Ammeter - How to?

[MASH]

I'd love to have an Ammeter to tell me how much I'm caning my batteries. Trouble is ammeters from swindleries come out at £30- 40 or so which is obscene. I know I can buy an ammeter (meter only) from an electronics supply company for £3-4 but these are too weedy to bung in-line to measure 200ma to 20A

Can anyone supply a circuit and components - plus a "how to" to enable me to view my battery drain at a sensible price?

 

[orizaba]

i use a cheap digital multimeter wired into my leisure battery system,i find its more important to know the voltage in your batteries than how many amps you are drawing. i don't leave it on all the time just when i want to check how things are,cost about 3.50 from maplins,

 

[DinghyMan]

If I remember this stuff from a good few years ago; there are two distinct ways to attach an ammeter, with or without a shunt. You are probably comparing a cheap ammeter with a proper ammeter plus shunt kit which is where the price difference comes in. You could just place your ammeter in line in your battery cable, if it is rated at a high enough current but that may cause you problems with your alternator.
Have a look at the TB Training pages for someone who knows about this stuff.

I'm sure someone much more up to date will come along and explain how shunts and this stuff works soon enough.

 

[tazzle]

Agree completely about TBTraining - excellent site. He's bang on the nail when he says the only useful place for an ammeter is from the B+ output of the alternator.

I looked at this when recently refitting Raedwald. A decent ammeter big enough to cope with the (new) alternator's rated output of 70A needs a shunt for def - and expect to pay £70 - £100 for a decent basic one.

In the end I settled for a clamp ammeter from
VWP for about £20 to check the alternator output under a number of different test loads - a worthwhile addition to any toolbox.

The TBTraining bloke has a good point; if you've sized your loads, batteries and alternator properly an ammeter is probably fairly low down on the list of essentials.

 

[MASH]

Ha! Thanks guys. The bit I am really interested in is the low amps stuff, say 500mA to 10A which enables me to judge low power - long term usage.

It's all this talk of shunts etc that bamboozles me - how do you do it in plain language/

 

[captainslarty]

Just to clear up any confusion with shunts, a shunt is simply a resistor, extremely low resistance in these cases. All standard ammeters (apart from the induction and other clamp ones) have shunts. When we use the term in a normal yottie sense it is simply a means of having the monitoring head away from the actual resistance.
There is a tiny voltage drop across the shunt, usually 1000ths of a volt, you can then measure the voltage drop across the shunt resistor and by knowing the value of the reisistor calculate the current using standard electric math.
However, this is only to clear that up, I think the small induction meter is a good start.

 

[halcyon]

Just to clear up a point, there is also a shunt that will measure the magnetic field around the main battery cable. This has no electrical contact with the boats system, measures what goes into / out off the battery, and cannot be overloaded, thus it will cope with engine start currant.

Brian

 

[orizaba]

would you not agree though that the voltage in the batteries is a far better and more accurate way of checking how much you are "caning "the batteries .an ammeter will tell you the load but ultimately you need to know whats in the battery , whether it needs charging and when,an ammeter will not give you this info..the only use as far as i can see for a ammeter is to see what your solar panels are putting out in relation to the sun.or to track down the source of a battery drain. i'm sure there will be some disagreement over the above statement but its based on living aboard using only 12v for 12 years, i know some of this may be taken as "not relevant "to the original post but i think it is relevant

 

[captainslarty]

Hello Mick,
I hope Brian doesnt mind me offering an opinion as I know the post was addressed from you to him,
but imho its a matter of having the info you need. Voltage gives you a good indication of battery state of charge, the monitoring of amps combined with that info gives you far more ie, the condition of the batteries and their ability to deliver what you expect from them. The other really useful tool is the hydrometer. I have a LOT of dosh invested in my battery system - apx 700 AH house and 220 Ah engine / reserve. Also in the charging systems. for me the info lets me make the most of a considerable investment.
It is best - again imho to have a handle on all three, volts amps and SG.
Joe.

 

[orizaba]

absolutley right captain,i think what i was trying to say was given the choice between monitoring voltage or amps i'd go for voltage first,

 

[halcyon]

Back in 1990 we started making a bar-graph ammeter and bar-graph / or digital voltmeter. The voltage side gave you a battery status reading, the bar-graph a red / green moving dot indicating net charge into, or out of the battery. The bar-graph also had the advantage of giving a visual indication from a distant glance, rather than having to watch a meter.

So I probably agree with you in broad terms.

Brian

 

[TonyS]

Just buy the NASA battery monitor complete with shunt and wiring loom. It shows amps in and out which is quite interesting when using the windlass with the engine on. It also shows at the same time a digital voltage as well as the battery capacity. It consumes a couple of milliamps so can be left on the whole time and at a glance you know everything about your batteries. IMHO when you have messed about buying shunts, wires ammeters, digital voltmeters and factored in your time you will have spent more than the £90 it costs.

 

[William_H]

High Mash... An ampmeter has all the current going through it. However it is common to have the shunt near the wiring or battery and the meter in an easily visible place. The shunt as already stated is a low value resistor. It in effect bypasses most of the current leaving just a small volt drop to go through or to your meter.
Now most digital meters require 0 to 199 millivolts. This means that if you set up a shunt to measure 0 to 20 amps (actually 19.99 amps then at 20 amps you have a volt drop of 200millivolts.
The resistor required is R=E/I or .2 divided by 20 = .01 ohms. This volt drop may be acceptable for your system but is not really good. The next step is 0 to 200 amps which requires a .001 ohm resistor. The smaller resistor results in the same volt drop at max current but less volt drop if you are actually only carrying typically 10 amps. Obviously with a 0 to 200 amp meter you do not get much indication or accuracy at .1 amp so digital is not allways ideal.

The shunt can be made using the smallest value wire wound resistors you can buy I think typically .1 ohm. You can mount 10 of these in paralell to get .01 ohm or 100 in paralell for .001 ohm. (a bit impractical)

If you have a multimeter with a 10 amp range then you can make your own shunt by putting the multimeter in series with your shunt, a battery and lamp load to compare the result.
For .01 ohms start with a piece of stainless steel strap fairly thin about 3 inches long and 1/4 inch wide. You will probably have insudffiient resistance so you grind a waste to make the strap narrower to increase resistance.
Put bolt holes in each end to carry the current carrying cables. Make smaller holes near the ends for seperate terminals to go to the meter. (If you use the main cable lugs the resistance of the bolt connection can vary which varies the resultant readout). You can interpolate the readings upwards to get the shunt correct. (or as accurate as you want)

Now using the moving coil meters from electronics shops. They might typically be 0 to 50 microamp they might have an internal resistance of 1000 ohms. You can measure the resistance with a multimeter and the meter should say what current it is designed to measure. Again a resistor is connected in paralell.
The resistance of the shunt is a ratio of the current of the meter (50uamps) to the current max that you want to measure as related to the resistance of the meter.

Or you can look at it as the meter requires E = IxR 50/1000000 x1000 (ohms) = 50 millivolts or .005 volt to swing it to full scale required to be dropped at our chosen max current eg 20 amps
Resistance required is R=E/I, R = 50/1000 (volts) /20(amps) = .00025 ohms. This value is difficult to get with off the shelf resistors (lots inm paralell) but is easier to get with a piece of stainless steel. By trial and error.

You can see that a moving coil analogue meter has the advantage of less volt drop in the system and of course doesn't need a power source.

There is one other little trick you can do. Which might be what you want. You design ( or make) a shunt that gives a 2 amp full scale reading on a 1000 ohm analogue meter. You fit a 9000 ohm resistor in series between the shunt and the meter (one leg) this then makes the meter 0 to 20 amps (ten times less sensitive) however if you fit a little push button wired to bypass the resistor you can change it to 0 to 2 amps to check your low drain items. Only push the button if you know the reading is going to be low.

Lastly analogue meters don't like to be over driven as they tend to bend the needle. You can limit some of the damage by fitting 2 small silican diodes each in opposite direction across the meter. Because of the inherrent volt drop of the diodes they will not affect anything until the voltage rises above .7 volt in which case they will conduct to protect the meter. Put a resistor (100 ohms) in series between the meter (and diodes) and the shunt.This protection is optional however.

So I hope this might answer your question and give you food for thought. PM me if you are confused. olewill

 

[roly_voya]

Have a look at the RS electronics site, wide range of meters and shunts but if you can get a 'proper' ameter from a chandlers for under £40 grab it becase a 1-200a shunt is more than that plus the head. I would suggest that buying cheap instruments may be a bad idea as inacurate info can be worse than no info. Cheapest way I know is to wire in a good digital volt meter (reletively cheap) then buy a 200a DC clamp meter for abaout £50-70 (posibly less if you need lower capacity) with which you can check the current in any wire, alt output, batt drain or any consumer item which is very useful.

 

[albineer]

If you still want an ammeter 60-0-60 ammeters are advertised here approx £7 postage extra but there may be a store near you
http://www.wilcodirect.co.uk/catalog/index.php?main_page=product_info&products_id=759

 

[William_H]

I would disagree that inacurate info is worse than no info.
An ampmeter that is eg reading only 50% of what the real current is or even 50% more than the real current will not matter too much if you use the readings on a regular basis for checking.
ie if your ampmeter always reads 25 amps of charge straight after starting the engine then accuracy doesn't matter if you find one day that it is double or half what it usually is.

Or again in the case of MASH he wants to check to see if there is n0o current leakage with everything off (or to confirm everything is off then actual accuracy is not so important.

Of course for some decisions accuracy is important you just need to know if you have accuracy or not. olewill