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Asked — Edited
Resolved Resolved by Rich!

Another Battery Question

Ok. so. If I am using 2 motors that run at 12vdc 1ah each, and I want to run them for 5 hours, do I need to get a 12v 5ah battery or a 12v 10ah battery?


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Total discharge will be 2x1Ah so 2Ah or 2 amps per hour.
To run for 5 hours with a drain of 2 amps per hour you would need 5 x 2 = 10Ah.

This is all assuming the motors will be running 100% of the time at full load. You may be able to apply a diversity. For instance, if the motors only work for 30 seconds every minute you can apply a 50% diversity. If they run for 45 seconds every minute you can apply a 25% diversity. Etc.
@Technopro to add to @Rich's comments, a very common 12V battery size (capacity) is a 7.2Ahr (these are the batteries they use in most UPS's on the market) which kinda falls between the ranges you specified so maybe it'll work nicely for you.

In addition to not running the motors full time, you could also apply PWM to the motors as well to not only slow the robot down a bit but to save on Battery life as well. PWM is often used in lighting applications to save on energy but the same concept can be applied to Robotics (motor control) as well.

If you want to measure the life you'll get out of the battery before it is completely discharged I found a nice explanation here. One thing to keep in mind is that you don't want to fully discharge a lead acid battery, you'll want to keep the battery over 10.5VDC I believe to prevent sulfation and prolong the battery life.

I ran into this info while researching Desulfation of lead acid batteries which I am doing a bit of these days at home. Sulfation is the #1 killer of lead acid batteries but good news is that they can be recovered most of the time (albeit it's a slow process) with some distilled water to top up the electrolyte and a desulfation circuit (with charger) to breakup the sulfate crystals.
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I have a script that can be used along with a battery monitor circuit (like the LiPo one) for SLA batteries, with the voltages for full down to "empty".

A 12v battery has the following;
100% = 12.7v
75% = 12.4v
50% = 12.2v
25% = 12.0v
0% = 11.8v

Anything below 11.8v may cause permanent damage to the battery. At least that's what I read anyway.

Gel are slightly higher but the 0% is still 11.8v
ok thanks. so I need to watch the discharge.
Hey! Don't some voltage regulators tell you the input voltage? I could just check the voltage every so often and make sure I don't go under.

If anyone knows of a regulator like this let me know!
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You need to watch the discharge with any battery if you want it to last a long time.

What's wrong with using a simple voltage divider and using the EZ-B to check the voltage through the ADC every few minutes? A 1/3 or 1/4 divider would be OK for a 12v SLA.

You could also use one of those cheap volt meters from ebay, just strap it across the supply, no need for a voltage regulator.
ok thanks. Both of you gave good advise but rich seemed to directly answer my question, so ill give it to him.