09-08-2020 06:24 AM
I'm attempting to measure cell voltages using an NI 9205 module configured for differential input. In total I need to measure 26 cells which I had planned to split across two modules. Connection to the battery is in blocks of four cells. When connecting the first block of four, the values (channels AI0, AI1, AI2 and AI3) in NIMax are correct (checked with a DMM). Connecting another block of four cells resulted in channels AI4 and AI5 reading correctly but channels AI6 and AI7 reading incorrectly. Connecting a third block of four cells (twelve in total) results in none of the channels giving representative values. I've learnt that this issue could be because I've exceeded the COM channel voltage limit (+/-30V) for the module but I'm not 100% sure.
As a work around, I've only used six out of the possible sixteen channels on each of the two NI 9205 modules. The remaining cells have been connected to 3x NI 9219 modules and 1x NI 9215. In this configuration, I haven't seen any of the behaviour from before.
My questions are:
Am I correct when I say that the COM channel voltage limit of the module has been reached, explaining why the values seen in the first configuration are not representative?
If the above is the case, is there a workaround to be able to use more channels on the module? (The maximum individual cell voltage that will be seen on a channel is 4.2V).
If there is no workaround to be able to use more channels on the NI 9205 module, what would be the best analogue input module option giving the most usable channels on a module for the test set-up described?
09-10-2020 07:56 AM
There's a few things to check first - you mention that we're using differential. Is this grounded or floating differential signals?
Secondly, it's a possibility that we're getting ghosting depending upon the speed that you're acquiring at. NI has a KB on dealing with this: https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019KzzSAE&OpenDocument=&l=en-GB
A quick check to see whether we're reaching the COM channel voltage limit - what happens if we connect 7 signals (7*4.2 = 29.4V)?
Finally, I think the best way to get a recommendation of a module is to contact NI sales.
05-27-2021 07:31 AM
I am facing similar issue, one worth to try option is to split the voltages to both direction + and minus voltages, with the COM across 1 Mohm resistor to the middle negative terminal of that or to the one negative cell terminal, that either of the one goes above 30 volts, then you could perhaps reach all in total 60 volts measuring range with the minus plus negative voltages measured in series connection of batteries.