Date: Sun, 30 Sep 2001 08:34:07 +0800 From: Raymond <[EMAIL PROTECTED]> Subject: Re: Homebrew battery packs
>>Since the extended pack is 2.4AH, I'm hoping to get a few hours runtime on this >3.5AH pack ... if I can figure out how to charge it without blowing it up! I'm not >too concerned if the external (12 volt) battery pack won't charge the internal (10.8 >volt) battery pack (although it'd be nice if it did) ... of course, if it DOES charge >each battery individually then why would it need 15 volts? (I was of the >understanding that older laptops needed 13-15 volts because they needed to have an >input greater than the packs they were charging which would have been all in series >and charged all together). >The L does contain battery charging circuitry, but nobody I know has a schematic, so >I wouldn't try to make use of it. It's designed to work with Li-Ion cells, and that's >a totally different game. Sorry, me being ambiguous again, I was meaning what are the chances that the Libretto charge its INTERNAL 10.8 volt pack if I put 12 volts into its EXTERNAL DC in port (ie. will the INTERNAL battery charge off any EXTERNAL battery I plug into the DC in port). The bit about 'why does it need 15 volts' was me just thinking out aloud ... sorry, I tend to type straight off the top of my head and things sorta get mixed up ;-) >>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? Until I woke up this morning (Australian time) and read the posts that accumulated here I was confident that I had a few ideas for using this pack (bear in mind whilst it'd be nice for this pack to last for a while, I'm trying to do this as cheaply as possible - I'm only a student and I already blew my budget getting the Libretto in the first place ;-) ... if anyone has a moment I'd appreciate any opinions or potential warnings ... sorry if anyone has already answered any of these and I've forgotten, I just woke up ... you reserve the right to abuse me in the list ;-) I've read the Maxim data sheets and a number of designs based around the MAX172/173 (as well as some designs based around PIC microcontrollers), unfortunately it is quite a bother trying to get components like that round here and besides which my propensity to overheat and destroy ICs seems to be directly proportional to how much they cost (I've destroyed about 20% of the PIC16F84's I've touched but I've never fried a 555 for instance!) so I don't think using a MAX173 or any dedicated monitoring chip is a viable option for me ... besides which I'm not sure I need the features on that thing (eg. I don't need to fast charge the packs, I don't need to be able to run the Libretto off the same pack that I'm charging and I'm quite happy with an overnight charge cycle). Anyway here's my idea (if some of my bits and pieces sound funny, it may be because I'm also trying to build as much of this as possible with stuff I've got lying around). I do realize my life would be a lot easier if I used a sealed lead acid pack but I'm trying to make use of the NiMH pack I already have and I'm trying to spend as little as possible ;-). 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 and hook them up in series using Xin's diode circuit in order to start the Libretto (I'll check the diode direction again but it looks right to me ...). I'll add a resistor and diode and wire it up so that I can get those cells to trickle charge when I apply the charging voltage for the main pack so they don't die on me. 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. 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). 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 (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. Phew! Well, if anyone has a moment to comment, I'd greatly appreciate it! - Raymond ************************************************************** 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 **************************************************************
