Does it make any difference how you put parallell

[AilSnail]

Option 1: put two and two cells in parallell, then put all the little two-packs in a long strin.

Option two: make two long strings of cells in series, and then parallell the two strings.

Does it make any difference?

 

[php_44]

Most packs of cells I've seen (from AA's to 800Ah lead acid batteries) are long strings only paralleled at the ends. I suppose this limits the current through any one cell to that which is flowing in series through the cell stack. If you paralleled at every cell, then it's possible for one weak cell to bear double it's rated maximum current. That could cause excessive heat and possibly a spectacular failure in the case of high current cells.

 

[evan9162]

Option 1 balances the voltages in the pack better (two long strings will be more likely to have voltage differences at the ends where they are connected)
Problem is that one cell in the pair will tend to pull the other one down, it also makes charging (if they are rechargables) less precise, as you only know when 1 cell has peaked (though many 8-cell chargers charge/check voltage in pairs only)

Option 2 allows you to switch the packs off from each other when not in use (prevents one pack from draining the other due to voltage differences), charging each string separately will result in better charge characteristics, and (depending on physical layout), it may be more flexible in terms of layout/battery placement, and/or the ability to run on only 1/2 the batteries.

I would say being able to switch the two strands off from each other (when not in use) would be a compelling reason to go with the two independent strand approach.

 

[LEDmodMan]

Also, this is a valid question when running Luxeons in parallel.

I'm not completely sure what the answer is myself, so instead of saying something incorrect, I'll pose the question to others more knowledgable than myself in this area.

What would be the advantages/disadvantages to each configuration when running Luxeons in parallel?

Based on what I know, I would think option #1 would be better for LS's (Luxeon uses this method on their flood parts).

 

[Harrkev]

Re: Does it make any difference how you put parall

If it was me, I would use two strings of batteries, and then put a diode on each string to keep from having a stronger string charge a weaker one. Use a diode with the lowest forward voltage drop possible (probably a germanium). You don't have to do this, but I am paranoid /ubbthreads/images/graemlins/ooo.gif

 

[php_44]

Re: Does it make any difference how you put parall

[ QUOTE ]
Harrkev said:
If it was me, I would use two strings of batteries, and then put a diode on each string to keep from having a stronger string charge a weaker one. Use a diode with the lowest forward voltage drop possible (probably a germanium). You don't have to do this, but I am paranoid /ubbthreads/images/graemlins/ooo.gif

[/ QUOTE ]

Good idea about the diode /ubbthreads/images/graemlins/thumbsup.gif, however I'd recommend a schottky diode. They are low forward drop, and are available at high amperage. They can have more leakage than silicon, so look at the specs for a particular diode to make sure you're happy with it. Germanium diodes also have low forward drop, but I don't see them around much these days and have never seen a high current version in person.

 

[EvilLithiumMan]

Re: Does it make any difference how you put parall

The problem with using a diode is that now each string has to be charged independently; there won't be any one point you can connect a charger to that will allow you to charge the whole pack at one time.

A common practice when working with large numbers of cells in series/parallel is to add a resistor to each string which is 2 to 3 times the impedence of the string. So if your strings had an impedence of 5 milli-ohms, you would add a resistor to each one of somewhere in the range of 15 to 20 milli-ohms. This helps to ensure (linearize) that each string has approximately equal impedence and will share the load equally. Indeed, a cell's impedence will change somewhat over it's useful lifetime. The resistor helps to minimize this effect.

 

[MrAl]

Re: Does it make any difference how you put parall

Hello there,

If i was going to do this i would use two parallel strings
if i was also going to run the LED(s) using series resistors.
That way i could use two resistors (instead of 1) each of
twice the required value (as for one string) with each one
in series with one of the strings. Each string would then
be supplying half the current for the LED(s).

For example lets say we're using two strings of 4
alkaline batteries. Each string comes up to 6 volts.
Lets also say we want about 270ma flowing though
one LS LED, which itself runs at about 3.3 volts.
Now one string alone would require 10 ohms, but
we want to parallel two strings, so we use 20 ohms on
each string and connect the two free ends of the two
resistors together and connect the LED there.

This gives us plenty of isolation between strings yet
allows us to parallel the two strings rather safely.

If i was using rechargeables instead of alkalines i would
do basically the same thing, except the resistor values
would come out a little less because of the reduced
voltage.
To charge the two strings, they really shouldnt be in
parallel anyway, but it might work to leave the two
resistors in place and replace the LED with a charger with
a slightly high voltage then usual to make up for the
loss across the resistors, or simply make a charger with
two outputs and connect the two strings independently
for charging only.

Another thing you could do is check the current between
strings after you connect them to see if one string is
trying to charge the other string. This will occur if
a cell in one string is low compared to the other string.
For this the Schottky diodes would work out pretty well.
At least with the diodes if one cell goes low the other
cells dont get 'charged'. This will be a problem if using
alkalines because the other cells will go bad also, but if
working with NiCds it probably doesnt matter much.

Take care,
Al