Confusing Readings

[DeanS]

A few days ago I bought myself a clamp meter so I could measure the amount of current travelling through various wires at various times. It's been a true eye opener and I'm really learning a lot, but it's making me question "why" "how" etc which I'm hoping this thread can focus on. I'll just put down some questions below, based on the measurements I've been taking:

 

1. When measuring the output from the alternator, I get readings like 150 Amps when the engine starts up, and this drops to about 50Amps after an hour. Would I be correct to assume that the average flow of current is therefore 150-50 divided by 2 = 50amphours. So in that single hour of running the engine, the batteries would have been given an extra 50amphours "back".

 

2. The demand placed on the batteries is in the region of 10Amps. So after 5 hours, I'm assuming that the 50amphour added to the batteries in point 1 above, has now been used up. If I run the engine again, and add another 50amphours, then during the evening, using 10amphour for 5 hours, by midnight, I've used up the 50amphour again. This effectively leaves the battery bank in the exact same state each day (empty) . Is my thinking correct.

 

3. Today I decided to run the gennie a bit so we could do some clothes washing. From 2pm I connected the cheapo Halfords "automatic" battery charger to the entire 660AH bank. When it began, it showed that the voltage on the batteries was 12.9, however I do think this was still the afterglow from the morning engine run. Anyway..I measured the current the charger was sending to the batteries. It was 8Amps. After a few hours, I remeasured and it said 6Amps. I expected higher than 8 to begin with. Why so low?

 

4. Lastly, I'm thinking that with 6 solar panels, it's possible to put around 150amphour into the batteries over the course of a day, which seems very good in comparison to what I'm currently feeding the batteries from points 1,2,3 above....and is my main reasoning for going onto point 5 below.

 

5. I'm considering getting 6 monocrystaline solar panels and an MPPT controller and hopefully avoid having to run either the morning charging or the evening charge.I'm hoping that the panels will cover the demand for the entire day, so I'd perhaps just run the engine once each evening if needed.

 

I'm very appreciative of all the help I've received so far, so thanks for anyone who responds to this new thread. I truly apologise for taking up so much forum resources, but having a larger boat with additional batteries and larger alternator comes with a whole new set of options.

[Paul C]

1. You've assumed an arithmetic mean, but got the sum wrong - it would be 100Ah. I'd say its more like an exponential decay though, so will be lower than 100Ah.

 

2. In theory, simply, yes. BUT there's something called Peurket's Law to worry about. For the sake of simplicity, just assume that the charging/discharging process of a battery is inefficient so you'd actually need to put slightly more back in, than you get out of it (the excess energy would be lost as low grade heat).

 

3. ...because that's how chargers behave?

 

4, 5. Without being able to predict the weather, can't say too much, but I'd have thought possibly a bit more than that. Don't forget that solar panels are permanently connected and currently, there's about 14 hours of daylight, but the first and last 2 hours are weak (its actually a sinusoidal relationship). If you're going to make a significant investment in solar panels, look seriously into tilting them (ie brackets/mounting to allow tilting). And look at MPPT controllers and their capability too

 

 

 

Personally I'd SERIOUSLY consider a proper battery monitor of some kind, at the moment you're taking "spot" readings of current and voltage, which gives a vague idea but no real accuracy on things.

[DeanS]

Point 1. Is it actually possible to shove 100Amps into batteries within an hour?

[nicknorman]

1. Very roughy (because we don't know the rate at which the current falls off) the average between 150A and 50A is 100A, so you are adding 100AH in an hour. In reality I suspect slightly less than this, because the current will probably fall off faster at first.

 

2. Well not quite right because in 1) you added nearer 100 AH

 

3. You don't say what current the charger claims to put out, but sometimes these cheapo chargers are not continuously rated. They may either reduce the current, or shut off altogether for a while, to prevent overheating.

 

4 and 5 - no experience of using solar but I suspect your figures relate to a sunny summer's day only.

 

Point 1. Is it actually possible to shove 100Amps into batteries within an hour?

Depends on the size of the bank, but in your case yes. Edited April 24, 2013 by nicknorman

[nb Innisfree]

3. Today I decided to run the gennie a bit so we could do some clothes washing. From 2pm I connected the cheapo Halfords "automatic" battery charger to the entire 660AH bank. When it began, it showed that the voltage on the batteries was 12.9, however I do think this was still the afterglow from the morning engine run. Anyway..I measured the current the charger was sending to the batteries. It was 8Amps. After a few hours, I remeasured and it said 6Amps. I expected higher than 8 to begin with. Why so low?

 

Probably because your batts were fully charged, or nearly.

 

Point 1. Is it actually possible to shove 100Amps into batteries within an hour?

If batts are in bulk charge phase they will take whatever amps you can throw at them limiting factor being overheating.

[nicknorman]

 

If batts are in bulk charge phase they will take whatever amps you can throw at them limiting factor being overheating.

Point of order - batteries are not in bulk, absorption or float. Only chargers have these different phases.

 

A charging device goes to absorption mode when the battery can no longer take the charger's full output at the regulated charging voltage. When this happens depends on the max output of the charger in relation to the size of the battery bank and the regulated charging voltage, there is nothing different happening inside the battery.

Edited April 24, 2013 by nicknorman

[dor]

Dean: Your clamp meter is a diagnostic tool. It is only giving you point readings whereas what you are looking at is a constantly changing analogue situation.

 

What you need to do is fit a proper meter with shunt into your system in a place where you can actually read what is going into and out of your batteries. You may (like me) need two or three or more to determine different aspects like net current going into the batteries, net 12V draw, net inverter draw, net solar input.

 

Only by glancing at the meters all the time will you get a feel and understanding for what is going on. For instance, in an earlier post you seemed to think that increasing revs didn't increase your charge rate. This is quite likely when maybe only 20 or 30 amps are going into the batteries, but when the batteries are low and 100+ amps are going in, there might be quite a difference between tickover and say 1500 rpm. You will also see how your amps drop (rapidly!) once you start charging after a heavy night of power use, and whereabouts it stabilises for a while.

 

Panel ammeters are cheap enough on ebay - you probably could have bought four with shunts for the price of your DC clamp meters!

[Tony Brooks]

For the sake of simplicity and with a tolerably well set up system you can assume that the average output of the alternator for the period of time between the first start and the charge dropping to (say) 10 to 15 amps will be 50% of the alternator's rated output.

 

After that point the average will go on reducing , making it a useless concept.

[DeanS]

The reason I like the clamp meter, is that I can move it around and measure whatever I want in the boat, on the gennie, on the charger, on appliances etc. It's quite handy and portable. Before I had it, I was completely blind to what was happening. So far I've discovered the following:

 

1. My alternators are working. Before they just looked pretty, but I had no idea if any current was flowing.

2. The invertor is pulling 10Amps most of the time. I've done a few spot checks, and it's typicaly close to that.

3. If the battery charger is only supplying 8 amps, then solar can replace that easily. You say "thats how chargers work" and "the batteries must have been full" , but if I had switched the charger off and gone back to running the engine, I am quite sure the alternator would have pushed more than 8amps to the batteries. 6 solar panels should do a better job than the battery charger. 6 solar panels will easily supply the 10Amps the invertor is needing most of the time.

[Chris Pink]

It's fascinating watching you stabbing at the subject. You seem like quite an intelligent person, it is unclear why you are struggling with some of these, fairly simple, concepts that are currently obscuring your attempts to understand the complex and tricky one underlying - the analysis of a dynamic system.

 

in your last: there's no point at all, at all in writing 6 solar panels. You should be thinking in terms of Watts, whether that's one panel or 20.

 

Passing that by, my comment is that the inverter is a constant load, solar is anything but a constant source.

 

1. yes you did, you have a voltmeter

2. how much current does the inverter draw with no load?

[WotEver]

... I connected the cheapo Halfords "automatic" battery charger <snip> the voltage on the batteries was 12.9

That's a pretty low charging voltage. Were the connections good? Does the charger have any settings to alter its voltage?

 

Tony

 

ETA: At 12.9v the batteries will be charged eventually, but it will take an awful long time.

Edited April 25, 2013 by WotEver

[DeanS]

It's fascinating watching you stabbing at the subject. You seem like quite an intelligent person, it is unclear why you are struggling with some of these, fairly simple, concepts that are currently obscuring your attempts to understand the complex and tricky one underlying - the analysis of a dynamic system.

 

in your last: there's no point at all, at all in writing 6 solar panels. You should be thinking in terms of Watts, whether that's one panel or 20.

 

Passing that by, my comment is that the inverter is a constant load, solar is anything but a constant source.

 

1. yes you did, you have a voltmeter

2. how much current does the inverter draw with no load?

 

I'm glad I fascinate you..lol.

1 When I say 6 solar panels, I'm talking about 6 x 100Watts at least, hopefully generating about 180amphour each day or thereabouts. (using the low value of 5amphour per panel for 6hrs a day) Of course I know that solar is affected by the weather. I'm hoping for wonderful bright days. Dont be a pessimist.)

 

The invertor never has no load. I'm assuming at least a draw of 10amps permanently. (as my spot checks back up)

 

Bear in mind, I am not trying to FULLY analyse a dynamic system, I am purely attempting to get a slightly better picture than the one I had before, so that I understand the benefits of adding more to the system...in the form of solar.

 

That's a pretty low charging voltage. Were the connections good? Does the charger have any settings to alter its voltage?

 

Tony

 

ETA: At 12.9v the batteries will be charged eventually, but it will take an awful long time.

 

Charger is an automatic one. It has a nice pic on the front showing a battery and a level indicator showing the batteries "level". I put it onto a bank of 6 batteries, to 8amps was only feeding approx 1.2amps to each battery...One of the "issues" I consistantly cant get my head around is....when you charge, the voltage shows 14V......and for the few hours after, the voltmeter shows an elevated...but decreasing value ...as the batteries "settle" . THIS reading is therefore not a great reflection on the actual state of the battery.......I use the reading first thing in the morning as a proper guide, because by then, all the "afterglow" from yesterdays charging has settled. Each morning my voltmeter on the wall says about 12.0...1...2......and thats with some load...

[Paul C]

What loads are (still) drawing power from the batteries first thing in the morning?

[smileypete]

If you want to understand exactly what's going on, it would help greatly to bite the bullet and install a shunt based batt monitor that can read amp-hours. Then you can see exactly what's going in and out of the batts.

 

Without that, all it really needs is to understand that batts are very roughly 85% efficient when discharged to 50% at a constant rate over 20 hours and then charged back to full again. So you need to put back about 15-20% more Ah than you take out. If you take out 300Ah then you need to put back up to 345-360Ah, yes, really!

 

The exact efficiency depends on a whole host of things like discharge rate, depth of discharge, state of health, bat chemistry and so on, but all you really need to know is how to use your clamp ammeter to tell when the batts have had enough charge.

 

cheers, Pete.

~smpt~

[magnetman]

It's fascinating watching you stabbing at the subject. You seem like quite an intelligent person, it is unclear why you are struggling with some of these, fairly simple, concepts that are currently obscuring your attempts to understand the complex and tricky one underlying - the analysis of a dynamic system.

 

in your last: there's no point at all, at all in writing 6 solar panels. You should be thinking in terms of Watts, whether that's one panel or 20.

 

Passing that by, my comment is that the inverter is a constant load, solar is anything but a constant source.

 

1. yes you did, you have a voltmeter

2. how much current does the inverter draw with no load?

I know it wasn't addressed to me but I found this post a little rude.

 

I would assume if someone said six solar panels they would be talking 80 or 100w each. Its hardly likely to be 10w panels is it

[DeanS]

What loads are (still) drawing power from the batteries first thing in the morning?

fridge and laptop, lights.

 

If you want to understand exactly what's going on, it would help greatly to bite the bullet and install a shunt based batt monitor that can read amp-hours. Then you can see exactly what's going in and out of the batts.

 

It's doubtful I'm going to wire in shunts etc, so the clampmeter is going to have to do I'm not trying to understand exactly what's happening, I just need to know that if I get 600W solar panels, they will:

 

1. Cover our usage during the day.

2. Go some way to contributing to the evening usage.

3. Remove the need to do 2 engine runs a day, and thereby save on fuel. I may have to do 1 run per day.

[Paul C]

fridge and laptop, lights.

 

 

It may be worth switching off the laptop and lights, and waiting for the fridge to not use any power (they run intermittently) to get a 'true' zero current reading. I know it will be affected by the fridge having consumed power a little earlier, but its better than a voltage reading with a current flowing.

 

I say "may" because you really need to get an amp-hour counter to properly understand what's going on. I know this involves fitting a shunt - but it would be worth it.

[DeanS]

ok, so here's a great example.

 

The last time I ran the engine was yesterday morning. I put in 100amphour to the batts.

During the day we did some washing, and I ran the gennie. I decided to hook up the battery charger and see what it was putting out. Discovered a measly 8amps flowing. Left it on for about 4hrs and the reading was 6amps flowing.

 

The voltage on the wall showed about 12.5 during the evening...dropping slowly.

I havent yet run the boat today, and it's 11:27am. The voltmeter on the wall is now showing....

 

11.9

 

shock and horror

[nicknorman]

What is probably happening is that the battery capacity is gradually dropping due to sulphation. I say "gradually" but this can happen in a week or two. Are you doing a weekly full charge, and if so when / how do you decide when to stop charging?

[DeanS]

What is probably happening is that the battery capacity is gradually dropping due to sulphation. I say "gradually" but this can happen in a week or two. Are you doing a weekly full charge, and if so when / how do you decide when to stop charging?

 

I'm struggling to do full charges. There's not enough canal length in the area to go on long cruises. Hooking up the battery charger with the gennie seems to only produce low amp readings from the charger (yesterday). It's a challenge.

[ditchcrawler]

ok, so here's a great example.

 

The last time I ran the engine was yesterday morning. I put in 100amphour to the batts.

During the day we did some washing, and I ran the gennie. I decided to hook up the battery charger and see what it was putting out. Discovered a measly 8amps flowing. Left it on for about 4hrs and the reading was 6amps flowing.

 

The voltage on the wall showed about 12.5 during the evening...dropping slowly.

I havent yet run the boat today, and it's 11:27am. The voltmeter on the wall is now showing....

 

11.9

 

shock and horror

Unless you sat and watched your ammeter for an hour and saw it didn't drop below 100 amps you don't know the alternator put out 100Ah and if you don't have a second ammeter you don't know how much of that 100 amps was being consumed unless you are measuring the current on the lead directly to the battery and not to any load. .

Edit

You said in an earlier post that your batteries were holding up OK, maybe they are not.

Edited April 25, 2013 by ditchcrawler

[Paul C]

I'd also seriously consider ditching the cheap Halfords battery charger (well, keep it as a reserve) and invest in a proper marine battery charger of 30A or 40A or so rating. Yes, I know they're costly - but they are built to do the job you need done.

[smileypete]

It's doubtful I'm going to wire in shunts etc, so the clampmeter is going to have to do I'm not trying to understand exactly what's happening, I just need to know that if I get 600W solar panels, they will:

 

1. Cover our usage during the day.

2. Go some way to contributing to the evening usage.

3. Remove the need to do 2 engine runs a day, and thereby save on fuel. I may have to do 1 run per day.

 

Fair enough, maybe hedge your bets and get a 40A MPPT and 2 or 3 170W panels off the Bimble Solar people

 

The panels are second hand, seems the backing has yellowed, but at least they're dirt cheap, if they get broken or stolen it's not the end of the world. Later on you could always sell them on if you really want something nicer looking, but they won't be seen that much on the roof of a boat:

 

 

In the meantime get a decent hydrometer from Halfords or a motor factor so you can check how the batts are doing.

 

cheers, Pete.

~smpt~

Edited April 25, 2013 by smileypete

[Naughty Cal]

I hope you have some money in the pot for some new batteries soon Dean boy

 

ETA: Its going to take a lot of engine running to recharge them from that low.

 

Also why is your laptop on 24/7?

 

If you are going to be living out of the marina you need to look at ways to reduce your power consumption or budget for changing batteries a couple of times a year. They wont stand up to the abuse you are giving them for long before giving up the ghost.

Edited April 25, 2013 by Naughty Cal

[magnetman]

If the laptop is on you could use a data logging multimeter and a shunt or millivolt output current clamp as a way to monitor all loads in and out of the battery.

 

The multimeter is about 30 quid and a current clamp can be got for about the same again.

 

You can produce graphs too /anorak off/