Question on amperage usage on 4D battery

Thread Starter


Joined Feb 21, 2008
I have two 4D batteries in parallel on my sailboat. I measured my voltage with ampmeter at 12.85 amps then turned on three DC cabin lights and then checked to see the draw on the batteries. They're 8 months old and completely charged. Over the course of 1 minute I watched the voltage drop as in 12.85, 12.83, 12.80, 12.75 etc. Is this normal to see this voltage change with what I would estimate probably 3 or 4 amps being drawn on it?

I'd appreciate any response or suggestions on this.



Joined Apr 26, 2005
I would expect such drops by a load when parallelling two 130 AH batteries.

What is the power rating of each of those bulbs?


Joined Dec 27, 2007
I think it is better to do the math in this situation, then if things aren't adding up in reality ,then there is a problem.

You need to find the power in watts that your bulbs are drawing when the switch is closed. Then, divide your voltage by the wattage, which will give you the current draw. Then, you now have the required current to drive your load. Batteries are rated in ampere-hours, which means that the battery can ideally provide x-amount of amperes for one hour. You can determine the running time of your battery in this mannar.

For example, if you have a 150AH battery and are powering three 24W bulbs on a 12V system. Ptotal=24*3=72W. Current=72W/12V=6A. then, 150AH/6A=25 hours

Of course, there are a bunch of rules and stuff when working with batteries, but this is just the basic idea.



Joined May 16, 2005
Is this normal to see this voltage change with what I would estimate probably 3 or 4 amps being drawn on it?
How are you estimating your current draw? 3 or 4 Amps at 12 Volts is only 36 to 48 watts. How big are your lightbulbs?


Joined Aug 8, 2005
4D batteries are generally 180A/hrs, two in parallel would be ~360A/hrs. They are more of a long-term battery, rather than short-burst high-current such as automotive batteries used for engine cranking.

Although you may loose a few tenths of volt upon turning on the lights, it shouldn't continue at that rate of 100mV per minute (if the batteries are in as good a condition as you think they are). At 100mV per minute, that's 6V in one hour, and I doubt your lights are drawing that much current. I think you need to make a more long-term measurement (10 minutes).


Joined Jul 17, 2007
Well, they're marine deep-cycle batteries. Their construction is different from automotive-type batteries; for one thing the plates are thicker.

Still, if the battery drops below about 12.4V, plate sulfation will begin. This will eventually cause the batteries to not accept a charge due to accumulated gook.

Overcharging batteries will likewise shorten their life. It's not a bad idea to use a "float" charger/maintainer, which will keep the battery fully charged to 12.7v while not overcharging it.

One really good way to extend your battery life is to use high-intensity LED lamps. The old-fashioned tungsten filament bulbs we all know and love/hate expend most of the energy as heat instead of light, and have a relatively short life span of around 800 hours.

In contrast, super-bright LEDs use far less current, generate very little heat, and have a life expectancy approaching 100,000 hours. A typical white super-bright LED might use between 25mA to 70mA current at around 3.7V-4.1V. You could use a couple of them in series with an adjustable current limiter or PWM circuit to get an efficient and adjustable (dimming) source of light.