Ammeter Confusion

richardpope50 · Post by **richardpope50** » Sun Dec 01, 2013 6:05 pm

I want to be able to measure the current going in to my fuse / relay / distribution box from the battery as I am unsure what the total loading is. Whilst only a one-off exercise, it could be useful to have such a meter to use occasionally. My calculated typical / maximum amp rating is around 60 amps assuming most things are on at the same time with a total maximum of 90 but then not everything will be on at one time (I won't have dipped, full, reverse light and hazards, etc., all on). I am happy with the distribution down the loom from my box so not overloading relays, loom, etc, but need to know this main feed current.

My alternator is rated at 100 amps but due to a partially melted (!) main fuse connection I need a meter in case the alternator is at fault or I do have too much of a current.

My digital multi-meter will not measure more than 10 amps and I see on eBay numerous panel volt / amp meters using 5 wires and a shunt such as
http://www.ebay.co.uk/itm/Digital-DC-4- ... 1189698480

As all I want to do is to test the big red cable's current from the battery to my main fuse, can I use one of these to do this? I kind of thought about using croc clips for the connections other than more permanent soldering type connections.

I'm not really sure what a 'shunt' is as most of these meters require one (but the above one includes one).

Any help / suggestions appreciated.

stevieturbo · Post by **stevieturbo** » Sun Dec 01, 2013 6:34 pm

http://www.ebay.co.uk/itm/MASTECH-MS210 ... 19d50a7261

http://www.ebay.co.uk/itm/UNI-T-UT203-A ... 56428579f5

Basically clamp it around the main wire to the alternator, as this will show the entire current draw whilst the engine is running.

mgbv8 · Post by **mgbv8** » Sun Dec 01, 2013 9:51 pm

As Stevie says. Just buy an Amprobe or Ampclamp meter. Much easier than trying to fit an inline Ammeter.

richardpope50 · Post by **richardpope50** » Tue Dec 03, 2013 8:34 pm

Thank you both for the pointer and it seems a much better approach than the route I thought of.

stevieturbo · Post by **stevieturbo** » Tue Dec 03, 2013 10:46 pm

And the clamp meter is very handy to have anyway

DaveEFI · Post by **DaveEFI** » Tue Dec 03, 2013 11:57 pm

richardpope50 wrote:Thank you both for the pointer and it seems a much better approach than the route I thought of.

If you fancy building something, I have a circuit which measures the voltage drop in the battery earth lead - either charge or discharge. I'm using it to drive a tri-colour LED which is amber when normal, green when charging hard or red if discharging. But it can also drive a meter. It would be possible to make it drive a standard car ammeter by removing the shunt. The beauty being it doesn't require heavy duty wiring to the meter.

I dunno how accurate the clip on types are for DC these days - at one time they were only a guide. Much easier to get an accurate one for AC.

stevieturbo · Post by **stevieturbo** » Wed Dec 04, 2013 12:01 am

Ive used a couple of cheap ones.

Have no reason to doubt their accuracy. They used to cost a fortune years ago too !

kev_the_mole · Post by **kev_the_mole** » Wed Dec 04, 2013 9:37 am

DaveEFI wrote:
richardpope50 wrote:Thank you both for the pointer and it seems a much better approach than the route I thought of.
If you fancy building something, I have a circuit which measures the voltage drop in the battery earth lead - either charge or discharge. I'm using it to drive a tri-colour LED which is amber when normal, green when charging hard or red if discharging. But it can also drive a meter. It would be possible to make it drive a standard car ammeter by removing the shunt. The beauty being it doesn't require heavy duty wiring to the meter.

I dunno how accurate the clip on types are for DC these days - at one time they were only a guide. Much easier to get an accurate one for AC.

Hi Dave,

I'd be interested please.

Cheers,

Ian

DaveEFI · Post by **DaveEFI** » Wed Dec 04, 2013 11:01 am

It's just an fairly old Maplin project - no longer available. But it works fine. If you PM me with an email address I'll send you scans of the construction details. I may also have some spare bare PCBs I made.

stevieturbo · Post by **stevieturbo** » Wed Dec 04, 2013 1:13 pm

You could probably rig up a remote hall sensor to a digital display for a proper current reading.

If it's on the alternator to battery main line, then if things are well it should always read current draw.

if it drops to zero, something would be badly wrong. At least it would give accurate and usable information about current draw

DaveEFI · Post by **DaveEFI** » Wed Dec 04, 2013 3:05 pm

The one I've got fitted to the SD1 - the Maplin project one - measures the voltage drop in the battery ground cable, both positive and negative going. So shows charge and discharge. With a tri-colour LED, it's a very good idiot light for charge/no charge. I've had a faulty alternator before now where the ignition warning light doesn't come on. This device shows positively if you are charging or discharging or normal. And takes up virtually no space on the dash - unlike a meter.

stevieturbo · Post by **stevieturbo** » Wed Dec 04, 2013 3:26 pm

But it won't let you know actual current flow which was the original request. To see what the alternator is doing

DaveEFI · Post by **DaveEFI** » Wed Dec 04, 2013 4:32 pm

True - but I thought you'd moved on with all that talk of hall sensors and digital displays.

In practice, all you really need to know is if the alternator is balancing the load. Except for fault finding, where an accurate ammeter will be useful. Sods law says there is never room to fit a clamp type where you need it.

stevieturbo · Post by **stevieturbo** » Wed Dec 04, 2013 5:23 pm

If you're trying te determine whether your alternator has adequate capacity. You need to know current draw. Both under normal use, and worst case scenario

DaveEFI · Post by **DaveEFI** » Wed Dec 04, 2013 5:43 pm

Simple surely to calculate the draw? You're only really going to need to know any added high current devices like cooling fans - and they tend to say how much they take.