divder network for each range.

A PSOC4 could easily handle the measurement and control. Software/compiler free.

http://www.cypress.com/products/psoc-creator-integrated-design-environment-ide

Board is cheap ($4) -

http://www.cypress.com/documentation/development-kitsboards/psoc-4-cy8ckit-049-4xxx-prototyping-kits

Has onboard reference, 12 bit Sequencing Sar for measurements, routable, OpAmps.....

Google "PSOC 101 Video" for a series of videos on working with PSOC.

Regards, Dana.
 

crutschow

Joined Mar 14, 2008
34,285
I could use a simple resistive divider and an ADC/Comparator to achieve the above by dropping the voltage down proportionately and measuring the same at the uP/Comprator directly, but the variation of the output of the divider with temperature will be far too huge to be applied to extremely sensitive Lithium batterries.
Good 1% resistors have a low temperature coefficient, so I don't think that should be a significant problem.
I wonder what is used for digital voltmeters that measure such wide and high voltage ranges?
They switch ranges, which is basically switching in a different resistive divider at the input.
 

AnalogKid

Joined Aug 1, 2013
10,987
Voltmeters use custom designed resistor networks that are laser trimmed to the requires precision. Because all of the resistors are on the same substrate, they ratio-track extremely well over wide temperature ranges.

0.1% tolerance resistors are surprisingly low cost these days.

A simple resistive divider will make for relatively noise measurements. For example, a range that captures the data in post #1 is 45 V to 60 V. If you divide the 45 V down to 1 V, that means the 60 V appears as 1.33 V. That is only 10% of a 3.3 V full scale A/D input range. However, if you subtract 45 V from the input, and then divide that by 5, the voltage range into the A/D now is 0 V to 3 V, over 90% of a 3.3 V input scale. Or divide by 3 for a 5 V input scale. More complex, precision voltage reference, blah blah blah - but better.

ak
 

ebp

Joined Feb 8, 2018
2,332
Resistors with 0.1% tolerance and 25 ppm/K temperature coefficient are readily available and inexpensive. Significant improvement on those spec's will get you into the one US dollar per resistor range. If you want really good resistors you buy something like Vishay bulk metal foil resistors (the thing that Vishay made its name in long before acquiring a bunch of other companies), but they are several dollars per resistor.

If you are going to spend money on good resistors you had better be certain your voltage reference is even better.

Merely monitoring the top end of a lithium ion battery is not even remotely adequate. The voltage at every node must be monitored.
 

Thread Starter

deepak4you

Joined May 1, 2010
51
Thanks a ton for your inputs guys. I found this on eBay:

https://www.ebay.com/itm/1000pcs-KI...680388?hash=item211c1e7b04:g:bAoAAOSw4GVYSbSl

Looks like a good deal. I'll look at my calculations and order accordingly.

@ebp - would it be that bad or inadequate to monitor the top end of the Lithium pack? I mean the voltage should eventually distribute out over all the connected cells, no? I read somewhere that a balance charger should not be necessary and I could charge the pack directly as long as the voltage/current source is precise, stable and set to the correct voltage.

For voltage reference, any easily available recommendations that I could use?

@AnalogKid - For substracting, say, 40-45V from my range, I'll again need a component like a zener, but that too has to be extremely stable. Anything else I could use?

Regards,
Deepak
 

ebp

Joined Feb 8, 2018
2,332
No, you cannot rely on putting a voltage across the entire pack and have the cell voltages balance. It is one of the major drawbacks of using lithium ion cells in series.

Everything is fine until an imbalance starts to develop. If a cell's capacity is reduced, for any reason, below that of the other cells in the string it will reach maximum allowable voltage while the other cells are still able to accept charge. It will also be the first cell to discharge and be taken below its minimum allowable voltage.

If you don't know EXACTLY what you should be doing with lithium ion batteries you should not be messing with them. Violent explosion is a real possibility.
 

Thread Starter

deepak4you

Joined May 1, 2010
51
@ebp - copy that. I'll get a 14s BMS module to ensure safety. Since I'm planning to put 14s4p arrangement to get about 10Ah battery capacity, I believe I'll have to use 4x BMS modules to cover each and every cell in the pack? I'll probably order 4x of these:

https://www.aliexpress.com/item/51-...or-51-8-V-lithium-ion-battery/1206415913.html

@AnalogKid - I understand resistors are too tiny, but I think I'll manage to solder it. More so because I don't have an option right now. Its extremely difficult to get these components in India. So I'll make-do with what I can get my hands on.

Also, it looks like the BMS module will do almost everything that I want to achieve. I just can't find the specifics that it will disconnect the batteries when each cell reaches 3.4V. Probably for that, I could use a bunch of resistors, comparator and a MOSFET to disconnect the whole pack when voltage reaches those levels.
 
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Thread Starter

deepak4you

Joined May 1, 2010
51
@danadak I am bit more familiar with Arduino IDE, so that way I could use an ADC with ESP8266/ESP32 which will give me networking capabilities as well. My concern was related to how to detect the high voltage from a uP board. Other's have helped me get some direction for the same. Thanks for your inputs
 

Thread Starter

deepak4you

Joined May 1, 2010
51
Considering the availability of resistors, below voltage divider seems to give me a good measurable range that I can easily use it on uP and comparator alike:

- R1 = 996K (0.1%)
- R2 = 49.5K (0.1%)

For a voltage range of 45V - 60V, R2 correspondingly and proportionately gives me 2.13V to 2.84V respectively. I can use an external ADC connected to ESP8266 (running on 3.3V), measure the voltage there, control some output pins for integrating few MOSFETs and also have the same available on ESP8266 web server to have the info available on a portable device like a mobile phone.

Does this sound too simplistic?
 

AnalogKid

Joined Aug 1, 2013
10,987
Depends on the voltage reference for the A/D. To resolve 0.01 V out of a 3.30 V range you need a 9-bit accurate *system*. That means that the converter non-linearity, voltage reference, and input divider *each* must be greater than 13-bit accuracy. Basically, 0.025% accuracy all around.

1 part in 330 = 9 bits
+/- 1 bit quantizing error (shows up in the output as an offset error)
+/- 1 bit integral and differential non-linearity (shows up in the output as an offset error)
+/- 1 bit reference error (shows up in the output as a gain error)
+/- 1 bit input attenuator error (shows up in the output as a gain error)

Note that offset errors appear as a larger percentage error at lower input voltages.

ak
 

Thread Starter

deepak4you

Joined May 1, 2010
51
Wow, that's quite detailed information. Thanks a ton. I'll be using a ADS1115 16bit external ADC with ESP8266 and also the voltage range in question is also in the mid to higher side of 3.3, so I guess it should be ok based on your inputs.
 

Thread Starter

deepak4you

Joined May 1, 2010
51
I got the resistors and the BMS chargers as well. I first need to get the chargers out of the way so that my vehicle can be tested. I'm having a little trouble trying to get the BMS chargers connected. Attached is the battery bank setup. Its a 14s4p arrangement. Here is the problem:

1. Since I have 4p arrangement, I got 4 BMS chargers
2. Each BMS charger will receive connections from each cell of the individual battery packs so that individual cell's charge/voltage can be monitored
3. Of what I understand, each BMS needs a connection across the whole single pack to monitor the total voltage across the single pack i.e. output across AA, BB, CC or DD in the attached diagram.
4. Now if my 4x14s packs are connected in parallel and if i connect all 4x BMS chargers to the +/- terminals of the entire battery bank, then this voltage might not be the same as the individual 14s pack due to imbalances across the packs.
bpack.jpg
Question:
What is the correct way to connect these 4 BMS chargers across my 14s4p battery bank when I need to charge the batteries?

Thanks
Deepak
 

Thread Starter

deepak4you

Joined May 1, 2010
51
I also just sqe a video where the guy connected individual set of cells in parallel like inside a battery bank, and then connected those units in series. This way, he just needed one BMS for charging

Is that a better way of making the whole battery bank, unlike mine?

I'm confused even more now. I don't wish to do rework on this battery bank later, so please advice which would be the best way to arrange the batteries and how to connect the BMS for charging it.
 

olphart

Joined Sep 22, 2012
114
May be a bit late, but a trick I've used is a negative voltage regulator.
Swapping "ground" and input puts the most positive and negative terminals in proper order.
So, ground is positive source input, negative input is put to ground.
This configuration precisely subtracts the regulator rating.
I've used 7908 on 13V source to get 8-13V as 0-5V. and a 7915 for a 24 volt version.
 

Thread Starter

deepak4you

Joined May 1, 2010
51
Thanks @olphart I'll try that as well. Right now I'm a little bit stuck with the battery configuration as to which one of the two configurations is better, to which I haven't received a reply yet. Once that part is done, I'll try the regulator trick to drop the voltage for sensing.
 
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