9v battery replacement circuit for Fluke 70 series and others

[robert.rozee]

am not really sure if this should be under test equipment or projects. we shall see how the discussion goes...

i've been playing around with some chinese MT3608 boards recently, these are a small boost converter. as the chip used has an ENABLE pin, i wondered how it may be used as some sort of 'automatic on/off' arrangement so that a Fluke 77 or similar multimeter (9v battery) could then be powered from a single LiFePO4 LSD cell. i came up with the following, a modification of the existing boards that i had bought 5 of:



when the F77 is switched off, the ENABLE pin is pulled low and the whole circuit draws 15uA less than 1uA. with a 500mAh LiFePO4 cell this would take several decades to completely drain the battery. while the cell's voltage appears at the output (via L1 and D1), with the F77 turned off this goes nowhere and so doesn't matter. the 4v7 zener + 18k resistor could be replaced with a 8v2 or 9v1 zener, and this may well reduce the 15uA drain (of which most is through the zener).

when the F77 is switched on, the draw from the LiFePO4 cell is 1.4mA, plus around 3x the current draw of the F77. because of the extremely low current through the 6v2 zener when the boost converter is running (270uA) the zener may need picking by trial and error if the 18k resistor is eliminated and a higher voltage zener used 8k2 resistor may need tweaking to get just the right output voltage. with components shown, the voltage presented between B+ and B- terminals of the F77 sits at about 9.1v 8.9v.

NOTE: i did try feeding the 4v7 zener from the other side of the power switch - this produced BAD results, with the boost converter occasionally glitching when the power switch was moved. i've not looked too closely at workarounds.

the LiFePO4 cell i'm looking at using is a 14430 size, and will easily fit within the envelope of the F77's existing PP3 battery, with space for the boost converter board alongside. Apart from the B+ and B- connections, just a single wire then needs to be run to the 'other side' of the F77's power switch.

i've NOT tried this in a real F77 yet, just on breadboard. does it look like an interesting solution?


cheers,
rob   :-)


EDITED: changed component values from 4v7 zener and 18k resistor to 6v2 zener and 8k2 resistor. this reduces 'off' current drain to less than 1uA. text amended variously to match new parameters.

[mqsaharan]

The downside I am seeing with the circuit is the unknown state of charge for the LiFePO4 battery. How would you know when to recharge or replace the battery?
Unless 9V batteries are hard to come by, for 70 series and I mean for up to series III the battery life is quoted as 2000 hours for alkalines and 600 hours for carbon zinc. Don't you think such long battery life makes this circuit/solution somewhat unnecessary.

[robert.rozee]

modern alkaline batteries leak, badly. many on here have had equipment destroyed by this, myself included.

a discharge monitor could no doubt be added, although the MT3608 does have a 2 volt under-voltage lockout built in.  so far the circuit/idea is just at the 'proof of concept' stage.


cheers,
rob   :-)

[mqsaharan]

Indeed, modern alikalines do leak. I have also had bad experience with that. Now I am using rechargeables in almost all my test equipment.
And I did read about 2 volt under-voltage lockout but it will not be a good idea if your meter suddenly dies during usage without any warning. So, as you said, some form of battery guage/discharge monitor/low battery indicator is a must for such type of equipment.

[bdunham7]

i've NOT tried this in a real F77 yet, just on breadboard. does it look like an interesting solution?

Well, maybe 'interesting' but not practical IMO.

First, the meter is designed to work with a battery (no noise), but surely there is at least some noise from this supply.  I don't know if it would matter.

Second, it seems inefficient.  Your zener circuit seems to be bypassing as much or more current than the meter itself uses, if my guess is right that the meter uses ~250uA.  Have you tested that? 

I think you'd get better results just using two cells in series and letting the meter do the rest.  You would need to see when the low battery indicator comes on to see what proportion of the charge is going to be available.  If it works to 6.0V, you'll use most of your cell capacity, but if it is 7.0V you'd want to switch to Li-ion cellls.

[BeBuLamar]

alkaline batteries are cheap enough for such a low power consumption like the meter. They don't sell the Fluke 70 series any more so may be you want to preserve them but for me I just use the alkaline. They may leak but I can get the battery manufacturer to buy me new meter.

[robert.rozee]

Well, maybe 'interesting' but not practical IMO.
First, the meter is designed to work with a battery (no noise), but surely there is at least some noise from this supply.  I don't know if it would matter.

the F77 et al are only 3¾ digits; i would be quite surprised if noise from the boost converter would be material in this instance, but will check when a little further down the path. this is just an idea that is at the early stages, proof-of-concept.

Second, it seems inefficient.  Your zener circuit seems to be bypassing as much or more current than the meter itself uses, if my guess is right that the meter uses ~250uA.  Have you tested that? 

250uA sounds about right. i've seen F77 meters run for 10 years or more on a single PP3 battery, so there is quite a bit of leeway for inefficiency while still maintaining a reasonable time frame between battery changing/charging.

1.4mA @ 3.2v -> 4.5mW wasted by the boost converter, while 0.25mA @ 9v -> 2.3mW consumed by the F77 itself. i'm comfortable seeing a 3x increase in power consumption. this is further multiplied by the difference in battery capacity: 3600mW/h for a zinc-carbon PP3 vs 1600mW/h for the LiFePO4 cell i'm considering. this effectively pushes it out to 6x.

i'd project a 250 hour run time per charge, and would be happy to trade this small inconvenience for the meter NOT being randomly destroyed by a leaking battery. oh, and the boost converter will effectively eliminate the F77's low battery indicator ever coming on!

I think you'd get better results just using two cells in series and letting the meter do the rest.  You would need to see when the low battery indicator comes on to see what proportion of the charge is going to be available.  If it works to 6.0V, you'll use most of your cell capacity, but if it is 7.0V you'd want to switch to Li-ion cellls.

i agree 100%. tell me where i can get 3S LiFePO4 PP3 batteries from, and i will be most happy to use them. unfortunately, they do not seem to be something that is available. while there are 9v PP3 batteries around that contain a single cell and internal booster, these have pretty awful self-discharge issues. and a 2S LiIon PP3 battery has issues with low terminal voltage.

if i could get LiFePO4 cells small enough to fit 3 into a PP3 casing, that would also be a great option. but, again, they do not seem to be available anywhere.


cheers,
rob   :-)

[robert.rozee]

They may leak but I can get the battery manufacturer to buy me new meter.

you must be living in a different reality to me.


cheers,
rob   :-)

[bdunham7]

i agree 100%. tell me where i can get 3S LiFePO4 PP3 batteries from, and i will be most happy to use them. unfortunately, they do not seem to be something that is available. while there are 9v PP3 batteries around that contain a single cell and internal booster, these have pretty awful self-discharge issues. and a 2S LiIon PP3 battery has issues with low terminal voltage.

3S would be too much, especially for Li-ion.  2S batteries are working fine for me--have you tried them or are you assuming they will be too low?  Even if your low battery indicator comes on at 7.0V (which would be too high, a bad design even for alkaline types) you will still use the large majority of the energy in a 2S Li-ion battery.

[iJoseph2]

I'm using Soshine 9v 650mAh Li-ion batteries in my Fluke 70 series meters, which I bought only earlier this year... so I'm not sure what happens when the battery gets low yet.

And I'm using eneloop/panasonic nimh batteries where ever possible.

Li-ion batteries:-
https://www.aliexpress.com/item/32808238857.html

Test:-
https://lygte-info.dk/review/batteries9V/Soshine%209V%20LiIon%20650mAh%20(White)%20UK.html

[iJoseph2]

i agree 100%. tell me where i can get 3S LiFePO4 PP3 batteries from, and i will be most happy to use them. unfortunately, they do not seem to be something that is available. while there are 9v PP3 batteries around that contain a single cell and internal booster, these have pretty awful self-discharge issues. and a 2S LiIon PP3 battery has issues with low terminal voltage.

I considered the Energizer Lithium 9v batteries but I wasn't sure if the open circuit voltage was safe for the meters. The soshine li-ion batteries I got looked like the open circuit voltage was lower than alkaline so that's what I picked instead.

(be aware that there is a 3cell soshine battery so be careful which one you use).

[wasedadoc]

No argument that alkaline AA and AAA leak but I've never see the 9 volt 6LF22 size do it.  Even the two I bought more than 20 years ago.  Tesco (UK supermarket) branded and marked 10 2009.  Were originally bought for use in microphone transmitter and camcorder receiver when making a wedding video.  Then unused until 3 or 4 years ago when I put them in a couple of those $10 LCR transistor tester thingies.  Had a look just now and no sign of leaking.  Voltage is well down but still enough to power the testers.

I have others in multimeters and security alarm control box etc.  Never seen any leakage.

As an aside a few weeks ago I wanted another for a newly acquired ESR meter.  Last ones I bought came from Poundland at £1 each because they were cheaper than Lidl. But neither store currently has any.  Not even a labelled empty space for them.  Is there a world shortage of those batteries or are they so little used now in comparison to AAs and AAAs that those shops have decided not to stock them?

[iJoseph2]

As an aside a few weeks ago I wanted another for a newly acquired ESR meter.  Last ones I bought came from Poundland at £1 each because they were cheaper than Lidl. But neither store currently has any.  Not even a labelled empty space for them.  Is there a world shortage of those batteries or are they so little used now in comparison to AAs and AAAs that those shops have decided not to stock them?

If I absolutely must use alkalines (such as smoke alarms) I only get GP Ultra/GP Super from toolstation ..
https://www.toolstation.com/gp-ultra-alkaline-battery/p37780

£8.99 for a 4pack
They seem to have one pack in stock in all the stores near a couple of locations I just tried and 20 for delivery.

[BeBuLamar]

They may leak but I can get the battery manufacturer to buy me new meter.

you must be living in a different reality to me.


cheers,
rob   :-)

All thre popular brands of batteries Duracell, Energizer and Rayovac will replace your meter if their batteries leak and damage your meter

[Black Phoenix]

They may leak but I can get the battery manufacturer to buy me new meter.

you must be living in a different reality to me.


cheers,
rob   :-)

All thre popular brands of batteries Duracell, Energizer and Rayovac will replace your meter if their batteries leak and damage your meter

Yeah right, still waiting for Duracell to replace my 54ll. I'm currently in 8 years and still waiting...

They basically ask me to pow sand, but they offer a replacement for the batteries who leaked, so a pack of 4 Duracell.

[james_s]

Seems like you're kind of reinventing the wheel. You can already get Li-ion 9V batteries and I've been using NiMH batteries in my Fluke meter for years, they cost about $6 and work just fine.