Battery for the Fluke 45 - Page 1
Forum section: electronics / testgear

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 21, 2025, 01:29:52 am
content: I know that the Fluke 45 has a battery option that includes the battery and the charge board. They are impossible to get. My question is if I use an 8V battery can I make the meter run on the battery. I don't mind if I have to remove the battery for charging.

author: J-R
author_rank: Super Contributor
timestamp: April 21, 2025, 01:59:27 am
content: This video may have some insights if you haven't seen it:

author: bdunham7
author_rank: Super Contributor
timestamp: April 21, 2025, 02:45:25 am
content: If you put the battery essentially across C27 (near the power input and switches) it should power up. I think a 12V battery will work fine, probably even 24V! This meter has one of the most convoluted charging and power supply systems I've ever seen! But despite all that, unless I'm mistaken the battery feeds directly to that spot when unplugged and on battery power. Unless you can find an original battery PCA, you might want to just use a 4S Li-Ion battery pack or something like that.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 21, 2025, 08:17:34 am
content: Quote from: bdunham7 on April 21, 2025, 02:45:25 am24V sounds improbable, but I guess it uses an SMPS? I'd still worry about voltage headroom on the components; any tants in that circuit would almost certainly be long past their bedtime...If the original battery was an 8V lead acid, I'd use a 2S LiPo (with integrated BMS); it would be much lighter than a lead acid pack, and likely give more run time. It's my intent to use LiFePO4s (I have a surfeit of NIB ones) in my Fluke 8125A, which has a primitive charger system (filament lamp current limiter) so I might have to add something to suspend charging rather than just letting it trickle constantly.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 21, 2025, 08:19:58 am
content: Quote from: BeBuLamar on April 21, 2025, 01:29:52 amIf you need to bypass the charging circuit in the meter, maybe you could add a dedicated charging connector in the back panel? Would save having to take it apart every time>

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 21, 2025, 10:52:46 am
content: Quote from: AVGresponding on April 21, 2025, 08:19:58 amI rather keep the meter original that is I can return it to original condition when I want. I don't want to punch hole in the case.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 21, 2025, 11:11:55 am
content: Quote from: bdunham7 on April 21, 2025, 02:45:25 amThank you. I have 3 LiFePo4 32700 which are rated at 6Ah. I think I will try that.

author: bdunham7
author_rank: Super Contributor
timestamp: April 21, 2025, 02:16:09 pm
content: Quote from: AVGresponding on April 21, 2025, 08:17:34 amThe system uses multiple SMPS blocks but the real oddity is the linear PSU at the input that uses a regular transformer in a CT-full wave configuration. It has a rated input of 90-264 volts without changing any connections, thus the secondary can have wildly varying output voltages depending on that input. I haven't tested one, but I think you end up with anywhere from 10 to 32VDC over the range of input voltages. The next stages in the PSU/Battery PCA are all designed to handle this.QuoteThis system is designed to charge the battery when the unit is plugged in but turned off and the power switch doesn't cut power to the initial transformer. Any replacement charging circuit can just take power from this. There's no need to bypass anything, without the battery PCA there's no charging circuits installed.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 22, 2025, 10:03:32 am
content: Quote from: bdunham7 on April 21, 2025, 02:45:25 amIf I plug the meter to AC power accross C27 always have 16VDC. If I have the battery connected there it would charge and damage the battery. When it's unpluged I think it's OK because the diodes prevent the current to go back into the transformer. With this setup I will have to remove the battery whenever I want to run on AC.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: April 22, 2025, 01:37:23 pm
content: the Li-Ion battery pack you make up should containTWOcells in series so as to give a combined outputnotexceeding9.6 volts (the maximum of 4 lead-acid cells at 2.4v each). further, the battery pack should be connected to the meter (across C27 is fine) via aDIODE. this diode is to prevent current flowing into the Li-Ion cells if the mains lead is plugged in.if you feed in a higher DC voltage, when run from 90v AC the battery may continue to be drained even though AC power is present.the fluke 45 service manual contains the schematic of the battery module PCB on page 9-13 (pg 177 in the .pdf i have here). as bdunham7 has pointed out, the design isextremelyconvoluted! if you do wish to charge your Li-Ion cells internally, you can pick off volts from pin 4 of SW1 (page 9-7) that is only live when the power switch is in the OFF position. but beware, the maximum power available for charging is rather limited due to the mains transformer being necessarily derated to allow for the wide 90-264v AC operating range.all connection points can alternatively be picked up on J6. this 10-pin connector is rather obscure, but you can unsolder it and replace it with 2 rows (5 pins each) of 0.1" pin headers (just had a thought - consider using one row of male and one row of female so that you can't accidentally reverse the plug).i have a fluke 45 myself, and have variously considered building a battery pack for it, so would be interested in seeing your progress.cheers,rob :-)

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 22, 2025, 01:52:53 pm
content: I intent to use 3 32700 LiFePo4 cells which gives 9.6V.

author: bdunham7
author_rank: Super Contributor
timestamp: April 22, 2025, 02:00:15 pm
content: Quote from: BeBuLamar on April 22, 2025, 10:03:32 amI was just pointing out that the meter runs on plain DC at this point and can be fed directly from the battery. I'm not suggesting you can just wire it in there and call the job done! You'd need to use an isolation diode to prevent back feeding as well as a separate charging circuit tailored for your battery pack. The power for the charging circuit is always there if the unit is plugged in so you don't need an external source, but you would have to do the work of converting that power to what the batteries need. And, asrobert.rozeepointed out, if your battery voltage is higher than the DC power supply, you'd need a switching circuit to disconnect it when line powered. So perhaps a 3-cell LiFePO4 would be good solution to minimize complexity.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 22, 2025, 05:16:07 pm
content: Quote from: bdunham7 on April 21, 2025, 02:16:09 pmYeah, I had a look at the schematics, it's an odd one alright. I'd guess you're right, though depending what gets used as a battery, the circuit will need to be tweaked to produce the desired voltages and currents. Seems doable though

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 22, 2025, 06:23:24 pm
content: Quote from: AVGresponding on April 22, 2025, 05:16:07 pm

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 23, 2025, 01:16:44 pm
content: I think I will solder 2 wires to the capacitor C7 and put a diode in the battery pack. If I can find a connector that fit the J6 that would be great but I don't think I can find that. Anyone has any idea what kind of connector it is?I think I can pop out the plastic cover that covers the HPIB port. The metal plate has holes so I can mount another piece in the with a barrel jack receptacle so I can charge the battery with an external charger. That way I can return the meter back to original condition when I want to.

author: J-R
author_rank: Super Contributor
timestamp: April 24, 2025, 11:22:05 pm
content: Instead of modifying this DMM, you might try a small battery pack with the Samlex PST-120-12 pure sine wave inverter.