Fluke 45 battery option, Li-Ion replacement

[robert.rozee]

hi,
    i recently bought a Fluke 45 with battery option, but of course the lead-acid batteries were long dead. now i'm thinking about ways of getting it running on battery power again, but instead using 18650 LI-Ion cells. has anyone out there tried this? did you modify the existing battery charging system?; build an alternative system?; just fit batteries and use an external charger?; or just bring out the 7-35v DC input to a pair of banana jacks on the back panel for using an external battery pack?

at the moment i'm inclined towards using 8x 18650 cells fitted internally, connected as 4 strings of cells (8v approx) combined together with 4x schottky diodes (one for each string). then a rather complex 'conservative' charging system using 8x B0505S isolating modules (5v DC-DC, 200mA each) and 8x TP4056 charge controllers feeding each cell separately via a schottky diode that drops the TP4056's 4.2v output down to a 3.95v full charge voltage. overall, charging would then be terminated based upon a micro monitoring the input current into each of the B0505S and shutting individual modules when current draw falls below a certain level or a timeout occurs.

the TP4056 modules i'm looking at using cost around 50 cents each and include onboard over- and under-voltage lockouts managed by DW01A's. while the TP4056 has a preset maximum charging voltage of 4.2v, with a series 1N5817 schottky diode in the -ve connection between TP4056 and the BMS circuitry each cell should see about 3.95v at a charging current of 200mA or less. the idea of this is to extend cell life while still yielding perhaps 70% of usable capacity. 8 x 2000mA.h x 3.7v x 70% = 40wh. the old lead-acid battery pack was only around 20wh.

any useful suggestions or other constructive input would be much appreciated before i proceed further...


cheers,
rob   :-)