Date: Sat, 29 Sep 2001 19:57:54 -0500 From: David VanHorn <[EMAIL PROTECTED]> Subject: Re: Homebrew battery packs
> > >>I take it therefore using a constant voltage to charge NiMH packs such > as plugging it into the Libretto's 15 volt regulated supply with 4x1N4004 > diodes to drop the voltage (in the hope that once the battery pack > reaches the charge voltage it'll stop charging due to a lack of voltage > gradient) would be a dangerous way of doing things? > >4.1 or 4.2V per cell, regulated within 1%. Current limited, temperature > limited, time limited. > >Too low, less than full charge. Too high, run away. >Umm ... I thought you only needed to have the voltage higher than cell >voltage in order for NiMH cells to charge (and just limit the current). >Where did 4.1 volts come from? That's the per-cell voltage for Li-Ion cells. >1: Use an external pack of 10 NiMH 3.5AH cells (13 if I can get my hands >on more 3.5AH cells but I very much doubt it) connected in series and >plugged into the external DC-IN port on the L50CT WITHOUT any buck or >boost circuit. The existing 10.8V LiIon pack is retained and left inside >the Libretto as backup. If I'm stuck with a 10 cell NiMH pack then I'll >add a couple of AA NiCad cells Be careful that those cells are OUT of the circuit during run-time. If they are left in, you could have very hot or exploding AA cells. >2: Use some voltage monitoring circuit (I just realized I've got a basic >bargraph voltmeter automotive battery warning kit lying around, I might >use that instead) so that the NiMH pack is disconnected (on the battery >side of the circuit so that it won't oscillate when it gets close to 11 >volts) if it falls below 11 volts (1.1 volts per cell), safely above the >10.8 volts of the internal pack so that the Libretto knows when to switch >to internal power. Not really needed. The L will switch automatically. >3: Recharge the external pack independently of (ie. totally disconnected >from) the Libretto using a current limiting circuit (the one at >www.uoguelph.ca/~antoon/gadgets/carnc12.htm looks good, opinions please? >I'm hoping I can get away without having to use the voltage booster if I >charge a 12 volt pack off 15 volts using a splitter cable or alternatively >off a 24 volt wall-brick as I have no idea where I can get my hands on >that IC). If you run at a low current, you can just use a resistor. A 3.5 AH cell will want roughly 350mA, take an 18V wall-wart, and figure the resistor accordingly. It will get warm, but it will work reasonably well. Disconnect after 15 hours. >4: Don't use any peak or temperature detection, instead use a basic 555 >timing circuit and a couple of relays to charge the pack overnight at a >rate of C/3 for 2 hours then C/10 for the rest of the night Dangerous.. If it retriggers, you cook the battery. >(5-7 hours, I might monitor it one time with a voltmeter and see if I can >set a time that's close enough to peak). I realize this won't fully >recharge a totally dead pack, I'm assuming that since I cut power draw at >11 volts (1.1 volts per cell) each cell doesn't discharge fully anyway. Of >course, I'll also make sure that before I charge the NiMH pack I'll >discharge it to 1.1 volts first so that I can be sure I won't overcharge it. 1.0 is generally considered discharged. DONT discharge below that. People do this, and all they are doing is ruining cells. -- Dave's Engineering Page: http://www.dvanhorn.org Got a need to read Bar codes? http://www.barcodechip.com Bi-directional read of UPC-A, UPC-E, EAN-8, EAN-13, JAN, and Bookland, with two or five digit supplemental codes, in an 8 pin chip, with NO external parts. ************************************************************** http://libretto.basiclink.com - Libretto mailing list http://libretto.basiclink.com/archive - Archives http://www.picante.com/~gtaylor/portable/faq.html - FAQ -------TO UNSUBSCRIBE------- Reply to any of the list messages. The reply mail should be addressed to: [EMAIL PROTECTED] - Then replace any text on the message's subject line: cmd:unsubscribe --------TO UNSUBSCRIBE DIGEST------ Do above but with this on subject line: cmd:unsubscribe digest **************************************************************
