"double diod system"

[Mr. Fixit]

Volvo sells a "double diod system" to charge both the "house and engine battery with the same alternator. I do not understand how this system works, hence the question" is it worth buying" ? I am keeping elec requirements to a minimum. Cabin lites,chartplotter, depth. radio, Sea water wash down system,will only be pressurized when using. The alternator is 60 amps. if necessary I will supplement with solar. I have not calculated elec demand yet, at this point I am still in the planning, purchasing stage. I do not want to run a alternator with a larger capacity as I read someware that 25 amps require 2 horsepower. I feel the engine I purchased is marginal and do not want to sacrifice engine output, or overwork the engine (Volvo MD2020). What am I missing here, appreciate any and all advice. To clear a point- I know how this "double diod system" is wired, I do not know how the components interact.Does the battery for starting get charged first, if it needs it? Can a system be set up this way?

[Rick]

First of all let's dispell the misinformation regarding just how much horsepower an alternator "pulls" from the engine. With NO losses one HP can theoretically deliver 62 Amps to a battery at 12.0 Volts. In the real world of belt losses, iron losses (from the stator and rotor of the alternator), diode losses, and copper losses one should figure a 50% efficiency for large frame alternators running fairly cool (less than 80 deg C). Small frame alternators are not as efficient, especially the "high output" ones which really push the flux density of the rotors in order to get the high output in the smallest volume. Small frame alternators get only around 40% efficiency at rated output.

So, in your case figure around a 2 HP engine load in order to get the 60A output at 12.0V. Your engine could easily drive a high output alternator yet you would want to install an "enable/disable" switch for emergency full-ouput-to-propeller mode of operation or some means of limiting the alternator output when you need all that you can get for boat propulsion.

Most "12V nominal" rated alternators are actually current rated at or near 14V, not 12 so be famaliar with multiplying voltage times amperage in order to get a meaningful comparison in Watts when considering different voltages.

I am fundamentally opposed to using diode isolators in order to split an alternator's ouput and here's why. An alternator regulator, whether external or internal, is capable of controlling only ONE output voltage period. If the regulator is internal then you will never develop sufficient voltage to recover a deeply discharged battery. If the regulator is externally regulated then necessarily the regulation "sense" line would have to go to the house battery (the one potentially deeply discharged) in order to properly recover any lost capacity with discharge/charge cycling.

If a diode isolator is used to separate the start-only start battery which is in very good condition then the fully charged start battery exhibits a very large capacitance and will not charge accept current. It takes very little current through the diode isolator to charge that capacitor equivalent of the start battery and, as a result, the start battery can rise to as much as 16Volts while the house battery is at 14V or 14.4V (where you need to be in order to recover capacity). Yes, this differential is not the usually "assumed" 0.7V drop of one diode. That assumed number does not work in this application. In fact the voltage drop of BOTH standard as well as Shottky diodes is between zero and 1 (or more) volts assuming direct current.

Now what we have in reality is BOTH direct current as well as a component of ac voltage variation on the alternator side of the diode isolator. The start battery charges its capacitor to the PEAK output voltage of the ac component that the alternator is capable of delivering. Meanwhile the house battery is stupidly happy pulling almost pure dc because it DOES charge accept current with not the same equivalent capacitance as the fully charged start battery.

About 18 years ago Powerline came up with the apparent bright idea of using two sets of internal diodes to give two isolated outputs from their high-output alternators. Their engineers failed to take into account that diode voltage difference, the equivalent circuit of the fully charged start battery, and the ac component of the alternator internal output. The result was a disaster to many people who literally cooked their start batteries. Not everyone had this problem, especially those with "low quality" leaky start batteries having constant loads.

One superior solution is to use a good quality battery combiner which can be programmed with voltages appropriate for your batteries.

Regards,
Rick

[sv_makai]

One small point. The alternator only pulls full HP at the intailly start of charging. Almost all regulated ALts will start to taper off as the battery charges. The tapering reduces the demand on the engine as well

We have 680 AMPs and a pair of 160 AMP & stock 55AMP on 2 27 HPengines. We have never seen full output when discharged to 50% on the house bank and the controller almost immediately startes to taper the charge as it charges.

Point is it is important to know the maximun HP draw, but be mindfull that it will no always take full HP..

If you are concerned that it may take full HP when you need it all of it for moving the boat you can put a disconect switch that can turn the alt off.

[Rick]

For trivia readers, I made a mistake...it was Lestek, not Powerline that came out with the dual output alternators.
Rick

[Rick]

Hi Capt. Bil,
As a note for checking on your battery's ability to charge accept, you SHOULD see that your alternator readily delivers its rated current output to the house battery for a significant period of time. During the bulk mode (by definition, the battery charge accepts as much current as the charge source is capable of delivering) if your 680 Amp-hour rated bank is depleated by 50% then it will charge-accept MORE than 320 Amps (I doubt that your alternators are capable of delivering that much unless they are "intelligently" controlled for a combined output) at 14.4Volts. If not, then either your battery bank is severly in need of replacing, or at least equalizing. One other possibility is that your regulator is not truly attempting to regulate the battery terminal voltage to 14.4V (or more).

If your alternators are of the large frame type and "hot" rated for 160A each then you should be able to read about 375A total combined output for at least 5 minutes, assuming that they are identically driven and controlled. They then "taper" off according to the copper heating to about 320A for a significant period of time (a half hour or so in your example). What is the use of having such hardware if you can't reap the ratings?

Look up my ex on the Fantasia 35 "Galadriel" either on the hard there in PLC or just splashed (Mike and Patty).

Regards,
Rick

[sv_makai]

You are correct. We do see Hi AMPS for a the first few minutes and then the taper. We use a Nextstep smart regualtor and a Link 2000 to monitor Both Alt are combined before the regulator. We are generally 90% charged by 1 to 1 1/2 hours when using the ALt when deep discharge, otherwise the recharge is very quick.

We have fresh batteries due to another problem and we equalize every 2-3 weeks via our solar system. Our bank rarely drops below 70% discharge. The number of times we have observered a deep discharge is limited.

The setup is builder (Voyage) installed and designed primarily for the charter world. Even at low RPMs the ALt will produce a decent output.. The Builder also put a delay in the alt setup so they do no start to charge for about 1 minute. This way the starter is not trying to turn them.

But even with deep to average discharge the engines Alts rarely draw all of the HP required to turn them at the max for a relatively short period of time.

We heard "Galadriel" on the net and will tell them Hola!