So why are batteries spec'd by the manufacturer to 80% DOD? Doesn't total Ah delivered over service life enter into the equation?
Mark K5LXP
Albuquerque, NM
Good question, and I don't know the precise answer. I chalk it up to marketing.
But if you follow the money...long cycle life for you, isn't really what the manufacturer
wants, but they'll never say that. What they want, from a cash flow perspective, is for
you to buy batteries more often. They're in business to sell batteries, not to have yours
last a long time. Success in business is all about revenue and margin, and that requires
sales volume. One other thing...never trust or believe a battery sales person. Very few
of them have any clue what they're selling, or how it works. But they all know how to
cheerfully ring up a sale.
As for total Ah over service life, I've never seen that on a battery spec sheet. If you have,
I'd like to know where to find it, because that would be interesting to know. As you have
mentioned before, it is totally reasonable for the battery capacity to degrade as the battery
ages.
With my portable setup, I don't think I've ever seen this charge controller go into bulk
charge mode. The charge controller was in definitely in absorption mode when the direct
sun first hit the panels in the morning. This is based on the output voltage coming
from the charge controller, and the amount of charging current the battery box was showing
on its bidirectional ammeter. By 1PM, the controller was in float mode, with it's output voltage
throttled back to about 13.8 volts, and the charging current a little below 1 amp. We kept
operating until 2PM local time, which is 1800Z. Once I got everything taken down and put
away, it took another hour of float charge in the house for the charging current to reach
zero, indicating a full charge on the batteries.
Also note, I'm using cheap rigid amorphous panels. Those don't need maximum solar
intensity to generate enough power to charge a battery, although like everything else,
more is usually better. Having used this setup for the last 3 Field Days, I've seen enough
output from these panels to charge batteries until about 45 minutes before local sunset.
I do make it a point to reposition my panels several times during the day to better face
the sun. By evening, they're pointing almost due west. Total daylight in northern Ohio
at the summer solstice is about 14 1/2 hours. Field Day around here usually sees sunrise
about 0630, and sunset around 2100 local time. The downside, is that the panel surface
area I need to generate 100 watts of solar is about double what I'd need if I had the
more efficient (and expensive) monocrystalline panels. A good 100 watt monocrystalline
panel from someone like Renogy is about half the size, and half the weight of my setup.
If the sky is cloudy, I still get enough output from the amorphous panels to charge the
battery, but not as much as if it's sunny. If the overcast is heavy, things get kind of tough,
although it will still float charge. A better MPPT charge controller would help too, because
my PWM controller is only about 75% efficient at best.
It's loads of fun to watch the whole setup work like it does. I get a big kick out of it.
I'm a bit excited to do the planned upgrades to my system though. But that has to wait
for available cash flow. Because I flat out refuse to incur indebtedness to pay for my toys.
73 de N8AUC
Eric