nickfabbri
Well-Known Member
The alternator on my Yanmar 3GM20 is rated at 60 A. Am I being a moron in assuming that it charges the batteries at 60 Amps per hour?If so can someone please enlighten me.
Many thanks
Many thanks
Charging rate of alternator
- Thread starter nickfabbri
- Start date
pvb
Well-Known Member
Not exactly a moron, more hopelessly optimistic! 
An alternator's rating is the maximum current it can produce. In practice, the current it actually produces depends on how much current can be absorbed. With batteries, the charging current can start off quite high, then it will gradually reduce as the battery becomes more charged. If you have a large battery bank which is very discharged, it's possible your alternator might get close to its 60A rating when first charging.
An alternator's rating is the maximum current it can produce. In practice, the current it actually produces depends on how much current can be absorbed. With batteries, the charging current can start off quite high, then it will gradually reduce as the battery becomes more charged. If you have a large battery bank which is very discharged, it's possible your alternator might get close to its 60A rating when first charging.
VicS
Well-Known Member
As said above.
You may see something approaching the 60 amps maximum when initially charging a discharged battery.
It will depend upon the size of the battery ... ie its capacity in amp hours .
A small battery may simply not accept 60 amps.
A very large one may be capable of accepting a higher current but the alternator will not produce more than its rated maximum.
NB
Current is measured in amps. ... it is the rate of flow of electricity.
Battery capacity is quoted in amp hours ... it is a measure of how much electricity the battery can store.
Amps per hour ..... is nonsense
You may see something approaching the 60 amps maximum when initially charging a discharged battery.
It will depend upon the size of the battery ... ie its capacity in amp hours .
A small battery may simply not accept 60 amps.
A very large one may be capable of accepting a higher current but the alternator will not produce more than its rated maximum.
NB
Current is measured in amps. ... it is the rate of flow of electricity.
Battery capacity is quoted in amp hours ... it is a measure of how much electricity the battery can store.
Amps per hour ..... is nonsense
cimo
Well-Known Member
if your total battery bank capacity was in the region of 240Ah and your bank was 50% delpeted, then perhaps your alternator could deliver close to 60A initially. but not for long.
The alternators current output will depend on the "charge acceptance rate" of your battery bank. The charge acceptance rate will depend on the current state of charge of the battery. The rule of thumb I use - maximum alternator output = 50% of discharged capacity.
So a fully charged bank that has given up 20Ah may accept 10amps initially upon charging. as the battery's state of charge increases, the charge acceptance rate will reduce accordingly - to a first approximation.
worth a search as there are loads of posts here on battery charging.
petlan
New Member
Not a moron! Well no. If you fit a sterling or similar advanced regulator then you will get max amps (60 in your case) until the batteries are 90% full. It will also take your batteries up to 14.7volts for a while which will prevent and or remove sulphation from the plates. My advice, as a livaboard with 2,000ah of batteries, is fit one now! I have on all of my boats over the last 20 years.
cimo
Well-Known Member
well lets see
...........the Sterling are good no doubt but if we were to learn that the OP has a total of 300AH in AGM then such a recommendation would look misleading, no? Must say 2000Ah is impressive - what on earth you doing with all that!
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William_H
Well-Known Member
Alternator rating
60 Amps is the max the alternator can supply. However this rating is not for charging batteries but rather for supplying 12v to systems ie lights radios heaters etc in a car or truck. I think you will find that at 60 amps the alternator can barely supply 12v.(there has been some discussion on this and really it depends on how the alternator manufaturer rates his alternator but obviously he is going to make it look as good as possible.)
Generally a battery will need 14v to accept a real charge. The max current that an alternator can provide while developing 14v will be much less than 60A rating. Even at 14v the current into the battery will fall rapidly as the internal voltage of the battery rises with charge. The current into the battery is dependent on the difference between the voltage delivered to the battery ie alternator voltage, and the internal voltage of the battery. (ie from 11v or less when flat to 13v when nearly charged.)
A smart regulator (stepped charge)(sterling etc) will get the alternator to produce more voltage than 14v so increasing the charge current. This kind of regulator can't make the alternator produce any more than max it can do (ie a lot less than rating) but it will increase the current into the batteries when with a normal regulator the current falls off after a period of charge.
What the op needs is an amp meter to measure exactly how much is going into the batteries at any time.
My guess is that typically with half discharged batteries he might start at 30 amps going in which after 1 minute will be down to 20 amps and after 5 minutes be down to 12 amps and after half an hour be down to 5 amps. Maybe more but still disappointing. Hence if you are running the engine only to charge the batteries you need a better regulator (sterling etc) good luck olewill
60 Amps is the max the alternator can supply. However this rating is not for charging batteries but rather for supplying 12v to systems ie lights radios heaters etc in a car or truck. I think you will find that at 60 amps the alternator can barely supply 12v.(there has been some discussion on this and really it depends on how the alternator manufaturer rates his alternator but obviously he is going to make it look as good as possible.)
Generally a battery will need 14v to accept a real charge. The max current that an alternator can provide while developing 14v will be much less than 60A rating. Even at 14v the current into the battery will fall rapidly as the internal voltage of the battery rises with charge. The current into the battery is dependent on the difference between the voltage delivered to the battery ie alternator voltage, and the internal voltage of the battery. (ie from 11v or less when flat to 13v when nearly charged.)
A smart regulator (stepped charge)(sterling etc) will get the alternator to produce more voltage than 14v so increasing the charge current. This kind of regulator can't make the alternator produce any more than max it can do (ie a lot less than rating) but it will increase the current into the batteries when with a normal regulator the current falls off after a period of charge.
What the op needs is an amp meter to measure exactly how much is going into the batteries at any time.
My guess is that typically with half discharged batteries he might start at 30 amps going in which after 1 minute will be down to 20 amps and after 5 minutes be down to 12 amps and after half an hour be down to 5 amps. Maybe more but still disappointing. Hence if you are running the engine only to charge the batteries you need a better regulator (sterling etc) good luck olewill
Andrew_Fanner
Well-Known Member
On my 24V system S/H ammeter reads about 20 or a bit less at startup and rapidly settles down to between 8 and 12A when running at river speeds. The P/H ammeter flips to about 15 at startup and then settles down to what looks about 5A or less at river speed. They are rated at 30A but I don't think I've ever seen more than 15 for any time over a couple of minutes, even in a derestricted area with the throttles opened up. No advanced controller, just a mysterious brass box atop each engine, full of what passed as high tech control circuitry in the late 1960s
