Battery charging current from 75A alternator

[jackmorgan]

Hi,

I have a 2007 four winns vista 258 with dual battery setup. I'm running a volvo penta 5.7GI with a 75A alternator. My question is, approximately how much current is available from the alternator to charge the batteries at idle / at cruise (3100rpm)? Assume the battery is discharged and will accept a large amount of current. Just wondering if i can discharge overnight and then run the engine at idle to charge back up (instead of a generator).

Thanks.

Jack

 

[sam am I]

Depends on the amp hours (ah) rating of your batteries, your specific type of alt, type of batt and internal resistance, the alt's specific shaft rpm at idle, depth of dis-charge, etc.......Guessing thou, idle is going to take a tad bit of time, if possibly not work at all!

As a rough example not knowing your particulars as listed above but to give you an idea where you're at using this chart's 72 amp alt w shaft speed running at 5K rpm and a 120 amp hour batt .........




120ah/60a= 2h roughly to get back topped off'ish...I think you can do the math from there.

Some guys I know will adjust their pulley's around to bump alt shaft speeds up higher at idle but, of course ya have to watch the top end then (mainly just faster wear out times at extra higher rpm's)......There is a happy medium when tweaking.




Maybe you could design a auto varying pulley system with neg. feedback that keeps the pulley's speed in the perfect power band independent of the motor rpm......

 

[bruceb58]

Typical pulley size ratios of the alternator to crankshaft are between 1.5 and 1.75 so if you fast idle at 1500, you alternator speed will be 2200 to 2600

BTW, never discharge your batteries more than 50% which his slightly less than 12.00V or you will diminish their capacity over time.

 

[bruceb58]

Problem with a generator is that you will likely have to have the generator drive a battery charger which is very expensive to get anything close to 70A.

You planning on a water cooled generator installed in your bilge? You have enough room?

 

[sam am I]

Here's what ya might do depending on your alts performance curve (or lack there of) as some manu's don't give them out, perhaps auto-zone might make ya one on yours if you don't know and ya can go from there, you could be good as it is and already be running a 7SI or similar, however........

If you find out yours is lacking and you have a few bucks to make this work, go with a 7SI, it's off the self ignition safe (SAE- J1171), runs at 105C, dual internal fans, yada yada. Adjust your pulley's if need be to say 3.64:1 (7.25" crank side, dodge has these as a reference to a 2" alt pulley)

At engine idle of 750 RPM, you'll have a alt RPM of roughly 2718.75 and accordingly have about 55 amps to play with now.

Cruising at 3100RPM = 11237.5RPM at alt and isn't too bad, won't last forever but, it'll work as the rule of thumb is not to run above 18K



22SI would work quite well but expensive and it's not ig. protected off the shelf. Might be able to find an aftermarket mod kit to make it that way and ya got the $$$......GL

 

[jackmorgan]

Problem with a generator is that you will likely have to have the generator drive a battery charger which is very expensive to get anything close to 70A.

You planning on a water cooled generator installed in your bilge? You have enough room?

True. was thinking about a honda 2000. My battery charger is only 20 amps though so yeah, it will take a while to charge! Might be better off with some of the other suggestions here about using fast idle / upgraded alternator / pulley.