[TriEmbed] Lithium Ion batteries

[Charles West]

Hello,

I'm trying to figure out the best way to power my pedestrian robot and both
Nathan and I are trying to determine the best way to power the
Astropresence telepresence drone.  The task is somewhat complicated by the
fact that we really want both robots to be able to recharge by docking
(rather than swapping out the batteries).

What follows is what we have learned so far mixed with questions we have
for anyone more experienced with Lithium Ion batteries.

Lithium Ion seems the way to go in terms of power density and cost.
However, charging and connecting them can be tricky and some sources on the
web seem to be in contradiction.  We focused in on 18650 cells because they
seem cheap and commonly used.

Everyone agrees that it is problematic to have the voltage on a battery get
too low or too high.  Some 18650 cells come with built in voltage
protection and most battery packs have PCBs to restrict the voltage.

Where people seem to disagree the most is charging battery packs.  Some
sources say that you have to have balancing circuits when connecting
batteries together and charging them.  Otherwise, one battery gets a lower
max voltage and the other batteries are are overvolted to try make up the
difference.  Other sources just connect the cells in parallel and leave it
be.

BatterySpace (
http://www.batteryspace.com/pcmforli-coli-mn-nibatterypacks.aspx) says that
balancing becomes important for >5 cells and that with >20 cells you should
get a more sophisticated battery control circuit.  In reading again, it
seems that they treat parallel batteries as one cell (Is that right?).
They sell voltage restriction PCBs, if anyone is interested.

It is also not entirely clear how sophisticated of a charging circuit is
needed.  I believe the consensus is that merely plugging the voltage
restricted battery pack into a similarly valued power supply is not a good
idea (Pete posted an awesome guide to the charging profiles:
http://www.digikey.com/en-US/articles/techzone/2012/sep/a-designers-guide-to-lithium-battery-charging).
They sell Lithium Ion chargers for different voltages but it is not clear
if they are suitable for batteries in parallel.  It is also not clear if it
would be possible to have an active circuit connected to the batteries
while they are charging.

It would be ideal if the robot could still be active while it was being
charged.  It is not clear whether or not that would confuse the charger
(also the charger might not be able to supply enough current to both charge
the battery and run the robot.

On a separate note, there are many 18650 cells for purchase on Ebay.  Many
sources say that the ones that are labeled 4000 mah are lying about their
capacity and often garbage.  Some of these cells have been reported to be
smaller batteries placed in larger cell packaging with conductive filler.
I believe the industry standard is on the order of 2200 mah to 2600 mah.  I
believe some of the "4000 mah" ones have been reported to have 2100 mah
capacity, so they are not all garbage (but be careful and test a few before
you order a bunch).

Another cheap source for 18650 cells is apparently used and laptop
batteries.  Many laptop batteries are composed of 18650 cells which can be
taken out (there are several youtube videos on the subject).

To some up the outstanding questions:
Can Lithium Ion batteries be connected and charged in parallel without some
special arrangement?  How important is balancing and when should it be used?

Can an active load be left in the circuit while a charger is working?

Thanks,
Charlie West
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