bidirectional ammeter?

When I was a lad, my family had a tractor with a center-zero ammeter. It swung to the right to show charging current when the engine was running, and swung to to the left to show discharging when running the lights with the engine off. How common-sense! I can't find *anything* similar in the modern digital realm. There are way-overkill battery-everything-meters for marine and RV use, but I don't need to count coulombs, and I'd rather not spend $400 on the thing. There are unidirectional LED-readout ammeters in a sensible price range, but none of those seem to grok the notion of negative numbers. I guess I could run two from the same shunt, just flip their sense leads... But am I missing something? Are there digital ammeters that would simply read positive-negative, and maybe the people selling them simply never mention that capability? Or is my best option really to grab an old analog bare meter movement in the center-zero style, lash it to whatever shunt is handy, and break out Inkscape to print my own scale to slip behind the needle? I do kinda like analog meters but this just seems anachronistic and silly. (Products I've found: Blue Sea makes some analog ones, I'm curious about digital. AltE store made a digital one but it's discontinued and out of stock everywhere.)

Oh, year-make-and-model. The application here is actually for an electric manlift, a Genie runabout machine. It's a 24-volt system but all the same techniques apply. I'm realizing that running two digital cheapies from the same shunt is probably the most sensible thing. I'm still curious what y'all come up with, idea-wise. Thanks in advance for the ideas!

There are electronic and digital equivalents - or ones that can be constructed - but the main reason they are rare or don't exist is that they are TOTALLY useless. Well, they will tell you if the battery is charging or discharging, but they won't tell you if the battery is undercharging or overcharging, not what state the battery is in (ie failing/collapsing), nor how discharged it is. And those old ammeters were a hazard - they ran the unfused alternator to battery connection thru the cabin. (Remote shunt & loop ammeters solve that issue.) The solution to all the above is a (3 digit) voltmeter. A voltmeter can also help determine if you are suffering an alternator OR battery fault, and - paraphrasing the above - provide warning of several battery damaging conditions. The only reason someone would want an ammeter is for experimental purposes - and that's really for set&forget systems (ie, ammeter is not permanent) - or battery capacity determinations (which is beyond most people, and unlike a voltmeter it won't tell what percentage of full capacity it has). Some modern vehicles use current monitoring incorporate current sensing for reasons that elude me (I know many reasons but I reckon they are flawed).    One genuine reason would be to current limit the battery's charging, but afaik no one does that for lead acids. If going the ammeter route, I'd suggest a current loop (donut) module that measures DC Amps, and one that uses a split (+/-) supply so it can be connected to a plain voltmeter module. EG DigiKey's Tamura Current Transducer which requires a +/-15V supply. That link is for 200A sensing and maybe smaller is better. FYI - Measure the hotside - ie, +12V to the battery from the alternator - NOT the GND and not including the starter heavy cable. And all loads (except startermotor) must be taken from from the alternator & fusebox etc side of the ammeter sensor - NOT from the +12V battery terminal. PS - the digit bobble in a 3 digit 12V or 24CV voltmeter is bad enough, but I reckon the general variance of a digital ammeter would drive you nuts.

One of the problems with the ammeter setup in an automotive application was that it wasn't clear in low-amp situations if the charging system was working.. when your only draw is the ignition system, the needle won't move far enough to accurately display that your charging system has stopped working. In your application, I would expect discharge only when the lift is running.. what's your intent in monitoring current flow? As oldspark mentioned, a voltmeter is a much more accurate way to monitor battery condition, which is why the automotive world seems to have gone to them. If you really wanted to pursue this, I'd consider a shunt and a voltmeter.. MS

Thank you both for the responses! I currently use a Kill-A-Watt on the charger's AC input to know when it's bulk charging, "almost done", or fully idle. The charger's own status lights only tell me "doing something" or "fully idle". An ammeter would help with this. During use, the stock systems are sweet in that, with your hand not on the controls, there's zero load. Really no need to turn the machine "off", it's off just sitting there. But I've been adding accessories -- an on-board inverter last week when windstorms knocked out the local grid, and I'm planning some platform-mounted work-lights. It would be nice to see their load in real time, the inverter especially. I could throw that Kill-A-Watt on its output, but that misses the idle current when the inverter's on and humming but not driving an AC load. You can't get state-of-charge info from a voltmeter unless the battery has been sitting idle for a long time; surface-charge takes a while to equalize: http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge That being said, terminal voltage will tell you whether it's charging or discharging, just not how rapidly unless the current is a significant fraction of 1C. I'm dealing with 225AH batteries, so a 3A load and a 30A load look basically the same on a voltmeter, but they yield pretty different runtimes! Questions I'm looking to answer:

• Is the battery charging? How fast?
• Is there any idle load left on as I walk away?
• How long can I run the machine at this load?
• If I leave this inverter load running, is it safe to walk away for 24 hours and will the battery hold up the load the whole time? Or should I hustle back with the gen-set to make sure the sump pumps don't fail?

SoC would be nice too, but really, if I know the SoC is at 100% whenever the charger stops, I can guesstimate down from there, assuming I know the current draw. A discharge-only ammeter would answer almost all the questions; the bidirectional question was at least in part curiosity, because it's a product that seems to have simply disappeared, or at least not been updated into the digital era.

Ammeters are bidirectional. You measure the voltage across a shunt resistor. That voltage is +ve or -ve depending upon the current direction.    "Fully charged" means the battery's charge acceptance is its float charge - maybe up to a few Amps for 225AH battery, but that varies with battery condition... Or that its terminal voltage is 12.67V etc (at 25C & rested... whatever its spec sheet states. Voltage is yet again the best indication of SOC however the voltage drop due to internal resistance is initially obtained from battery datasheets and thereafter - or otherwise - by testing and datalogging. For a given wet cell, removal of surface charge usually involved a MAXIMUM load-time discharge. Surface charge should reduce with age and non-fully charged batteries. As I recall, surface charge is far less significant for an AGM. But as you point out, an AH counter may be required for low discharge rates. IMO it looks as if you want a BCM - Battery Condition Monitor. Good luck with that! (Low voltage cut outs or battery protection switches are bad enough. Settings of (say) 10.6V may be fine for high currents but disastrous for low currents.) Tho many BCMs can alert you to a failing battery, I doubt the "simple" BCMs can accurately forecast remaining capacity; they merely compare collected data. And I don't think batteries have changed in that their next discharge may fail (even if that is less than a 1 in 500 probability for near new cyclic battery).   

As you pointed out, they do make devices for this to monitor RV battery banks.. but they ain't cheap. In your situation, I'd probably install an accurate ammeter and voltmeter. Take datapoints (or datalog) and graph from there, keeping in mind that discharge rates will determine runtime. AH ratings are often based on a 20 hour discharge rate.

Agreed. Noting that AH capacities (ratings) can be obtained from battery spec sheets or reasonably estimated from any C-rate. And for highly variable loads, an AH counter, or powermeter, tho relating that to varying AH capacity depending on discharge rate is complex. Of course a BCM will scale the remaining capacity by the battery's actual terminal voltage as calculated from its internal resistance and current compared to what it should be given its SOC points and intermediate discharges. (Impedance is usually determined using AC waveforms.) And don't forget temperature as a variable. People wonder why BCMs are so expensive...! But you should be able to make your own AH counter or powermeter using a CPU or PICAXE - ie, 2 or 3 analog inputs (battery voltage; shunt voltage drop else shunt_V+ & shunt_V-) and a display - eg, a $3 PICAXE 08M2 with i2c driven display. Your brain is the best remaining capacity estimator until you translate that to coding (and probably more program space than the ~2,000 line 08M2!)

I work on boats and yachts. I've installed a bunch of Blue Seas meters. The Blue Seas digital amp meter reads bi-directional. There is a (-) in the display when the current is in the discharge direction. Blue Seas meters may only be installed in the negative wire. To monitor a (+) circuit like a alternator output, requires another device they have available. In most yacht installations, I install the shunt in the battery negative wire, with nothing connected to the battery-side of the shunt. Installed in this manner, the shunt will read either + or - into the battery, regardless of how the rest of the system is wired. Which is what I think you're after. You will also see starter loads on the meter, and the shunt has to be sized appropriately to carry the maximum load. If your starter exceeds the 500 amp capacity of the standard shunt supplied with the meter, you can install a larger capacity shunt, just double the Mv rating of the stock shunt. Ie if the stock shunt is a 500A/50Mv, install a 1000A/100Mv. The shunt will carry 1000 amps, the meter will still be accurate within its range, and if the current exceeds 500A, the meter reads "OS" of Off Scale, but won't be damaged. I've done this with big diesel engines that have huge starter draws. IIRC the Blue Seas single function 500-0-500 amp meter with shunt is about $100. Not bad considering a 100 amp SW analog with shunt is more than that.

The Link 10 will monitor amp hours consumed and do state of charge calculations based on Perkurt (sp?) Exponent. It also has volts and amps, and indicates forward and reverse current flows. I'm not sure it's still available. I installed one about 24 months ago, and it was about $175. Neat device, and good value for what all it can do.