In the last chapter I explained Ohm’s Law, that wonderfully elegant and simple relationship between three variables that allows us to clearly understand just about any problem we have with our boat’s electrical system.
Now let’s look at some practical applications: how batteries charge and voltage regulators work—two of the most misunderstood pieces of gear on a voyaging boat.
To make this easy, we are going to use lead-acid batteries in all or our examples. Yes, I know, you want lithium. That’s fine, we will get to those in a later chapter and the stuff we learn in this chapter will make understanding lithium systems way easier.
First we need to do one of those algebra trickery things (that we all slept through in school) so Ohm’s Law will get us the answer for any one variable as long as we know two others:
That is:
- Amps equal volts divided by ohms.
- Ohms equal volts divided by amps.
- Volts equal amps multiplied by ohms.
Real World
Now let’s say:
- Our system is 12 volt.
- Our lead-acid battery bank has 400 amp hours capacity and is half discharged.
- We are sailing along using 20 amps for various loads.
- We have 200 watts of solar panels and it’s sunny so they are putting out 100% of their capacity—unlikely, but it doesn’t matter for our purposes.
- We have a 100 amp alternator on the engine.
- Both the solar panels and the alternator have regulators.
Well done, John. I think this is going into the log book to edify the rest of the crew.
Hi John
Good article.
There is one thing that I do not understand. Where du you get the 120A from in point four under “Get charged”? Or ist this rounded up from 116.66A?
Hi Enno,
Woops! I rounded up, but should not have as it makes things confusing. I will fix it. Thanks very much for the correction.
Hi John
A small point. Watt hours or kilowatt hours are a measure of the amount of work done or energy expended. Power is measure of work done in a period of time or Work divided by Time. So multipying it by Time , the two “Times”cancel out and you are left with Work.
Hi Bryant,
You are, of course right…although I have to admit that I had to look it up. However, given our goal here to only burden people with what they need to know to manage their electrical system I think I will leave it as it is, rather than introduce another term.
Hi John
I was just trying to help your Science teacher to stop revolving by pointing out a small thing that in no way detracts from a really useful Article . You are right that demystifying seemingly unnecessarily complicated electrical systems by the application of some simple science basics will benefit us all.
Thanks for the valuable article
Hi Bryant,
No worries, I just didn’t want the discussion to veer off into the more esoteric aspects…particularly since I might get out of my depth!
I would vote to fix it because it is at the root of some other misconceptions. In the true engineering sense, kW-hr is a ridiculous term, similar to saying I filled up my boat with 20 gals/hr-hrs at the fuel pump. It’s just gallons. For energy, it’s just joules (or kJ or MJ). A joule is a Watt-second so that is the right term, but I acknowledge that multiplying/dividing by 3600 is hard and kW-hrs are here to stay so…
But the other confusions driven by this lack of understanding are things like “I need 100 kW-hrs but I only have a 50 kW engine so I have to get a bigger engine” and other foolishness. Right up there with people saying they have a “500 amp battery” when they mean amp-hr. But power as a *rate* of energy flow is just as important a distinction as amps vs. coulombs, therefore my vote.
Great site!
Hi John, I know it would add a small complication to the equations, but I think you’ve assumed the solar panels are operating at 100% efficiency. In reality, its about 80% I think.
Hi Andy,
I wrote” We have 200 watts of solar panels and it’s sunny so they are putting out 100% of their capacity—unlikely, but it doesn’t matter for our purposes.”
Hi All,
A gentle reminder of what I said at the bottom of the first ohms law post: