Battery question - two 7.2 V packs to get 14.4 V

Werehatrack wrote: (clip) If a higher-current charger was obtained, the two
would almost certainly not charge at the same rate...thus one would
begetting too much current, and one too little, to charge
properly.(clip)^^^^^^^^^^^^^^
Jasper wrote: And *that* is the real problem.
^^^^^^^^^^^^^^
It may not be as big a problem as it seems. If the two batteries are
reasonably well matched, and they are connected in parallel to a charger
which provides a voltage a little higher than the batteries, each will draw
some current, in accordance with their internal resistance and voltage
difference. The one which draws more current will charge more rapidly, so
it will taper off more rapidly, and they will eventually both reach a fully
charged state. This is a situation where the unbalance is self-correcting.

On Sun, 27 Nov 2005 02:48:15 GMT, "Leo Lichtman"
wrote:

Werehatrack wrote: (clip) If a higher-current charger was obtained, the two
would almost certainly not charge at the same rate...thus one would
begetting too much current, and one too little, to charge
properly.(clip)^^^^^^^^^^^^^^
Jasper wrote: And *that* is the real problem.
^^^^^^^^^^^^^^
It may not be as big a problem as it seems. If the two batteries are
reasonably well matched, and they are connected in parallel to a charger
which provides a voltage a little higher than the batteries, each will draw
some current, in accordance with their internal resistance and voltage
difference. The one which draws more current will charge more rapidly, so
it will taper off more rapidly, and they will eventually both reach a fully
charged state. This is a situation where the unbalance is self-correcting.

Except it's not. It'll kill your batteries in fairly short order. I'm not
entirely sure what the mechanism is, but that's both received wisdom and
my experience.

Jasper

Jasper Janssen wrote:
"Leo Lichtman" wrote:

It may not be as big a problem as it seems. If the two batteries are
reasonably well matched, and they are connected in parallel to a charger
which provides a voltage a little higher than the batteries, each will draw
some current, in accordance with their internal resistance and voltage
difference. The one which draws more current will charge more rapidly, so
it will taper off more rapidly, and they will eventually both reach a fully
charged state. This is a situation where the unbalance is self-correcting.

Except it's not. It'll kill your batteries in fairly short order. I'm not
entirely sure what the mechanism is, but that's both received wisdom and
my experience.

I think at least one issue is that the battery which is in
worse shape will usually have higher internal resistance,
so more current will flow through the better battery.
However, I don't believe that its internal resistance
increases much (if at all) as it reaches a charged state,
so the two batteries don't equilibrate. In the meantime,
you have the good battery which is capable of delivering
7.2 volts (or whatever) and the worse battery which is
at a slightly lower charge and voltage. But these two are
tied together in parallel, so they're pinned to the same
voltage and you may actually be discharging the good battery
through the worse one. Lord knows what dV/dt a smart charger
is measuring off this setup. It seems like a false economy
to me.

Jasper Janssen wrote:

Except it's not. It'll kill your batteries in fairly short
order. I'm not entirely sure what the mechanism is, but that's
both received wisdom and my experience.

The reason is the voltage characteristics of NiMH cells as they
charge. The voltage increases slowly as the cell charges, and
then drops a little as full charge is reached. In a parallel
charge situation, the one to reach full charge first then gets
all the charging current (plus some more from its undercharged,
higher-voltage neighbour).

John

"John Henderson" wrote: (clip) In a parallel charge situation, the one to
reach full charge first then gets all the charging current (plus some more
from its undercharged, higher-voltage neighbour).
^^^^^^^^^^^^^^^^^
I can see no way that one battery can supply current to the other while
they are both hooked to an outside source that exceeds their voltage. How
can the higher voltage battery supply current to the lower one, while at the
same time drawing current from the charger? Current can't flow both ways
through the battery at the same time. If you left the two batteries hooked
together in parallel and disconneced them from the charger, this would be
possible, but that's not what we're talking about here, and it would be a
dumb thing to do.

Leo Lichtman wrote:

I can see no way that one battery can supply current to the
other while
they are both hooked to an outside source that exceeds their
voltage. How can the higher voltage battery supply current to
the lower one, while at the
same time drawing current from the charger? Current can't
flow both ways
through the battery at the same time. If you left the two
batteries hooked together in parallel and disconneced them
from the charger, this would be possible, but that's not what
we're talking about here, and it would be a dumb thing to do.

How could the supply voltage exceed that of the lowest-voltage
battery? In any unimpeded current-limited situation, it's
(lowest) battery voltage which sets supply voltage.

John

Sharp wrote:
Hello

Can anyone tell me can I connect two 7.2 V Ni-MH packs in series to get
14.4V, use it for powering 20 W halogen lamp, and after power runs down
disassemble it and charge 7.2 V packs separately, first one and then
another.
Is it possible to do this and not damage the batteries?

I own smart battery charger that can charge only 10 cells (MW 7168 omni) and
I need 14.4 V battery.

This is indeed a common problem, since there are only a few NiMH
chargers that can charge 12 or more cells in series at a time.

What you want to do is to have a two pin connector across the + and - of
each pack. These connectors are for charging. The connector from the +
of one pack to the - of the other pack, goes across all six batteries,
and is what connects to the light.

You'll have to manually move the charger from one pack to the next.
There are ways of automating the process of charging two packs,
sequentially, using a timer and relays. See
"http://nordicgroup.us/s78/chargers.html" and look for the section
entitled "Unattended Sequential Charging of Multiple NiMH Battery Packs
with a Single Charger."

Note that the Maha MH-C777PlusII
"http://thomas-distributing.com/mh-c777plus.htm" will charge 12 cells in
series. This would almost certainly be a better solution than futzing
with connectors, timers, and relays.

Werehatrack wrote:
On 24 Nov 2005 08:05:05 -0800, wrote:

Werehatrack wrote:

The drawback to trying to charge two packs with one charger is the
human factor; try as you will, there is going to be an instance when
one of the packs will have been charged and the other isn't; in that
case, you'll probably discover the problem in short order, but it
could still damage the uncharged pack in the process. Get a second
charger, so that you're always charging both at the same time, and
this potential pitfall becomes much less likely to obtain.
Is there any reason not to charge the two packs in parallel off one
charger? ISTM it should be pretty easy to build a Y adaptor to allow
this. I don't know any reason this would confuse a smart charger -
although, admittedly, I'm not expert on those things.

Modern rechargable battery packs require a constant current, not a
constant voltage; it's pretty much guaranteed that hooking the two
packs in parallel will halve the current for each, which is wrong
anyway. If a higher-current charger was obtained, the two would
almost certainly not charge at the same rate...thus one would be
getting too much current, and one too little, to charge properly.

One charger, one pack, of matching characteristics. That's the way
it's designed to work.

It should be noted that with NiMH cells, it's perfectly acceptable to
hook packs in parallel, with no isolation, for discharging. However it
is unacceptable to charge the batteries in parallel.

SMS wrote:
Sharp wrote:
Hello

Can anyone tell me can I connect two 7.2 V Ni-MH packs in series to get
14.4V, use it for powering 20 W halogen lamp, and after power runs down
disassemble it and charge 7.2 V packs separately, first one and then
another.
Is it possible to do this and not damage the batteries?

I own smart battery charger that can charge only 10 cells (MW 7168 omni) and
I need 14.4 V battery.

This is indeed a common problem, since there are only a few NiMH
chargers that can charge 12 or more cells in series at a time.

What you want to do is to have a two pin connector across the + and - of
each pack. These connectors are for charging. The connector from the +
of one pack to the - of the other pack, goes across all six batteries,
and is what connects to the light.

You'll have to manually move the charger from one pack to the next.
There are ways of automating the process of charging two packs,
sequentially, using a timer and relays. See
"http://nordicgroup.us/s78/chargers.html" and look for the section
entitled "Unattended Sequential Charging of Multiple NiMH Battery Packs
with a Single Charger."

Note that the Maha MH-C777PlusII
"http://thomas-distributing.com/mh-c777plus.htm" will charge 12 cells in
series. This would almost certainly be a better solution than futzing
with connectors, timers, and relays.

ISTM that spending $50 to $100 on a charger, and/or cobbling something
together from timers and relays and connectors doesn't make sense
unless your hobby is building electronic toys, not bicycling.

I've played around a bit with electronics, but it's not really my
thing. I certainly never subscribed to "Popular Electronic Stuff To
Build In Your Basement." So I prefer to use lights that simply
_work_.

- Frank Krygowski