Inverter Use

kloudie1 · Jan 26, 2012

Yes, Or a generator

Bill Roosa · Jan 27, 2012

You nailed it Russf. Since you can't store AC current you have to take what you have and invert it to AC or generate it with a genset.
There are several of reasons for attempting to run as much equipment off DC as possible.
1) Less expensive as you don't need as large an inverter
2) Potentially less complex as a small inverter might be able to be run off a DC cigarette outlet and you can avoid the whole transfer switch and inverter instal etc.
3) Provides more AH from the same setup as the inverter is inefficient (90% ish) but more importantly the high currents an inverter needs cause the batteries to actually operate in a mode where they have less AH. Basically a batt used for a high amp load will deliver fewer AH than the same batt powering a low amp load.

Joe · Jan 27, 2012

Russf said:
I have researched inverters and know that they convert DC to AC current. Now if a boat has AC outlets but no inverter does that mean that the AC only delivers current when hooked up to shorepower and that if you want AC while underway that you need an inverter?

Thanks

Yes

LloydB · Jan 27, 2012

Bill your statement:
#3) Provides more AH from the same setup as the inverter is inefficient (90% ish) but more importantly the high currents an inverter needs cause the batteries to actually operate in a mode where they have less AH. Basically a batt used for a high amp load will deliver fewer AH than the same batt powering a low amp load.
Do you mean that if I draw some battery (X) down at a 5 amp per hour rate for 9 hours thereby reaching 50% state of charge, then if the draw down had instead been a 15 amp per hour rate then a 50% SOC would exist before 45 amps had been used? What formula do you use to determine how much?
thanks Lloyd

Maine Sail · Jan 27, 2012

LloydB said:
Bill your statement:
#3) Provides more AH from the same setup as the inverter is inefficient (90% ish) but more importantly the high currents an inverter needs cause the batteries to actually operate in a mode where they have less AH. Basically a batt used for a high amp load will deliver fewer AH than the same batt powering a low amp load.
Do you mean that if I draw some battery (X) down at a 5 amp per hour rate for 9 hours thereby reaching 50% state of charge, then if the draw down had instead been a 15 amp per hour rate then a 50% SOC would exist before 45 amps had been used? What formula do you use to determine how much?
thanks Lloyd

Lloyd,

This is called Peukert Effect...

A 100Ah battery at the 20 hour rate is rated to deliver a 5 amp load, for 20 hours, before dropping to 10.5V/dead.

The formula is; 20 hour Ah capacity divided by 20.

100Ah Batter / 20 = 5A

or

125Ah Battery / 20 = 7.5A

Any draw above 5A will result in less than 100Ah out of the battery before hitting 10.5V. The higher the load, further away from 5A, the less Ah's you'll have...

Any draw below 5A will result in slightly more than 100Ah's from the battery before hitting 10.5V...

This is why it is sooooo critical to properly program a battery monitor with your manufacturers Peukert value, if you want it to be close to acurate..

mitiempo · Jan 28, 2012

Russf said:
Thanks to all. The responses were very helpful. A follow up question. When you see a sailboat advertised with say a 30 HP diesel and a nice suite of electronics but no inverter how does it power those electronics when underway? Several noted having a generator for AC current when underway. Does that imply a separate generator or do engines sometimes also have a generator powered by the inboard diesel?

Thanks again.

They are DC powered.

Items on a boat that need AC are water heater, any space heaters, microwave, electric kettles and toasters, and battery chargers. Navigational electronics and lighting is DC, as is refrigeration - batteries are charged by the AC charger when plugged in and alternator and solar/wind gen when away from the dock.

Inverters are typically used for AC outlets only when away from the dock - small appliances and power tools. Never the water or space heaters, or charger.

Bill Roosa · Jan 28, 2012

Think of a sailboat as a car and you'll get it. All the lights and motors are 12 volt. But it is a really big car so you might want to run a blender. There are 12 volt blenders at RV outlets but most of us just use a household one and an inverter.

The Peukert effect has a formula 1) complicated and 2) not very accurate. You can make some assumptions that simplify your SOC calculations though. If you watch the ammeter for a while you will note that it tends to stay in on place with occasional high current loads of short duration. For SOC calculations you can ignore the perkert effect of the high amp short duration loads as you are not talking about many AH. Once you can determine an average amp usage you can calculate the C factor and know how many AH are actually in the bank at that rate of discharge.

Benny17441 · Jan 28, 2012

I agree with Bill, the formula generally used to describe the Peuker Effect is not very accurate as it fails to consider a number of variables inherent in temperature and battery chemistry not to mention load induced fluctuations. I also advocate the use of an average figure based on actual experience as a guide to determine the state of charge of your batteries. The use of an average figure will take into account all those variables and neutralize their impact on a final indicative result. I do the same when calculating the fuel burn rate so I do not have to concern myself with "for how long I ran at what RPM". By using the average I can regularly approximate my fuel consumption to 1/4 of a gallon. Calculating electrical consumption is a lot more difficult than fuel consumption so a good approximation may have to do. I believe a good smart battery monitor will provide a better approximation than perhaps we could and it will do it automatically. For the rest of us we can resort to the old time solution of carrying a back up battery or source of recharging.

mitiempo · Jan 28, 2012

And if you use a battery monitor make sure it has the correct Peukert entered for the batteries you have - as well as bank AH.