# Measuring individual cell voltages in a series cells Battery

I'm making a 600V battery, and I'm trying to design a Battery Monitoring system, that measures (and keeps log of) each cell's voltage turn by turn, in a series configuration of 162 lithium cells. 162 cells x 3.6 volts per cell = 600V battery. A simple Arduino Analog input reads each cell in sequence, here's how: Each two cells (+ve to -ve) connection has a third wire going to an optocoupler collector and its emitter is going to either of two common lines. All of these 162 OCs are switched by a 162 bit shift register. The two common lines go to Arduino Analog Inputs (A0) and (A1).

Keeping all OCs off, when I switch on only 1st two OCs, (O1 and O2) Cell 1 (C1)'s +ve comes to A1 via Line 1 and -ve comes to A0 via Line 2. Keeping A0 digitally Low, thus making it GND, can read voltage of C1 at A1. Then switch on only OC3 and OC4 to read C3's Voltage at A1, and so on for C5, C7... C161. Then digital low at A1 and read at A0 for C2, C4,... C162.

Should this work?

• May I ask a question? What are you trying to do with a 600V battery pack? – Long Pham Sep 16 '18 at 3:45
• Warning: magic smoke may appear and you could kill yourself. – Long Pham Sep 16 '18 at 3:48
• Yes, this is an extremely dangerous project, so do take every precaution. I assume the reason you want this circuit is as a precaution against overcharge and overdischarge. Lithium ion batteries are potential firebombs and a great amount of skill and effort can be required for large banks. Aside from the intrinsic risks and challenges of Li-Ion, 600VDC is an extremely dangerous voltage, especially in the case of a battery, which cannot be "shut down" as a motor can. Take every precaution because your life and possibly the lives of others are on the line if you proceed with this project. – K H Sep 16 '18 at 4:07
• clever idea except 600V exceeds the highest rated one on D-K site of 400V and reverse CE voltage fails without a blocking diode, so 2:1 division maybe possible – Tony Stewart Sunnyskyguy EE75 Sep 16 '18 at 4:31
• I'm making the battery to drive a 3 Phase Induction motor through a VFD that accepts DC input. The one I have is Siemens Micro-master, that needs 600v DC at input to produce 220V variable frequency output. This is a hobby e-bike project. True it is dangerous, and I try to be as cautious as possible. – Abdullah Sep 16 '18 at 12:17

This approach might work... but it is far from optimal or safe. This is not how things normally done.

First, you need to minimize the drain on the battery form your monitoring solution. Optocouplers are not good for this.

Second, there are many more equally important things that need monitoring, like temperature, for example. Industrial battery monitoring solutions measure all that, estimate each cell impedance, life time etc. And they also monitor the integrity of the monitoring system itself.

Finally, there are proven solutions for this. Specifically - multicell stackable monitors, that can be daisy-chained for much simpler wiring than in your proposed solution.

Take a look at TI lineup, for example. The monitor chips like bq76PL455A-Q1 can monitor 16 cells and can be daisy-chained for up to 16 devices, for 256 cells total. Some of them even include balancing circuits (which you have no way of doing in your 162 cell battery).

So, my advice would be - stop trying to reinvent the wheel. Use proven industrial solutions with automotive grade components and your will save yourself a lot of grief.

Oh, and one more thing. I cannot imagine anyone assembling this kind of battery without thorough testing and matching cells first. After all, the battery is only as good as its weakest cell. So, make sure you get more cells than you need, label them (or write down serial numbers if they have them), keep a spreadsheet with all cell data. Buy a bunch of charger/discharger modules if you want this done under a month.

• Thanks for the link, I'm going to give it another detailed read after this. I have made a Jig to charge up 18 cells in parallel, for this purpose. And yes I am keeping logs of all these cells, its tedious job. You pointed a very right concern of High drain, I have been thinking for a solution to that. – Abdullah Sep 16 '18 at 12:08