Battery Packs, Parallel connection, current load balancing?

[Jon.boston]

I'm looking at connecting three battery packs in parallel to get more
current carying capability. If each battery has a maximum current
draw of 4 amps is there a standard circuit to use to help with current
sharing? I'm worried that all the current will come from the pack
with the higher voltage.

I'm looking at getting something like 9 Amps from the three packs in
parallel?

Battery packs are li-ion with saftey circuits.

Thanks,
Jon

 

[Winfield Hill]

Jon.boston wrote...

I'm looking at connecting three battery packs in parallel to get more
current carying capability. If each battery has a maximum current
draw of 4 amps is there a standard circuit to use to help with current
sharing? I'm worried that all the current will come from the pack
with the higher voltage.

I'm looking at getting something like 9 Amps from the three packs in
parallel? Battery packs are li-ion with saftey circuits.

If you use three high-current Schottky diodes for current sharing, that
should work well. For example, the TO-220-package MBR1545 or MBR1645
diodes drop about 0.45V at 3A each. Some larger parts do slightly better.
E.g., a 30CTQ045 has two diodes in a three-lead TO-220 package. With two
sections in parallel, you get a lower roughly 0.4V drop at 3A per package.

You can do even better with FETs and comparators for current-sharing.

If one battery is more charged than the rest, it'll deliver more current
but this will stop as it approaches the same charge state as the others.
In the meantime its internal resistance and resistance of the current-
sharing diodes will help equalize the currents sooner rather than later.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[James Meyer]

I'm looking at connecting three battery packs in parallel to get more
current carying capability. If each battery has a maximum current
draw of 4 amps is there a standard circuit to use to help with current
sharing? I'm worried that all the current will come from the pack
with the higher voltage.

I'm looking at getting something like 9 Amps from the three packs in
parallel?

Battery packs are li-ion with saftey circuits.

Thanks,
Jon

You don't have to do any additional balancing for batteries in parallel
as long as the batteries are all the same type.

Once they're in parallel, NO battery will have a higher voltage than any
other.

Jim

 

[Mac]

I'm looking at connecting three battery packs in parallel to get more
current carying capability. If each battery has a maximum current
draw of 4 amps is there a standard circuit to use to help with current
sharing? I'm worried that all the current will come from the pack
with the higher voltage.

Not for long. ;-)

I'm looking at getting something like 9 Amps from the three packs in
parallel?

Battery packs are li-ion with saftey circuits.

Thanks,
Jon

AFAIK you don't need to do anything special to discharge in parallel.
There are battery groups that know more about this question. I think there
are, or can be, issues with charging in parallel. Because you don't
actually know what the current into each individual pack is.

regards,
Mac

 

[Jerry G.]

We've assembled large gel battery systems using batteries in parallel. The
key is to purchase matched batteries from the manufacture. This way there is
even loading and charging of the batteries.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


I'm looking at connecting three battery packs in parallel to get more
current carying capability. If each battery has a maximum current
draw of 4 amps is there a standard circuit to use to help with current
sharing? I'm worried that all the current will come from the pack
with the higher voltage.

I'm looking at getting something like 9 Amps from the three packs in
parallel?

Battery packs are li-ion with saftey circuits.

Thanks,
Jon

 

[mike]

I'm looking at connecting three battery packs in parallel to get more
current carying capability. If each battery has a maximum current
draw of 4 amps is there a standard circuit to use to help with current
sharing? I'm worried that all the current will come from the pack
with the higher voltage.

I'm looking at getting something like 9 Amps from the three packs in
parallel?

Battery packs are li-ion with saftey circuits.

Thanks,
Jon

LiIon cells are routinely hooked directly in parallel inside laptop
battery packs. Seems to work ok as long as they're matched.
So, what's your definition of "parallel"? If they're hard-wired,
you shouldn't have any trouble if you start with matched cells.
If you have swappable packs that can be inserted in any state of
charge, your problem is isolation more than sharing. You have to
add some (variable) voltage drop to the highest voltage pack so that
it delivers only 3A at the voltage of the lowest pack. You don't have
a lot of headroom between the 3A and 4A numbers. The most important
spec might be the internal resistance of the cell pack (over it's
lifetime). Every bad Lithium laptop pack I've seen has failed due to
high internal cell resistance. If you want long battery life, you need
to make sure your circuit will work with lots of battery internal
resistance. Diodes may provide isolation, but may not provide enough
resistance to keep the max A/pack below 4A (over the useful life of the
pack).

And you have the same problem charging. Have to keep the max charge
current below the max rating at each cell. May be an issue for
high rate charging.

What are you doing to keep the temperatures down? I've dissected
several laptop packs. Seems that there's often only one pair of dead
cells. They're the ones nearest the hot part of the laptop. So, even
if you start with matched packs, they may not stay that way for long.

Where did the 4 amp max current come from? The 75% of the ABSOLUTE
maximum number may be horrible for cell life. A pair of 18650s is
typically used in a laptop that idles at 800mA or so. Even then, life
ain't that great.

Sure would be nice to start with bigger cells.

mike


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