Alternator Shunt and Meter

[14p]

I have never heard of negative installed shunts or ammeters until now, positive shunts and ammeters have been used in multiple charging systems for decades, but used analog ammeters. What is rubbish, the reason I posted here is google found this when I searched the subject, tell me what is rubbish so I know what to forget.

[Dsanduril]

In the old days with analog gear you could put the shunt anywhere, all that was measured was the differential voltage across the shunt.

Low-side sensing has become quite common in the battery management business with digital gear. I suspect, without seeing a full set of specifications, that the digital battery monitor contains an integrated circuit (IC) battery fuel gauge (Texas Instruments, Analog Devices, etc.). These usually have a DC-DC converter built-in or at the input and actually run at 3.3V or 5V (Vcc). The analog input pins are limited to something like 0.3V above Vcc. If you install the shunt on the high side you exceed that limit and you burn out the input. The inputs are probably single-ended and referenced to ground, then a difference is calculated (rather than using a true differential input).

It is entirely possible to build an IC with fully-differential isolated inputs, and they do exist, but they are more expensive.

[goboatingnow]

Jelly bean instrumentation op amps tend to have common mode range only to about 0.6 v of their power rails. This means that sensing shunts that are on the high side , while also powering the sensing circuit from the same source can’t be easily done. ( easy in electronics means cheap )

This is especially true if you are using some of the low cost ICs used in panel meter systems ( this is different to using self powered analogue meters )

However in recent years this situations has improved , I use the ST TSC2010 current sense amplifier ,this can withstand ( and work correctly ) -15 to 50 v on its inputs while being powered from 3v3 .

Note this is a $2 IC. This is an expensive component. Then you have to protect the inputs from 12v alternator driven systems , inductive pulses , current spikes etc. This require large TVS diodes , and other components.

It’s all adds up to cost, in a market that expects electronics to be cheap.

So today it’s entirely practical to build a robust high side sensing system ( both uni and bi directional ) but it’s certainly not present in a cheap digital ammeter from China !!!

[Cadence]

It would appear to me you have the shunt in the alternator supply line.

[noelex 77]

This is an old thread, but for anyone looking for an answer to the original question, the Pico battery monitor can use and display multiple shunts.

Mine is set up to show the output of each of my solar panels separately, as well as the net input/output, but alternator output would be possible.

There are also numerous cheap ammeters available. The reliability of these units is sometimes poor and be careful with the supplied shunts. The current rating for the shunts often looks a little optimistic to my eyes. Hall effect shunts are available and while less accurate they remove the concern about the current carrying ability.

[14p]

Originally Posted by Cadence

It would appear to me you have the shunt in the alternator supply line.

Yes thats how I need it in order to view alternator output, thats the way the electrical system is wired, two alternators feeding one bus and charging the battery. Each alternator has a shunt in the + cable before the bus, and than on the other side of the bus is the battery but there is the 3rd shunt between the bus and battery, 3 shunts, and a ammeter selector switch allowing you to select each alternators output individually, and the battery charge current or discharge current through its shunt.

[goboatingnow]

Originally Posted by 14p

Yes thats how I need it in order to view alternator output, thats the way the electrical system is wired, two alternators feeding one bus and charging the battery. Each alternator has a shunt in the + cable before the bus, and than on the other side of the bus is the battery but there is the 3rd shunt between the bus and battery, 3 shunts, and a ammeter selector switch allowing you to select each alternators output individually, and the battery charge current or discharge current through its shunt.

Good idea. I have high side sensing on all charge sources (Mains charger , solar and alternator ) and battery supply sensing on the high side “ consumer “ supply from the batteries.

Very interesting to track what’s going where, my next step is to use this info to force the mains charger and solar into and out of absorption /float mode properly ( via ve.direct )

Going to add sensing on the inverter next and ultimately move to lithium but I need to design a BMS first


Ps. Just ensure your digital panel meters are capable of high side sensing the cheap ones rarely are.

[14p]

Yes thanks, probably wont try it on the hot side after reading this.. at least not until I try it out on the low side and see if it works. Than I might blow it up LOL