
ContinuousWave Whaler Moderated Discussion Areas ContinuousWave: Small Boat Electrical Calculating Battery Capacity in AmpereHours

Author  Topic: Calculating Battery Capacity in AmpereHours 
brainstormer 
posted 09212010 04:48 PM ET (US)
How do you [calculate] how much battery is used up in X hours of use? [A particular] machine uses 12Volts at 4.2amperes, which it presently gets from a transformer from 120VAC . A good sized battery would stand how many hours of 4.2ampere use? 
Hoosier 
posted 09212010 05:12 PM ET (US)
Divide the battery capacity in amperehours by 4.2, then divide by 2. You don't want to use more than 50% of your capacity. If that doesn't get you where you want to go then hook up a second, identical, battery in parallel so you double the amphour capacity, but keep the system at 12 V. For this application I'd get good deep cycle batteries. 
Chuck Tribolet 
posted 09212010 05:31 PM ET (US)
There are three (at least) types of inverters: Square wave. This is either full positive, then full Modified sine wave: This is full positive, then zero for a True sine wave. This is a graceful curve from full positive Chuck 
AZdave 
posted 09212010 06:02 PM ET (US)
I'm probably displaying my ignorance, but could you simply make up a line to run the machine off your existing 12Volt battery? That would save losses in the inverter and transformer. 
jimh 
posted 09212010 08:22 PM ET (US)
You can run a 12volt DC device from a 12volt battery. This is much more efficient than using a battery to create 120VAC via an inverter, then running an ACoperated power supply to make 12volt DC. To do that would be very inefficient. The capacity of a battery is specified in its amperehour rating. The amperehour rating is specified at a particular discharge rate. Generally the smaller the discharge rate the longer the battery will last. This looks like a good article on the topic: 
jimh 
posted 09212010 08:48 PM ET (US)
An example: A battery is rated at 42amperehours (Ah). How many hours can it run a device that draws 4.2amperes? A battery of 42Ah could theoretically supply 4.2amperes for 10hours. In actual practice, the performance would depend on other factors, including the temperature. The 42Ah rating assumes the discharge rate is for 20hours, typically. Therefore, to reach the 42Ah battery rating, we would have to only draw enough current to force a 20hour discharge. Such a current would therefore be 42amperehours / 20hours = 2.1amperes Since the load is double the rating, we would find that the battery would not provide its rated 42Ah performance. A load of 4.2amperes will discharge a 42Ah battery in less than 10hours. Exactly how much less would be determined from a curve provided by the battery manufacturer. 
jimh 
posted 09212010 08:54 PM ET (US)
If asking about battery capacity while the battery is being charged by an outboard motor, most outboard motors have charging current capacities greater than 4.2amperes. Thus you could run a machine that consumed 4.2amperes from a 12volt battery as long as you wanted while the outboard motor was running and charging the battery. 
David Pendleton 
posted 09222010 11:26 PM ET (US)
Also keep in mind that AC appliances draw ten times their rated amps when running on 12V (using an inverter). 
brainstormer 
posted 09242010 11:21 AM ET (US)
OK, the machine in use is a Cpap machine, helps momma breathe and needs to last 810 hours every night. I have an Optima battery rated @ 55ah . I could tie in another battery if necessary. The machine right now, in the house, uses a transformer from 110v to 12v (its not here, so I can't tell if its AC or DC) @ 4.2 amps of useage. On the boat, I can recharge either/both batteries during the day, but it'll be needed every night. I just want to make sure (not much of a math person) that it'll do the job without killing machine or batteries. 
jimh 
posted 09252010 10:08 AM ET (US)
If you want to be certain to be able to operate for 10hours from battery power a machine that draws 4.2Amperes at 12volts, you should use a battery with about double the calculated ampere hours needed. The calculated ampere hours are 4.2 Amperes x 10 hours = 42Amperehours A battery with double that rating would have 2 x 42Amperehours = 84Amperehours A battery with a rated capacity of 84Amperehours will typically be in the battery size Group24 or Group27. Of course, you will want to use a battery designated as a deepcycle battery. Each overnight 10hour period of discharge will be pulling the charge on the battery to 50percent or lower. A battery designed for deepcycle use will tolerate this sort of chargedischarge cycle much better than a starting battery or a marine battery. Another good source of information on deepcycle battery operation can be found in this FAQ: Deep Cycle Battery FAQ If you are intending to recharge the battery from an outboard motor, you must consider the time needed to fully recharge the battery. In my own use of my boat, I often see that my daily time underway with the engine running is often only three hours. Using three hours as a typical running time, we will need to recharge the battery completely during that time. Since we used 42Amperehours of charge, we must restore all of that charge. Because we will only have three hours of charging, the average charging current must be 42Amperehours / 3hours = 14Amperes It is typical that the efficiency of the battery at accepting a charge is not 100percent. The precise factor for efficiency of charging will be determined by many factors, such as the battery itself, the temperature, the charging voltages, and so on. However, we should consider that the efficiency might only be 70percent, that is, for every 10Amperehours of charging current, the actual charge on the battery increases only 7Amperehours. We apply this to our average charging current for the three hours of charging we will have: 14amperes / 0.7 = 20Amperes We now have to consider that in most situations the charging current will not be uniform during the charging period. Typically as the battery charge begins to reach fullcharge level, the charging current will taper off. The exact curve of charging current as a function of battery state of charge is not known, and it will, again, depend on several factors such as the battery, the charger, the temperature, and so on. We should assume that we will need a much higher charging current during the initial phase of charging if we are going to maintain an average charging current of 20Amperes over the three hours. Here we will use a factor of two, and we will need a source of charging current of 40Amperes if we want to expect to be able to restore all of the charge to the battery. A charging current of 40Amperes is a substantial charging current, and it is more than most outboard motors can supply. Many popular outboard motors can only supply a net charging current output of 10Amperes or less. It would not be reasonable to expect that an outboard motor with limited charging current will be able to replace the charge lost in 42Amperehours of discharge current in only three hours. 
jimh 
posted 09252010 10:19 AM ET (US)
Another consideration in charging batteries is the rate of charge. One source suggests that the rate of charge should be limited to the battery capacity at the 20hour rate divided by 8. For our hypothetical 84Amperehour battery, the recommend limit for the charge rate is therefore 84/8 = 10.5Amperes If we limit the charging current to 10.5Amperes, we can assume that this will taper over time, giving an average of only 5.25Amperes charing. If we reduce the charge by our factor of 70percent efficiency, we will obtain only a net average charge of 5.25 x 0.7 = 3.675amperes Thus to restore 42Amperehours of charge at a net average rate of 3.675Ampere, we should plan to have the battery on the charger for 42Amperehours / 3.675Ampere = 11.4hours That is a lot of time underway with the outboard motor running on a daily basis. Cf.: http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Charging 
jimh 
posted 09252010 10:21 AM ET (US)
If this machine uses 4.2Amperes from a 120Vac source, it would not be reasonable to operate this machine for 10hours from a battery. You would need something like 840Amperehours of battery capacity. A battery of that capacity would be so large and have so much weight that it would not be practical to install in a small boat. 
Hoosier 
posted 09252010 03:08 PM ET (US)
According to this info from Interstate batteries http://www.interstatebatteries.com/cs_estore/content/product_info/ marine_f.asp all deep cycle/cranking 12v batteries can do the 5 amp load for 10 hours. It would have been more useful if they had listed the same parameters for each battery. 
brainstormer 
posted 10052010 09:14 PM ET (US)
sorry for taking so long...doctor visit again. I have no problem hooking 3 or 4 55ah batteries or better together to secure the discharge and once in dock, I can put the batteries on a land based charger. wouldn't that fairly well cover the few overnight trips? I appreciate all the knowledge sharing Jim, I'm just hoping not to have to get a generator and ruin the night. 
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