Using multiple 9v NiMH batteries in PARALLEL

[hollowman]

Alkaline impedance

Alkalines are RELATIVELY safe to parallel because their ... they have a very high internal impedance ....

They do???
https://www.youtube.com/watch?v=q58E8DdNgDs
R_int_Alkaline = 8.4/1.0 - 5.6*1.0 = 2.8 Ohm
R_int_ZincCarbon = 8.1/0.55- 5.6 = 9.1 Ohm

 

[audioguru]

A little 9V alkaline battery has a fair amount of power.

The controller of my company went to the stage and began her speech. Then she screamed and jumped around.
The spare 9V alkaline battery in her pocket (for her wireless mic) was shorted by her keys and made things very hot.

 

[hollowman]

A diode is not needed and not used because a 9V battery has a connector that cannot be connected backwards.

WRONG!
Sometimes matching the plug to the batt terminals -- in a dark room, or if you have poor eyesight -- can lead to bass-ackward cond. (they just need to make contact momentarily, not plug in, to do damage downline). The pwr switch may be off, but if you've got electro caps --- boom. Also for this device, I have DC adapt. input -- and one of those universal adapters with switchable polarity. I have piece of tape over it ... but you never know.

 

[Sceadwian]

I don't believe there are any conflicts about the safety of using cells in series/parallel configurations at all, anyone that says it's unsafe has no idea what they're talking about. It's common practice in lead acid battery banks, not to mention Lithium power packs for radio controlled application and those are a far cry more dangerous than 9V NiMH's. Cell matching is important and so is proper charging, in your case as long as the cells were closely matched before hand (if you bought them in the same pack this is a good bet) and you charge them separately this is good practice.

I would personally allow the batteries to rest after charging for a few hours and monitor their no load voltage at this point as the packs are probably close to the end of their lifespan you'll get a cell reversal any week/month after this point, I'd go so far as to replace them now just to not have to worry about it.

Given the saftey of all of this being out of the way it's still a horrible idea, the energy density of those 9V NiMh's is a crime to sane engineering. I'd scrap the whole battery setup you have now and use a three series lithium pack, you'd get many many times the run time out of the pack for the same size and a significant weight reduction. Lithium cells can be bought in virtually any physical size you can imagine just Google a bit to find one's that will fit your needs. Despite the bad rep Lithium has gotten with modern cells and chargers the risk is nothing of the explosion/fire fears that come to mind when people hear Lithium.

 

[bountyhunter]

Originally Posted by bountyhunter View Post
Alkalines are RELATIVELY safe to parallel because their ... they have a very high internal impedance ....

They do???
https://www.youtube.com/watch?v=q58E8DdNgDs
R_int_Alkaline = 8.4/1.0 - 5.6*1.0 = 2.8 Ohm
R_int_ZincCarbon = 8.1/0.55- 5.6 = 9.1 Ohm

Wow.... this thread was about paralleling rechargeable batteries. Compared to those, alkaline battery internal impedance is orders of magnitudes higher.

Want to learn something?

Look up the internal impedance of a NI-MH or NI-CD battery... which is what this thread was supposed to be about.

Maybe you should read the original post.... oh wait.... you wrote it. (?) Oh, well.



As for internal resistance of cell types:

While alkaline cells generally more energy per-cell than rechargeable types (NiMH or NiCd) this energy may not be accessible to the appliance using that cell - particularly if it is a device that draws a lot of current. /// Internal resistance is the culprit.

NiCd and NiMh cells, on the other hand, typically have a much lower internal resistance over their charge life and this resistance (which varies depending on state of charge, temperature, age, condition of the cell, the cell's internal chemistry and its construction) is typically lower than that of an alkaline cell - even when the NiCd or NiMH cell is significantly discharged.

https://www.ka7oei.com/nicds.html

class dismissed

 

[bountyhunter]

I don't believe there are any conflicts about the safety of using cells in series/parallel configurations at all, anyone that says it's unsafe has no idea what they're talking about.

Really.

Tell you what: take a fully charged "8.4V" battery and a discharged one and strap them in parallel......

then get back to me about how there is no danger.

BTW: don't hold them in your hand while they "equalize"....

 

[WTP Pepper]

And when the weak one starts going down, the others will be pumping current into it to hold it up essentially "recharging it". The real problem comes when yu get a cell that loses A-hr capacity or just dies altogether and looks like a short. That is a common failure mode for NI-CD and NI-MH when the internal insulator gets a tiny hole and a crystalline shorting bridge grows across.

Not if you diode OR them. The Diode prevents current from any one cell being backfired into another.View attachment 62708

 

[Sceadwian]

Tell you what: take a fully charged "8.4V" battery and a discharged one and strap them in parallel......

What does this have to do with anything I said? You're comparing a worst case intentional fault scenario with proper usage, there's no basis for that sentence to even be rationally uttered.

Why don't we just go around with an awl and stab fully charged Lithium cells and wonder why they burst into flames, and compare that to how a Lithium pack is typically treated in the real world!

Cell matching is important and so is proper charging

You would under no circumstances ever use a fully charged NiMH with a fully discharged one in parallel if you used proper charging methods. The packs would always be discharged in parallel, always charged separately, and their final charged states would be equalized before being use, as I said again towards the end of life individual pack voltage would have to be monitored for cell reversal which would cause a chain effect to an eventual pack failure, NiMh's aren't well known for smoke fire or explosion especially with the horrible energy density of a 9 volt.

I stated quiet clearly that using NiMH batteries in this manner although it can be safe is horrible practice even then your fault scenario would be blamed on a stupid user not a fault in the concept of paralleling series battery packs, the best solution is a simpler sane battery setup which I suggested.

 

[hollowman]

Not if you diode OR them. The Diode prevents current from any one cell being backfired into another.View attachment 62708

But one can't EASILY re-wire the CELLS w/diodes in a commercial 9v BATTERY ...
**broken link removed**
... and of all the 9v NIMH batteries I've seen INTERNALLY, none -- IAC -- have diodes. And a diode(s) between parallel 9v BATTERIES does barely anything ... you've still got the full current-promoting potential across them.
Bottom line: diodes, as you've suggested, are impractical and unnecessary for this config. (NiMH).

 

[hollowman]

Look up the internal impedance of a NI-MH or NI-CD battery... which is what this thread was supposed to be about.

As for internal resistance of cell types:
http://www.ka7oei.com/nicds.html

The point is: the internal R (or Z) of an alk 9v is not all that much more compared to NiMH:

Alk 9v -- about 1.5-1.7 Ω according to **broken link removed** and http://www.electronicspoint.com/do-calculate-battery-internal-resistance-t10177.html and https://www.electro-tech-online.com/custompdfs/2012/03/PC1604_US_PC.pdf

NiMH, Li-po, etc -- about 0.8 Ω for NiMH according to **broken link removed**

 

[audioguru]

An Energizer 9V alkaline battery does not have much power in it. A new one gets hot if shorted.
An Energizer 8.4V 175mAh Ni-MH battery has much less power in it. I never tried shorting one.

Here is how they compare with a load: