need some assistance with digital ammeter

I picked one of these up from ebay, for my vehicle. http://www./itm/0-100V-200A-DC-Digital-LED-Voltmeter-Ammeter-Amp-Volt-Meter-200A-shunt-12v-24v-/281514343083?pt=LH_DefaultDomain_0&hash=item418b8ffaab I have an older vehicle with the stock ammeter and connected the shunt between the line coming into the old ammeter and the lines that were on the other side - basically I put the shunt in place of the old ammeter. Now I am not sure how to hook up the digital ammeter/voltmeter? I see where it says it should be on the negative leg of the load, but this is not possible with it taking the place of the ammeter circuit as it goes into the fuse box. I saw this schematic: http://suns-power.en.alibaba.com/product/560302938-213540574/Blue_Panel_Digital_DC_or_AC_Current_Ammeter_Blue_Digital_Ammeter_Digital_DC_Ammeter_72mA_100A.html and that is basically how I have it hooked up with the load after the shunt. the ammeter wires for the ammeter I got have a COM, IN+, and POWER+. I am not sure if the POWER+ wire go before the shunt and the IN+ after or vis vera. Also, is the COM a ground wire? Any help on this would be greatly appreciated. Thanks

Hmmm, for some reason the first link got messed up and I wasn't able to edit. It should have ebay[dot]com after www. not sure why it was removed... Second link is fine...

Firstly I'd question why the Ammeter? IMO - dump it. What good does it do or what information does it give? Unless you are an experimenter, and even then ammeters are usually only temporary insertions. The voltmeter will tell you all you need to know in practice and it provides far more info than an ammeter. Traditionally OEM vehicle ammeters are placed in the alternator-to-battery line with all loads taken from the alternator (maybe at the main fuse/relay box) and NONE from the battery (except that starter motor). Hence the ammeter merely measures whether the battery is charging or discharging. (A voltmeter tells you the same but also if you are over- or under-charging the battery, or about to blow your electrics.) The ground sensing ammeter is wrong for automotive applications. It either needs a isolating dc-dc converter else must be wired in what is solely an alternator GND (engine) to battery -ve WITHOUT including cranking current (the starter GND - ie, maybe a bypass relay) and that's hard to do due other engine to chassis to battery GND paths thru engine mechanicals, cables etc, and sensor and other wired engine grounds. But I do suggest omitting the ammeter. Your OEM ammeter wire can simply be looped (reconnect without the ammeter), or better still, bypassed using a shorter (and these days, fused) alternator B+ output to battery +12V or main fusebox etc. PS... - I fitted a $6 (now $1) mini blue 3-digit LED voltmeter (in lieu of my preferred backlit LCDs which weren't small enough) to my old vehicle's dash (the ute 50 years old this year; purchased 1989 for "temporary" 12 months duties and finally discarded last December) and it's one of my mandatory additions to any vehicle that does not incorporate battery monitoring with alarms. And IMO forget analog including bar-graph meters, or 4 digits. A 3-digit "12V" meter is a reasonable compromise between accuracy versus annoying flickering due to natural variations of typically +/- 0.1V... ie a flickering "14.x" x-digit is one thing, but to have 14.xx or 14.xxxxx... and where flicker means digit bobble - the 0.3 second interval updates with [i[random readings between (say) 14.15 & 14.25V, or 14.10 & 14.30. Of course filtering could be added but I found unfiltered instant response with its greater digit bobble quite acceptable. (But a dimmable display that doesn't wash out in sunlight...) And FYI - IMO a 0.1V resolution for critical auto 12V battery voltage measurements is acceptable. (Real tests should involve at least 10-bit voltage conversion or resolution better than 0.01V.) But EOM analog or bargraph displays are rarely that fine scaled, and for automotive purposes a mere difference of 0.2V (if not 0.1V) can reflect some problem, or a 20% battery discharged etc. It's also one of the few automotive displays that IMO is better digitally than analog (tho digitals like speedo and maybe tacho should also have a analog (bargraph or needle) displays). A desirable addition to any such voltmeter is alerts - ie, red when out of spec; amber if near limits - but that varies in situ - eg, a 12.6V or higher battery is fine, but that voltage with engine running (charging) is a RED ALERT situation - like an overload, or dropped alternator output, etc. Alas its alerts need "intelligence" and just like the humble (no) oil-pressure light is probably still delegated to the "best" intelligence - the driver. Until all related intelligence is programmable & economic, drivers will probably still decide when to kill the engine if the oil light illuminates.    And IMO vehicle electrical systems are far more complex. But fine - alert the driver to the dip to 13.1V or 12.3V and let them decide if it's because of the newly added amp or rare use of spotlights, all is cool. (Disregard the latter if smoke appears...)

Sorry, Oldspark. TLDR. I'll just answer the guy's question. How to wire his meter, and point out his mistake. The shunt for that meter can ONLY be placed in the NEGATIVE side of the circuit. Placing the shunt in the positive leads of the old ammeter won't work unless you also install an isolator as shown in "Connection(OPTION 1: SHUNT on POSITIVE OR NEGATIVE, need to connect a isolated DC Power )" Placing the shunt in the negative cable right at the battery is actually really easy, and will tell you exactly what is going into or out of the battery. You can run very small gauge wires from the shunt to the meter. 18 gauge will be fine. And they don't need to be fused. If they short to ground, so what?, they are already at ground potential. If there are more than one wire on the NEG battery terminal, remove them all and place them on the "engine" side of the shunt. There can only be ONE wire from the shunt to the battery neg terminal. If you miss one, it will "short" your shunt, and the shunt won't read accurately. Connect your meter: Pin 1 = B+ power to meter. For the display. Put on a switched ignition source, so it comes on with the key. Pin 2 = Ground. Again, for the display. Pin 3 = Signal to the shunt. Place this pin on the side of the shunt that goes to the battery negative terminal Pin 4 = Signal to the shunt. Place this pin on side of the shunt that goes to the engine block. If the meter reads backwards, ie minus when it's really charging, reverse the connections on 3&4. If you didn't isolate the display power pins 1&2 with an additional device, it won't work. I can't recall if doing this damages the meter or not. Hook up pins 1&2 to power. If the meter powers up, it's probably OK. That should get you going.

But beware shunting the only -ve from the battery. Car ammeters are usually not designed for cranking currents.

It won't hurt it. There aren't many autos that draw over 200 amps cranking, especially with modern gear-reduction starters. He'll be fine. His shunt is rated at 200 Amps. Shunts are fairly robust, and survive some over-current. The meter itself will survive an over-current too. It will read "OS" or sometimes just go blank while the over-current exists, but the meter won't be damaged. I've been installing remote-shunt digital ammeters in yachts with huge diesel engines, multiple 150+ amp alternators, and large battery banks sometimes exceeding 2,000 AmpHr capacity for 30 years. Negative-sensing is simple to accomplish and reliable if your main objective is to monitor the battery bank current flow. I used shunts rated for 1000 Amps in an installation with 12 Cyl Detroits with a 12 volt system I did 20 years ago. The cranking current in that boat with cold engines is 500+ amps per engine. I was on that boat about 2 years ago. The metering system was still present and functional. 20 years, not bad for a marine installation. Cheers

I use a 50A shunt to test starters but I have no problems... But others don't have the same knowledge, and ammeters & shunts are not a good fusing device. (I recall one motorcyclist that used his ammeter as his only fuse!) Also being 200A is pretty good. Many I have seen use smaller shunts (intended for alternator-battery monitoring. But again I ask why would anyone want an ammeter in a normal or typically accessorised vehicle?

oldspark wrote: But again I ask why would anyone want an ammeter in a normal or typically accessorised vehicle?

You're right. Most people don't need an ammeter. Voltage indication that the system is "normal" with the engine running is enough for most drivers. I like to monitor my battery's health. Batteries in good condition accept high charging rates for fairly long periods of time, and gradually taper to zero rate. Batteries near the end of their life accept much lower initial rates, and the rate rapidly falls off to zero. Ammeters are good indicators of charging system performance with the engine running, or RATE of discharge, while the engine is NOT running. Voltmeters are good indicators of system voltage, but can't be easily extrapolated into rate of change over time. Example: 60 amp alternator output max, "set voltage" 14.1V. System load 40 Amps. (We have lots of stuff on!) Only 20 amps left for battery charging. Most healthy batteries can accept more than 20 amps. In the above scenario, a volt meter will slowly rise until the battery acceptance rate is <20amps. The voltage indication will now be set voltage = 14.1V. As the battery continues to charge, the amperage will decline, eventually to 0 amps, but voltage will stay at a constant 14.1. Once charge rate = zero, the battery is fully charged. At that voltage and temperature, at least. So an ammeter will give you state of charge for the battery while the engine is running. A voltmeter will give you system voltage, which can be extrapolated to give limited indication of battery state of charge. But once voltage reaches set point, voltage doesn't tell you squat about battery state of charge while the charging system is operating. IMO, Both is best. Cheers

You're preaching to the teachers... But I see some things differently - eg, an ammeter tells me the current being discharged but gives no indication of "rate" wrt to its capacity. (The same applies to charging and is more obvious with end-of-life batteries.) However a voltmeter tells how much of its capacity remains. Re spare capacity, IMO so what? If someone did know their alternator output at that RPM deducted their load current (thru another ammeter?) - is the spare capacity enough, or not enough? A voltmeter tells you, and it's simply two fine wires to the battery. There is no need for ammeters with high-current shunts. And no calculations. Battery recharge current quickly decays in normal use. Even my peaks of 45A (after minutes of cranking) were below 10A within 60 seconds. Most batteries will not take "all spare capacity" available from the alternator. Exceptions can include high initial charge currents as above or older or worn alternators at low RPM. Vehicle system voltages have nothing to do with the SOC of a battery - except of course when discharging beyond their surface-charge... Most systems rise immediately to 14.2-14.4V. Older systems used 13.8V and - especially with external or elecro-mechanical voltage regulators - were more prone to voltage dipping at low RPM. The whole system is geared to voltage as required by the battery. Voltage generally has the greatest effect on battery lifetime and is via simple & fast regulation. Essentially battery current is irrelevant... or at least it has been considered such thru automotive history. It is only recent times that manufacturers nor impose battery charge current limiting. For traditional and especially AGM batteries that's simply squeezing the extra %age of life (the common exceeding of the battery-spec's max recharge current was not detrimental enough to impact warranties & probably only impacted battery life by 5% or so) whereas current limiting for modern >200V vehicle battery systems is a different issue altogether. The only people I know that have justified ammeters are like me - experimenters or designers that need to test or measure systems. But it's only temporary & only used for sizing purposes; they are essentially useless as any sort of performance or problem indicator. Such meters were designed to measure the hotside current by default since dedicated GNDs could not be guaranteed (and were uncommon anyhow).    Historicals with ammeters that I know of measure only the battery current - usually hotside due to multiple GNDs. I can't think of any that measure alternator output except for alternator testers They're great for certain discharge measurements - like knowing the discharge current from my (isolated) secondary battery to my 12V fridge. But even then a battery protector (low voltage disconnect) should be installed before worrying about that. (Yet again, voltage sensing does the protection.)     An ammeter will tell you the battery is at 100% SOC when it drops to float current - ie, typically under 2A for automotive batteries. Otherwise it tells the current charge rate, so marry that to its recharge tables else assume "flat" behavior based on its C10 or C1 rating.     I did the latter after my last alternator failure - abeit bottom-about: I estimated load (engine & headlights etc - assume 15A); Battery was 38AH (probably at C1 rate since it was a UPS AGM); I calculated 2 hour reserve if it was at 100% of rated capacity - after all, it was only 13 years old). One hour home so I should get there with 1/2 to spare. Assume 0.1V drop per 10% SOC (conservative). IGN on & crank and watch the (battery=system) voltage drop to ~12.2V. So 12.2V is my "100% SOC" voltage reference. It should be 11.7V in 1 hour... I considered the voltage stable after a few minutes... good - no evidence of a weak dead battery... yet! The rest of the trip saw the voltage drop as hoped. (Battery voltage was ~12.2V the next morning confirming a discharge of ~0.5V.) An ammeter would have told me (accurately) what my load was but nothing about the battery's real SOC nor "crash imminent" rapid voltage decay.    Alas that segwayed... Resolution tends to be an alternator issue. On the older common 30A or 60A ammeters it was difficult to judge float currents. (And what if it's 2A but at 15.5V?) Newer electronic displays tend to overcome that if they have enough segments. Voltage is simply a 3 digit display covering at least 8V-16V. When applied to batteries, ammeters are merely "predictive". IE if the battery is within specs (& at 25C or 50C etc) then that current for 1 minute equates to c% of capacity recharge. The physical test is still the cell voltage - unless a hydrometer can be used.    For those that like the lights or already have ammeter/s or have a specific reason, ammeters are fine. Maybe use it to measure HU or amp load, or lights(?!)...?   (I'd consider that - or a fridge - if I had a 2 in 1 meter, but otherwise I'd probably not use the ammeter - not with a shunt.) But to fit an ammeter to a non-metered vehicle or think you are somehow getting valuable information from it...? Make sure you get voltmeter first! Else do like me - use DC current transducers; just loop the cable - no need to cut & join. And they're zero-Ohm to boot! But use the split DC supply versions for meters.) But let's see if pityocamptes has anything to reply.