Ammeters questions

[booster]

All the talk about second alternators and fast charging by them got me wanting to have an ammeter in the wiring from the engine to the coach batteries. We currently have 2 #4 wires to the rear fused at 160 amps total, but may switch to a single 200 amp breaker in the future.

I have been looking at ammeters and it is a bit confusing, as it appears that nearly all the digital ones with shunts need to be in the negative battery cable, unless they use a "shunt reverser" type thing to be able to put it in the positive wiring. Only Blue Sea seems to have one and it is very spendy. When I look at the plain old fashioned analog ammeters, they seem to show them in the positive wiring in their wiring diagrams. They are also pretty inexpensive.

Anybody got any insight on these things, or recommendations of what would work the best?

[markopolo]

Can't help much. I have small digital ammeters wired in on the negative cable.
My portable clamp-on ammeter goes around the positive cable.
Will your Trimetric give you the info you want? Is their shunt on the negative cable also?
Were you thinking of putting something in or on the dash?

[booster]

Yeah, the goal would be to have something on the dash, and to have it read the entire amount of current from the alternator to the coach. The positive line thing is what really caught me off guard.

The Trimetric gives us some of the information, but it is in the negative line to the coach batteries, so it only shows the current that is going to the batteries. There can also be various amounts of current going directly to run things in the coach, and those don't get seen by the coach battery negative cable.

This is one of the "I wish I knew then" things, I think. When I ran the new wiring to the coach from the engine alternator, I just figured I needed to cover enough amperage to run the microwave when we wanted. That was less than 110 amps, so I just doubled up the #4 Roadtrek had with another one, both breakered at 80 amps for a total of 160. Since the stock alternator would only do about 85 amps to the coach at idle, I got the bigger alternator to get the 110 amps at idle to cover the microwave by itself and save the batteries, which it does very well.

Now comes the not so good (maybe). To get the 110 at idle, I needed to get a 250 amp max alternator. I figured that was no big deal, as Roadtrek uses only one 80 amp breaker on 220AH of AGMs in the new units, as far as I have been able to find out. I have double that wiring, and less than double wet cells AH. It seems to work OK, as we only pull about 100 amps to the batteries when they are at 30% charge, so that is good, but if we ran the microwave at that point, we could trip the breakers as we would be over 200 amps total. If we ever decided to go with AGM batteries, they probably would pull more than the wiring could handle by themselves when low.

From what I have read, the two #4 wires could handle 200 amps for a reasonable amount of time, which would be fine as the extra voltage drop might help limit current a bit, so I think I will rebreaker at 200 amps total, which will help, but best would be to be able to limit the alternator to 200 amps, which I haven't been able to find a way to do at this point. I really don't want to run even bigger wiring, as 200 amps is all we really need, and it would be a huge PITA to do. If the alternator limited there, we could run the micro and any extra would go to the batteries, and the wiring would be fine.

For now, I just have the original alternator in place, so we can't overload anything. Only downside is the low idle output, so the batteries have to contribute to microwave running.

Long explanation, but that is why I want the ammeter on the wiring to the coach. If I am going to be messing with this stuff, I want to be sure we don't do anything dangerous, as 200 amps is plenty enough to cause severe hazards.

As a sidenote, I think this may point to some things that hadn't made a lot of sense to me concerning some recent electrical failures I have heard about on Roadtreks. There seems to be more separator failures of late, so maybe it is related to a similar issue. The Chevies have a 145 amp alternator, that will put out about 110 amps hot, so it can easily overpower the 80 amp breaker in the coach lines. They are also using AGM batteries now at 220AH, so they have a place to put a lot of amps, if they are low. Ours had one of the 80 amps breakers before the separator, so it the breaker tripped the separator would also disconnect. The breaker would cool and reconnect and in the process reconnect the separator. Repeat and repeat until the batteries got full enough to reduce the current. That could put a lot high amp switching cycles on the separator and cause early failures.

I think this is probably something we all need to be aware of, especially with so many folks going to AGM batteries. The old wet cells just won't take lots of amps, so the problem would be not there or much less with them.

[markopolo]

Google: Hall Effect ammeter or similar. That might be the easiest way to add an ammeter without having to add a shunt.

Something like this: http://www.fixmyambulance.com/product-p ... -300-1.htm

[booster]

Originally Posted by markopolo

Google: Hall Effect ammeter or similar. That might be the easiest way to add an ammeter without having to add a shunt.

Something like this: http://www.fixmyambulance.com/product-p ... -300-1.htm

Thanks Marko, very interesting. Spendy to be sure, but would do the job if I got the whole meter and shunt kit from Inpower. What I haven't found is what the accuracy of them is, which gets pretty important when you get to the low readings as they usually are % of full scale.

It appears, if I am reading things correctly, that only the digital shunt type meters need to go in the negative cable, analogs go in the positive. I found this kind of neat design while searching:

http://metersandinstruments.yokogawa...se-dc-ammeters

2% accuracy (better than Blue Sea), available in a 300 amp size, and a very interesting mounting and shape. About $80 delivered from the mfg, without shunt. This style of mount would allow an interesting stack of them (they are made to stack) for various volts and amps readings.

[Bruceper]

Current flows from negative to positive. The reason (most) ammeters go on the negative cable is because you want to measure current from the source (output).

One other item to consider of course is safety. If you ever get into an older vehicle built in the 60's or 70's you will see many had an ammeter. This necessitated running the current from the negative on the alternator up through the firewall and to a meter in the dash and back to the battery to charge it. Many fires ensued when old wiring frayed, wore, degraded, lost insulation and shorted out. It also ended up with expensive repairs when the meter broke and current no longer flowed through the meter.

For many years there were a lot of articles in car magazines on how to replace an ammeter with a voltmeter. Newer vehicles use a voltmeter in the dash to avoid this issue and of course now it's still a voltmeter but computer controlled.

The hall effect ammeter is probably your safest bet. Yes it may cost a few bucks, but doesn't require additional high current wiring which in itself is also expensive.

[booster]

Thanks for info Bruceper. I agrèe on the indiuctive, but even with cost ourt of it, I still wonder why they don't put anything abcout accuracy in the specs.

i find it interesting that most of analog, shunt type, meters, say to put the shunt in the positive line, while the digitals say to put it in the nefative ( because they can't handle voltage on the shunt lines).

The good news on safety is that the shunt is quite safe way to wire it in, and meter wires are millivolts, so totally safe into the cockpit.

I will have to call on the accuracy of the indctive one before I finally decide what to do.

[booster]

We finally finished this off, after collecting a bunch of information and starts and stops. I was all set to order the analog thin meter and a shunt, when I ran into a $100 minimum charge issue, so I would need to get two of them. They wouldn't even let me pay $100 for the $80 meter. Did get accuracy info from Inpower on the digital inductive and it is actually very good at the low end compared to most meters. They don't publish accuracy because it is a mulitlayered thing of basic measuring, plus significant digits, plus lot of other geek speak. Bottom line was I asked if I would be within two amps at 5-10 amps and they said yes.

Got the Inpower and put it in, and it works very well, it appears. Nice to have the voltage at the input to coach wiring at the engine to see how much we lose to the rear. If just the batteries are in the circuit, it matches the Trimetric extremely well. I was very surprised at how much the amperage to the inverter changes with voltage when running the microwave. Off full batteries (wet cells), they drop to under 12v when running the microwave, and the inverter is pulling over 105 amps. The big DC Engineering alternator can hold that voltage to 13.5 or more, even at idle, and the inverter pulls mid 70 amps.

With full batteries and at 14.4 volts, engine running, the amps go down to the 3 amp range, which is the ending amps I see from the shore charger, so when I get there, I just shut off the separator to prevent over charge.

I would recommend a big ammeter from the engine charging system to anyone who is considering an engine generator and running high loads off the engine. It is even more useful, IMO, is if you are going to be consistently charging your batteries while driving. Knowing when the batteries are full is critical to not overcharging them. AGMs are particularly susceptible to damage, wet cells will mostly use lots of water and not be as severely hurt.

Just mounting your Trimetric where you can see it while driving would give you all the battery saving benefits, but you wouldn't see you amps to big loads.

[markopolo]

I saw the photos you posted here: http://classbforum.com/phpBB2/viewto...tart=11#p23101 in with the cooling topic.

I agree that amps in is valuable info both for engine generator setups and traditional Onan generator rigs. Batteries really take too long to fully charge (100%) using a generator so it's much more fuel efficient to shut down the generator (engine or Onan) when the amps in slow to a relative trickle.

I can measure amps in on my van but can't see it from the drivers seat. Having the display on the dash like you did is ideal.