# 72volt battery pack - voltage drop?



## Coley (Jul 26, 2007)

What kind of batteries are you using and do you have an ampmeter?


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## Randeb56 (Apr 6, 2015)

I have used 150a/h UPS AGM batteries. Huge ones at 106 lbs ea. all hooked together in series. I have an amp meter and generally I'm pulling between 150 to 200 amps in the 45 to 50 mph. It can pull over 400 amps on some hills. Can't shift gears on the go so I've tried every gear from 3rd to 4th. I hit 55 mph with 4th but the pull from stop is too slow so I'm back to 3rd. More testing to come.
Really appreciate the help.


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## dougingraham (Jul 26, 2011)

Randeb56 said:


> How low can one go when under load as long as it comes back when not?


There is no generic answer to this question. It depends entirely on the particular battery chemistry and cell construction. However once you go below half the unloaded voltage you have reached the point of diminishing returns as far as power output goes. A 72 volt battery generally has a full charge voltage of around 86.4 so don't go below 43.2 volts because your overall power output goes down when you get into this area even when you draw more amps. Voltage goes down faster than amps go up.

You mention AGM UPS batteries, these are not going to have a long service life in an EV as they are intended to have fairly infrequent use and no deep discharges. They are intended to sit around in a standby capacity kept at full charge. When you daily deep cycle them that is not the intended use.

This question probably should be under the battery section.

You should probably figure out a way to shift gears while in motion. Even if you didn't keep the clutch, you can almost always still shift. Coast down regen is the exception to this as the motor comes to a stop as soon as you put it in neutral. But careful application of throttle should allow you to rpm match and push it into gear. That is just a guess because you don't mention what kind of drive system you have.


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## poprock (Apr 29, 2010)

Hi, I am intrigued too about the not shifting. My 72v system in my Daihatsu Sirion will start from rest in 3rd and only drop from 75v to 71( no ampmetre). Only having one gear will either limit top speed if low or pull excessive amps if high and at the first steep hill will send forth the dreaded smoke. It is virtually direct drive at forklift ratios or Tesla ratios.


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## Randeb56 (Apr 6, 2015)

Thank you Doug Ingraham for your clear answer. I will keep a keen eye on the voltage. Shifting will be quite the accomplishment for me. The Smart car was shifted by a computer thru a stepper motor of some sort via gears. I'm going to attempt at some point trying to get it to shift at least from 3rd to 4th via a mechanical device. No hurry on that one.


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