Measuring multi voltages on multi cells

Hi guys, I've just built a 13 channel multimeter. It works fine checking individual cells, but what I intended it for is to check 13 individual cells (4.2v each)in series (54.6v total). When I connect the first set of wires it's fine. But if I go to connect the Next set the earth wire is shorting. I have made a standard voltage divider, what would I need to change to get this to work if possible? Any help will be greatly appreciated.

Thanks

Don't use a GND wire for each ;) Connect GND to battery- of the first. and just use a voltage divider for each next cell. Each divider must work with the series voltage at that point. Or, easier, you can all build them to handle 60V (as safety). Now you can measure the voltage of cell 1, and the combined voltage at each point. But you know the serie voltage of cell 1 and cell 2 is just V12 = Vcell1 + Vcell2. So to get the voltage of cell2 you simply do Vcell2 = V12 - Vcell1. And for the next Vcell3 = V123 - V12 = V123 - Vcell1 - Vcell2 etc

But 13 in series a is a lot! The resolution is not going to be that great as you have now (but it might be okay) 60V / 1024 = 0,059V per step (assuming a Uno-ish aka 10-bit). I think this is still plenty for battery voltage monitoring :)

Ok, awesome thanks I understand a little of what you have said. So I'll have to change the voltage divider for each cell to allow for the voltage increase? And change the code to read the end voltage and divide buy each cell! Thanks again

Voltage is also measeured across something or with respect to something. On a Arduino (in normal mode) this is referenced to GND. And you can't connect GND to multiple places if the batteries are ins eries because that will short them out. So just read the voltage of battery1 (with respect to GND), read voltage of 1 and 2 combined with respect to GND etc. And then just do the math to seperate it for each battery.

If you make the same (60V tolerant) voltage divider (for example 120k and 10k, save for 65V) for each stage math is pretty simple. Then the scale of each measurement is the same.

Yep, got it, thank you for your help. I'm very new to this so takes a while to sink in.

Once again thanks

I think measuring a battery stack with 13 voltage dividers is not a very good option.
The 'top' cell is being discharged by only one divider, while the lowest cell is discharged by 13 dividers.
There are special chips to measure individual cells in a battery stacks, but they are expensive and hard to get.
I have been working on a system with relays.
Small capacitors, one on each cell, are switched between the cell and Arduino's A/D with a small DPDT relay.
The caps are in turn disconnected from each cell, presented to the arduino A/D for measurement, and returned to the cell. Fully isolated, and zero battery drain. This 'scanning' can be done e.g. every 15 seconds.
Measuring e.g. 24 cells takes ~1sec. Only downside so far is relay chatter.
Leo..

That's a nice option as well. But I would find that way to big (and loud) to be useful. You could do something similar only with mosfets or BJT's. But then you still discharge the cells a bit uneven but not all the time.

But I don't think the divider is really that problematic. If you make the divider values a bit bigger, say around 500K, the load on the first cell is less then 1mA. And if you're really concerned about un-balancing the cells you can always mirror the divider to bat+ to evenly load each cell :D

13 cells is an odd number. Your application required 13 and not 12?

12 is much more common to find in industry. You can buy ICs that will measure the 12 cells very accurately → http://cds.linear.com/docs/en/datasheet/680412fb.pdf

You could still use the IC for your 13cell stack, however. You’d need to hook it up to the bottom 12cells and measure them the typical way. You then have a couple options how to measure the 13th cell using the LTC6804 GPIO and Auxilary ADC channels.

Edited to Add: The next question prompted by your post is are you planning to balance these 13 cells? I would guess so since you’re going through the trouble of measuring them. If so, then the IC also can do the balancing for you (although again you’ll need to play some games for the 13th cell).

Yes 13 cells, 13x4.2=54.6v x 4., nominal voltage is 48.1, im making a large battery bank to power my house. So in each pack will be 40 cells. So 4.2v max and 88 amps each. I am running a battery management system to keep the cells in balance. But I want to monitor the voltage on each cell bank. I’m using lithium cells so the charging and discharging is important and I want to be able see that its working correctly. Im not to concerned with discharge as you can tell I will have a lot of power.

For the noise problem, relays can be replaced with an Opto FET.

http://www.farnell.com/datasheets/1644106.pdf

.

Thanks guys for all your help. I like the linear LTC6804-1 that would be perfect but I feel over my head to get it to work. But ill give it a go anyway!!! Lol.

LarryD: For the noise problem, relays can be replaced with an Opto FET.

http://www.farnell.com/datasheets/1644106.pdf .

Only 30volt peak...

Craig_Minca: Thanks guys for all your help. I like the linear LTC6804-1 that would be perfect but I feel over my head to get it to work. But ill give it a go anyway!!! Lol.

So you are an advanced electonics tech with the ability to design a circuit board. And are able to write an Arduino library for that complicated chip.

I'm not, so I opted for relays. Leo..

Wawa:
So you are an advanced electonics tech with the ability to design a circuit board.
And are able to write an Arduino library for that complicated chip.

I’m not, so I opted for relays.
Leo…

I wish Leo, just a pleb mate.

Use 13 high-resistance dividers into an 16->1 analog switch, buffer the output of the switch with an opamp follower. The high resistance means the cells don't discharge too quickly, the buffer allows faster switching between the channels (with high resistance it takes a little while to stabilize due to stray capacitance). Having the dividers first means the analog switch isn't over-voltage, and 2 resistors per channel is a lot cheaper than a relay.

Craig_Minca: Thanks guys for all your help. I like the linear LTC6804-1 that would be perfect but I feel over my head to get it to work. But ill give it a go anyway!!! Lol.

That's sort of the beauty of LTC chips. They may cost a lot, but they take circuits that would be difficult to build discretely and put it in an easy-to-use IC. They also have an Arduino sketch for the LTC6804 on their website that will do the basic functions (read voltages, activate balancers, etc) -> http://www.linear.com/product/LTC6804-1#code

The discrete circuit that it implements is actually pretty similar to what Wawa is doing, except it uses FETs instead of relays. Above 4 cells nobody really uses the voltage divider method. The tolerance/resolution for the cells at the top of the stack make it impossible to get cell voltages to compare to each other, and when it comes to balancing Li-Ion cells a few mV really matter in the flat part of their SOC-vs-voltage curve.

Awesome. This answers a question that I had not asked yet.

Did not know about the Arduino sketch that lineair provides. That makes using the chip a lot easier. Leo..

polymorph: Awesome. This answers a question that I had not asked yet.

Heh...I'm sure you know there are people who are now wondering what your question is?

BigBobby: That's sort of the beauty of LTC chips. They make may cost a lot, but they take circuits that would be difficult to build discretely and put it in an easy-to-use IC. They also have an Arduino sketch for the LTC6804 on their website that will do the basic functions (read voltages, activate balancers, etc) -> http://www.linear.com/product/LTC6804-1#code

The discrete circuit that it implements is actually pretty similar to what Wawa is doing, except it uses FETs instead of relays. Above 4 cells nobody really uses the voltage divider method. The tolerance/resolution for the cells at the top of the stack make it impossible to get cell voltages to compare to each other, and when it comes to balancing Li-Ion cells a few mV really matter in the flat part of their SOC-vs-voltage curve.

Wow you guys are a great help, thanks to all the positive input