110v fridges! Gen all day?

Brock Chobar

New Member
Jan 9, 2018
18
Long Beach, CA
Boat Info
1992 370DB "Kara Jane" twin 7.4 Merc's. 10'6'' INMAR rib tender with a 20hp Honda.
Engines
7.4 454 Mercruiser with Hurth trans
Curious to what everyone with 110v fridges do when on the hook? My last boat had a 110v/12v fridge, new boat has two 11ov fridges. I was thinking id install a dedicated bank of golf cart batteries and inverter to keep them on overnight or when we are off the boat when on a mooring. Bummer to have to run a gen all day just for the fridges. What do everyone do? Have you done batteries or do you just run the gen as the factory set it up?

Thanks!
 
We have ten 6 volt golf cart batteries with a 2800 watt inverter. It runs lights and 2 refrigerators. I can go 2 days before the batteries need charging. To full charge them I need 8 hours. Inverter is also 120AMP charger. One refrigerator is bar type other is 12/120 volt. I do not use he 12 volt feature on the refrigerator as it is set up to automatically use 120 volt when ever it is detected and that include the inverter power. Normally we run the generator 2 hours a day to use eclectic stove, kettle, heat water for shower and make water using watermaker, that keeps all batteries above 11 volts, most of the time they are 12+ volts.
 
As soon as I'm away from shore power, my genny is running......nonstop. It is very typical on extended weekends, like Memorial, that I dont shutdown for three days. My point is that the boat has the equipment, and is made to run.

If you dont need to run the AC, an inverter is a nice quiet option.
 
Inverters –

Let’s say your refrigerator requires 70 watts at 120 volts AC

That equates to about 0.58 amps in current at 120 volts AC (70 / 120).

Now we need to convert from 120 volts AC to 12 volts DC for the current (amps) which will be about 6 amps (120/12) X 0.58 = 5.8 amps

Now to not run the generator in the evening and through the night (say 10 hours) then you will require 60 amp hours (Ah) which is 6 X 10 = 60. The refrigerator will not always run so let’s say it runs 50% of the time which now sets the requirement to a total of 30 amp hours (Ah) less motor starting current and the inductive load power factor so let’s adjust to 35 Ah to be safe.

Lead Acid and AGM batteries are limited in four areas: 1 - the useable amp hours, 2 – the allowable rate of charge, 3 – the rate of discharge, and 4 memory. Of these three 1, 2, and 4 are probably of most interest.

Battery useable amp hours – Let’s say you have a lead acid/AGM 12 Volt deep cycle battery that is rated at 90 Ah’s. The life of the battery is dependent upon how that battery is charged and discharged. From a discharge perspective lead acid / AGM battery life is determined by Depth of Discharge (DOD) and cycles. Typically with limiting DOD of 45 to 50% the battery life will be around 2000 cycles or you should get a good 6 to 8 year service of recreational use. So, what this means your available Ah is not 90 but rather half of that or 45 Ah. Should, for example, a lead acid/AGM battery be cycled routinely to say 80% DOD you will be lucky to get a season out of the battery. This, by the way, is where Lithium Ion batteries really become economically viable as they can be cycled to 80 to 90% DOD with minimal voltage drop; consequently your battery bank needs only be half the size.....

Now that we know our battery has 45 Ah available to use and we need 35 Ah then we know that we need 1 battery 35 / 45 = 0.78 to run the refrigerator for 10 hours.

Inverter and wiring efficiency – Naturally we cannot expect 100% efficiency from the inverter and wiring system. Typically, inverters are between 90 to 95% efficient then allow 5% in loss to heat in the wiring system. So let’s say we can assume that we have a 90% efficiency of the inverter and wiring. So simply stated we need another 10% in battery capacity to drive the inverter and wiring; That’s another 3.5 Ah required for a total of 38.5 Ah which will still allow the use of one battery.

Charging the batteries – There are three phases of charging for lead acid and AGM batteries; Bulk charge, Absorption, and Float. For the sake of discussion you have a 40 amp battery charger which in bulk charge supplies 40 amps. So, that means in one hour the charger in bulk mode can provide 40 Ah to the battery. After the battery achieves approximately 80% charge the charger switches to absorption mode which then the amps reduce to around 8 amps so that rate is now 8 Ah until the battery reaches full capacity. This means at 50% DOD the battery needs 45 Ah. 80% of that 45 Ah will be recharged at the bulk charge rate of 40 amps so that phase is around 1 hour (45 X 0.8) / 40 = 0.90. Then absorption at 8 amps for the remaining 20% which would be 1.125 hours for a total charge cycle of around 2 hours. You can quickly see that recharging the batteries is not insignificant. This, by the way, is why the boat’s charging system should always be active and maintaining the batteries. Regarding Lithium Ion batteries as a side note the charge rate can be the full discharge rating and there is no absorption phase; so, you can hammer hundreds of amps to the Li battery then instantly to float - a big plus for the Li technology.

Battery memory – Keep in mind that lead acid and AGM batteries can take a memory. For example if they are not brought to float in the charging cycle then the capability of the battery (Ah) will diminish. The batteries memory is permanent and cannot be returned to the previous capacity.


There you have it – not as simple as most think to properly set up an inverter system….. But if done right, you will get a long serviceable life.

As an edit - a bit more on inverter efficiency - Any good inverter manufacturer will provide an "efficiency curve" with the unit. A large inverter with a light load will require more Ah due to being on the low end of the efficiency curve. So when sizing your inverter make sure your nominal demand is no lower that 20% of the inverter rating. For example if you have a Brand X inverter that has a maximum operational rating of 2800 watts then plan your nominal loading to be no less than 560 Watts or 4.7 amps at 120 VAC. With that said, however, the latest designs have power factor correction (PFC) so they can be driven at lower percentages. The message here is to understand the efficiency of the inverter and it's PFC when sizing.
Tom
 
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And those gentlemen, are the words of a “steely eyed missle man” - with apologies to ripping off a line from Apollo 13.

Thanks Tom,

Great post - ought to be a sticky.
 
And those gentlemen, are the words of a “steely eyed missle man” - with apologies to ripping off a line from Apollo 13.

Thanks Tom,

Great post - ought to be a sticky.
Ha - not quite but aspirational. In the days of early Apollo and Gemini programs there was a local "club" of those hard core test pilots, engineers, scientists, and technicians called the "Steely Eyed Rocket Scientists". Those folks worked and played hard and were the leading edge to where we are today.
 

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