Increasing Alternator Output

[cm]

I have a newish 80 amp genuine Hitachi 12v alternator in my boat. It charges 1 engine battery and 3 house batteries, each is 120 a/h AGM. There is a diode isolator. Everything is working fine.

I want to increase the amps output from the alternator to charge the batteries faster. I can run the alternator at the optimum speed and get initially 60 amps output if the batteries are fairly discharged. The 60 amps drops gradually to 35 amps in 30 minutes with no equipment running, except the 4 amp engine blower. I know that this drop is caused by the resistance of the batteries as they become charged and the voltage difference between the batteries and the alternator output diminishes. However, 30 minutes at an average of 45 amps is only 7 amp hours for each house battery (assume the engine battery is quickly recharged after starting). I would have thought it would stay higher for longer.

My question is:

Will replacing the 80 amp alternator with a 120 amp model charge the batteries any faster? Will it not start at around say 100 amps and quickly drop down to 35 amps to suit the batteries take up as is the case now? OR will the drop in amps output be proportional to the higher maximum output? Eg if the 80 amp alternator drops to 35/80 = 44% will the 120 amp alternator drop to 44% = 53 amps output after 30 minutes?

[nornaj]

Norna J (an SO39i) was commissioned with five house NG31s (a mix of Exides and Tudors) and a single NG31 starter battery. A Balmar 100amp alternator and a smart regulator were also installed.

The results were underwhelming.

We recently had the system reviewed and upgraded. To cut a long story short, the isolator was replaced with a combiner. The wiring for the alternator had not been upgraded, and was replaced with heavier cable. A number of other commissioning "errors" were found, they included not running all negative battery feeds to the shunt for the Xantrex Linklite, which meant we were not measuring either the charging or discharging correctly.

So far, the results have been excellent. The alternator charges a 12.2v battery bank to 12.9v in less than 3 hours motoring.

Might be worth changing out the isolator for a combiner and checking the alternator wiring for proper sizing.

Hope this helps
Norna J

[pbunning]

My last boat (SO34.2) used a relay to connect engine and boat batteries when charging from the alternator - relay coil energised from the alternator secondary output.

My present boat (SO33i), new last year, has a diode isolator although the manual drawings show a relay.  My guess is that Jeanneau now use diode battery isolators. 

Diodes do have a small volt drop, which is significant when it comes to using them in a charging circuit.  The alternator regulator maintains a constant output voltage, hence there are a number of external regulators on the market, which monitor the battery voltage to control the alternator.  Added advantage to these is that they can cycle the voltage to maximise power into the battery.

I had an Adverc regulator fitted to my last boat. www.adverc.co.uk  They have some useful information on their site.

Diode regulator fitted to my SO33i - Cristec MOSFET.  MOSFETs have a low voltage drop - Cristec info seems to indicate it is negligible. I do however intend to fit an Adverc regulator.

In conclusion, if your present alternator current output is dropping to 35 amps within 30 minutes, fitting a larger alternator will have no discernible effect, other than to shorten the time to below 30 minutes.

[electricmonk]

As you have AGM batteries it would be possible to increase the charge amps and hence decrease the charging time.

In theory at least you could charge your AGM's at 480 amps. and have them up to 100% in an hour. That's a 7 KW charger!

The issues you will face are these.

1. The system for providing power to the alternator will need to be upgraded the current single V belt will run out of puff at around 100amps.

2. You will need a better charge monitoring system including temperature sensors along with a charging system that is matched to the charging characteristic's of your AGM batteries.

3. You wont have any power left for propulsion.

4. And lastly; In my humble opinion "nerves of steel".

[Trevor]

This is a great debate because it is the most annoying thing to know you have an 80 amp alternator but rarely is that capaicty used to charge the batteries. A normal alternator with internal regulator is generally only designed for simply start battery recharging. If you have an external regulator you have a better chance of greater output from the alternator but the point is you still need to run the motor for an extended period to recharge those house batteries.

I understand this thread is about alternators but the end goal is to have charged batteries so I think solar panels are extremely effective. You can get great flexible panels now that are so easy to install. 2 x 100 watts solar panels can provide up to 14 amps. Ok, that will not happen all of the time but even half of that for an extended period tops those batteries up and hopefully stops the need to run the motor. They are operating from the moment the sun come up until sunset so during the day they are constantly doing the job.

Nothing worse than being in a beautiful quiet location and having to run the motor to charge those batteries.

Happy sailing

Trevor

[rxc]

I agree with all the above. I have done this on two boats so far, and all the recommendations are good. First, replace the diode isolater with a combiner. Make sure that the wiring sizes are correct (i.e., as large as you can afford). Check that connections are clean and tight, and properly installed. Then comes the more expensive stuff. (1) Install an external regulator for the alternator, and make sure that it will deal with your battery chemistry (AGM) properly. (2) Make sure that you have appropriate monitoring for the batteries (something that actually measures the current going in/out of the house batteries, as well as voltage). (3) I assume that your house batteries are all in one bank - if not, make it so, because you will get more capacity from them (see Peukert's law for details). If the batteries are old, this might require replacing the batteries, first. (4) Upgrade your alternator if the Hitachi cannot charge fast enough, but realize that this can be problematic - side loads on engine pulleys can be an issue, and belts need to be upgraded. (5) Install solar, which involves both panels and wires and controllers and more wires. (6) install windmill - more supporting structures, wires, controllers, plus noise.

If you do it gradually, over a few years, you can think of it as a long-term project that won't be TOO expensive, and you will understand how the electrical system works MUCH better. The down side is that your spouse will not understand it at all, unless she is an engineer/techie.

[cm]

Thanks for the info.

I have fitted a Stirling fast charger. I don't recommend them but I have persevered and got it to work. It burnt the diodes inside my Hitachi 80 amp but now works with 3amp diodes. Nothing in their instructions to warn you. I now get around 40 amps at 1500 rpm which holds up rather than drops down. Barely worth the cost and drama but now it's done.

I thought about fitting a 120amp Bosch Marine grade alternator but you have to make your own support bracket and it will shorten fan belt life and possibly that of the bearings too. Advice received is that the higher capacity may not make a big difference for my boat and would require upgrading all the wires.

What I learned on the way was that the alternator must be suitable for a marine environment, ie no sparking to ignite gases that may have accumulated in the bilges. Although some big name brands are now manufactured in China, the Chinese no name look-alike alternators are not likely to be suitable and, like their starters, won't last as long.