Battery voltage monitoring questions.

jdolecki

I have a mobile robot that has two 12v batteries in series for 24v drive system. It will be out away from its charging station so i would like it to know that it can only go so far and still return in time before it does not have the required battery life left to make it all the way back.

From what I have read rechargable batteries usally provide a consistant 12v but at a point the voltasge drops off rapidly. Until they need to be recharged,

So if I measure voltage and it reaches it discahrge point I may not have enought battery to get back.

Or should i measure the ammount of time current is flowing and possiblly how much current is flowing would I get a better idea of battery life?

thanks john

Leon

I'd measure the current and voltage. You should be able to come up with an algorithm to calculate how long you have before the batteries need to be recharged, knowing the characteristics of the batteries. Your battery manufacturer should have some data that will help.

RobotWorkshop

jdolecki wrote: »

I have a mobile robot that has two 12v batteries in series for 24v drive system. It will be out away from its charging station so i would like it to know that it can only go so far and still return in time before it does not have the required battery life left to make it all the way back.

From what I have read rechargable batteries usally provide a consistant 12v but at a point the voltasge drops off rapidly. Until they need to be recharged.

Calculating the remaining capacity can vary quite a bit depending upon the battery chemistry. What type of battery are you using? Is it built up from Nicad's, NiMH, LiPo, or SLA type batteries?? From you post I would guess that yo may be using a pair of 12V SLA batteries to get your 24V power for your robot. Find out for sure what type of battery then you can figure out what you need to monitor for. Also, the remaining time will vary depending upon what load your putting on the battery vs the Ah rating of the set.

As I recall the Nicad's hold their voltage well and then drop off rather quickly as they are getting close to discharged. On the other hand the SLA type batteries taper off more slowly.

If you measure the voltage make sure you factor in that when the main drive motors turn on the voltage may drop below your threshold for a moment when they start up but then be ok. You may want to consider that and average out the readings (or ignore a quick drop). It all depends on your platform and you'll need to experiment to see what works best for you.

Robert

kwinn

jdolecki wrote: »

I have a mobile robot that has two 12v batteries in series for 24v drive system. It will be out away from its charging station so i would like it to know that it can only go so far and still return in time before it does not have the required battery life left to make it all the way back.

From what I have read rechargable batteries usally provide a consistant 12v but at a point the voltasge drops off rapidly. Until they need to be recharged,

So if I measure voltage and it reaches it discahrge point I may not have enought battery to get back.

Or should i measure the ammount of time current is flowing and possiblly how much current is flowing would I get a better idea of battery life?

thanks john

Most batteries have an Amp-Hour rating and I would use that as the basis of estimating when to start back. A typical lead/acid battery with a 10AHr rating should provide 1 amps for 10 hours and 2 amps for somewhat less than 5 hours when it is fully charged. The amp hours available decreases with increasing current. Measure the current drawn once per time period (second?) and keep adding it to a variable so you can calculate the amp hours used.