Well if there is it’s been faulty from new ! Sorry to me it comes down to simple ohms law. You connect two sources in parallel as happens when the powered charger is on and connected to the battery a proportion of the load current must come from both sources. The amount obviously depends on the relative resistance of the supply leads but if my charger output fuse is 15A and the total load is well over 100A at initial start it is easy to see how it pops it’s clogs!
So how's that any different to flicking a switch to attach a discharged battery? Chargers only produce so much current, blowing a fuse cos its putting out 100% of rated capacity sounds implausable, bad design not able to cope with a voltage spike maybe.....
Sorry Paul but I did not say the starter load of 100A was going through the charger. I indicated a PROPORTION of it was, the relevant amount being determined by the resistance of the starter motor cable from the battery and the resistance of the cable from the charger to the battery. Do not try this at home (!) but try connecting a charger across a battery and then shorting the battery terminals with a jump lead. Well apart from a Big Bang and possible burns the output fuse in the charger will obviously blow, you have after all just put a dirty great short across it. To a lesser extent this is what happens when the starter motor is dropped across the battery. A very large initial current flows due to lack of back EMF from the motor and the added load of the pre engaged starter. IF the cables between battery and starter had absolute zero resistance, yes, there would be no possible current flow though the parallel connected charger but in practice of course this does not happen, it only takes a small amount of this total high load to blow any protective fuse in the charger. I agree there should be no reason why running the shoreside charger once the engine is running is really a problem (although not really certain why you would really want to), it’s the initial engine start that can cause problems. Of course if the charger is not powered up, all is well because it is not trying to contribute to the starter supply. Perhaps a higher output charger than the 15A one I have sited could withstand this transient load but it is not possible to say it is totally without problems to run a shoreside supply charger and expect the charger and fusing to survive on initial start up. Again sorry, ohms law is important here, it is after all the basic theory that electronics work on and we all learn on day one in our careers in electronics.
Completely different scenario. Discharged battery will have residual voltage and present some resistance to current flow. A starter motor is not “soft start” and will present a very high initial load, it is after all why starter motor fuses are rated in hundreds of amps.
Is it? So a battery connected to a starter has no voltage left across it when the starter motor kicks in? And a charger blows when it's producing 100% of the current it can?
To be honest I never indicated “a flat battery would likely cause a battery charger to implode”, although if it (the battery) was unable to supply sufficient starting current a point might be reached where the charger could have problems trying to help and support the load and self protect by tripping its output either by fuse or trip.
I think you perhaps mis read the original post ? The Question was, :I regularly run the 100amp mastervolt battery charger from the generator when maneuvering into a berth. The bow thruster is a 5kW unit, much bigger than a typical starter motor and runs for much longer. The engine is generally on tickover and not producing very much power and the battery charger provides a bit of extra power to assist the batteries after a long sail when they aren't anywhere near fully charged. It also means I can put the electric kettle on for a much needed brew.
I've been doing this for years without causing any faults on the battery charger.
I my case it's an old wives tale.
Sorry cannot understand your point here at all...
That just sounds weird. So I have a severely depleted battery- (lets say down to 10.6v as that's close to what my batteries voltage go down to when running the starter motor), easily will accept more current that my charger output. Attach the 2 and the charger produces 100% of the current it can produce and doesn't blow a fuse. But somehow when the starter is running the charger sees the same voltage but tries to produce more than it can and blows a fuse?
Well you say when the starter motor is “running”. The problem is the initial inrush of current. My pre engaged starter motor on my car has a pull in and hold coil , that, until the starter motor contact close will draw in itself 50 amps, this on top of the high, no back EMF, load of the motor. Unless the output fuse in the charger is of a very slo blow type it is not going to withstand even a small amount of this total load if 100% of it does not come solely from the battery.
Peak hold meter shows just under 11v when my starter runs. 50A ain't that much. Of course power will flow from the charger if the voltage it sees is less than the voltage set point, just like if the battery is discharged.
Perhaps I shouldn't admit this .... but I have an old-fashioned bulk charger which is rated at 6A. It has no one-off switch and is "on" as soon as I plug it into the mains.Peak hold meter shows just under 11v when my starter runs. 50A ain't that much. Of course power will flow from the charger if the voltage it sees is less than the voltage set point, just like if the battery is discharged.
So your saying a charger will deliver more power than it can deliver and blow a fuse if the voltage is pulled down by a starter motor but not when the voltage is lower because the battery is discharged?
So the power the charger is physically capable of producing isn't a factor then? ............
Lots of interesting theory chaps. My initial response was to disagree with Paul that it was an "Old wives tale"
