Fluctuating Alternator Output

[TradewindSailor]

I have a catamaran with two Yanmah 3YM30's with the stock 60 Amp internally regulated alternators.

Recently I have noticed that the output becomes unstable when the charging voltage approaches 13.5V ...... i.e. it oscillates between approx. 35Amps and 25Amps with the engine tending to race a little as the load comes off the alternator. This effect appears to be independent of any load I am placing on the electrical system ... i.e. running the refridgeration.

The effect however does appear to be dependent on engine revs to some extent ..... I tend to charge at 1200-1400 rpm. Decreasing the revs to 900rpm or increasing them to 1800rpm appears to steady the alternator output.

The effect is also engine independent ....... I run each engine separately and get the same effect.

My house bank is 450 Amp Hour.

I don't know whether I am just seeing a normal regulator switching process ..... or whether it is a symptom of a fault with the charging system or batteries. Any comments would be appreciated

 

[savageseadog]

A standard alternator regulator regulates its output to 13.8V or so. As the batteries approach the regulated votage the charging current should gradually taper off to lower values. Your problem could be caused by gassing of the cells. Hydrogen could be bubbling finely off the plates, reducing the current and eventually the a biiger bubble wil form and sweep a large are of the plates clear of gas and allow the cell to conduct. Your battery or batteries could be defective. It's a possible explanation.

 

[halcyon]

Are both alternator charging the house bank ?, do you have any charge splitting ?

May be alternator hunting at regulation, one cuts out other one drops voltage as the load increases, other alternator kicks in, volta go up.

Can you give more detail of the charging system , battery banks etc.

Brian

 

[2Tizwoz]

Could you just have a loose connection?

Is the wiring warm anywhere?

 

[seamaster30]

I had a similar problem and cured it when (Ahem) I added some distilled water to cover the plates in the battery /forums/images/graemlins/smile.gif
I also had to replace the batteries shortly afterwards, but found the cause was a faulty charger cooking the batteries on shore power, not the alternators.

 

[TradewindSailor]

Thank you for the quick responses.

Firstly this effect happens when either engine or both engines are on.

I have a small cranking battery within 3 feet of each engine (max 4 feet cable run). The house bank consists of 4 D8's and is some 30 feet cable length distant from the alternators .... using 1/0 cable (53mm^2). All batteries are flooded lead acid's of 2006 vintage.

The charging current runs from the alternator to the engine isolator where it is teed to the house bank (through a relay ... acting as an automatic isolator) and to that engine's starting battery. I've removed the usual manual T-switch isolator as I was getting a 0.3v voltage drop across it ... which is important as the alternators are internally regulated (3-stage regulators).

The big house charging cables are fused.

I have no diode isolators in circuit as the voltage drop would cause significant under-charging of the house batteries.

 

[TradewindSailor]

Dry Cells ..... none of the cells are dry. I checked them yesterday.

I must get a hydrometer to check each cell for faults.

The house bank is connected as one bank with the charging and supply cables connected across all 4 batteries ....... i.e. the positive charge and supply cables are connected to battery 1, the negative to battery 4.

 

[halcyon]

If you keep running does it go over 13.5 volt ? or just stops there ?

What is the voltages the 3 stage regulators run at ?

Have you checked water ?

Brian

 

[pampas]

Before going mad and ripping things apart ,check that the Negatives are clean and tight, also check that the cables are tight( in the Crimp)or on the fittings by giving the cable a good tug.
Hope this helps.

 

[TradewindSailor]

Hi Brian
The charge voltage will eventually get to 14.0 V at the house bank, a loss of 0.2 V from the alternator.

The float voltage is set at approx 13.5 or 13.4 volt.

I made one mistake .... these 8D's are rated at about 225 Ah each .... i.e. I have a 900 Ah house bank! /forums/images/graemlins/cool.gif

I will have to carry out a discharge test and see if the battery/ies are in a poor condition.

Thanks all for your help .....

 

[TradewindSailor]

Thanks .... yes all the terminals seem okay with no heat. I've had terminals work loose before.

 

[TradewindSailor]

I have finally solved the problem ...... after re-cleaning all the contacts, and re-newing the battery bank /forums/images/graemlins/frown.gif ....... the problem was due to BOTH internal alternator regulators being damaged.

Both alternators are about 1000 hrs run time ...... Why should they both go down at the same time? What would damage them? Are they getting damaged by the heat of the alternator?