[TriEmbed] TriEmbed Digest, Vol 10, Issue 5

Thu Mar 6 20:56:00 CST 2014

Charlie, 

I don’t have much experience in charging mulit-cell packs but I am a bit curious so, I did some reading and found some information you may be able to use.  I happen to be a fan of the 18650 battery as I prefer rechargeable batteries and LiON has some great characteristics such has high current and relatively low leakage.  There are some downsides as have been well documented in the press.  

As to your first question about powering your circuits while charging a multi-cell pack, the answer is yes.  The 18650 batteries are often the building block of laptop batteries as you pointed out below.  Laptop batteries can be charged while the laptop is powered so perhaps that is the model to follow.

The batteries used to make a multi-cell pack are unprotected.  Based on tests documented here, you may want to look at LG or Panasonic cells.  Many of the brands with a commercial presence are simply taking a Panasonic cell, adding a voltage regulation circuit and putting their brand name on the wrapper.

The circuits used to charge these multi cell packs are complex as they need to monitor and balance each cell individually while measuring temperature, current and voltage.  As this articles shows, there is a fairly complex logic to charging these batteries as the level of discharge, age, balance and cell health determine the correct charging mode.  One way to simplify the process would be to use an evaluation board like the ones here.  They are not cheap but they may help you get something running quickly. 

I hope this information is helpful,

Chip

On Mar 6, 2014, at 1:00 PM, triembed-request at triembed.org wrote:

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> Today's Topics:
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>   1. Lithium Ion batteries (Charles West)
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> Message: 1
> Date: Wed, 5 Mar 2014 23:48:04 -0500
> From: Charles West <crwest at ncsu.edu>
> To: TriEmbed <triembed at triembed.org>
> Subject: [TriEmbed] Lithium Ion batteries
> Message-ID:
> 	<CAGojqSmmjp8OE5kZhFQdOEJBtzq1Ca_aKtdxrHPR8NPbv+KK5A at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> 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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