Break out the slings and arrows you battery monitor lovers. Here, again, is another view.
- I don't like 'em.
- I don't believe they can be REALLY REALLY accurate over time.
- I believe they spawn false feelings of knowing the real state of charge.
- I believe that they represent an unnecessary complication and expense.
- I believe that a calibrated digital voltmeter -- properly used and interpreted -- will pretty accurately guage the state-of-charge (SOC).
- I believe there are a lot of folks, including techies, who'll disagree with the above.
Battery monitors are supposed to act like an automatic calculator of your bank balance. With money
, that's pretty easy. Take out $50, put back $50. Easy to measure and monitor and report.
It's not so easy with energy transfers between your boat's equipment
and your batteries, however. It's not a 1:1 equation, not by a long shot.
First, there's Peukert's Equation. I believe all modern battery monitors attempt to incorporate Peukert's factor in their calculations, adjusting the numbers depending on the rate of discharge and charge.
Next, there are sometimes loads which don't go thru the shunt, and are therefore not seen or counted. Among these could be SSBs, pumps, etc. wired directly to the batteries.
Perhaps the greatest problem is that of unseen and uncalculated losses, i.e., those occuring as a function of the battery chemistry itself. Sandia National Labs has done an excellent job of highlighting some of these, noting that charge efficiency
varies greatly depending on state of charge. Getting from, e.g., 20% SOC to 60% SOC is pretty efficient...well over 90%. However, as the battery SOC begins to approach higher levels, like 80-85%, efficiency drops dramatically, often to less than 50%.
This means that if your battery system is operating in the 80-90% SOC range, it's going to take about twice the power fed to the batteries as they will actually absorb.
Another complicating factor is the rate of charge
. Charging efficiency is actually higher with lower rates of charge than it is at higher rates of charge. However, on a cruising boat due to time and other constraints one wishes to charge the house bank as fast as possible, using a high rate of charge (even though more supplied power is wasted this way).
Got all this in your head
It's enough to make you crazy! And we haven't even spoken of the other variables which can affect energy transfers and storage
Now, take the lowly digital voltmeter.
IF you let your batteries sit awhile after charging with little or no load on them; and
IF you measure voltage at the batteries with a calibrated digital voltmeter;
THEN you will have a pretty good idea of the SOC of the batteries.
For flooded lead acid batteries:
12.6-12.7VDC is nearly full charge
12.4VDC is about 25% discharged
12.2VDC is about 50% discharged
What about loads you can't really turn off? Like the frig? Well, you'll get to know your boat over time. If a few hours after charging (with only the frig running @ 4-6A draw) you're now reading 12.55V, that tells you the batteries are pretty well up.
Yeah, it's approximate. But so, too, is the battery monitor. It just makes you THINK it's more precise.
Notice that nothing in the above has been said about battery capacity
. The voltmeter won't tell you that. The battery could be fully charged, but having lost
half it's capacity thru sulfation, stratification, contamination, etc.
Guess what? The battery monitor won't really tell you that, either! But if you know your boat and you pay attention, you'll be able to guage battery capacity pretty well over time. Hey....I used to be able to go for XXX hours without charging; now I can only go half that time!
Doesn't take a $200-500 device to tell you your batteries have lost
a lot of capacity!
OK....let go the SLAMs :-)