Electricky Dicky wrote:
On Fri, 05 May 2006 15:42:34 GMT, chuck wrote:

Snip

Chuck,
Thanks for the URL above, most interesting.

The author of the site that I quoted does not follow this group these
days, however we had a conversation and the following is copied from a
mail to me

(Start quote from mail)

With battery internal resistance of 0.01R (a typical figure for a
brand new fully charged deep cycle 100AHr battery) the figures come
out at:- (End wired)

Battery share of current

1 44.2
2 26.9
3 17.6
4 13.5

The figure used on the website of 0.02R represent a battery at about
half life. It doesn't matter what figures are used. The end result is
the same. Just the magnitude changes.

With batteries with an internal resistance of 0.05R (i.e. totally
knackered - 100 Ahr batteries unable to support a 50 amp load without
the terminal voltage falling to 10.5 volts) the figures come out:-

Battery share of current

1 29.8
2 25.6
3 22.9
4 21.6

To suggest that the internal resistance of each battery
represents a larger percentage of the total is completely irrelevant.
Each battery's internal resistance is in series with it's own battery
current and NOTHING ELSE.

The resistance of the battery interconnecting leads is in series with
different batteries and each one has hugely differing currents through
it if the batteries are not wired corner to corner.

If one battery is slightly older or more tired than the others then
it's internal resistance will be higher. This leads to that battery
producing less current during discharge and accepting less current
during charge. It is therefore worked gentler. This therefore works
the other batteries harder. This gives a natural balancing effect
between the different batteries in the bank with the result that each
battery is worked by the same amount.

This of course, is on the assumption that the battery bank is wired up
correctly. If it is wired "all from one end" then this doesn't happen
and the first battery is worked harder.
(End of quote from mail)

If you have the opportunity then I suggest doing the actual physical
test (preferably with a new battery set) to see if the numbers crunch
as above.
I am just a simple soul but it seems to me that working a battery set
"equally" is the best way to go.

Regards

Richard

Nb "Pound Eater" Parkend G+S

Thanks again for the thoughtful reply, Richard. More than anyone, and
more than ever, I need and welcome reality checks on what I say!

Starting with the end, I do agree with your sentiment, as I noted in my
initial post on this thread, although I continue to doubt that it is
very important.

The example given on the web site you referenced is really not the
situation posed by the OP.


Granted the OP was asking re 2 x 24v in parallel which is no different
to 2 x 12v in parallel for the calcs


It is probably more instructive (for me at
least) to approach this from the most simple abstraction, to wit, two 12
volt batteries with different internal resistances (0.01 ohm and 0.0115
ohm), in parallel with each other and with a resistive load. The
difference in internal resistances is due to the added resistance of the
connecting wires as we have discussed. We need not know the load
resistance, but we assume the load current is 100 amperes.

Each battery has the same voltage, so setting them equal:

V1 = V2
12 - I1 x 0.01 = 12 - I2 x 0.0115
I1 = 1.15 x I2

Since I1 + I2 = 100,

I1 = 53.5
I2 = 46.5



Ok with this simplistic calculation you are now agreeing that there is
a difference!


I believe this is a pretty insignificant difference in current between
the two batteries. It is in the realm of battery-to-battery differences
in internal resistance and open-circuit voltage at full charge.


Come on, you did a calculation, now you are trying to ignore it by
changing variables, you have seen that there is a difference. If you
had done the calc with 3 batteries you would have seen a greater
difference. Therefore surely always connecting a set cross corners is
the correct method, whether it be 2 3 or 4, all it takes is a couple
of feet of cable.


Moreover, since a 100 ampere load can reasonably considered a worst
case, in practice, and for most of their operating lives, these two
batteries will experience an even lower imbalance than that suggested by
the above analysis.

On the other hand, if I've overlooked something or made one of my
notorious arithmetic mistakes, please let me know.

Chuck


As it happens I am replacing 4 x 230Ah (24v 460Ah set) batteries next
week. These are currently wired off one end. I will do a test with a 2
or 3kw load after installation, then again after I change to cross
corners. Interestingly it is the current first pair that have failed
after 5 years ;-)

BTW just to be really really pedantic, you suggested in another post
that 0.040A over 24hrs was 1AH. Sorry no, its more like 0.24Ah
assuming a 20hr 100Ah battery. Do not forget that at low currents Mr
Peukert is your friend ;-)

Thanks for the discussion, nice to chat with no flaming and snide
comments.

Best regards

Richard
Edelec - Design and Manufacture of Control Systems
Boat Electrical repairs and Installations.

Sorry about the misunderstanding, Richard. I've always acknowledged that
the farther battery will see the additional resistance of the connecting
wires. My position has always been that the imbalance caused by that
difference is insignificant in practice a) because it is relatively
small, even at high currents, and b) because it is swamped by battery
differences at more commonly seen low currents.

I understand your point about Peukert and the manner in which batteries
are rated for capacity. Nonetheless, one AH removed from the battery is
exactly that, no matter how the battery is rated.

Yes, the chat has been a pleasant one. Thanks again for your comments.

Chuck

chuck wrote:


Sorry about the misunderstanding, Richard. I've always acknowledged that
the farther battery will see the additional resistance of the connecting
wires.

simple pleasures seeem exotic

My position has always been that the imbalance caused by that
difference is insignificant in practice a) because it is relatively
small, even at high currents, and b) because it is swamped by battery
differences at more commonly seen low currents.

for what happens next,in a strange way i feel exhilaration more then
fear


I understand your point about Peukert and the manner in which batteries
are rated for capacity.

this must be it,now i understand you being a poet,you know????

mk5000

"Hey you know what you remind me of the sun that shines above I think
that I'm in
Love cuz every time I look in your eyes they be dumb bright like the
sunlight when I look in The sky and you always look fly, you stay with
a mean outfit and got shoes that didn't even Come out yet."--cassidy,
get no better