<div dir="ltr"><br><br><div class="gmail_quote">---------- Forwarded message ----------<br>From: <b class="gmail_sendername"></b> <span dir="ltr"><<a href="mailto:kschilf@yahoo.com">kschilf@yahoo.com</a>></span><br>Date: Thu, Mar 6, 2014 at 2:42 AM<br>
Subject: Re: [TriEmbed] Lithium Ion batteries<br>To: Charles West <<a href="mailto:crwest@ncsu.edu">crwest@ncsu.edu</a>><br><br><br><div><div style="font-size:10pt;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif">
<div><span>Hi Charlie,</span></div><div style="font-style:normal;font-size:13.3333px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif"><br><span></span></div><div style="font-style:normal;font-size:13.3333px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif">
<span>I am not an expert on batteries, but I can share a few pointers. :-)<br></span></div><div style="font-style:normal;font-size:13.3333px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif">
<br><span></span></div><div style="font-style:normal;font-size:13.3333px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif"><span>In general, it is not a good idea to connect voltage sources in parallel without current management. With large loads, you want the sources to share the load to prevent one source from overheating. You should also be concerned about a higher voltage source dumping current into the lower voltage source. Electrons follow potential not human desires. With batteries this is especially critical. If a battery shorts internally it will happily take current from its parallel neighbors. If you are lucky, this will just discharge the other batteries. If not, this large current through the faulty battery can cause a fire. Higher energy density batteries are great under normal circumstances, but they also make a bigger bang when they fail. :-)<br>
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<span>The simplest approach to parallel batteries to to connect them in parallel through diodes. This does nothing for current sharing but does prevent a bad battery from bringing down the system. The cost is a loss of energy = current * the forward voltage of the diode.<br>
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<span>If you have cells in series, charging slows/stops when the first cell reaches its maximum charge. On the other end, the string stops discharging (providing energy) when the first cell is depleted. Balancing the cells helps to age them at a uniform rate. You want all of the cells to have similar charge and discharge characteristics to maximize energy storage/delivery and provide greater reliability.<br>
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<span>Can you charge and run at the same time? The first requirement is a charger large enough to provide charge current and the running current simultaneously. That gets you to the party. The next question is how the power supply in your specific devices are designed?</span></div>
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<span>Be safe. Large battery packs should not be treated lightly. Even modest batteries can provide large peak currents. If you want more information on specific charging profiles or load sharing, I would point
you to one of the semiconductor vendors that make battery charger chips (Texas Instruments, Linear Tech, Analog Devices, etc.). I am kind of partial to Linear Tech (<a href="http://linear.com" target="_blank">linear.com</a>). Any of these vendors will have a great deal of technical data.<br>
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<span>Good luck,</span></div><div style="font-style:normal;font-size:13.3333px;background-color:transparent;font-family:HelveticaNeue,Helvetica Neue,Helvetica,Arial,Lucida Grande,sans-serif"><span>Kevin Schilf<br></span></div>
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<div><div class="h5"> <div dir="ltr"> <font face="Arial"> On Wednesday, March 5, 2014 11:48 PM, Charles West <<a href="mailto:crwest@ncsu.edu" target="_blank">crwest@ncsu.edu</a>> wrote:<br> </font> </div> </div></div>
<div><div><div class="h5"><div><div dir="ltr"><div><div><div><div><div><div><div><div><div><div><div><div>Hello,<br><br></div>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). <br>
<br>What follows is what we have learned so far mixed with questions we have for anyone more experienced with Lithium Ion batteries.<br><br></div>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.<br>
<br></div>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.<br>
<br></div>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.<br>
<br>BatterySpace (<a rel="nofollow" href="http://www.batteryspace.com/pcmforli-coli-mn-nibatterypacks.aspx" target="_blank">http://www.batteryspace.com/pcmforli-coli-mn-nibatterypacks.aspx</a>) 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.<br>
<br></div>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: <a rel="nofollow" href="http://www.digikey.com/en-US/articles/techzone/2012/sep/a-designers-guide-to-lithium-battery-charging" target="_blank">http://www.digikey.com/en-US/articles/techzone/2012/sep/a-designers-guide-to-lithium-battery-charging</a>). 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. <br>
<br>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.<br>
<br></div>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).<br>
<br></div>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).<br><br>
</div>
To some up the outstanding questions:<br></div>Can Lithium Ion batteries be connected and charged in parallel without some special arrangement? How important is balancing and when should it be used?<br><br></div></div>Can an active load be left in the circuit while a charger is working?<br>
<br></div>Thanks,<br>Charlie West<br></div></div><br></div></div>_______________________________________________<br>Triangle, NC Embedded Computing mailing list<br><a href="mailto:TriEmbed@triembed.org" target="_blank">TriEmbed@triembed.org</a><br>
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