Please,I need some advice for measurements 38 cells on string,for forklift battery,each cells is 2V DC,38x2V=96V with CD4051 multiplexer and if possible Arduino mini PRO and WiFi modul for remote measure cells.If you have experiance with this combination or some sugestions please halp,but just with this components.MUST be a very,very cheap!Thanks
Hi and welcome.
Do you need to measure each cell separately, or just the whole battery? If each cell, why is it important to do this remotely? Could you not measure the whole battery remotely, and if there is a problem, bring the forklift to the repair area and then measure each cell by hand?
Instead of a pro micro and wifi module, you could consider an esp8266 board such as a wemos d1-mini.
You will need at least 5 x 4051 multiplexers (plus one more if you choose an esp module, they have only one analog input). You will need a voltage divider for each cell to bring the voltage down to a safe level for measurement.
Paul
There are special ICs for battery stack measurements, but 38 small DPDT relays with "flying capacitors" could be the easiest/safest/cheapest DIY approach.
A (100n?) cap is connected to the two common contacts.
In rest, the relay connects the cap to the battery cell.
Activated, the relay connects the cap to the analogue input and ground.
Cells can be scanned one by one.
Leo..
Hi,and thank for you supports.I must measure each cell separately,and do this on the road when everything on forklift works(all motors,pump,electronics.......and use max energy from battery),and must send this via wifi on PC and save on memory like graphics.Why?When you have problem with battery,you dont have problems with all cell,just with 2-5 max,but when you want to find this cell you must put a big load and discharge all,and measure.For this i spend 7-10 h,and dont have graficons,just measure with DMM every 15-20minuts!Big jobs for nothing:)Just I need this project.I know for other IC for measure cell,but in Serbia,I have a very big problems too buy,and imports,and........develops....Arduino have everyvhere for small price!I dont need to measure all cells in same moments,maybe 2-5 cells on secon be OK.This project for forclift in company verehouse where I works and I have problem with menagers about meintenance!
The problem is the high voltage of the stack. >=95volt when (fast)charging.
No common IC is able to handle that.
That's why I recommended looking at small/cheap DPDT relays and "flying caps".
You could measure two cells at once. That would halve the number of relays to 19.
Leo..
Hi,
What type of cells are they?
If lead acid the 38 * 2.15 = 81.7V
38*2=76V
Tom... 
I assume deep-cycle lead/acid is used in forklifts.
2volt/cell when flat, 2.135volt/cell when charged and left to rest overnight, 2.3volt/cell when charging, but can be ~2.5volt during balance charging.
So two cells can be measured at the same time with Arduino's 5volt Aref without using a voltage divider.
Important to not "load" the cap with a voltage divider in a "flying capacitor" system.
Leo..
Lead acid batteries while charging can rise to 2.7 volts per cell depending on the type of the charger that is been used or equalisation method specially in forklifts. There is away that it can be done individually to measure each cell is start at cell one then resistor divider on cell two subtract cell one then the same for cell three subtract cell one and two but this becomes messy and complicated towards the end and lots of coding. The trouble with measuring the total voltage is the you could have some weak cells and have lower voltages than the others which would reflect on the total voltage making it harder to detect how much charge remains in the battery. The only true way to test the battery is load test and measure each individual cell plus the SG readings. I worked in the forklift industry with batteries and chargers for 10 years. Some forklifts already have this type of system fitted sending information who's driving how many hours it's been operated and the voltage/remaining charge with various points installed around the factory but these are not cheap and not an easy task for an arduino. Companies have spent thousands of designing them.
That's my input/ 2 pennies worth
A resistor divider gets messy and inaccurate with a stack of a few cells, and discharges the cells unequally.
There are opamp circuits that bring each cell voltage down to the A/D input range, but that also gets messy over 30volt, and also discharges the cells unequally. DIY Lithium Balance Charger - Project Guidance - Arduino Forum
A "flying capacitor" design takes a sample from each cell, and presents it to a single A/D input.
So zero load for each cell, and same measuring error for each cell.
Resolution/stability can be maximised by using 2.56volt or 3.3volt Aref.
A Mega already has a 2.56volt Aref, and enough ports to control 38+ relays.
Other Arduinos need e.g. 38-output shift registers.
A small DPDT relay and logic/driver could cost only ~$1 to $2 per cell.
Leo..
Yes Steveiboy,when charge battery on equi. go to 2.7V per cells and we have 38x2.7V,but......may be with some resistors or...?MGE have a 20years old project with relay and........I dont like this,big dimmension,.....I must everithing save of acid......
zambezi5:
MGE have a 20years old project with relay and........I dont like this,big dimmension,.....I must everithing save of acid......
I'm talking about 1x1x2cm relays. some of them are factory potted/sealed. Not hard to do that yourself.
Leo..