Is my battery charger a 'bottleneck'?

[magictime]

51 minutes ago, Tony Brooks said:

 

1. Absolutely, with a 30 amp rated charge source it would probably only be supplying 30 amps for an hour. After the the  batteries will have been charged enough to start reducing their demand and thus charging current.

I know batteries reduce their demand as they get towards being fully charged, but what if an 800Ah bank was still going from say 60% of capacity/480Ah to 70% of capacity/560Ah? Would they still demand less than 30A? I didn't think charging was that slow, that early in the process.

51 minutes ago, Tony Brooks said:

 

2. I thought you had you had read Wotever's piece on batteries. As far as charging concerned you CAN NOT just say "a 30 amp charge source running for three hours will put 90Ah into the batteries". It will not on two counts.

 

a. After a comparatively short but unknown time the batteries themselves will start to reduce the current they will accept and a s time goes son the current gets lower and lower (at a stated voltage).

Yes, I understand this, which is why I wrote 'even if' they could take the full 30A.

51 minutes ago, Tony Brooks said:

b. Battery charging uses some of that current to produce heat and possible gas so not all the current supplied by the charge source actually charges the batteries.

 

Four hours to charge to 95%? No way, it will be much longer in the vast majority of cases. Four to six hours MIGHT get to 80% of fully charged. I( will take emaany hors longer to get to 95% charged.

Get to 80% of fully charged starting from where, sorry? And on a day-to-day basis, how close to fully charged do I want my batteries to be (assuming they get more like a 'full' charge say once a week)? I keep reading about 'typical' battery charging regimes being, say, two hours a day plus more like 8 hours once a week, but maybe that's because people are generally only trying to top up from say 70% to 80% most days?

51 minutes ago, Tony Brooks said:

 

On ANY engine fuel consumption is dependant upon load and as I explained above the load on the change source will be continually reducing after an initial short period of full load. Even so because of starting surges the charger load will have to be less than the generator's maximum by some margin so even at 30 Amps charge the generator will not be at full load so fuel consumption will be less.

 

I have no idea where you get the consumption figures for the main engine from unless you are working form the manufacturer's full throttle consumption figures. Typically narrowboat engines consume between 1 and 1.5 litres per hour when cruising. The load when battery charging will be less so the fuel consumption will be less - considerably less at the later stages of charging.

OK, this is where I'm really lost - I thought an engine at tickover basically just burns a set amount of fuel, which (as Alan suggests above) I've always understood to be 1 litre plus per hour? Surely the engine speed can't go below that minimum, whatever the load is when battery charging? Hence the temptation to switch to a gennie where the speed/consumption can dip right down.

[WotEver]

2 hours ago, magictime said:

Really? So most of the time I wouldn't even be putting 30Ah back into the batteries per hour of running?

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?

19 minutes ago, magictime said:

on a day-to-day basis, how close to fully charged do I want my batteries to be (assuming they get more like a 'full' charge say once a week)?

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. 

[Tony Brooks]

22 minutes ago, magictime said:

I know batteries reduce their demand as they get towards being fully charged, but what if an 800Ah bank was still going from say 60% of capacity/480Ah to 70% of capacity/560Ah? Would they still demand less than 30A? I didn't think charging was that slow, that early in the process.

Yes, I understand this, which is why I wrote 'even if' they could take the full 30A.

 

Get to 80% of fully charged starting from where, sorry? And on a day-to-day basis, how close to fully charged do I want my batteries to be (assuming they get more like a 'full' charge say once a week)? I keep reading about 'typical' battery charging regimes being, say, two hours a day plus more like 8 hours once a week, but maybe that's because people are generally only trying to top up from say 70% to 80% most days?

 

OK, this is where I'm really lost - I thought an engine at tickover basically just burns a set amount of fuel, which (as Alan suggests above) I've always understood to be 1 litre plus per hour? Surely the engine speed can't go below that minimum, whatever the load is when battery charging? Hence the temptation to switch to a gennie where the speed/consumption can dip right down.

 

1. I have no idea about how long the batters would remain in so called bulk charge, especially as 800Ah of batteries is a lot for a typical narrowboat but on my boat's 330Ah bank a 70Amp alternator stays at maximum output for less than half an hour from about 60% charged. That includes recharging the engine battery but its charge is insignificant and will not alter the time in bulk much. So 800 Ah bank will probably take less than an hour before the current starts reducing.

 

2. For maximum life you want your batteries back to 100% charged as soon as they are discharged to any degree but that is not practical. Ideally to maximise battery life by minimising sulphation you would recharge to 100% each day but again that is not really practical unless you use solar and even then not in the winter. So it comes down to the individual boater deciding the balance between battery like and charge time/fuel use they are willing to accept. I have no idea who says two hours a day for 80% UNLESS they also have a solar. Two hours engine charging and leave the rest to a decent solar array in summer will probably get to 80% charged or more. With just the engine I would have expected it to take at least four hours running. You are probably correct that they are only topping up from a high state of charge. The only way you will find the answer to these questions for your own boat with your own level of electrical demand is to got battery monitoring. The basic setup is an ammeter and voltmeter and learn how to interpret them. More expensive monitors may well encourage you to destroy batters unless you ignore things like % charged, Ah left and such like or set them up correctly and recalibrate them regularly.

 

3. Lets look at how diesels control their speed in the simplest form. A long toothed rod called a rack can move back and forth top twist pumping elements. The degree of twist controls how much fuel is injected. The throttle control squeezes a compression spring at one end that in turn pushes the rack towards full fuel (flat out). The other end has a pair of centrifugal bob-weights that act against the spring trying to push the rack to no fuel. The faster the engine more so goes the harder the bob-weights push.

 

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.

 

The heavier the load the more the engine tries to slow down so the more the spring can push the rack to deliver more fuel. In the extreme on a test bed one can load the engine sufficiently for it to still be at tick over speed but maximum fuel is delivered. However the engine may well smoke a bit in that condition.

 

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..

 

[Paul C]

31 minutes ago, magictime said:

 

OK, this is where I'm really lost - I thought an engine at tickover basically just burns a set amount of fuel, which (as Alan suggests above) I've always understood to be 1 litre plus per hour? Surely the engine speed can't go below that minimum, whatever the load is when battery charging? Hence the temptation to switch to a gennie where the speed/consumption can dip right down.

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.

[Alan de Enfield]

3 minutes ago, Tony Brooks said:

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..

Just out of interest - what would you suggest is the 'minimum' fuel usage for a typical 4-cylinder 35-40hp engine running at tickover / fast tickover with only the alternator & water pump as the loading ?

 

I have read that the fuel usage is pretty constant (and proportional) in the terms of HP generated

"1 gallon per hour per 20hp developed".

About 0.2 litre per hour per hp developed.

 

That would suggest to me that if an engine if running at around tickover on mimimal load that the consumption (if it was simply turning an alternator & water pump, it could be using as little as 0.4 litres per hour).

 

Does that make sense ?

Is it correct ?

[Tony Brooks]

8 minutes ago, Alan de Enfield said:

Just out of interest - what would you suggest is the 'minimum' fuel usage for a typical 4-cylinder 35-40hp engine running at tickover / fast tickover with only the alternator & water pump as the loading ?

 

I have read that the fuel usage is pretty constant (and proportional) in the terms of HP generated

"1 gallon per hour per 20hp developed".

About 0.2 litre per hour per hp developed.

 

That would suggest to me that if an engine if running at around tickover on mimimal load that the consumption (if it was simply turning an alternator & water pump, it could be using as little as 0.4 litres per hour).

 

Does that make sense ?

Is it correct ?

Sorry Alan, no idea because I have never done a specific fuel consumption test without a load at idle.

 

As at idle without an alternator the only loads would be friction, water pump, fuel delivery system, and pumping the air then you are probably not far out. It would depend to a degree on engine design though so doubt there is a published figure.

 

I have a gut feeing it could be less.

Edited August 31, 2019 by Tony Brooks

[Tom and Bex]

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. 

Edited August 31, 2019 by Tom and Bex

[Alan de Enfield]

13 minutes ago, Tony Brooks said:

Sorry Alan, no idea because I have never done a specific fuel consumption test without a load at idle.

Ok thanks.

I find it a very interesting subject - particularly when my manufacturers engine specs (graph) show pretty much 'flat' (constant 0.195 l/hp/hr) fuel consumption all the way up to about 1600 rpm.

I wonder how many HP I am developing at 800rpm ?

 

At about 1400 - 1500rpm I am at my cruising speed of 5knts

 

[Paul C]

Fuel consumption is not proportional to engine rpm its proportional to load. Its possible to run a diesel engine at vastly different loads but the same rpm.

 

ENERGY BALANCE

[Alan de Enfield]

4 minutes ago, Paul C said:

Fuel consumption is not proportional to engine rpm its proportional to load. Its possible to run a diesel engine at vastly different loads but the same rpm.

 

ENERGY BALANCE

Are these three statements incorrect ?

 

Fuel consumed is proportional to the power developed

Power developed is approximately proportional to the cube of the engine revs

Hence fuel consumed is proportional to the cube of the engine revs.

[Paul C]

Yeah you've fundamentally misunderstood something along the way, the second statement is wrong. (Leading to the third being wrong). The first one is right if you assume an engine is the same efficiency throughout its operating range, which is a fair enough approximation for now.

[cuthound]

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.

 

This ^^^^.

 

I worked with diesel engined standby generators and batteries most of my working life 

 

The generators ranged from 10kVA 4 cylinder engines to 2.8MVA 16 cylinder engines. Fuel consumption against load was always measured during commissioning. It was always around half a pint of fuel (0.284 litres) +10%, per hour for each kW of load, irrespective of size of engine or number cylinders.

Edited August 31, 2019 by cuthound
Decimal point in wrong place.

[Alan de Enfield]

8 minutes ago, Paul C said:

Yeah you've fundamentally misunderstood something along the way, the second statement is wrong. (Leading to the third being wrong). The first one is right if you assume an engine is the same efficiency throughout its operating range, which is a fair enough approximation for now.

I lifted the 'formula' from a book called "Basic Ships Propulsion" (printed 2004), it also seems to be being used all over the world, including :

 

My Hanse Yacht forum

Dutch Barge Forum

Newtonian Mechanics

The RYA Handbook

Marine Propulsion - Physics (website)

 

Example :

 

"RYA Navigation Handbook"

 

 

 

 

 

[TheBiscuits]

1 minute ago, cuthound said:

 

This ^^^^.

 

I worked with diesel engined standby generators and batteries most of my working life 

 

The generators ranged from 10kVA 4 cylinder engines to 2.8MVA 16 cylinder engines. Fuel consumption against load was always measured during commissioning. It was always around half a pint of fuel (2.84 litres) +10%, per hour for each kW of load, irrespective of size of engine or number cylinders.

That's more like half a gallon ...

[TheBiscuits]

 

Which is the correct figure please?

 

[cuthound]

6 minutes ago, TheBiscuits said:

That's more like half a gallon ...

 

I know I put the decimal point in the wrong place, ?, corrected now.

[TheBiscuits]

2 minutes ago, cuthound said:

 

I know I put the decimal point in the wrong place, ?, corrected now.

Thanks.  That was what I thought had happened, but you know what they say about assume!

 

 

[Alan de Enfield]

12 minutes ago, cuthound said:

 

This ^^^^.

 

I worked with diesel engined standby generators and batteries most of my working life 

 

The generators ranged from 10kVA 4 cylinder engines to 2.8MVA 16 cylinder engines. Fuel consumption against load was always measured during commissioning. It was always around half a pint of fuel (0.284 litres) +10%, per hour for each kW of load, irrespective of size of engine or number cylinders.

With 1kw = 1.3hp

 

0.284 litres / kw = ~0.21 litres per HP

 

Which is (virtually) the same as I was using (0.2 litres per HP)

[Paul C]

13 minutes ago, Alan de Enfield said:

I lifted the 'formula' from a book called "Basic Ships Propulsion" (printed 2004), it also seems to be being used all over the world, including :

 

My Hanse Yacht forum

Dutch Barge Forum

Newtonian Mechanics

The RYA Handbook

Marine Propulsion - Physics (website)

 

Example :

 

"RYA Navigation Handbook"

 

 

 

 

 

They haven't stated the assumptions though, I suspect its something like "while the engine is turning the (fixed pitch) propellor"; and we'll also assume this propellor is always underwater. When an engine isn't driving the propellor, and might/might not have other loads such as AC, alternator, etc etc.......

[cuthound]

5 minutes ago, Alan de Enfield said:

With 1kw = 1.3hp

 

0.284 litres / kw = ~0.21 litres per HP

 

Which is (virtually) the same as I was using (0.2 litres per HP)

 

Probably derived the same way, by observing lots of engines at different loads or power outputs.

[Alan de Enfield]

1 minute ago, Paul C said:

They haven't stated the assumptions though, I suspect its something like "while the engine is turning the (fixed pitch) propellor"; and we'll also assume this propellor is always underwater. When an engine isn't driving the propellor, and might/might not have other loads such as AC, alternator, etc etc.......

So my calculations for fuel consumption on my displacement boat with a fixed pitch prop will in fact be pretty close (ignoring the comparatively small amount of power used to drive the alternator)

 

WOT = 2600 RPM

WOT = 143 HP

Fuel Consumption = 0.195 Litres/HP/Hour (from manufacturers graph)

 

At a 5 knot cruise at 1500 rpm that is 58% of WOT

So : 58% x 58% x 58% = 19.5%

ie 19.5% of 143 hp = 27.9hp

 27hp @ 0.195 lt/hp/hr = 5.4 litres per hour.

[Tony Brooks]

50 minutes ago, Paul C said:

Fuel consumption is not proportional to engine rpm its proportional to load. Its possible to run a diesel engine at vastly different loads but the same rpm.

 

ENERGY BALANCE

 For clarity I said RPM because certain things like the water pump may increase the power demanded as the speed increases to a minor degree. Pumping loses on indirect injection engines will definitely increase the pumping load on the engine as speed increases. Naturally this is all load but in these cases it load as a result of an RPM increase. I suspect the power demanded by the injector pump will also increase as the speed increases.

[Paul C]

4 minutes ago, Alan de Enfield said:

So my calculations for fuel consumption on my displacement boat with a fixed pitch prop will in fact be pretty close (ignoring the comparatively small amount of power used to drive the alternator)

 

WOT = 2600 RPM

 

WOT = 143 HP

 

Fuel Consumption = 0.195 Litres/HP/Hour (from manufacturers graph)

 

At a 5 knot cruise at 1500 rpm that is 58% of WOT

 

So : 58% x 58% x 58% = 19.5%

 

ie 19.5% of 143 hp = 27.9hp

 

 27hp @ 0.195 lt/hp/hr = 5.4 litres per hour.

 

TBH I didn't even look at that, but its only relevant for cruising (in that particular boat), the figures can't be applied to an engine not turning a propellor; and can only cautiously be applied to other boats of different sizes etc. IMHO its the wrong "direction" to approach the problem from. You need to approach it from an energy balance perspective, ie using science rather than simply measuring things then trying to apply empirical formulae or rules of thumb.

Edited August 31, 2019 by Paul C

[TheBiscuits]

2 minutes ago, Paul C said:

using science rather than simply measuring things then trying to apply empirical formulae or rules of thumb.

Isn't that pretty much the definition of the scientific method?

[Alan de Enfield]

1 minute ago, Paul C said:

TBH I didn't even look at that, but its only relevant for cruising (in that particular boat), the figures can't be applied to an engine not turning a propellor; and can only cautiously be applied to other boats of different sizes etc. IMHO its the wrong "direction" to approach the problem from. You need to approach it from an energy balance perspective, ie using sciene rather than simply measuring things then trying to apply empirical formulae or rules of thumb.

Ok - thanks I have learnt something today.

 

However - it is a well known & used formula (guidance / estimation, call it whatever) that is used for calculating range and fuel consumption for displacement boats, there is another for planing boats which is far more complicated.