Cool, it is interesting to hear about your progress. Thanks for sharing! On Sunday, April 14, 2019, Bob Blick wrote: > Just a quick followup. I put 8 LiFePO4 AA cells (Tenergy 400mA) in my HP > 403B, using an 8-AA holder with extra taps I added. The charge current on > the 403B is adjustable, HP suggests 6.5mA for normal use, 11mA for cyclic > use with the standard 225mAH NiCd pack. I set the charge current to 11mA. > > Individual cell voltage was quite balanced during charge until about 3.45 > volts, at which point the voltages started diverging. Based on this guy's > analysis I decided that I was fine stopping charge at that point: > https://www.powerstream.com/lithium-phosphate-charge-voltage.htm > > Although the HP 403B retains pretty good balance on the loads from the > different voltage taps on the battery pack, there will be some imbalance, > so I wanted to find out what will happen as I use and recharge the meter. > > Taking a couple of spare cells that were at slightly different charge > levels and charging them in series at about 8 to 12 mA, one cell's voltag= e > rose quickly once it hit 3.45 volts, and I just let it happen. Would the > two cells eventually equalize, or would one of them melt into a puddle? > Maximum charge voltage is supposed to be 3.65 volts, but "that guy" teste= d > his all the way to 4.2 volts. > > The low cell was at 3.42 volts and the high cell rose quickly to 3.78 > volts, at which point the world shifted and the low cell started catching > up. I stopped the test at 3.78 on the low cell and 3.81 on the high cell. > Charge current at that point was about 8mA. > > Anyway, my conclusion is that LiFePO4 cells do self-equalize, but you nee= d > to take them to a higher than normal charge voltage. I think (actually I > know) it's not good to leave them charging too long after they are full. > But I think that for my purposes I will get an adequate number of cycles > out of them and won't ever need to replace them or manually equalize them > in my HP 403B. > > Cheerful regards, Bob > > > ________________________________________ > From: piclist-bounces@mit.edu on behalf of Bob > Blick > Sent: Wednesday, April 3, 2019 2:02 PM > To: Microcontroller discussion list - Public. > Subject: [EE]: Refitting HP boat anchor with LiFePO4 cells > > I have an HP 403B AC millivoltmeter that is battery powered and AC > charged. It's only about 40 years old and the original NiCd batteries hav= e > already quit working. Unfortunately it uses a tapped-battery configuratio= n > with a total of 20 cells, delivering typically +13V, -13V and -6.5V. Othe= rs > in my situation have used a string of 6.5V Zener diodes and electrolytic > capacitors to operate their meters on AC power only, without batteries. > Unfortunately when plugged into AC there is enough transformer magnetic > field or whatever to make the lowest ranges inaccurate, so I'd like to > restore battery operation. > > My hate for NiCd batteries is legendary, so I was thinking about a tapped > string of eight LiFePO4 batteries. At first glance 3.2 to 3.4 volts per > cell looks promising. I don't use this meter very often, so I don't mind > every few months popping the side cover off and checking the charge volta= ge > and cell balance while charging. > > The current drain is not perfectly balanced, it's about 13mA on the -13, > less on the +13 and under 1mA on the -6.5(I forget which way the current > flows). I can add some load resistors to bring it closer into balance. > > The two things that I know least about are the discharge voltage of a > LiFePO4 cell when it's in its happy range, say 50% to 80% charge, if it'l= l > get me close enough to the required voltages. And also, how is > self-discharge compared to lithium-ion which are excellent. > > I know I could just use three 9V alkaline batteries and some > semiconductors to give me a center tap and -6.5, but the battery life wou= ld > be miserable and I don't like using disposable batteries. > > Thanks, Bob > - > --=20 Jason White --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .