Alternator Questions

[BillBerner]

Educating myself on Alternator and charging stuff.

1. Does anyone know the pulley ratio for the original Alternator provided with the M35B. Or pulley size at the crankshaft & alternator

2. I've checked my tach and it's accurate. What I don't understand though, is that since the rpm's for tach are measured at the alternator, how is the speed adjusted down to that of the shaft. Simply in tach adjustment?

3. Now, let me know if I've got this wrong. I've got 2 4d's in my house bank. At 200 amps ea., I've got a 400 amp bank. If I'm running the bank from about 90% capacity to 50% capacity my math tells me I've got to replace 160 Amps. (400*90%=360, 400*50%=200, 360-200=160).
I believe the OEM tach is either 51 or 55amp. So, at 6000 rpm of the alternator rotor, it should take 3ish hours to bring the bank from 50-90%.

4. I don't have an external voltage regulator. Does the internal adjust voltage as the batteries approach full to protect against overcharge?

Have I got this right? Of course I have to factor in charging of my starting battery, too. But off the top of my head I have no idea how much each start of the engine takes down the battery.

Thanks.

[leigh weiss]

Bill,
Using a standard alternator and an internal regulator, your calculations assume the output of the alternator is linear over time, I don't believe it is.
For this and other reasons the recharge time will be longer. In MHO your best and shortest recharge times can be achieved with an external multistage (smart) regulator with an external battery voltage sense and temp probe mounted on the battery bank.
As I checked my alternator output vs engine rpm, I changed the pulley to the smallest diameter that would fit my serpentine belt. This brought the rpm of the alternator higher without going beyond the max rpm for the alternator.
I found the pulley on e-bay for about $5. I had to change the belt size to compensate for the new pulley size.
At my cruse speed of about 2200 rpm, my output is over 100 amps, more than enough to run the microwave with the 2 kw inverter and also recharge the batteries.
My change to a serpentine belt solved the belt ware and subsequent belt dust issues at this high load.
I cruse the boat to New England for the summer and anchor most of the summer. I have 2 AGM 4d batteries and recharge them in about 2 hrs. as we move to a new location or with the generator and a 100 amp 3 stage charger/inverter.
I changed the alternator output wire to a lower gauge wire connected directly to the battery terminal of the starter.
I added an external ground wire from the alternator to the main engine/battery ground. This was to eliminate voltage drop/loss.
I have an amp gauge in the output of the alternator to keep tabs on the output and a volt meter to check the state of the batteries.
I hope this helps and is a starting point for your system design.

[Triumph]

Leigh is correct. The math is not that simple. The first thing is that your 50 amp std alternator will yield about 40 amps max. You may start out at 40 amps going in, but you will see the input slowly drop as the batteries begin to accept the charge. I would guess that after an hour, you'd be down to accepting about 25 amps going in, after the second hour, 16 amps going in and so forth. Of course you've probably read that in trying to go from 80% charged to 90% charge it will take you hours, or as much time as to go from 60% to 80%. For this reason, most of us run between 50% and 80% charge while cruising and not running the engine. That last 20% just takes more time than it's worth.

while out cruising ....Most boats with a standard factory set up like ours run their engine for an hour in the AM and an hour in the PM, and everything is pretty good. Maybe after a few days of this you'd have to run the engine more (w/ 50 amp alternator) but by then, maybe it's time to move on the the next island.

I personally do not use my main engine to charge the batteries. That is, I do not ever run it while sitting on the hook for a while. I have solar (day), wind(night), and a Honda 2000 generator that I hook up a 90 amp battery charger to if needed.

I'd experiment with living unplugged and running your engine for an hour AM & PM and see how you do with all the normal loads.

Bill Cullen
TRIUMPH
www.thebookofsail.com