Shunts and digital metering

[BabaYaga]

Inspired by the thread on shunts/resistors and current measurements running on Scuttlebutt I started thinking about this project:

For monitoring the DC system on board I have two analogue needle meters by the switch panel. The V meter scale goes from 9 – 15, the A meter scale covers -30 to +30. These are connected to two shunts, one in the domestic battery circuit, the other in starter battery charging circuit. Both shunts are on the positive side (unlike most modern monitoring systems). There is also a connection to negative, for the voltage reading.
On the display unit there is a toggle switch which allows the meters to show V and A in either the domestic OR the starter battery circuit.
The system is 40 yrs old, but works.
I have been thinking about replacing the analogue meters however, partly because the scales are small and it is hard to get an accurate reading (especially the crucial 1/10 of volts), partly because of the 30 A limit of the amperes scale – I have a 70 A alternator.
Ideally I would like to keep the shunts and only replace the needle meters with digital displays. Is this possible and what should I be looking for?
I am fairly certain the shunts have a voltage drop of 1 mV/A.
A digital voltage meter is probably not hard to find, but I would like an A-meter that matches in size and appearance.

Any ideas or pointers are most welcome.

 

[pvb]

It's probably feasible for you to find matching displays, but you may find they require a separate power supply. Surely a more elegant solution would be to fit a proper battery monitor - this will give you voltage and current readings and, importantly, show the Ah in and out of your battery bank. You don't necessarily need a shunt on the starter battery, as in normal use this battery will usually be fully charged, but many battery monitors allow you to check the voltage of the starter battery as an extra function.

 

[VicS]

Lots of digital meters on ebay.
Voltmeters, combined volts and amps meters. Perhaps not so many amps only.

Make sure you get ones which don't require a separate power supply as pvb mentions

I would not expect to necessarily be able to use old ammeter shunts, rather use ones provided with and matching, the meters you buy. You might have to put shunts in the negative battery connections but as pvb says no need to consider measuring the starter current. ( IIRC shunts for digital ammeters are usually 50mv/ max reading)

No sure about digital ammeters necessarily reading + and - you will have to check that out.


Nasa BM1 or BM2 will monitor charging/discharging and volts of the domestic battery and also give a volts reading for the starter battery.
Compact versions also available but IIRC they do not measure the starter battery volts.

If you opt for a Nasa battery monitor wire it EXACTLY as per the instructions. No ifs or buts EXACTLY

 

[BabaYaga]

Thanks for the replies so far.
I agree a proper monitor with Ah reading would be elegant, but it would also involve more of a rewiring because of the need of placing the shunt on the negative side (NASA and Blue Sea...). Not quite there yet.
The present shunt in the starter circuit only meters charging current, not starter motor current, but clearly this is less relevant to monitor.
The ability for a digital ammeter to display both + and -is essential for my purpose. Does anyone know if these are rare, as VicS hints?
- 50 to + 50 would be OK, but +/-70 would be better.
Any further input is most welcome.

 

[William_H]

The existing amp meter shunt can be itself shunted with a resistor of suitable value to increase the range of the amp meter.
So another identical shunt in parallel will double the range from 30-0-30 to 60-0-60. You would have to remark the dial of the amp meter. Alternatively and much easier is to add a series resistor in the wire (either wire) from the shunt to the meter. Even have a switch to change ranges.
If this resistor equals the internal resistance of the meter (coil) then it will halve the reading. You can check the resistance of the meter with a digital mulitmeter.
There is much to be said for keeping the existing centre 0 amp meter where you don't need great accuracy.
One of the difficulties of digital amp meter apart from often the need to connect in the negative line is that the shunt tends to need to be a relatively large resistance compared to a moving coil meter. Once digital meter chips tended to need 200mv for max reading this meant that the shunt dropped 200mv at max current. It means that for typical readings some significant voltage/power is lost to the system. This can be overcome with an amplifier and perhaps some modern digital amp meters do this. It is most likely that you will need another shunt to suit the digital amp meter so check actual resistance.
Digital volt meter yes good idea go for it look to China Ebay for cheap. good luck olewill

 

[BERT T]

Have a look here: http://www.ebay.co.uk/itm/DC-0-20V-...18846093&tpos=top&ttype=price&talgo=undefined

 

[BabaYaga]

Thanks for the additional comments.
Given the reasonable cost for a dual A & V meter with shunt included (thanks Bert T), I am now leaning towards keeping what I have for normal use and installing one of those on a switchable power supply for the occasions when I am really interested in numbers. Which would save me 0.7 Ah/day if I understand correctly.

 

[VicS]

Thanks for the additional comments.
Given the the reasonable cost for a dual A & V meter with shunt included (thanks Bert T), I am now leaning towards keeping what I have for normal use and installing one of those on a switchable power supply for the occasions when I am really interested in numbers. Which would save me 0.7 Ah/day if I understand correctly.

I'd think BertT has found something as close as you will get to what you actually wanted.

Downsides are that it will require the shunt in the negative wiring and does not monitor the starter battery. A simple changeover (SPDT) switch will enable it to read the starter battery volts in the same way that a battery monitor like the Nasa ones does. Alternatively fit a separate volts only meter for the starter battery, or keep the existing voltmeter to do that.

 

[Graham_Wright]

Have a look here: http://www.ebay.co.uk/itm/DC-0-20V-...18846093&tpos=top&ttype=price&talgo=undefined

Beware!

I bought (a similar) one of these.
The instructions (for mine) were in Chinglish and very misleading.

After three purchases, I find one (for our camper van) that appeared to do what I wanted. The voltage readout is ok but even with the current sensing leads shorted together, the indication was around 10 times actual. Eventually, I opened the unit and found the possibility of introducing a shunt directly across the input terminals. Even with the leads from the battery shorted at source and a near zero resistance shunt internal to the meter, I still see twice the actual.

The excuse offered by the seller was tha it was not very accurate at low readings!