Measuring Amps - for Liveaboards :)

[DeanS]

I've been living out of the marina again for about a month (bliss), except we use a lot of elec, and I wasnt sure what was going on with amps supplied to batts and amps supplied from batts etc. I didnt know if running the engine on idle was better than running it on higher revs (nothing to measure it with). Last week I went to Maplin and bought a clamp meter....for £14...brought it back and got wonky readings, only to find the one I had bought only measured AC amps, whereas batteries are DC amps, and the supply to the invertor is also DC amps. I went back today and bought the bigger one, and got back to the boat, to do some measuring.

 

Now...the boat had been on idle for about an hour, so the test was to see:

 

1. What amps the alternator is giving out, if any.

2. What change is there if I turn up the boat revs.

3. Test to see what amps were being drawn by the boat appliances right now.

 

Answers below:

 

1. 30Amps

2. No change when upping the boat rpm.

3. 10Amps

 

So what did I learn. I learnt that I can now manage my recharging better, and not waste fuel running the engine on fast rpm (1300) when on idle (900) the Amps from the alternator is the same. (BECAUSE IT IS THE BATTERIES THAT REGULATE WHAT THEY RECEIVE NOT WHAT THE ALTERNATOR DECIDES IT SHOULD PUSH OUT)

 

My tests to come will be:

 

1. How many amps does the 175Amp alternator supply first thing in the morning, after the kids have sapped the batteries into the late morning hours.

2. Does idle and higher rpm change the amps or not.

3. Can I reduce my fuel bill. (it's eating diesel at the moment)

 

I have 6 batteries. 660AH. That means about 330AH capacity. If I use 10Amps each hour (with a fridge and 2 laptops running), thats approx 200Amps a day. Since I run my engine for an hour in the morning, and 2hrs in the afternoon/evening, it's putting in a charge of about 60Amps during that time (30Amps minus 10Amps usage x 3 hours = 60Amps ). Not really enough. Luckily I've been doing some cruising lately. which has helped. This little clamp meter has blown my mind...I now have lots of thinking to do...do I spend money on petrol (gennie) or diesel(boat) or solar. Clearly if I had 10Amps per hour output from solar panels it would meet our basic needs (during the day). My mind is in a quandry. All I can say is...I love my new little clamp meter.

 

Anyone know what size solar I would need to give me 10Amps per hour for most of daylight hours.?

Edited April 23, 2013 by DeanS

[Chris Pink]

Please Dean go back and write amp hours where you mean amp hours and not amps

[DeanS]

Please Dean go back and write amp hours where you mean amp hours and not amps

 

I think it's now correct?

[by'eck]

Unfortunately your clamp on meter won't tell you which way the current is flowing. However if clamped to alternator B+ cable before any connection to batteries, you can assume (allowing for inaccuracies) that this is the current its providing.

 

If you could clamp onto the main service battery cable adjacent to them before alternator connection, it would indicate current flowing into or out of batteries. Depending whether engine charging is available or not you can make an assumption which way the current is flowing. This would be the most meaningful figure since you are concerned about battery charge current.

 

Whilst its the batteries that present a load to the alternator, don't dismiss the fact that under high demand (discharged batteries), more engine revs will be needed (above idle) to provide the current they will draw. Your present finding suggests that the alternator will produce 30 amps at idle but that batteries are sufficiently charged not to draw more when its available.

[nicknorman]

No it's not! Eg "I use 10 amps each hour". Amps is a flow rate, not a quantity. You could say "10 amp hours per hour" but why bother when "10 amps" means exactly the same thing.

 

Anyway, whilst your alternator was giving 30A at idle when you checked, you will find its giving nearer full output when you first start to charge in the morning. I think you have a Beta? These are pullied to give full output pretty much at idle, so you may see 175 A at idle, however that is not the only consideration. Running the alternator at the minimum speed to get max current means it will be running very hot due to the slow fan speed. It also means the stretch force on the belt is at a maximum and the belt will be on the point of slipping. Running the engine faster helps cooling and reduces the load on the belt.

 

Therefore I would recommend running the engine above idle, say 1200rpm, until the initial current has fallen off. Then by all means reduce to idle.

 

Unfortunately your clamp on meter won't tell you which way the current is flowing. However if clamped to alternator B+ cable before any connection to batteries, you can assume (allowing for inaccuracies) that this is the current its providing.

 

If you could clamp onto the main service battery cable adjacent to them before alternator connection, it would indicate current flowing into or out of batteries. Depending whether engine charging is available or not you can make an assumption which way the current is flowing. This would be the most meaningful figure since you are concerned about battery charge current.

 

Whilst its the batteries that present a load to the alternator, don't dismiss the fact that under high demand (discharged batteries), more engine revs will be needed (above idle) to provide the current they will draw. Your present finding suggests that the alternator will produce 30 amps at idle but that batteries are sufficiently charged not to draw more when its available.

DC clamp meters do show direction of current flow, well they show + or - and you have to put the meter clamp on in the same orientation each time. The Beta engines are pretty good at putting out max current at idle, though as I mentioned this is not the best way to treat the kit.

[DeanS]

Thanks..great comments.

I still get confused with Amp Hours, Amps etc. When I say Amp Hours, I'm normally talking about the battery capacity...but when I say Amps, I mean...the measurement...at that time....on the meter. When I say I'll use 10Amps per hour....I mean that the invertor is on permanently...so if it's using 10Amps when measured...it would draw 10Amps in an hour from the battery (which is rated in AmpHours). Does that make sense:)

 

If I measure 10Amps on the input to the invertor, and continue that demand for 1 hour...the batteries would have lost 10 "whatevers"...from a capacity of 330 "whatevers".

[nicknorman]

Thanks..great comments.

I still get confused with Amp Hours, Amps etc. When I say Amp Hours, I'm normally talking about the battery capacity... Amps hours is merely a measure of charge - using the water analogy, a volume of water. Either a volume of water that you have taken out of the tank (battery) or a measure of how much water the tank can hold (battery capacity) depending on context. .. but when I say Amps, I mean...the measurement...at that time....on the meter. Amps is the rate of flow, like litres per hour for water. When I say I'll use 10Amps per hour Thats like saying when I turn on the tap, the water flows out at 10 litres per hour per hour....I mean that the invertor is on permanently...so if it's using 10Amps when measured...it would draw 10Amps in an hour from the battery No, it would draw 10 AmpHours in an hour (which is rated in AmpHours). Does that make sense:) No! Well we know what you mean, but when you get the dimensions wrong it stores up confusion and trouble for later!

 

If I measure 10Amps on the input to the invertor, and continue that demand for 1 hour...the batteries would have lost 10 "whatevers"...from a capacity of 330 "whatevers". They would have lost 10 AH out of a capacity of 330 AH.

 

Amps is flow rate, AH is quantity of electricty or battery capacity. Simples!

[DeanS]

 

Thanks..great comments.

I still get confused with Amp Hours, Amps etc. When I say Amp Hours, I'm normally talking about the battery capacity... Amps hours is merely a measure of charge - using the water analogy, a volume of water. Either a volume of water that you have taken out of the tank (battery) or a measure of how much water the tank can hold (battery capacity) depending on context. .. but when I say Amps, I mean...the measurement...at that time....on the meter. Amps is the rate of flow, like litres per hour for water. When I say I'll use 10Amps per hour Thats like saying when I turn on the tap, the water flows out at 10 litres per hour per hour....I mean that the invertor is on permanently...so if it's using 10Amps when measured...it would draw 10Amps in an hour from the battery No, it would draw 10 AmpHours in an hour (which is rated in AmpHours). Does that make sense:) No! Well we know what you mean, but when you get the dimensions wrong it stores up confusion and trouble for later!

 

If I measure 10Amps on the input to the invertor, and continue that demand for 1 hour...the batteries would have lost 10 "whatevers"...from a capacity of 330 "whatevers". They would have lost 10 AH out of a capacity of 330 AH.

 

Amps is flow rate, AH is quantity of electricty or battery capacity. Simples!

 

Thats what I said/meant

[smileypete]

I have 6 batteries. 660AH. That means about 330AH capacity. If I use 10Amps each hour (with a fridge and 2 laptops running), thats approx 200Amps a day. Since I run my engine for an hour in the morning, and 2hrs in the afternoon/evening, it's putting in a charge of about 60Amps during that time (30Amps minus 10Amps usage x 3 hours = 60Amps ). Not really enough. Luckily I've been doing some cruising lately. which has helped. This little clamp meter has blown my mind...I now have lots of thinking to do...do I spend money on petrol (gennie) or diesel(boat) or solar. Clearly if I had 10Amps per hour output from solar panels it would meet our basic needs (during the day). My mind is in a quandry. All I can say is...I love my new little clamp meter.

 

AMEN BROTHER!!!

 

Also to do a weekly FULL charge for the 6x110Ah non sealed leisure batts, the current should drop to 2% of 660Ah which is about 13A, it may be you reach this level during cruising.

 

As for solar the easiest way to 'get some skin in the game' would be to buy a 100w 'camping' type panel for £100ish, connect directly to batts via a diode and a fuse next to batt +ve.

 

That way you'd get 4-6 amps when the sun has got his hat on, but need to watch the voltage verrryy carefully and disconnect the panel if the batt volts gets above say 14.8V or if you're going out and can't keep an eye on it.

 

Then in the mean time see if you can beg or buy a cheap PWM controller, longer term it'd be better to get more 100W panels and a decent MPPT so no point spending a lot.

 

If you have a big roof then you could get the big 200-250W domestic sized panels but bear in mind their output voltage is all wrong for connecting them directly to batts, doing sol will still only get 5-6 amps until you fork out for an MPPT which should see you get 13-15 amps in full sun.

 

cheers, Pete.

~smpt~

Edited April 23, 2013 by smileypete

[DeanS]

Does it mean that IF we are very heavy elec users, and there is a VERY slim chance the solar panels will EVER drive the batteries into float mode, that we dont need a controller. We never leave the boat for any lengthy time, and I could put an isolator switch on the wall for times we do leave the boat. Would it work? I'd rather put cash into the panels rather than the controller and get an MPPT controller down the line. We're using probably 200Amps a day at the moment. (probably more if we add in PS3..(the XBox is sadly not seeing as much usage since the new appliance arrived)

 

just a thought.? 2 panels would be nice as a start?

[Chris Pink]

200 amp hours - c'mon Dean. It's not that difficult and until you understand why it's important just take our word for it.

 

200 ah is a lot, btw, and without some way of measuring could be an over-estimate but better over than under

[DeanS]

Would this be a decent solar panel option - Ebay Link

[smileypete]

Yeah, 6 amps into a 660Ah bank is pretty well a trickle charge if the batts are 'unsealed' ones, but still worth keeping an eye on the volts and also the acid levels in the batts.

 

There's plenty of solar panels in the 100-125W range on Ebay though check delivery times. There's also a few panels on Ebay that are mid way between the camping panels and usual roof panels in terms of voltage output, bear in mind the 1.5m by 1m size:

 

http://www.ebay.co.uk/itm/Solar-Panels-Ideal-Thermasol-TSPV-200-PE-Solar-PV-Collectors-200W-/271122529531?pt=UK_Gadgets&hash=item3f20297cfb

 

According to a google search that'll give 8 amps into batts in full direct sun, with an MPPT that should go up to 12-15 amps. Down the line it'd then be best to buy an identical one to go with it, rather than tryyyy to mix and match with the usual camping and domestic panels.

 

cheers, Pete.

~smpt~

[DeanS]

Measured 10A on the meter.

Fridge on.

Laptop.

 

Those are on 24hrs. So thats 240Amps.

TV goes on for a few hours a day. 5Amps x 6 = 30Amps.

This excludes water pumps, cell charging, IPad charger etc.

This excludes PS3...about 2hrs a day.

 

So most likely looking at over 200Amps per day usage.

 

This excludes washer and desktop, which run off gennie.

[smileypete]

Would this be a decent solar panel option - Ebay Link

 

Best look for a UK supplier, and in your case fast delivery, for example these Ebay item nos:

 

150955494600

251200229645

170997859150

161006036887

200917871847

181005557723

 

ETA: So going for 100W ones give a wider choice of suppliers, choice of frame colours black/silver/white and more flexibility down the line, bit higher amps when direct to batts, but going for the bigger panels gives you less choice and that, BUT more watts for your £££ down the line with MPPT.

 

Also amps are like speed in MPH, amp-hours are like distance in miles. So you say your ammeter is reading 10 amps, in one hour you'll cover 10 amp hours. Bit like cycling at 10mph, you'll cover 10 miles in an hour, and cover 240 miles in 24 hours.

 

If you're still tempted to say amps-per-hour, just say AMP HOURS instead, or you'll incur the wrath of some people!

 

cheers, Pete.

~smpt~

Edited April 23, 2013 by smileypete

[DeanS]

Running engine on idle again.

Whereas earlier I caught it at the end of a 1hr run, and the amperage from the alternator was 30Amps, it is now showing 130Amps (flowing through the wire to the batteries). In 1 hour I will measure it again, and work out the average, over 1 hour, and will assume that the batteries would have received X number of whatevers in that single hour. I am tempted to say it would receive X amps, but I think I should say it will receive X AmpHour over the next hour.

 

?

[smileypete]

Problem is the current sometimes drops drastically within the first hour. Now say the current is 30 amps after one hour and 20 amps after 2 hours, in the second hour the average is about 25 amps, so the amount of charge supplied in the second hour is about 25 amp-hours

 

The only thing really worth noting for now is the finishing current before you switch off, that helps you gauge how full the batts are. Bit like when you fill a tank and the sound changes as the level rises toward the top.

 

Now just have a break and enjoy the sunshine!

 

cheers, Pete.

~smpt~

Edited April 23, 2013 by smileypete

[DeanS]

This is like reading your future in a tea cup.

 

When I started the engine 20mins ago, the alternator was pushing out 130 whatevers.

20min later it is pushing out 35 whatevers

 

WHAT'S THAT MEAN My understanding is that batteries tell the alternator how much they want. Why dont they want more? I very much doubt they are full....the voltmeter on the wall was showing 12.0V this morning (under load). By the evening, (and 2 charging sessions during the day) it normally reads about 12.8 and is about 12.3 when I go to bed around 1am.

[nicknorman]

As the state of charge increases, the batterys want to take less and less current. Think of it as the surface charge effect. If its charging at 30A and you stop charging and run a heavy electrical load for 5 minutes, say an electric kettle, you have not really taken that much % charge out of the battery, but you have depleted the surface charge - that means the chemical reaction taking place to deliver that current has been mostly at the surface of the plates, in good contact with the electrolyte. So now when you put the engine back on, its back to 130A or whatever, even though at the same state of charge a few minutes ago it was only charging at 30A. But soon the current will fall back to 30A as the surface charge is restored and the chemical reaction to absorb the charge has to take place deeper inside the plates and is hence slower. That just how batteries are!

[smileypete]

If you watch the volts when you start charging, you'll probably see it zoom up to 14.4V with your size alt, then it'll stay there and as it does so the amps will drop right off.

 

But what sort of volts do you see in the morning before you start switching on the loads? 12.2V 'off load' means the batts are about 50%. I wouldn't worry if they go a little lower, especially if they get some charge later in the morning.

 

Maybe for a normal daily charge just do it until the current drops to around 25A, then at least once a week keep going until it drops to 10 or 12A.

 

cheers, Pete.

~smpt~

Edited April 23, 2013 by smileypete

[nicknorman]

If you watch the volts when you start charging, you'll probably see it zoom up to 14.4V with your size alt, then it'll stay there and as it does so the amps will drop right off.

 

But what sort of volts do you see in the morning before you start switching on the loads? 12.2V 'off load' means the batts are about 50%. I wouldn't worry if they go a little lower, especially if they get some charge later in the morning.

 

Maybe for a normal daily charge just do it until the current drops to around 25A, then at least once a week keep going until it drops to 10 to or 12A.

 

cheers, Pete.

~smpt~

10 or 12A is quite a high terminating current for bats in good condition - our 440AH bank of cheapo leisures gets down to below 1A at 14.6v after a long day's cruise. I would go lower on the once a week charge, especially if you can be doing something else useful with the engine (you could try cruising!)

[DeanS]

well then.....interestingly......it might not take THAT many solar panels to achieve the same 30Amps (for 3hrs). That would save 3hrs diesel and 100hrs a month saved in engine running. Solar is looking attractive if the 175 alternator is only actually a 30 alternator.

 

100W solar = 5amps.

600W solar = 30amps.

 

Right?

[nb Innisfree]

OP. Work out how many amps on average are flowing through a wire over a length of time

measured in minutes, then dividing those minutes by 60 will give you amphours that have passed through that wire.

 

Amphours going towards the battery will be amphours added to that battery and amphours going away from the battery is amphours used out of the battery so you will need to take that into account somehow. A proper amphour counter is the best way.

[nicknorman]

well then.....interestingly......it might not take THAT many solar panels to achieve the same 30Amps (for 3hrs). That would save 3hrs diesel and 100hrs a month saved in engine running. Solar is looking attractive if the 175 alternator is only actually a 30 alternator.

 

100W solar = 5amps.

600W solar = 30amps.

 

Right?

Roughly, in summer and assuming a reasonable day. Don't forget that when you first run the engine in the morning it will be 175A slowly dropping off, so over your 3 hours the average will be a lot more than 30A. I would suggest that a modest amount of engine running (say 1-2 hrs) topped up with moderate solar capacity is the way to go - the engine is good for belting in a fast charge at the beginning, the solar is good for pushing in a moderate charge rate for the rest of the day - that being all the batteries want to take. So probably best to think of them in combination. Running the engine presumably gives you hot water as well, so you are killing 2 birds...

[smileypete]

well then.....interestingly......it might not take THAT many solar panels to achieve the same 30Amps (for 3hrs). That would save 3hrs diesel and 100hrs a month saved in engine running. Solar is looking attractive if the 175 alternator is only actually a 30 alternator.

 

100W solar = 5amps.

600W solar = 30amps.

 

Right?

 

Ish.

 

You get about 25-50% more with MPPT depending on batt charge voltage and MPPT efficiency of say 95%.

 

100W/14.5V * 95% = 6.6A

100W/12.2V * 95% = 7.8A

 

With PWM or direct connection you just get the current the panel is capable of, so if your 100W camping panel gives 6 amps, and another domestic 200W panel give 8 amps, that's what ya get. But with the same panels on MPPT it's a different story of course...

 

cheers, Pete.

~smpt~