isolated single-cell Li-Ion charger, advice on how to fix leakage problem - Page 1
Forum section: electronics / projects

author: robert.rozee
author_rank: Frequent Contributor
timestamp: November 12, 2023, 02:50:16 pm
content: hi,i'm working on a plan to repower a Fluke 45 bench multimeter using 8x Li-Ion cells, arranged in 4 strings of 2 cells each. the original meter used 4x lead-acid cells, which are long dead. to achieve this i need a means of charging half of the cells (ie, four of them) from isolated charging circuits. each cell is to be charged from a 4v, 200mA charger with a small micro used to shut off charging when the charge current into a cell drops to below 100mA or so. before anyone makes the obvious comment - i am using a 4v maximum charging voltage and 200mA maximum current to ensure the Li-Ion cells a long service life with minimal risk of any 'exothermic events'. by my calculations the design will yield around twice the run time of the original 4x lead-acid cell solution and around half the charging time.below is adraftof the business end of the circuit i've come up with:the problem is, as designed there will be a leakage path (shown inRED) when not charging, from the +ve end of the Li-Ion cell, back through the B-C junction of the two transistors, 12k resistor, schottky diode and down through a second 12k resistor. this path may also partially turn on the MOSFET used to isolate the 5k1/8k2 voltage divider. end result - a slow battery drain.i've been mulling over this problem for a week or two with little success in finding a workable fix without excessively complicating the circuit or sacrificing precise voltage control - ideally, i'd like to avoid the obvious of moving the schottky diode (and at the same time making it a 1N4001) to the battery side of the charging circuit and boosting the charging voltage to compensate. i also want to stick to easily obtained through-hole components.any suggestions?cheers,rob :-)

author: CosteC
author_rank: Regular Contributor
timestamp: November 13, 2023, 07:59:34 am
content: You put lot of limitations onto yourself. Li-Ion charger with only easy to obtain THT components for start.You can put low leakage diode between BD136 and 5.1k (BTW no designators on parts make communication rather difficult)But, of course, it will force you to increase charging voltage. Alternative solution is use two back-to-back mosfets as 'low leakage' bi-directional switch.You can think of small relay too, working only when charger is enabled. There is no discharge protection if I see.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: November 13, 2023, 01:16:34 pm
content: Quote from: CosteC on November 13, 2023, 07:59:34 amgood point! without considering the reverse leakage current they have, i had wanted to use a schottky blocking diode (on the left) to minimize losses due to voltage drop - there is very little power available from the mains transformer in the fluke 45 as it has been designed for operation on anything from 90v to 264vwithoutswitching of windings. across 8 cells using schottly instead of regular silicon diodes would have saved nearly half a watt.i'm now warming to the idea of just using a silicon diode on the right,afterthe 5k1 resistor, and putting up with the (small) variable voltage drop as the charging current goes from 200mA down to 100mA. this then allows me to throw out the 2N7000.Quote from: CosteC on November 13, 2023, 07:59:34 amproblem is, anything other than a 2N7000 that has reasonable RSD(on) is relatively expensive. i'd prefer the associated components to be well below the cost of the Li-Ion cells they are charging.just did a quick test of Vfon a random 1N4002 diode:50mA - 0.74v100mA - 0.78v150mA - 0.79v200mA - 0.80v250mA - 0.81vi think i can live with this. revised schematic:Quote from: CosteC on November 13, 2023, 07:59:34 ami did consider small relays, but with 8 of them the coil current starts to mount up. also there is the expense.each cell will have a DW01A protection circuit (the assembled boards for this are around 50 cents each) that provides for under-voltage lockout.