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Is my battery charger a 'bottleneck'?


magictime

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Sorry, I know there are loads of threads on battery charging on here and I have had a good trawl through them, but I still don't exactly feel on top of the subject!

 

My immediate question is this: I have a whacking great Bank of Trojans, 800+ Ah when new, two alternators rated at (I think) 110A for the domestic bank and 70A for the starter, a solar charge controller rated for 40A (though not yet enough panels to deliver that).... BUT my Sterling battery charger is rated at 30A. I understand that as my batteries get past 80% charge or so they won't be able to take loads of amps anyway, but does that 30A rating mean they're never going to get more than 30A, regardless of how many amps the alternator or solar panels are able to supply during the early stages of charging? I'm wondering if my charger is a bit of a weak link.

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2 minutes ago, magictime said:

Sorry, I know there are loads of threads on battery charging on here and I have had a good trawl through them, but I still don't exactly feel on top of the subject!

 

My immediate question is this: I have a whacking great Bank of Trojans, 800+ Ah when new, two alternators rated at (I think) 110A for the domestic bank and 70A for the starter, a solar charge controller rated for 40A (though not yet enough panels to deliver that).... BUT my Sterling battery charger is rated at 30A. I understand that as my batteries get past 80% charge or so they won't be able to take loads of amps anyway, but does that 30A rating mean they're never going to get more than 30A, regardless of how many amps the alternator or solar panels are able to supply during the early stages of charging? I'm wondering if my charger is a bit of a weak link.

Your alternators and your solar panels shouldn't be going through the mains charger - they should be connected directly to the batteries ...

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Simple answer - No

 

More complicated answer - a battery charger that remains connected to a battery, when there is more than one connected, will not affect other  battery charger(s) connected to the battery. The reason being, the current doesn't take a path through it and it doesn't act as a "current-limiting" device in any way.

 

NOW.....had you asked a more complex question regarding 2 chargers (eg the alternator AND solar) operating together, and with a sufficiently discharged bank, does both together mean then can input eg 110A + 40A = 150A...................

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That 30 Amp charger is only active when you are plugged into the shoreline and it charges from mains electricity or a generator if that is plugged into your shorepower socket. It is completely independent of the alternator and solar charging.

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All of your various chargers will share the charging duty, but not equally.

 

The charging source with the highest voltage will supply the most current, and so on.

 

However it is the battery that determines how much current it wants.

 

When flat it will take all the current your charging sources can supply (and probably then some), so all will give their maximum outputs. Then as the battery charges up the one with the lowest voltage output will back off, then the next  etc until only the one with the highest output is left.  Then that one will reduce its output and go into float charge.

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I've often wondered if there would ever be a niche market for a device which can "turn down" the alternator's charge, thus reduce its load, thus the fuel consumed, when the solar is charging reasonably well. It could operate as a simple thing - if the sun's shining and the solar is charging, turn down the alternator. Or it could get as complex as you like - if it knows the sun will be shining for a while yet (eg buit in calendar and weather forecast receiving, possibly combined with GPS) then it deliberately turns it down knowing the solar will suffice later.

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13 minutes ago, Paul C said:

I've often wondered if there would ever be a niche market for a device which can "turn down" the alternator's charge, thus reduce its load, thus the fuel consumed, when the solar is charging reasonably well. 

As a 130A alternator flat out will be using less than 2hp I doubt you’d notice the difference. 

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Ah, OK, so the alternator and solar won't be 'routed' through the charger. I guess the alternator is routed through my Sterling 'Alternator to Battery Charger' instead. That makes sense.

 

So, next question: I'm considering buying a very small suitcase-style petrol generator, the 700W Impax model from Screwfix. Am I right in thinking that this should be more than adequate for the 30A charger I have? Over the winter, when solar is no longer contributing anything worthwhile, I'm thinking along the lines of running the engine for say an hour a day, to do the 'heavy lifting' during the bulk stage of charging, then switching over to a generator for a further hour or three a day, as needed, plus six or eight hours once a week. We'll be CCing 'off grid' so shoreline is not an option. Am I right in thinking this should be a lot more economical with fuel? Last winter we were running the engine three hours a day and the amount of diesel we got through, between that and our Lockgate stove, was outrageous!

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9 minutes ago, magictime said:

Am I right in thinking that this should be more than adequate for the 30A charger I have?

Assuming it’s the Pro Charge Ultra then yes, it’s over 94% efficient. 

 

Read this to answer lots of your questions:

https://www.canalworld.net/forums/index.php?/topic/95003-battery-charging-primer/

Edited by WotEver
Checked the PF for the charger
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Just now, WotEver said:

I’ll check the power factor for your charger in a mo, but I’d expect it to be fine. 

 

Read this to answer lots of your questions:

https://www.canalworld.net/forums/index.php?/topic/95003-battery-charging-primer/

Thanks. I've read that post - it's one of the reasons I have at least some shaky grasp of the basics around multiple stages of charging!

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1 hour ago, WotEver said:

Assuming it’s the Pro Charge Ultra then yes, it’s over 94% efficient. 

Thank you. Am I right in thinking the relevant figures are 700W divided by (say) 14.5V charging voltage = 48 amps going to the batteries from the gennie via the charger, (or say 45 amps allowing for inefficiency) - suggesting I'd want to run it at about 2/3 load? (Only presumably the gennie would actually be putting out more like 3A at 230V, which the charger then has to transform?)

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Your conclusion is fine, but for the wrong reasons.  You need to come at it from the other end.

 

You cannot get more than about 30 amps from the charger, ( 'cos that is its rated max output.)  At say, 14.4 V absorption charge voltage , this is roughly 450 W output. Divide this by the efficiency of the charger (94% is àsuggested above) to get an input requirement  of say 500 W.

  That is within the 700 W capacity of your proposed generator.

 

So, no problem provided the generator really does deliver 700 W continuously.   The generator's speed, frequency and voltage regulators will sort things out so everything is happy.  All you will need to do is tip in petrol, frequently,  and save up for the next generator, cos it probably won't last long running at 70% of max continuous all the time.  If it was a Honda then things might be different, but

so would the costs.

I suspect your batteries won't last long either,  but the regime you suggest is about the best of a bad job.   See MtB's experiences wih his Trojanoids on a similar routine.

 

N

 

 

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43 minutes ago, magictime said:

Thank you. Am I right in thinking the relevant figures are 700W divided by (say) 14.5V charging voltage = 48 amps going to the batteries from the gennie via the charger, (or say 45 amps allowing for inefficiency) - suggesting I'd want to run it at about 2/3 load? (Only presumably the gennie would actually be putting out more like 3A at 230V, which the charger then has to transform?)

Yes and no. Your approach is slightly muddled. 

 

The 30A charger will put into the batteries whatever they demand, up to a maximum of 30A. So the most power the charger will ever supply is 30 x 14.5 = 435W. That’s the max current at the max voltage. 

 

Divide that by the quoted efficiency of 94.4% and you get 435/0.944 = 461W. That’s the maximum that the charger will pull from the genny.

 

461W @ 230V is nearasdammit 2A. 

 

That maximum won’t be for very long, if ever, depending on how discharged the batteries are. 

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13 hours ago, Paul C said:

 

NOW.....had you asked a more complex question regarding 2 chargers (eg the alternator AND solar) operating together, and with a sufficiently discharged bank, does both together mean then can input eg 110A + 40A = 150A...................

 

I have 2 separate mains chargers, 30 amp and 40 amp output. I use the 30 amp charger when I'm on shore power and both together from the generator when I'm away sat for more than a few days without moving. They do indeed add up to 70 amps when they're first switched on, but I'm not sure if that's the case if you try to combine alternator and solar as you suggested?

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9 hours ago, BEngo said:

All you will need to do is tip in petrol, frequently,  and save up for the next generator, cos it probably won't last long running at 70% of max continuous all the time.  If it was a Honda then things might be different, but

so would the costs.

 

 

It looks nice but I also suspect the generator won't last long. Not at that price.

 

https://www.screwfix.com/p/impax-im800i-700w-inverter-generator-230v/15760

 

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12 hours ago, magictime said:

Am I right in thinking this should be a lot more economical with fuel? Last winter we were running the engine three hours a day and the amount of diesel we got through, between that and our Lockgate stove, was outrageous!

According to the data sheet the fuel tank is 2.1 litres and running at 75% power the tank lasts 2 hours.

 

Ergo - using 1 litre per hour.

 

Your boat engine at tickover / high tickover will be using between 1 and 1.5 litres per hour - BUT you will also get hot water 'free'.

 

Your fuel costs will be very close irrespective of running the generator or the boat engine.

Domestic 'red diesel' is around £0.90 / litre

Petrol is around £1.28-£1.30 / litre

 

I would suggest that the inconvenience of :

Having to go and find a petrol station every couple of days

Having to lift the generator off the boat to run it.

The possibility of it being 'pinched' or kicked into the canal

The hassles of finding somewhere safe and 'legal' to store both it and the petrol

The fact you will have to top it up every couple of hours. and,

The fact you have got to 'fork out' £180 to buy it

 

Would mean that it is not really a financially viable option (assuming the decision is being made on a financial basis)

 

You can buy a lot of 'extra' diesel for £180.

Edited by Alan de Enfield
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9 hours ago, BEngo said:

All you will need to do is tip in petrol, frequently,  and save up for the next generator, cos it probably won't last long running at 70% of max continuous all the time.  If it was a Honda then things might be different, but

so would the costs.

I suspect your batteries won't last long either,  but the regime you suggest is about the best of a bad job. 

Sorry, to be clear, do you mean 'about the best you can manage in the absence of shoreline'? 

 

Also, presumably it wouldn't actually be running at 70% of max continuous for long at a time if this is right...

 

9 hours ago, WotEver said:

Yes and no. Your approach is slightly muddled. 

 

The 30A charger will put into the batteries whatever they demand, up to a maximum of 30A. So the most power the charger will ever supply is 30 x 14.5 = 435W. That’s the max current at the max voltage. 

 

Divide that by the quoted efficiency of 94.4% and you get 435/0.944 = 461W. That’s the maximum that the charger will pull from the genny.

 

461W @ 230V is nearasdammit 2A. 

 

That maximum won’t be for very long, if ever, depending on how discharged the batteries are. 

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

 

I'm just thinking, very roughly, if the batteries were at 80% when I switched from engine to generator, that would put them maybe 160Ah below capacity, which would suggest running the generator for four hours at 70% of max load just to get them to 95% even if they were capable of taking 30A of charge throughout that time. That seems a heck of a lot. But then it is a big battery bank; when they do conk out on me, I'm wondering if I should be replacing them with something half the size really.

 

(Actually I wouldn't be at all surprised if their capacity is already half what it started at; they are almost six years old.)

 

On the point about 'saving up for a new generator', yes, I'm sure a Honda is better, but it's going to be four times the price and I suspect just the wrong side of being small enough to easily stow out of the way on the back deck. And if I can save maybe £50 a month by burning (say) 7 litres of petrol over the course of 18 hours, rather than (say) 25 litres of diesel, it would pay for itself in its first winter.

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What sort of back deck so you have? Petrol container storage on the back deck of most narrowboats isn't advisable, and a petrol generator is a petrol container unless you run it completely dry. 

 

Treat petrol fumes as you would lpg. i.e. heavier than air and liable to settle in bilges waiting for a spark. So you need to store petrol containers in a locker with overboard drainage.

 

If you do purchase a suitcase generator then you should always refuel and run it on the bank, not on the boat. Also even when it's running on the bank, take care to see which way the wind is blowing and avoid exhaust fumes and carbon monoxide blowing into your boat or neighbours boats through open doors or windows.

Edited by blackrose
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42 minutes ago, Alan de Enfield said:

According to the data sheet the fuel tank is 2.1 litres and running at 75% power the tank lasts 2 hours.

 

Ergo - using 1 litre per hour.

 

Your boat engine at tickover / high tickover will be using between 1 and 1.5 litres per hour - BUT you will also get hot water 'free'.

 

Your fuel costs will be very close irrespective of running the generator or the boat engine.

Domestic 'red diesel' is around £0.90 / litre

Petrol is around £1.28-£1.30 / litre

 

I would suggest that the inconvenience of :

Having to go and find a petrol station every couple of days

Having to lift the generator off the boat to run it.

The possibility of it being 'pinched' or kicked into the canal

The hassles of finding somewhere safe and 'legal' to store both it and the petrol

The fact you will have to top it up every couple of hours. and,

The fact you have got to 'fork out' £180 to buy it

 

Would mean that it is not really a financially viable option (assuming the decision is being made on a financial basis)

 

You can buy a lot of 'extra' diesel for £180.

The 2-hour figure is wrong. One of the reviewers on the Screwfix site picks up on this, and indeed the instruction manual quotes 4 hours. And that's at full load; I'd never need more than 70% to 'feed' my 30A charger. So a third of a litre per hour's running shouldn't be unrealistic; say 43p per hour vs. maybe three times as much per hour of running the engine.

 

It's good point about the 'free' hot water, but I was still thinking in terms of running the engine for an hour a day, which we generally reckon on being enough for a day's hot water, to handle the first stage of charging.

 

Yes, there are inconveniences involved, but last year it was pretty inconvenient having to get hold of so much diesel every month, given that our winter cruising range was only 30 miles or so and had to fit around stoppages. I don't have a problem with walking to a petrol station once a week or less to fill two 5 litre cans (which used to be a daily task when cruising on our old petrol-outboard boat with a 13 litre tank!)

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9 minutes ago, blackrose said:

What sort of back deck so you have? Petrol container storage on the back deck of most narrowboats isn't advisable, and a petrol generator is a petrol container unless you run it completely dry. 

 

Treat petrol fumes as you would lpg. i.e. heavier than air and liable to settle in bilges waiting for a spark. So you need to store petrol containers in a locker with overboard drainage.

 

Also, if you do purchase a suitcase generator then you should always refuel and run it on the bank, not on the boat. Also even when it's running on the bank, take care to see which way the wind is blowing and avoid exhaust fumes and carbon monoxide blowing into your boat or neighbours boats through open doors or windows.

It's a semi-trad.

 

As I mentioned in my reply to Alan, we used to have a petrol-outboard boat, so I am conscious of the way fumes can creep into the cabin from the back deck. This used to happen sometimes, setting the carbon monoxide alarm off, even when doing nothing 'wrong' - usually in a lock.

 

I wonder if a re-jig of the gas locker to accommodate two 5L cans and the gennie might be our best option.

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17 minutes ago, magictime said:

The 2-hour figure is wrong. One of the reviewers on the Screwfix site picks up on this, and indeed the instruction manual quotes 4 hours. And that's at full load; I'd never need more than 70% to 'feed' my 30A charger. So a third of a litre per hour's running shouldn't be unrealistic; say 43p per hour vs. maybe three times as much per hour of running the engine.

After a bulk charge with the main engine I used to follow up with an absorption charge once a week with a Kipor 2kva. On tickover with half a 3.7 litre tank of petrol it would run for approx 8hrs before running dry. 

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47 minutes ago, magictime said:

I wonder if a re-jig of the gas locker to accommodate two 5L cans and the gennie might be our best option.

From the BSS Section 7:1:1

 

…….cylinder housings may be used in open locations. Cylinder housings are ventilated enclosures intended
solely for storage of one or more LPG cylinders,
(my bold) pressure regulators and safety devices and located on the
exterior of the craft, where any leakage would flow overboard. [ISO 10239]

 

If you were unlucky enough to get an examiner who knew the rules you would have a 'BSS fail'.

 

1 hour ago, magictime said:

............it would pay for itself in its first winter.

There was a thread recently about this type of "Jenny"

 

Someone had bought one (from memory a 1000w version) - it broke immediately, they had it replaced it broke within a matter of weeks, they had it replaced and it broke again. I'll do a search and see if I can find it (and if I got the facts correct)

 

EDIT to add - it was a different brand.

 

It may not last 'the 1st Winter'.

 

I do realise the constraints of having sufficient disposable income but it really is false economy to buy 'cheap'.

Buy 'right' and buy once - buy 'wrong' and buy it many times.

 

Have you considered buying a 'decent' second hand one ?

I picked up a Kipor 2000i (almost as good as 'good' as the Honda 2000) for £300.

Edited by Alan de Enfield
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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?

 

I'm just thinking, very roughly, if the batteries were at 80% when I switched from engine to generator, that would put them maybe 160Ah below capacity, which would suggest running the generator for four hours at 70% of max load just to get them to 95% even if they were capable of taking 30A of charge throughout that time. That seems a heck of a lot. But then it is a big battery bank; when they do conk out on me, I'm wondering if I should be replacing them with something half the size really.

 

(Actually I wouldn't be at all surprised if their capacity is already half what it started at; they are almost six years old.)

 

On the point about 'saving up for a new generator', yes, I'm sure a Honda is better, but it's going to be four times the price and I suspect just the wrong side of being small enough to easily stow out of the way on the back deck. And if I can save maybe £50 a month by burning (say) 7 litres of petrol over the course of 18 hours, rather than (say) 25 litres of diesel, it would pay for itself in its first winter.

 

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.

 

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).

 

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.

 

3. I can stow my Honda Eu1000 in a front cockpit side locker with space to spare for a bag of coal. I can't see a Screwfix special being much smaller although it might be a different shape.

 

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.

 

The last paragraph makes little sense to me and to suppose a Scrwwfix special generator will last a whole season running 18 hours a day/week is in my view highly suspect. I think you will stand a good chance of it failing with engine wear.

 

Please study more about battery charging and grasp the fact that the charging current from any source will reduce over time however large that charge source is.

 

 

Edited to add:

 

petrol engines are inherently less efficient than diesels and two stroke petrol engines even less efficient. Although a four cylinder diesel will probably suffer more frictional, parasitic and pumping losses than a single cylinder petrol engine I suspect that at a given load the diesel will still return a better economy that a small petrol generator engine. I suspect its the main engine wear, vibration and noise that persuades many people to use a petrol generator but the cost of using a series of cheap ones I suspect will be far more than the cost of using the main engine, especially as the main engine will typically also heat water for you.

Edited by Tony Brooks
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