Is my battery charger a 'bottleneck'?

[magictime]

26 minutes ago, WotEver said:

There’s a thing called the ‘amp-hour law’ which isn’t really a law at all, more a rough and ready guide. It states that the charging current demanded by a battery will be roughly equivalent to the depth of discharge. So let’s assume your six year old bank now has a capacity of 400Ah. You’ve charged it to 80%, leaving an 80Ah depth of discharge. Given a big enough charge source your batteries would take 80A charging current. Your charger is only 30A, so (ignoring charge inefficiencies for now) it will give 30A for an hour, reducing the DoD to 50Ah. After another 45 minutes or so the DoD will be down to 30Ah. From this moment on the charge current will be reducing. 

 

Does that help?

I think so... 

 

So, say that 400Ah battery bank is at 60% capacity, DoD 160Ah. I run the engine for half an hour and the 100A alternator puts back 50Ah, leaving a DoD of 110Ah. Another half hour and I've gone past the point of 'diminishing returns', where the batteries will no longer take 100A from the alternator because the DoD is now below 100Ah. I switch to the alternator at about the 80% charged/80Ah DoD you illustrate above, and two hours later I'm past the point of diminishing returns for the gennie but with the batteries now close to 95% charged - DoD 20A or so. 

 

I'm just trying to square that with what Tony's saying about it typically taking at least four to six hours of engine running to get even to 80% charged on a typical battery bank, and much longer to get to 95%. There's quite a difference between 'one hour' and 'four to six hours' (to get to 80%), and between 'two more hours' and 'much longer' (to get to 95%)!

26 minutes ago, WotEver said:

As I wrote in that ‘primer’ post, for maximum life the batteries should be fully charged after every discharge. Clearly that’s both impractical and expensive in terms of time and fuel cost. So ‘as often as possible’ becomes the best advice. At a very minimum, once a week. 

OK, so once a week for a 'full charge', but what about the daily top-up? Are we talking about trying to stay above 75%, 80%, 90%?

 

1 hour ago, Tony Brooks said:

You set the engine to tickover (or any others et speed) and the combination of spring an bob-weights will balance the rack in a position that  delivers just enough fuel to keep the engine running at that speed. Now put a load on the engine . The load will slow the engine down but that causes the bob-weights to push less hard so the spring pushes the rack so more fuel is delivered until the engine speed/bob-weight speed again balances the spring...

 

At any speed or load combination the fuel consumed will differ according to both the speed required and the load. I am ignoring things like volumetric efficiency etc. to keep things simple..

 

 

1 hour ago, Paul C said:

This is important, and there's some complete bollocks out there. Fuel burn at idle is directly related to load, which varies according to the demand on the alternator.

OK, this is new to me. So there's no efficiency to be gained by switching to a generator that's designed to generate some fraction of one horsepower by burning a litre of petrol every two to four hours, because a 38Hp diesel engine will burn fuel just as slowly if the demand from the batteries is the same? I have to say this doesn't square with my experience of feeding diesel into the engine last winter to charge the batteries, or with any 'guesstimate' I've ever read of what an engine consumes while battery charging. Of course the whole picture looks very different from what I've been assuming if my engine is only going to burn say half a litre of diesel per hour of charging after the first hour. It really would be just wear and tear/servicing costs I had to worry about, rather than fuel costs.

 

1 hour ago, Tom and Bex said:

I would strongly advise not to go for the cheap Impax generator from screwfix. We took ours back 3 times under the 1 year guarantee - the 3rd time we got a refund and put the money towards a Kipor instead that so far has proved much more reliable.

 

The cheap Impax generator would start tripping into overload with less and less power being used, getting worse and worse until it would not power anything. I suspect it was the very cheap Chinese electronics failing, the actual engine continuing to start and run very well.

 

As to powering your 30a charger from a 700w generator, while in theory it should work, it may well not in practice. We find our that while our 1kw generator will just about run our Sterling pro charge ultra 40a charger, it will trip our generator if starting charger at full power. The only way around this is to start it at 75% and switch to 100% whilst charger is running. Unfortunately this is not an easy thing to do on the Sterling, involving holding buttons down and multiple button presses in conjunction with the manual!

 

Having said all that, none of it is really relevant to us now, and we only charge our lithium batteries with main engine or solar. We find this much more convenient than messing with generator and petrol, but does depend on what engine you have. 

Thanks for that... no shortage of food for thought here!

[Alan de Enfield]

1 minute ago, magictime said:

OK, so once a week for a 'full charge', but what about the daily top-up? Are we talking about trying to stay above 75%, 80%, 90%?

That is entirely down to your own commercial decision.

The more hours the batteries remain less than 99.99% charged the more they become sulphated and loose some capacity.

 

If you get charged to 70% every day and 99.99% once per week - you may loose (for illustration only) 65% of their capacity in 12 months

If you get charged to 80% every day and 99.99% once per week - you may loose (for illustration only) 50% of their capacity in 12 months

If you get charged to 90% every day and 99.99% once per week - you may loose (for illustration only) 25% of their capacity in 12 months

 

It comes down to how much does getting the last few % cost in fuel compared to buying a new set of batteries every 6 month / 12 months / 2 years

[magictime]

48 minutes ago, Alan de Enfield said:

That is entirely down to your own commercial decision.

The more hours the batteries remain less than 99.99% charged the more they become sulphated and loose some capacity.

 

If you get charged to 70% every day and 99.99% once per week - you may loose (for illustration only) 65% of their capacity in 12 months

If you get charged to 80% every day and 99.99% once per week - you may loose (for illustration only) 50% of their capacity in 12 months

If you get charged to 90% every day and 99.99% once per week - you may loose (for illustration only) 25% of their capacity in 12 months

 

It comes down to how much does getting the last few % cost in fuel compared to buying a new set of batteries every 6 month / 12 months / 2 years

Well yes, but I need to base that decision on something. (Something along the lines of your 'illustration only' figures, but without the disclaimer - and some reasonable assumption about whether running the engine for an extra hour a day is going to cost me 30p for a third of a litre of diesel, or £1.20 for a litre and a bit!)

[Paul C]

53 minutes ago, magictime said:

 

OK, this is new to me. So there's no efficiency to be gained by switching to a generator that's designed to generate some fraction of one horsepower by burning a litre of petrol every two to four hours, because a 38Hp diesel engine will burn fuel just as slowly if the demand from the batteries is the same?

That's another matter. In my posts above, I've simply corrected the incorrect assumption that fuel usage is directly related to rpm. Fuel usage is related to load, but that assumes the same efficiency across all operating conditions; and also applies to one engine. 

 

If you start comparing eg a diesel operating at a very low load idle, with a petrol generator operating at/near maximum, its quite a difference in efficiency. Generally, an engine is most efficient at max load. But then also a diesel in inherently more efficient than a petrol engine (especially at idle - no losses due to the throttle). And that's only part of the equation - the fuel cost is very different, because you have to pay duty on petrol. And as you rightly point out, there's all the other costs eg servicing, capital cost, etc etc. 

 

I've tended to slightly simplify things by doing a "differential cost analysis". You work out the additional cost of charging batteries by running the existing diesel propulsion engine, over (say) a year. Then you work out the total costs of charging batteries by buying a generator (and maybe a battery charger too, if you don't already have one). This is worth doing.

 

Its also worth doing the costs for solar too; this is where the idea of modifying a boat's charging system so it does the lion's share of charging by solar, when it can, only using fuel when its needed. (The running costs of solar are easy to work out.....)

[magictime]

25 minutes ago, Paul C said:

That's another matter. In my posts above, I've simply corrected the incorrect assumption that fuel usage is directly related to rpm. Fuel usage is related to load, but that assumes the same efficiency across all operating conditions; and also applies to one engine. 

 

If you start comparing eg a diesel operating at a very low load idle, with a petrol generator operating at/near maximum, its quite a difference in efficiency. Generally, an engine is most efficient at max load. But then also a diesel in inherently more efficient than a petrol engine (especially at idle - no losses due to the throttle). And that's only part of the equation - the fuel cost is very different, because you have to pay duty on petrol. And as you rightly point out, there's all the other costs eg servicing, capital cost, etc etc. 

 

I've tended to slightly simplify things by doing a "differential cost analysis". You work out the additional cost of charging batteries by running the existing diesel propulsion engine, over (say) a year. Then you work out the total costs of charging batteries by buying a generator (and maybe a battery charger too, if you don't already have one). This is worth doing.

 

Its also worth doing the costs for solar too; this is where the idea of modifying a boat's charging system so it does the lion's share of charging by solar, when it can, only using fuel when its needed. (The running costs of solar are easy to work out.....)

Thanks. I suppose some such 'differential cost analysis' is what, in very rough terms, I'm trying to come up with, but I'm increasingly at a loss as to what assumptions to make about how much diesel per hour my engine is going to be burning while charging batteries. Half a litre, one litre, more? My hunch is 'quite a lot', because apart from anything surely it's generating an awful lot of heat all the time it's running, which is energy that's coming from somewhere. Whereas a suitcase petrol generator should (I think!) be burning something between 250ml and 500ml an hour, ish, to satisfy my batteries' demand for 30A or less of charge.

[Paul C]

Directly measure it.

[Tony Brooks]

2 minutes ago, magictime said:

Thanks. I suppose some such 'differential cost analysis' is what, in very rough terms, I'm trying to come up with, but I'm increasingly at a loss as to what assumptions to make about how much diesel per hour my engine is going to be burning while charging batteries. Half a litre, one litre, more? My hunch is 'quite a lot', because apart from anything surely it's generating an awful lot of heat all the time it's running, which is energy that's coming from somewhere. Whereas a suitcase petrol generator should (I think!) be burning something between 250ml and 500ml an hour, ish, to satisfy my batteries' demand for 30A or less of charge.

The heat only comes from the fuel that is burned. the amount of fuel burned depends upon the load. Diesels have a better thermal efficiency than petrol engines while two stroke petrol engines normally have an even worse fuel efficiency. What prevents us giving you a definitive answer is that to do so would require test data and as someone above said such test data is done at full throttle with the speed being controlled by the load applied. There is no way to extrapolate from this to the fuel consumed with a partial load because of you look at the specific fuel consumption for a diesel it tends to get a little better for as while as the speed increases and then gets worse again. If you can find all the efficiency and fuel consumption figures at differing loads for both engines I suppose you could work out the theoretical consumption. In the real world I think it would be easiest to actually measure the consumption of both engines while driving the charger. However remembering that apart from an undefinable period of bulk charging the load on the engines should be gradually dropping even that figure would not be accurate with a different sate of battery charge. I have never used a generator to charge my batteries and so far, touch wood, they are in their eighth year but I do have 165 watts of solar and do not live aboard.

 

A quote from The Fundamentals of Motor Vehicle Technology by VAW Hillier: Compared with a petrol engine of the same size, a CI (diesel) engine has the following advantages:

1. Fuel economy - the high compression ratio gives good thermal efficiency (30 to 40% indicated) and provides the operator with approximately 30% more miles per gallon.

 

 

So if we accept that then for the generator to return a better fuel consumption  the extra frictional losses caused by the extra diesel cylinders would have to require more than 30% of the power developed. I simply can't see that. Things may swing even more in favour of the diesel if you have an indirect injected engine because at higher speeds frictional and pumping losses caused by the throat between the per-combustion chamber and cylinder would have been factored into the percentage while at lower speeds the efficiency may well be better.

 

 

 

[magictime]

Thanks Tony.

 

At a bit of a loss now, and trying to think back to last winter for some 'real world' figures on consumption. I remember spending about £200 a month on diesel, so was probably burning a little over 200 litres - say 220. Maybe 20 of that for propulsion (say 12 hours' cruising), about 100 for heating (Lockgate stove), so about 100 litres for battery charging; 25 litres a week for about 25 hours' charging = 1 litre an hour. I think that's about what I'd concluded from monitoring the contents of the tank.

 

BUT maybe it was actually 1.5 litres in the first hour each day, 1 litre in the second, and 0.5 litres in the third, in which case there'd be little if anything to gain by switching to a petrol gennie for those later stages of charging.

 

...except that you then have the wear and tear/servicing on the engine to factor in. 25 hours a week for maybe 20 weeks is 500 engine hours!

 

Hmm. I still think there should be a practical way to use a TEG or something to keep the batteries topped up using the stove - three or four amps, 16 or 24 hours a day, would do nicely - but I suspect that's a topic for another thread!

[Tony Brooks]

27 minutes ago, magictime said:

Thanks Tony.

 

At a bit of a loss now, and trying to think back to last winter for some 'real world' figures on consumption. I remember spending about £200 a month on diesel, so was probably burning a little over 200 litres - say 220. Maybe 20 of that for propulsion (say 12 hours' cruising), about 100 for heating (Lockgate stove), so about 100 litres for battery charging; 25 litres a week for about 25 hours' charging = 1 litre an hour. I think that's about what I'd concluded from monitoring the contents of the tank.

 

BUT maybe it was actually 1.5 litres in the first hour each day, 1 litre in the second, and 0.5 litres in the third, in which case there'd be little if anything to gain by switching to a petrol gennie for those later stages of charging.

 

...except that you then have the wear and tear/servicing on the engine to factor in. 25 hours a week for maybe 20 weeks is 500 engine hours!

 

Hmm. I still think there should be a practical way to use a TEG or something to keep the batteries topped up using the stove - three or four amps, 16 or 24 hours a day, would do nicely - but I suspect that's a topic for another thread!

 

I think that you have grasped it, the consumption drops as the batteries charge and you get hot water as well. That should bed deducted from the fuel consumption for charging even though it is waste heat. If it was from a stove, gas or diesel boiler you would be paying for the hot water.

 

I agree the wear  on the engine comes into it but there will be wear on any generator engine and I fear many of the cheaper generators are more a disposable item that one suited to long hours of running.

 

Remember that when using the generator the fuel consumption will also reduce as the batteries charge. Not a lot of help though. I think the only way to find out is to buy a generator and monitor the fuel consumption and see how it works out for you but I suspect for much of the year the cost of a generator would be more effectively spent on more solar. The more you have the more output you get in low light conditions so adding more extends the months in which it makes a contribution. It also starts charging earlier in the day and finishes later.

 

[magictime]

1 hour ago, Tony Brooks said:

 

I think that you have grasped it, the consumption drops as the batteries charge and you get hot water as well. That should bed deducted from the fuel consumption for charging even though it is waste heat. If it was from a stove, gas or diesel boiler you would be paying for the hot water.

 

I agree the wear  on the engine comes into it but there will be wear on any generator engine and I fear many of the cheaper generators are more a disposable item that one suited to long hours of running.

 

Remember that when using the generator the fuel consumption will also reduce as the batteries charge. Not a lot of help though. I think the only way to find out is to buy a generator and monitor the fuel consumption and see how it works out for you but I suspect for much of the year the cost of a generator would be more effectively spent on more solar. The more you have the more output you get in low light conditions so adding more extends the months in which it makes a contribution. It also starts charging earlier in the day and finishes later.

 

Yes, we have 200W of solar and are planning to add at least another 165W. Plus we cruise a lot from spring to autumn. It's purely over the winter, when solar drops off a cliff and we cruise much less, where I'm wondering if a generator might be an economical option compared to endless running of the engine while stationary.

[Richard10002]

4 hours ago, magictime said:

Yes, we have 200W of solar and are planning to add at least another 165W. Plus we cruise a lot from spring to autumn. It's purely over the winter, when solar drops off a cliff and we cruise much less, where I'm wondering if a generator might be an economical option compared to endless running of the engine while stationary.

My Honda EU20i is out of commision at the moment, (I'm packing it up to send it to pete power for a service and a few other bits), so have been using solar and engine for a few weeks. 500w of solar, and an alternator that seems to charge at a max of about 35A at 1500 revs.

 

My 240v charger is 60A.

 

On the days when there has been little/no sun, and I have run the engine I think I run the engine for an hour to 90 minutes longer than I would the genny to get to 100% charged. I'm quite motivated to get the genny sent away and back.

 

I havent read every post here, but you can only compare apples with apples if your engine alternator is as powerful as the 240v charger you would use with your genny.

 

ETA just read the OP..... ignore what I've said  your alternators are more powerful than your charger.

Edited August 31, 2019 by Richard10002

[TheBiscuits]

50 minutes ago, Richard10002 said:

an alternator that seems to charge at a max of about 35A at 1500 revs

Get a better alternator.  A standard V belt should be able to drive about a 90A one.

[Richard10002]

1 hour ago, TheBiscuits said:

Get a better alternator.  A standard V belt should be able to drive about a 90A one.

I've got to get a round tuit first

 

In reality, I prefer to use the genny and charger, rather than the engine and alternator. I tend to believe that it is better to wear a genny than the main engine, although I have no empirical evidence as to why  

[TheBiscuits]

1 hour ago, Richard10002 said:

I've got to get a round tuit first

 

In reality, I prefer to use the genny and charger, rather than the engine and alternator. I tend to believe that it is better to wear a genny than the main engine, although I have no empirical evidence as to why  

Understood.   But I still think that would change if you had a bigger alternator! 

 

I have a 110A domestic alternator and wouldn't have a petrol generator on my boat.  I don't like the idea of having petrol onboard.   Fairly short story that involes a burns unit ...

 

 

 

 

[MtB]

11 hours ago, TheBiscuits said:

Get a better alternator.  A standard V belt should be able to drive about a 90A one.

 

But its the batteries that determine the charge current, not the alternator. Or the inadequate tension of the drive belt! (DAMHIK.)

 

Once out of bulk charge, a 90a alternator will progressively tail off towards zero for hours on end.

 

 

 

[TheBiscuits]

47 minutes ago, Mike the Boilerman said:

 

But its the batteries that determine the charge current, not the alternator. Or the inadequate tension of the drive belt! (DAMHIK.)

 

Once out of bulk charge, a 90a alternator will progressively tail off towards zero for hours on end.

Sure, but it will drop out of bulk charge quite a bit sooner than a 35A alternator when charging flat batteries.

 

Richard's comment was that he has to run his englne for an hour and a half longer than his genny to charge his batteries.  I think that would go the other way with a bigger alternator.

[Richard10002]

43 minutes ago, TheBiscuits said:

Sure, but it will drop out of bulk charge quite a bit sooner than a 35A alternator when charging flat batteries.

 

Richard's comment was that he has to run his englne for an hour and a half longer than his genny to charge his batteries.  I think that would go the other way with a bigger alternator.

Maybe.... but I have a permanent mooring with a little storage shed for the genny, and dont go far. I seem to have taken on board the fact that it's better to run a genny for domestic power, than the engine with no load.

 

having said that - could I just buy a 90A alternator, and pop it in. Or are they not standard, such that a 5 minute job will become a big job taking weeks  

 

 

[Alan de Enfield]

21 minutes ago, Richard10002 said:

Or are they not standard

Yes - except for whichever boat you happen to try to fit one on.

Then the fixings spacings will be all to cock, the pulley dimensions wrong, the belt alignment 2mm out ……………………………...

[Richard10002]

4 hours ago, Alan de Enfield said:

Yes - except for whichever boat you happen to try to fit one on.

Then the fixings spacings will be all to cock, the pulley dimensions wrong, the belt alignment 2mm out ……………………………...

Exactly my expectation  

[Alan de Enfield]

23 minutes ago, Richard10002 said:

Exactly my expectation  

Just get a 90a version of the one you have.

 

When I was looking for new one an 'alternator / starter motor specialist' said just bring your old one in and we can match (identically) the various features, then just choose the out put you want.