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alonso
10-14-2012, 01:32 PM
Hey everyone!

Quick noob question, I bought a LiPo battery and charger right here from Trossen and I was wondering if it's normal that my 14.8v 4S battery charged to 16.5v when using the balance charging?

Here are the battery and charger I'm using:

14.8V 2000mAh Pro Lite Military Spec LiPo Battery
Imax B6 Multi-Function LiPo Balance Charger

I'm using a Dynamixel MX-28T servo which is rated for 14.8v, I imagine running 16.5v would be very bad for it right?

-Alonso

tician
10-14-2012, 02:34 PM
General rule for Lithium chemistry batteries: The max charge voltage of a cell is ~0.5V higher than the nominal voltage, and the max discharge voltage of a cell is ~0.5V lower than the nominal voltage. Most LiPo/Lion cells are 3.7V nominal (4.2V max and 3.2V min), so a 4S pack should go up to 4*4.2V = 16.8V at full charge and go down to no less than 4*3.2V = 12.8V at full discharge. You absolutely must not permit the battery voltage to drop below the lower limit or the cell will be permanently damaged (the charger should prevent the voltage going too high during the charge cycle).

There was a recent post here about using a 4S pack with MX-series dynamixels and the individual followed up at the Robotis Q&A (http://www.robotis.com/xe/qna_en/154542). Apparently the Nimbro-OP (DARwIn-OP derivative/upgrade) uses a 4S pack safely, but the firmware of the MX-series servos has the default highest voltage limit set to 16V so will produce over-voltage errors until the pack voltage drops a bit. The support site (http://support.robotis.com/en/product/dynamixel/mx_series/mx-28.htm#Actuator_Address_0C) claims the range of valid values for the voltage limit is 50 to 250, so it should be possible to change it and prevent the error.

jwatte
10-14-2012, 11:23 PM
Also, if you want your packs to last longer, don't charge over 4.05V, and don't discharge below 3.35V. This is, AFAICT, what electric cars do to get 10 years projected lifetime with daily use of the batteries. Note that most chargers will just cut off at 4.2V, which is the "safety threshold," rather than the 4.05V limit, so you may have to roll your own if you want best lifetime. Or go with a longer-life chemistry, like LiFePO4.

Also, the curve for "voltage vs charge" is not linear at all -- at the very left (low charge) it rises quickly from 3.2V to 3.5-3.6V, and on the right, it takes a long time before it rises above 3.8V/3.9V and shoots towards 4.2V. This makes a "gas gauge" for a LiPo battery pretty hard to build, but it also means you could just use the 3.4V - 4.0V range as your design limit, and not use the extreme values, because the amount of charge (running time) represented by those areas is actually pretty small.