12VDC Voltage Level

Hello,
This should be an easy one for all the experienced hands out there...
I have a AB PLC running on a mobile unit that has 12 VDC batteries on it. I am using a voltage convertor to bump up the 12 volts to 24 to run the controller and instruments. What I want to do is monitor the battery voltage on the 12 volt battery. The problem is that the AI cards common is from the 24 volt side. I have some resistors for a voltage divider to scale the battery voltage down to 0-5volts so that is not a problem. What would be some recommendations for accomplishing this? Do I need to isolate the signal somehow?
Thanks!

If none of your analog inputs is isolated then you need to use an isolator. Not sure if joining DC commons will be a problem.

I'm not 100% following your setup. What are you using to monitor your battery level? Are you using a voltage divider to drop the voltage down on the 12volt battery and then reading that voltage on an analog card?

As far as the two separate commons (-) are concerned, you should be able to bond them together along with the ground if you need to. As long as your power is clean, it shouldn't hurt anything.

Re

Thanks for the replies!

I was planning on using an analog input on the PLC to monitor the voltage and I got some resistors and created a voltage divider to scale the battery voltage down and read it on a analog voltage input. So, I wired the 12 volt battery through the voltage divider and into an AI on the PLC. I found that the PLC was not reading the voltage (I checked the input voltage and it was at ~3.5volts and I configured the analog channel for voltage input) and I realized that the PLC "wants" to see 24vdc common wired to common on the AI channel. I hope that makes sense...I am using an AB Micro 850 PLC.

What Analog input card? Most can be configured and wire for differential inputs.

there are small DIN mount devices that convert voltage to 4-20ma. Possibly a bit pricey for this application but they do exist.

Why not using 2 battery to get 24v from the beginning.

The first thing is to make sure the battery negative and the 24v converter negative can be connected toghether. (Some are isolated, some would usually be connected and some could also be connected on the positive side so making a connection on the negative side could lead to a 12v short circuit)

then with s12 and 24 at the same negative potential, your resistor voltage divider would work. In the other hand you would need an isolated input that usually need to convert 2 AI into 1 differential AI
and use a 0-10v input.
resistor for division could be 2/3 like 10k+5k for 15K to read voltage out from 10k (66%) so if battery charge to 15v, you would read 10 at the plc input

Why not using 2 battery to get 24v from the beginning.

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It is a battery on a piece of mobile equipment, and probably Cricha has not been authorized to re-design the electrical system of the mobile equipment. Converting the device from a 12 volt system to a 24 volt system would create lots of new problems to overcome. It may have a warranty that would be voided, and other operational problems may result from adding a second battery, even if there was room for the new battery.

Also, the mobile charging system would need redesign, unless another switching system was installed to allow easily switching the existing system from Bat 1 to Bat 2. But that would also introduce another failure point on the device. Redesigning equipment that has to be reliable in the field is not a small undertaking. It may be that many of the existing 12-volt electrical mobile devices (lights, fans, solenoids, sensors) would have to be replaced. How could you guarantee that the new system would be as reliable as the old?

I realized that the PLC "wants" to see 24vdc common wired to common on the AI channel.

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What it really "wants" is to see a complete circuit (both sides of your analog input. If your input does not and can not share a common with the normal PLC 24 VDC, then you need to use a PLC Isolated Analog Input Module (where each analog input can have its own power supply), or simply use a normal analog input module, but limit its use to only inputs that share the new voltage divider 3.5-volt-level input.