Inverter/Charger/batteries/genset

[Knucklehead]

Am considering the purchase of a previously owned motor yacht for the purpose of cruising to places where I avoided with my sailboat (6ft draft).
The boat I am looking at, has a generator 7.5kw, that the previous owner admits to not maintaining and suggests replacing with a 11-12 kw generator.
My thoughts lead me to not going to a bigger generator, but, installing an advanced inverter charger with an automatic on/off capability when battery parameters are met.
The demand for power would be the normal demand; refrigerator/freezer, lights (LED), one AC unit, anchor light, radio, etc. when anchored.
Additionally, instead of Li house batteries ($$$$), I could continue to use the current AGMs.
This could lead me towards replacing the generator with a smaller footprint generator with larger house batteries.
Possible????? Logical ?
Karl (the KISS guy)

[john61ct]

A generator is only required when away from shore power, and many (most?) cruisers in smaller boats make do without completely.

A charger is still required, so not an either / or scenario.

An inverter, whether one big expensive one, or a collection of small/cheap ones, is only needed to the extent you "need" AC powered appliances.

Again, orthogonal to the generator question.

Finally, a bigger storage bank does not enable a smaller genset.

Ah out must be lower than Ah in averaged every day.

The size of the bank just gives a larger "time shifting" window, more hours/days between charges.

You must have solar to keep a lead bank healthy when off grid, getting back to 100% Full takes at least 6-7 hours no matter how big the gennie.

Hope that helps, as you can see overall design takes a systems approach, and lots of specific quantified data about your Ah per day usage, sailing patterns, use of shore power etc.

[smac999]

A fridge and ac would probably run off a 3.5k. An 11k is rediculas for that.

How big is the ac unit?

[Dockhead]

I think you are thinking about "Power Boost", a function of certain charger/inverters which turns the inverter on to supplement generator power during peak demand when loads exceed what you want to put onto your generator.


If I'm understand you correctly, then in my opinion your thinking is absolutely correct, and using Power Boost is a great way to avoid oversizing the generator, which has many disadvantages.



Why you don't want to oversize a constant speed AC generator:


1. Constant speed generators are much more vulnerable to wet stacking and bore polishing than main engines. Because they are always running at the same speed regardless of load, and pumping through the same amount of air. Below about 25% load, depending on the particular generator, and it may be worse for a high speed one (3000 or 3600 RPM), this gets to be dangerous, and generators needing overhauls because of polished bores is actually a pretty common and pretty expensive problem. So you don't want the generator to be oversized; you want it sized so you can arrange loads to keep it at 50% or more most of the time. This issue is especially a problem on boats which run their generators all night running air conditioning, which cycle off most of the time when the night is cool.



2. Oversized generator is heavier


3. Oversized generator takes up more space


4. Oversized generator is more expensive.




SO -- it is inefficient and unhealthy to size the generator for the peak loads; using Power Boost can shave the peaks and allow you to reduce the size of the generator to something corresponding to the shoulder loads.


I do this on my boat. I have a 6.5kW heavy duty 1500 RPM Kohler generator, which I derate to about 85% of maximum using the current control on the charger inverter. I don't have air conditioning, but when I'm doing power intensive things like electric cooking while running laundry, I use Power Boost to avoid loading the generator above 85%.



ONE CAVEAT:


Lead batteries don't really like being slightly discharged from 100%, then being put through another absorption phase, over and over again in quick succession. They can overheat and you can corrode the positive plates by too much absorption. As a result of this, I limit the amount of Power Boost I use and I turn down the fixed absorption time in my Victron Multiplus. Power Boost is brilliant, but it would work much better with lithium batteries, which don't have any absorption phase and don't mind quick cycling like this.

[Dockhead]

One more thing --


If your existing 7.5kW generator is a low speed, heavy duty one --


These things last almost forever; they are almost bulletproof. I would not be in a hurry to rip it out. If the power head is good but there are problems with the diesel engine, these can often be fixed for much less than the price of a new generator. These are KISS devices with overbuilt everything and simple electronics -- often good for 20 000 hours if not abused. A little love might go a long way, and you could spend what you save on a nice lithium battery bank.



And on a M/V which doesn't care about windage much, since it's already a windage nightmare, you could add a solar array too.

[ranger58sb]

Originally Posted by Knucklehead

The boat I am looking at, has a generator 7.5kw, that the previous owner admits to not maintaining and suggests replacing with a 11-12 kw generator.
My thoughts lead me to not going to a bigger generator, but, installing an advanced inverter charger with an automatic on/off capability when battery parameters are met.
The demand for power would be the normal demand; refrigerator/freezer, lights (LED), one AC unit, anchor light, radio, etc. when anchored.
Additionally, instead of Li house batteries ($$$$), I could continue to use the current AGMs.
This could lead me towards replacing the generator with a smaller footprint generator with larger house batteries.

I would first postulate the existing 7.5 kW genset might be fixable, and will then last another 20-30 years. And that's usually the easiest solution to aircon, and that size could probably run a couple ACs simultaneously if necessary.

A decent invert/charger can be a good thing, depending on what you need. Fridges might already be AC/DC, no need for inverter to run.

Adding battery capacity can be a good thing, depending etc.

Adding solar can be a good thing, depending etc.

I've had good service from quality AGMs.

-Chris

[GordMay]

I often wonder why folks think they need a larger capacity generator, than their shore power supply.

[Dockhead]

Originally Posted by GordMay

I often wonder why folks think they need a larger capacity generator, than their shore power supply.

My generator has about double the normal maximum capacity of my shre power, 30 amps (@230v) vs 16 amps.

This is common, and there is logic to it. Shore power is continuous; generator gives power only when running, and you want to shut it down. Therefore, when using the generator we bunch up the loads to do everything all at once to the extent possible, and besides that, we may be bulk charging batteries at the same time, using another 2kW of power which you would not be using on shore power. A watermaker might add another 1.5kW or even more to that, another task you would never do with shore power. On top of your other loads.

So a 3.6kW generator would really be too small for me (YMMV of course), even though I have no air conditioning. That would leave only 1.6kW of power available when the batteries are bulk charging and only 1.1kW of power when a watermaker is running, and nothing at all if doing both.

With lithium batteries which did not mind frequent cycling under "Power Boost", I might be able to use Power Boost more aggressively and somewhat reduce the size of the generator, but I wouldn't want less than about 5kW. Although it would be easier to load up a 5kW (or smaller) generator, I wouldn't be able to bunch up the loads to such an extent, and I would end up with longer generator runs.

All this despite the fact that 3.6kW (16 amps) of shore power is generally enough at the dock. But since we're rarely in a hurry to shut off the shore power, there is no need to bunch up loads, so you don't mind deferring the washing machine run until after electric cooking is finished, don't use a vacuum cleaner while the dryer is running, etc. etc. When the generator is running, on the contrary, you want to do everything at once to the maximum extent; hence most people will want a bit more capacity.

Another reason to size the generator a little bigger is that no generator is really happy running at 100%. This will kill light duty high speed generators pretty fast. Heavy duty low speed ones will take it, but it is healthier to derate them to 80% - 90% of maximum power (more for high speed generators). So 3.6kW of theoretical generator capacity is NOT equivalent to 3.6kW of shore power in any case.


I derate my generator to 25 to 27 amps. That's probably excessively conservative, since it's continuous duty rated anyway, and besides that is already derated by my running it in its 1500 RPM, 230v regime, instead of its 8kW, 1800 RPM, 110v regime, under which the same hardware has to handle more than double the amps. But why not, if you can? The generator is not something you want to push to its limits.

[john61ct]

Excellent explanation there, thanks much.