Don't ask me why but, many moons ago I opted for a 75amp alternator. Never had any issues but, now wondering how it might affect the batteries. When all running through a vsr split charge what will the alternator charging current be? Surely, its not charging at 75amps? But, what actually controls it? Sent from my VOG-L09 using Tapatalk
The alt will provide whatever current stuff on the the bus needs: things that are turned on, plus current into the batteries depending on their state of charge. It can't "force" current into the batteries. Having a 75 amp job just means thst it's capable of supplying more than a 55 amper. I'd doubt that it's ever generating 75 (or 55) amps. If you fit 47 spotlights etc, the regulator won't be able to keep up and the charging voltage will drop. You're ok.
One thing you can be sure of is that it will never actually provide 75A and if it did, after 10 minutes it would be so hot smoke would be coming out of it.
Not what boaty people with lithiums are finding, they find if you have say a 90A alt and try to get more than 40-45A out of it they do not like it. Like many things, they're headline specced for a short burst. That's fine for LA batteries as even a nearly flat one will only take high current for a few minutes, but lithiums will take as much as you can ram into them until fully charged and then the problems start...
Well, boat people are strange Prob shows that car alternators intended to drive lead acid batteries aren't really suited to other types.
Yes, but it's interesting all the same to see what happens when you push one a bit and find that the headline rating is just a dream.
So I take it that if the battery voltage is quite high (nearly fully charged) then the charge current won't be very high and if flat then essentially it will have a higher charge current because of the higher voltage difference? Sent from my VOG-L09 using Tapatalk
Correct. The regulator on the alt tries to keep the alt voltage higher than a fully-charged battery voltage, so that current flows into the battery if it needs it. You can't force a battery to accept charge that it doesn't want. Your higher alt current rating just means that its physical construction (no of turns on windings etc) means it can supply a higher current if required ie extra lights etc. Don't forget that when you're driving along, it's the alternator that's supplying all the electrical power, not the battery.
This may help understand the charging. The amount of amps produced by the alternator is determined by the battery. A battery works a bit like a sponge in a bucket of water. The sponge is the battery and the water is the amps produced by the alternator Put a dry sponge in a bucket of water and it will soak up the water fast and as it gets saturated the speed and amount of water it soaks up slows down until it can’t take anymore.
I did a quick read around. That current rating is a maximum capability at 6000RPM at the alternator. In general the voltage on the battery quickly comes up to meet the alternator, so 55 Amps from a 55A alternator will only appear for a few minutes after starting, tailing off . Headlamps full on and tail lights, traditional maximum continuous loads are about 120 watts or around 10 amps. Plus a couple of amps for ignition and choke heater. Hence a 50A or 55A alternator is a reasonable match. And its about 20A capacity max at idle too.. so the alternator doesnt ever have to give its rated output 100% of the time.. Also given that on a boat the air being sucked into the alternator is probably being warmed up in an enclosed box around the boat engine that sits around 85-100C and which is water cooled without a fan.. so it suffers about the same as the air cooled engine - noticed today on the M25, the air going from the engine bay into the carb is 49C from the "cold" side of the air cleaner.. So the alternator will be running hot there too. As an alternator regulator is also designed to tail off charging voltage with temperature to avoid boiling batteries, the max available current also drops away with heat. So boaty types and others with B2B and other brutal chargers may find that drawing max current reduces the life expectancy of their hot alternators, that are trying to protect themselves by backing off ...
They will also be rated in Chinese AH which is basically multiply the AH rating of one cell by the number of cells in series. This unit is used on a few of those lithium car starters as sold on eBay So three 10AH cells in series for 14.4 volts is a 30 Chinese AH battery.
My Sprinters has a 220amp alternator, it virtually runs the bloody thing, when it went as I pulled in home I arranged to have a new one fitted the next day. I put the battery on charge over night, set off. Made it 4 miles, the fault lights started and it died 30 seconds later, jump leads out the window to winch battery onto the jump point. Off I went and made it the 2 miles.....amazing how much power the van uses to run
