snip

Four 6 volt golf cart batteries in series parallel will give you about
440 amp-hours of capacity but in actual practice it is a bad idea to
discharge below the 50% level since that will greatly shorten batter
life. You can get a rough idea of discharge level by looking at a
digital voltmeter when the batteries are under load. 12.6 volts is
fully charged, 11.6 volts is about 50%, and 10.5 is fully discharged.

I thought 12.2v was 50% charged (12v lead acid battery). No idea were I got
that idea from, but it would explain why I struggle to keep my batteries
above it. I have 4 x 12v 135amp batteries connected as two pairs of 12v
batteries wired in series then wired together in parallel, to give 24v.
Something I have often wonder about is if I should connect them to the
electrical system with both positive and negative coming off one pair, or
the positive off one pair and the negative off the other.

"krj" wrote in message
...
4. If you re-charge your discharged 12v battery using a 48watt
(power: watts = volts x amps) solar panel, you should get 4
amps out of it (48/12), and if it keeps up its 4a (which it
won't) you'll get 4 x (the number of hours) of AH back into the
battery.

A 100 amp hour battery. 100/5amps = 20 hours. Yes. 100/50 = 2
hours.
NO! Maybe 20 minutes.

Meltdown!

Well, he was talking in the context of solar panel re-charging.

JimB
http://www.jimbaerselman.f2s.com/sw_peloponese.htm

the math is easy enough to figure out, a lot of people gave you some
good information on this thread. for my own deep cycle systems i've
noticed it never works out like the math says it should though, never,
not in actual practice. you kind of imagine all these perfect numbers,
12 volts and 100 amp hours and that means you can do this task for
such-and-such a time and it'll take your solar cell such-and-such time
to recharge and all this. and there are even charts that give you an
idea of the danger areas for depth of discharge, that is, discharging
the batteries too much and too often, and how many times you can go to
certain levels before you have to get new batteries, etc. my
experience has been that in the end the math is ok for like a really
general idea, but that's about it. the math quickly turns into
non-sense in the face of harsh reality. in actual practice your
battery bank is always in some weird state of charge that's hard to
measure because it varies so much with temperature, battery age, what
angle the batteries are sitting at, how much water you put in it last
time you serviced them, etc. and who knows what your solar panels are
doing depending on time of day, cloud cover, etc, and the same for your
wind generator, and even your alternator, none of it has very much to
do with what it's rated at, or what the specifications claim it's going
to do. then your loads are so variable too, which lights will you have
on at which times ? how often is the compressor going to flip on in
your refrigerator ? how many amps is your starter going to draw ?
does humidity affect any of it ? etc. in reality it's a crap shoot,
and nobody really has any idea what their system is doing most of the
time, and it's even hard to measure the voltage on the batteries
because power is going into them from the solar panels and things most
of the time, even on shady days. and even if you could measure the
battery banks voltage it doesn't give a real measure of the state of
charge. so you design it the best you can, fit in as many batteries as
you have space for, try to minimize how much power you are wasting, try
to keep everything maintained (especially your batteries), try to put
in as much power generation (solar and wind) as you can, then cross
your fingers and hope for the best. i mean, really, does anyone really
have any idea how many amp hours are in their battery bank right now ?
lol. yes, yes, i know, your battery power meter tells the whole truth
G. my personal experience has been that it's a bunch of voodoo.

In article dz7Df.1473$UY3.1361@trndny05,
"Thomas Wentworth" wrote:

I get so confused when it comes to electric power, especially battery
output. How many AMPs can a battery put out before the battery is dead?
Say, a 12 volt battery. Could you put this as .. "if you have on a light
for xxx hours the battery is gone".

I know AMPs are the flow of power and volts is the possible amount ..
measure of flow and time . and light bulb.

Voltage is the pressure, amps is the volume per time-period. Amp-hours
(AH) = the total volume.

Why am I asking? I was looking at this boat on a web site and the owner had
added a bank of small batteries. About half the size of the marine ones I'm
use to. The batteries were tied together so they became on big battery in
power possibility.

Now,, if he had 6 batteries and each one is an 8 volt .. does this mean he
has 8x6 = 48 volts of possible power? And how does this work out to amps. ?

I suspect they're small 12v, possibly 6, unlikely to be 8. Others
covered the series-parallel stuff.

batteries are heavy. how many do you carry. wouldn't a bunch be bad for
the boat??

also.. solar power.. how much power does one of those things give you?

These last depend on your usage, though I almost guarantee that one
large battery will do better than a "bunch" of little ones.

We have simple needs, so are comfortable with a group 31 (120 AH) as our
primary, with another kept in reserve. Our 12w solar panel keeps up with
our needs most of the time and can bring the battery up enough to start
the engine by mid-morning. When we're away for a few days, the panel
brings the battery to full power, which is a couple of days' normal
usage.

When we next upgrade, I'm going to move the better 31 to right next to
the engine and "hardwire" it to the starter with short cables; No
switch, though it'll be fused of course. The house bank --probably two
6v-- will primarily be solar charged, though we will be able to parallel
it to the starting battery for a fast recharge or extra starting power.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/