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Old 06-04-2008, 18:27   #1
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Inverter and Alternator Questions

I plan to install a Xantrax Pro xm1800 inverter in order to occasionally run a small fridge and other electrical components while underway or at a non power anchorage etc.

My question is this - I have a Perkins 4-108 with the standard alternator which I believe is 55 amp output - so in order to get the maximum 15 amps out of this inverter, do I need to upsize the alternator and if so to what size and can it be done on this Perkins?

Any advice is appreciated.
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Old 06-04-2008, 18:38   #2
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Randy, a lot of it depends on the size of your battery bank. The alternator doesn't have to be able to keep up with the peak current draw. If an inverter load requires more than the alternator can deliver, you will just have a drain on your house battery bank.


What size is your battery bank?

Also, keep in mind your loads will not often be at the maximum 15 amps, unless you are running a microwave, toaster oven, heater, hair dryer or something like that.

When you run that fridge, it may have a high starting current, but that is momentary, and will pass quickly, having no effect on your battery bank or alternator.
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Old 06-04-2008, 19:03   #3
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Benny, you need to design your electrical system as a system. Start with your load analysis to determine what size battery bank, inverter, alternator, etc.
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Old 06-04-2008, 20:10   #4
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Benny,

I certainly agree with the above two posts.

But to get directly to your question:

1. the Perkins 4-108 can easily handle a 100-110A alternator on the same single belt; I have had one for many years;

2. you will definitely need a smart regulator, such as the Balmar MaxCharge, to get the most efficiency out of your alternator; the internal and external regulators which are usually fitted as standard on 4-108's are not sufficient by any means.

That said, you do need to think of the whole system: battery bank size, anticipated loads, time between charging, alternator size, smart regulator, battery charger for dockside or with an onboard generator, etc., etc.

Unless you do this thinking and planning, you are unlikely to come up with a satisfactory solution, and it could be quite costly.

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Old 07-04-2008, 19:47   #5
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Thanks for your replies - I have 3 type 27 wet cells - 2 in series and one as a starter battery.

I will not be relying on the inverter for long periods because my cruising is Lake Ontario however, I do have a microwave/convection/toaster oven combo and a 2 burner hot plate. I will probably only run the microwave to warm something while underway or possibly the hot plate but mostly the fridge if needed to cool it down. Apart from that, I have a DC on demand pressure water system,DC stereo, DC dvd/cd player and small DC TV and the usual DC lights and several AC lights. My biggest concern is draining the batteries too far etc or having to run the engine all the time to keep the fridge cold or use the microwave or charge the batteries.

I understand the suggestions about doing a system design but for occasional use, I am looking to the vast knowledge of the members of this forum to give me some guidance.

BTW - I have a Norcold cold machine top loading fridge box which is DC only and can be used if on a long trip. It has been cycling on and off and not cooling if I try to set it on a colder setting - for example to keep food frozen so I am installing a small GE spacesaver fridge for my main fridge requirement and I can leave it on at the dock without fear that it will shut itself off which is what the Norcold does or it just keeps cycling on and off. I am in the process of having this problem diagnosed by the Norcold guru.

I keep my charger on while at the dock when I am away for the week.

Of course beside the appliances, all those power tools I have may need to be used away from the dock so it wouldn't hurt to have the extra amps delivered from the 1800 watt unit.

It would be nice if I didn't have to replace the alternator for these needs.

Also is the inverter mounted between the battery and the selector switch or is it wired directly into the AC part of the panel after the batteries?

Thanks all for your help.
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Old 07-04-2008, 20:06   #6
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Benny,

Basic Ohms law. Amps X volts = watts. So in round numbers you need 10 amps @ 12 volts DC to make 1 amp in 120 volt AC plus the penalty paid for the conversion. To run AC appliances check the watts required or the amps and do the math.

1800 watts of AC power is going to drain those batteries dead as a stone in short order. That means 150 amps DC to make 1800 watts AC! If you take the amp hours from the battery ratings and divide by 50% that will give the amount of power you could drain the batteries and not trash them fast. If you drain a battery dead you will have to replace them a lot sooner than if you drained them only half then recharged them fully.

It's a total picture as noted by Sullivan. What you take, you need to give back. It goes out a lot faster than it goes back in. Batteries don't accept charge quickly for long periods. When they are low they can take a lot of amps in but as they recharge more they can't take as many amps as fast. Suddenly the 50% changes to 40% as the last 10% is a lot longer charging time at a lower rate.

To fix this you need more batteries plus the ability to charge them 100%. Normally, you don't run a fridge "occasionally". They generally run a duty cycle of 50% in really hot weather.

The key to the cheap solution is the power you don't need never has to be recharged. You need to really compute what you really need or else bite the bullet and buy far more battery capacity than you have now. The bigger alternator aside I doubt it will bridge the gap you have in mind.
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Old 07-04-2008, 20:14   #7
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I'm figuring this out also...120 volt fridge?

First, it really depends on how long you are willing to run the engine. A big battery bank is necessary, I would guess at least two more group 27's. But the bottom line is that you can only put the power back in so fast. The batteries accept less charge after the first 20 minutes or so even with a good alternator and regulator. My experience with a 100 amp alternator is that the fridge, occasional tv use, is 1- 1.5 hours engine running a day in the carribean with a 100 amp alternator and 3 X 4D batteries. Do not buy over 100 amp alternator unless you can put a 2 belt system on it.

Here's my question: BACK TO USING THE SMALL HOME FRIDGE IN LIEU OF THE $900-1300 nORCOLD. The $159 fridge at Lowes with freezer is rated at 335 KW per year. By my calcs that is 920 watts per day. at 12 volts that comes out about 75 amps per day. add 10% for invertor losses (when on the hook) and we're talking 82 amp per day. That seems comparable with my 12v fridge in the carribean... have it got this right?
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Old 07-04-2008, 20:30   #8
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Cheechako - thanks for the input - so if the calculations are right, then there should be no problem with the fridge and if I only use the microwave occasionally to warm something, there should be not too much drain.

I am thinking that a good battery monitoring system is in order - I have readouts for volts and amps on my panel but I would prefer an automatic warning system - the Xantrax inverter I mentioned has a remote control panel that gives warnings and charge and output levels - is this good enough?
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Old 08-04-2008, 09:35   #9
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you will need more batteries...

I think you said you have two group 27's "in series", I believe they must be in parallel? unless you are running 24 volts. With any refrigeration going while you are away from the dock you are probably going to need two more in the house bank... otherwise you could end up with those batteries too low overnnight. If you are just out for a few days, you could try switching the fridge off while you sleep and then monitoring it during the day based on voltage. Or stick a block of ice in the fridge when you start out to help also. To truly cruise your boat for more than a week's vacation, you need the High out put alternator, regulator and large battery bank. If your alternator has an external regulator I suppose you could try using a H.O. regulator on it to get the most out of it.
Sooner or later you have to pay the piper and get the amps back into the batteries... and it takes a while. see chart, notice that your batteries are 50% discharged at 12 volts! the first 50% is the easiest to recharge, if you pull them down to 11.5 volts or so it takes quite a while:

(Temperature: 57 degrees Fahrenheit)
Percent Hydrometer Unloaded
charge reading voltage
100 1.258 12.59
75 1.203 12.26
50 1.153 11.96
25 1.113 11.72
0 1.093 11.60
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Old 08-04-2008, 14:50   #10
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A typical Group27 deep cycle battery would have a capacity around 90-100Ah at the 20 hour discharge rate, probably more like 60-75AH at higher rates, giving you maybe 150AH capacity from the pair you have. And then for longer battery life, you never want to exceed a 50% discharge, so you are down to about a 75AH working capacity for the two batteries.

At a nominal 12 volts, that's about 900 watt-hours, or 30 minutes at 1800 watts. And then, all else being equal, it may take you 4-5 hours to put that charge back INTO the battery bank, figuring on a very rough but typical charge acceptance of 1/5th the rated capacity. (If I got my numbers right.) That's pretty much regardless of how big an alternator or charger you use, because trying to pump power into the batteries any faster is somewhat self-limiting, all you'll do is boil off electrolyte by trying to charge them faster.

Offhand? I'd say using the microwave for a fast five minute burger is about all you'd want to do, figuring that might need 1/2 hour to an hour of engine time to restore that power. Yowser, a butane camping stove, a kettle, and a skillet become much more attractive, in terms of the noise and the fuel cost both.

There really is a logic to these things, we may each debate which set of numbers works best in any particular installation--but when you run the numbers and design a whole system as a balanced system, right down to matching the alternator pulley size to the engine speed, you can either save or spend a whole lot of money. Sometimes both at once.[g]
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