What do dead Nicads do ?

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...

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 (

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.

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| > 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.