Measuring forklift battery's voltage

[ahmedragia21]

I'm trying to measure a fork's lift battery by an ADC. The problem that the output voltage isn't predictable and not constant, so that I can make a constant voltag divider. What I have measure is it goes from 7 to 30VDC, and the max input voltage of my adc is 1.2V. How would I design the voltage dividers and the op-amps in that case?

 

[gary350]

I'm trying to measure a fork's lift battery by an ADC.

Are you saying Amp DC????? Amp meter measures amps not volts. Clean the battery terminal so it looks like new metal. I worked in a battery manufacturing factory for 16 years when a battery gets OLD it will give you crazy voltage and amp readings.

 

[crutschow]

You would use a voltage divider that reduces the 30V to 1.2V. Then 7V will give you 0.28V at the ADC input. Even if the ADC is only 8-bits that will still give a minimum digital reading of 60 (out of 255 total) which would seem to be reasonably good resolution for measuring battery voltage. A higher resolution ADC will give you even more resolution, of course.

 

[tcmtech]

If your 30 volt battery is dropping to 7 volts under a load its time for a new battery or at least a trip to the battery charger and right proper charging.

 

[MikeMl]

If your 30 volt battery is dropping to 7 volts under a load its time for a new battery....

because you screwed it up by over-discharging it. A lead-acid battery should never be discharged to less than 1.8V/cell. I have never seen a 30V forklift battery. I have seen 24V (12cells), 36V (18cells) or 48V (24cells). You may have a 12cell battery which shows ~30V under charge. If so, you should recharge it when it has discharged to 22V.

 

[dr pepper]

You'll get reasonable accuracy by dividing a little over the max voltage say 40v down to 5v for the adc, then you'll have 40/1024 (for a pic on full res) which is 40mV resolution, should be more than enough for a truck driver.
And if you take fairly slow adc readings say twice per second and then average them by 64 (right shifting 6 places) every 32 seconds you'll have a reasonable average of battery voltage, it will still vary if the trucks moving or lifting but if you look at some of the commercial ones made by curtis they do the same.
Put a fuse in the sense wire, current from these batteries is scary during a short.

 

[ahmedragia21]

Thanks for the replies.
The problem that I get that 7-9V when igniting the engine. That's really the main problem also.

Regarding the voltage divider:
Should I count and recalculate the power rating of the resistors based on the total current that will be drawn from all the circuits or the current will be branched to the resistors on its on, like kirchof's law ?

 

[wiliswene]

the battery charger and right proper charging.**broken link removed**

 

[MikeMl]

Most likely, you have a corroded high-resistance connection at the battery terminals, at the jumpers between batteries, etc. Have you measured the voltage at the battery terminals while cranking?

 

[dr pepper]

Are we talking about an engine truck or an electric truck?

I assumed electric as engines are usually 12 or 24v, except for some big ugly trucks which have 36v marine engines.