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Old 05-12-2019, 14:51   #1
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Alternator Output Question

Hello all. Could you please help me understand which aspects of the output behaviour of my alternators is normal? We have a 120ah Volvo alternator on each D2-55 engine in our 2 year old catamaran and a 700ah gel house bank. The engines each have about 200 hours on them.
The rule of thumb is that running an engine for an hour will charge your batteries. Normally in the morning we are at about 70-74% charged (fridge, freezer, lights pumps, alarm and occasional fan).It takes us about 6 hours of engine running to fully charge the batteries, albeit that could include raising the anchor and mainsail as well as powering the fridge, freezer, autopilot and full instrumentation. We are also experiencing approximately 20ah difference in output between the alternators at any given state of charge. Is such a variance normal between 2 alternators of the same specification and age?

I attach three graphs of data we collected on motoring days. In each case we kept engine revs at 1900rpm and switched motors to give us direct comparisons of output. The fridge, freezer, instruments and autopilot are about 20ah, meaning at 75-80% charge we are typically getting 70a from our port alternator and 50a from our starboard. At this level of charge on gel batteries is this the sort of charging rate people would expect? What sort of diminution in charging rate are others experiencing as the batteries reach higher levels of charge?

Any comments would be gratefully received.


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Old 05-12-2019, 15:19   #2
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Re: Alternator Output Question

There is no rule of thumb that says charging for an hour will charge your batteries. Unless it is a smart phone.

Your alternators, and I assume you mean 120A not 120amp/hr ?... You will rarely if ever see that sort of charge current using an internal regulator... perhaps 70 or 80A?.. (guessing) and even then the charge current will reduce as the batteries accept charge.

I will often find it takes 3+hrs to charge to 100% from an overnight usage to 100%..and that is with a 100A alternator & 3 stage regulator and overnight consumption of 40AHr. so while i feel your 6hrs may be high, your consumption is also significantly higher than mine and charging performance possibly less

You did not state if your alternators are controlled with an external 3 stage regulator, but if not, then you will see greatly improved charge performance if you add these... as the internal regulator of an alternator is set to approx 13.5V, whereas an external regulator follows a charge profile to improve efficiency.

if you do not have a clamp on ammeter and a battery monitor with shunt they are good investments as you get real solid data on consumption and charging performance
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Old 05-12-2019, 15:34   #3
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Re: Alternator Output Question

Thank you for your response. We do have a battery with shunt. I did not get to attach an ammeter to the alternators but will do so when I am next onboard. I have separate three phase regulators on shore and solar, so would imagine that is the same for the alternators. My take from your response is that my port-side figures are not spectacular but nor are they unreasonable, and I should focus on the cause of the difference between the alternators.

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Old 05-12-2019, 17:03   #4

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Re: Alternator Output Question

A deeply cycled lead bank, say down over 30%, takes at least 5-6hrs to get back to 100% Full, even if it is at 90% in the first hour.

Would be better to state specifically what batteries, also the VR and maximum total alt output of the circuit feeding the bank.

The endAmps spec varies but that is the definition of 100% Full, e.g. 0.05C or trailing amps accepted dropping to half an amp per 100Ah capacity, measured at the posts.

Failing to reach this point will reduce lifespan.

Once the acceptance rate from an ICE source in the early AM has dropped to below say 0.1C, then can shut it down and use your solar for the rest of the day to finish "the long tail" and get to Full
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