Harbor Freight Multimeter Add-on - Page 1
Forum section: electronics / projects

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 19, 2019, 06:23:12 pm
content: I use the freebie Cen-Tech multimeter (Harbor Freight gives away from time to time) for testing batteries because it tests them under load. I have a few of them. So I designed this 3D-printed add-on so that this meter is a dedicated battery tester. Still thinking of ways to improve it but works well in the current configuration.

author: tooki
author_rank: Super Contributor
timestamp: June 21, 2019, 06:47:56 pm
content: That’s cool I guess, but what is the scale used? The manual’s description makes no sense to me:1.5V at 4mA is 6mW, not 370mW. And I’ve never heard of displaying battery capacity by current, always by voltage under load.

author: Gyro
author_rank: Super Contributor
timestamp: June 21, 2019, 07:07:21 pm
content: A load current of 25mA for a 9V battery and only 4mA for an AA cell doesn't sound right. That's a moderately heavy load for the 9V but insignificantly low for the AA.I assume it's using the same resistor for both. The AA reading is going to give a misleading (high voltage) result.

author: Stray Electron
author_rank: Super Contributor
timestamp: June 21, 2019, 07:11:17 pm
content: How/why is it reading 3.5 with only a single 1.5V battery in the adapter?

author: Gyro
author_rank: Super Contributor
timestamp: June 21, 2019, 07:14:22 pm
content: 3.5mA?It looks weird. It should be measuring the batteryvoltagewhile it is under load, not asking the user to try to extrapolate from the current.A pretty useless function.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 21, 2019, 08:19:52 pm
content: Geez1) the battery being tested is bad (less than good). That is why it reads 3.5.2) the red knob in the center can be rotated left to Volts DC and measure open-circuit voltage (add a load resistor of your choice)3) Given Ohms law and a fixed known resistor, measure current or measure voltage--does not matter4) 25mA and 4mA makes sense to me. A bad battery is application dependent, so there is no PERFECT test.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 21, 2019, 08:20:38 pm
content: Quote from: Stray Electron on June 21, 2019, 07:11:17 pmIt is current, not voltage

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 21, 2019, 08:26:43 pm
content: Quote from: Gyro on June 21, 2019, 07:14:22 pmDoes the user know what is a good voltage or a bad voltage? This is just one metric...maybe good...maybe not so good.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 21, 2019, 08:28:30 pm
content: Quote from: tooki on June 21, 2019, 06:47:56 pmI think the manual should have said 370 OHMs, not mW.

author: tooki
author_rank: Super Contributor
timestamp: June 22, 2019, 10:38:09 am
content: So again, what’s the scale? With a full AA it should read 4+ mA? Just making sure I understand.As I said, I — and others, it seems — have never heard of measuring battery capacity by current. Always just voltage under load. So it’s weird.But again, the holder itself is cool. Perhaps it’d make sense to put a load resistor in it and then just use it in volts!Did you DIY the contacts, or use COTS ones like from Keystone?

author: Gyro
author_rank: Super Contributor
timestamp: June 22, 2019, 12:16:28 pm
content: Yes, just to be clear, I was commenting on the poor meter battery test functionality, not your 3D printed add-on.Adding (switchable) load resistors, with appropriate loads for the two types of battery to your add-on would make a great deal of sense, using the voltage range instead.Voltage under load is the natural measure of battery health. A battery labelled 9V should read in the vicinity of 9V, likewise a 1.5V marked AA cell.Appropriate test current for an AA cell would probably be in the 200mA region.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 22, 2019, 02:26:09 pm
content: Quote from: tooki on June 22, 2019, 10:38:09 amI used banana jacks (crappy ones that I modified) with the plastic cover removed.I believe these here from Ebay:https://ebay.to/2Fr4wmyThese are really pretty bad. You have to solder the lantern to the post otherwise you have intermittent connectivity.Regarding voltage/current...it is natural for us to think in terms of voltage. However, as I said, it does not matter for a given (known) resistance as long is there is some quality metric (e.g., 4mA is good, 3mA is bad) to inform the test result.Voltage is no more informative than current because the usefulness of your battery depends heavily on your application.The designers of the meter decided to add this feature...load the battery with a 370 ohm resistor and measure its current. It could just as well have measured the voltage instead. That was their choice--not mine. I would argue that the ONLY bad thing about measuring current vs. voltage is that we don't have an intuitive sense of what is good and what is bad (like we do with voltage--as I said...because we think in voltage terms). So the designers of the meter decided to give you a number, a threshold, on the dial.I am posting a page from Linden's Handbook of Batteries which illustrates that there is no "one size fits all" method for testing a battery.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 22, 2019, 02:27:01 pm
content: Quote from: Gyro on June 22, 2019, 12:16:28 pmSee my post with the Linden attachment

author: tooki
author_rank: Super Contributor
timestamp: June 22, 2019, 02:57:46 pm
content: Quote from: Wimberleytech on June 22, 2019, 02:26:09 pmI meant the battery contacts. I wasn't asking about the banana plugs.Quote from: Wimberleytech on June 22, 2019, 02:26:09 pmBro... I wasn't asking for a lecture on the principles of methodology. I was just asking what the scale was, since it's not self-explanatory, and their method deviates from the one commonly used, and it's not explained in the manual, which as you saw is so wrong that it actually adds to the confusion. It says nothing like "≥4mA is good, <2.5mA is empty" or whatever it would be. That the "only" downside to showing it as current is that we have no sense of it isliterally my point... That's not a minor little disadvantage!

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 22, 2019, 03:10:12 pm
content: QuoteThese, as I recall:https://ebay.to/2L8b3pZQuoteOK...we can still be friends

author: Gyro
author_rank: Super Contributor
timestamp: June 22, 2019, 03:22:52 pm
content: Quote from: Wimberleytech on June 22, 2019, 02:27:01 pmSure, there are all sort of load profiles and the useable battery capacity will vary with each one. Actually, rather than the Linden attachment, you're better going to the battery manufacturer websites particularly Duracell and Energizer and downloading the technical datasheets. There you will find quoted capacities and discharge curves for all sorts of conditions, constant current, constant resistance etc. together with the sort of effect storage and pulse loads have. Very informative when you're designing something that uses them and establishing realistic battery life.Certainly testing the battery voltage under some sort of dummy load gives some sort of idea, as it reflects the internal resistance, which tends to increase as the battery ages. Hopefully we can agree that a 'one size fits all' approach doesn't really work and that maybe Harbor Freight's implementation of sticking a 370R resistor in series with the battery, regardless of voltage, and displaying the resulting current is probably the worst possible implementation.P.S. That's where an external add-on, with integrated push-button loads would come into its own.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 22, 2019, 03:39:28 pm
content: QuoteIt is a big world...we don't have to necessarily agree--might be boring if we did.QuoteYes, indeed...this would be a nice feature and it is something I am pondering.Maybe this is the proper test to perform, no?

author: Gyro
author_rank: Super Contributor
timestamp: June 22, 2019, 03:49:10 pm
content: Haha, maybe. I think I'd put that one slightly ahead.I've seen cheap meters with individual 1.5V and 9V battery test positions. Maybe they do a better job of reading voltage under load. Of course that still doesn't differential between an AAA and a D cell.I remember many years ago, back when I was a teenager, I used to work Saturdays in the photographic department of a big store. We had a battery tester supplied by (I think) Ever Ready. It had a 3 zone moving coil meter - bad, ok, good, switch positions for all the various battery sizes HP2, SP2, SP11, HP11, HP7, PP3 etc. and a whole bunch of load resistors inside. This was in Zinc-Carbon days, 'copper tops' were only just beginning to surface.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 24, 2019, 01:49:52 pm
content: Well, I redesigned the battery tester so that it now looks prettyI printed wiring channels at the bottom of the battery holder.I also shrunk the X dimension so that it does not hang over the edge. I am done with this now!

author: tooki
author_rank: Super Contributor
timestamp: June 24, 2019, 04:46:43 pm
content: That looks great!

author: mikerj
author_rank: Super Contributor
timestamp: June 24, 2019, 05:38:26 pm
content: Quote from: Wimberleytech on June 22, 2019, 02:26:09 pmThis is true for a single cell, which is all your adapter appears to cater for. For a multi-cell battery voltage under load is a more useful measurement since a battery with a shorted cell may still be able to supply sufficient current, but you'd have no idea the voltage was incorrect.

author: floobydust
author_rank: Super Contributor
timestamp: June 24, 2019, 06:51:55 pm
content: This works for me, a few seconds to test. 50mA AA load and the manual has the pass/fail voltages.

author: rsjsouza
author_rank: Super Contributor
timestamp: June 24, 2019, 07:51:22 pm
content: That is a nice adapter. I have been meaning to create something like this for my meters.It's been a long time since I have used my pair of HF meters, but it seems this function is identical to the Aneng mini: it has both 1.5V and 9.0V battery testers that only display the measured current - a bit of a cop out. IMHO the Aneng uses much more useful currents of 40mA and 25mA respectively.I have another meter (Mestek DM91A) that actually reports the voltage under load (30Ω and 300Ω for 1.5V and 9.0V respectively).

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 24, 2019, 08:32:01 pm
content: Quote from: rsjsouza on June 24, 2019, 07:51:22 pmThe Aneng mini is $3.34 on Amazon with free shipping...lol

author: tooki
author_rank: Super Contributor
timestamp: June 25, 2019, 12:40:02 am
content: And...? The DT830B like your harbor freight one are available for a dollar, including free shipping from China. Nothing to brag about.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 25, 2019, 01:11:27 am
content: Quote from: tooki on June 25, 2019, 12:40:02 amLooks very similar. Missing battery test, but my jig would fit most likely

author: rsjsouza
author_rank: Super Contributor
timestamp: June 25, 2019, 08:19:25 am
content: Quote from: Wimberleytech on June 24, 2019, 08:32:01 pmHehehe... Yes, it is that cheap. I got it as a curiosity together with a few bits from a banggood order and the only real use I give it is as a battery tester.The red HFs I have were freebies, got at different times to see how the quality eroded over the years (there's an ancient thread around here where me and a few others do a comparison).

author: Brutte
author_rank: Frequent Contributor
timestamp: June 25, 2019, 11:45:13 am
content: Very nice battery tester jig.You could add some feature to ease pulling battery out once the test is finished. Otherwise one has to use some tweezers (possibly shorting + and -).I would also cover third banana socket (10A) for safety.Imagine a scenario where someone does 100V DC current test, then unplugs (-) terminal leaving (+) in the meter and apply your jig "temporarily". If it is easy - make it foolproof.I own the DT830B - it is a very nice multimeter, 0.5% with 1M input impedance.

author: Wimberleytech
author_rank: Super Contributor
timestamp: June 25, 2019, 12:57:16 pm
content: Quote from: Brutte on June 25, 2019, 11:45:13 amGood comments.The depth of the housing is shallower than a normal battery holder. As a result, a AA battery sticks up above the plane of the top so it is very easy to pop out. A AAA sits a little deeper. In one iteration, I put a pad below the AAA to address that very issue, but somehow forgot to add it in this final iteration.I am the only "someone" using this, and this meter is dedicated to battery testing--I will never remove the jig to use it for anything else. My Fluke 87V is for those "other" tasks.There have been numerous comments on this thread (all appreciated) devolving into battery-testing philosophy. All very interesting and thought provoking. But this project started in the following way.I have a bunch of this freebie meters scattered around the house, tool box, glove box, etc. One sits on the bench in Lab #1. I had a couple of stock battery holders wired to it (dangling). I am always thinking about doodads I can design and print with the 3D printer, so it popped in my head one day, "Hey, design a jig for this." So I did.I am not going into production--I am sufficiently wealthy already. This was simply a fun, creative distraction between pushing over trees with my tractor and preparing for a deposition for one of my clients.If someone wants one of these and has a 3D printer, I am happy to give them the stl file, or even the Freecad design file--go to town big boy!

author: Wimberleytech
author_rank: Super Contributor
timestamp: July 24, 2019, 09:47:37 pm
content: After all the carping about load resistance, I added this Phoenix connector/plug that allows for easy attachment of different load resistors.No modification to the 3D print...just JBWeld the Phoenix socket to the test jig. Then screw-mounted a resistor on the plug.