Discussion in 'Electronic Basics' started by LED Man, Feb 27, 2005.

1. ### LED ManGuest

I have a solar powered torch, battery 2.4v 280mah nicad, hard pcb wired.

I am thinking of wiring up a second nicad battery 750 mah in parallel.

My question, when one nicad, probably the 280 mah dies, what happens ?

Does it go open circuit, closed circuit, how does it affect charging and
useage of the higher capacity nicad

Any experience of same ??

TIA

2. ### Don BruderGuest

Depends on failure mode - Did it die of pure old age? It probably went
short, then. Did it die of abuse (rough handling/vibration?) Likely it
went open, but it's impossible (without having it in hand, anyway) to
say it didn't short.
Gonna depend on how it fails. If open, probably very little, if any,
effect. If short, it'll probably drain the other one quickly, and/or
prevent charging at all.
No actual experience this way, but common sense says it isn't likely to
be a good idea. I'd pick one or the other, and use that one exclusively.
No point in messing around, I'd say...

3. ### CWattersGuest

It's not generally a good idea to connect two voltage sources in parallel.
If the voltage of one is slightly different to the other (due to charge
state) what limits the current.....Hint I=(V1-V2)/R but R is very small if
it's wire.
What do you mean by "dies"? flat? old? faulty?

Most likely the larger battery will supply all the current when the smaller
on is empty but batteries can be unpredictable. Mostly it works if you
charge them both before connecting them but watch out for melted wires and
fire if one is empty!
With two voltage sources in parallel the results are unpredictable. Old
batteries tend to have a slightly higher voltage on charge so most of the
current may go into the larger external battery leaving the internal one
uncharged/less charged. If the larger battery is a different make it might
have a higher on charge voltage and all the current may go into the smaller
internal battery. Personally I would remove or disconnect the smaller
internal battery.

Caution: I've see batteries explode due to missuse. It's can be dangerous.

Ta

5. ### PeteSGuest

As other have noted, depends on dies. If we are speaking simply of
discharging, then the (fairly) simple answer is that as the smaller
capacity battery gets more discharged, it's terminal impedance
increases, and the other battery supplies more of the current. Note
that NiCads in this state have rather horrendous leakage
characteristics, so the smaller battery may be taking charge from the
other battery too. In effect, the larger battery is charging the small
one, to a certain extent. Parallel batteries should always be of the
same type (type, construction and capacity).

Other things to watch out for are the exact type of battery. There are
different types of NiCad (optimised for a particular issue) and it is
very unwise to mix them. High temperature types, for example, can't
supply very much discharge current (<C/10 for some of them). The list
is rather lengthy.

Charging is a real issue - for ordinary (see note about different
types) NiCads, standard charge is C/10 for 14 hours (where C is the
capacity) so the 280mAh battery would need 28mA, but the larger one
75mA. If they were in parallel, it's very difficult to know the
effective terminal impedance of each (meaning you can't really properly
regulate the charge).
Some NiCads can never be fast charged (normally C/3 for 4.5hours)
although some can be boost charged (C rate for about 2 hours), but
again, you can't mix capacities in a charger without the distinct
possibility of killing the battery(ies), with a possibility of making
them come apart at the seams quite spectacularly.

Cheers

PeteS

Ta

7. ### Doug McLarenGuest

| | > I have a solar powered torch, battery 2.4v 280mah nicad, hard pcb wired.
| >
| > I am thinking of wiring up a second nicad battery 750 mah in parallel.
|
| It's not generally a good idea to connect two voltage sources in parallel.
| If the voltage of one is slightly different to the other (due to charge
| state) what limits the current.....Hint I=(V1-V2)/R but R is very small if
| it's wire.

It's not so bad with two rechargable batteries (as long as it's the
same chemistry of cells, and each battery has the same number of
cells) in parallel. At least not with regard to discharging ...

| > My question, when one nicad, probably the 280 mah dies, what happens ?
|
| What do you mean by "dies"? flat? old? faulty?

For the record, most NiCd and NiMH cells seem to die open rather than
short.

| Most likely the larger battery will supply all the current when the
| smaller on is empty but batteries can be unpredictable.

This isn't so unpredictable -- it's generally the way it works.

| Mostly it works if you charge them both before connecting them but
| watch out for melted wires and fire if one is empty!

Nah. If one is empty, the current coming from the full NiCd/NiMH cell
will bring it's voltage up to match the full one very quickly, and
then the current will stop, even though one battery is almost
completely empty and the other is almost completely full.

(Corilary: You can't tell how full a NiCd or NiMH cell is just based
on it's voltage. Adding a small load helps somewhat, but even so, you
really can't tell just based on voltage.)

| > how does it affect charging
|
| With two voltage sources in parallel the results are unpredictable.

Charging will be totally screwed up. What will generally happen is
hinted at what I mentioned earlier -- one cell will get most of the
charge and the other won't get charged much at all.

It probably won't melt down, unless you charge at a really high rate
expecting equal amounts to go into each battery, but you're likely to
really only charge one battery.

| Old batteries tend to have a slightly higher voltage on charge so
| most of the current may go into the larger external battery leaving
| the internal one uncharged/less charged. If the larger battery is a
| different make it might have a higher on charge voltage and all the
| current may go into the smaller internal battery. Personally I would
| remove or disconnect the smaller internal battery.

That's probably best.

Note that this applies to NiCd and NiMH cells. I do lots of R/C plane
stuff, and putting NiCd and NiMH packs in parallel works fine for use,
but you need to remove them and charge seperately.

For LiPo and Pb cells, you *can* put them in parallel and you can even
charge them like that and it works fine, but you want to make sure
that the voltages are similar before you connect them (otherwise it'll
melt down like Cwatters said. It generally won't melt down with NiCd
or NiMH, but will with LiPo and Pb batteries.) But once connected,
you can just treat them like a larger cell.

| Caution: I've see batteries explode due to missuse. It's can be dangerous.

Yup. Especially the Lipo/LiIon cells.

In any event, I agree with Cwatters -- remove the original battery,
and just use one.