Using multiple 9v NiMH batteries in PARALLEL

[hollowman]

There are conflicting views on whether one can "safely" use 9v NiMH batteries in PARALLEL -- by "safe", I mean mostly for battery longevity as opposed to fire/explosion risk. One may want to do parallel as the mAh of most NiMH 9v single batts kinda sucks, so multiple parallel batts may be better ?? Let's say two for the sake of argument.

Some say PARALLELING is not a good idea ...

From head-fi.rg DIY thread on this topic:
https://www.head-fi.org/t/601247/is-it-ok-to-put-two-9v-battery-in-parallel#post_8229450

"It's okay if they are alkaline batteries, but not okay if they are rechargeables. It has to do with the difference in internal impedance. The effective impedance of alkalines is high enough that batteries in parallel will effectively "share" the load, while rechargeables will fight each other unless they are very well matched, and that state won't last forever even if you manage to match them at the start."

Your fdbk appreciated -- thx!

 

[audioguru]

9V Ni-MH batteries are different.
I can buy an Energizer made in Germany seven-cells 8.4V Ni-MH "9V" battery at Wal-Mart and I can buy an Energizer made in Japan six-cells 7.2V Ni-MH "9V" battery at Shoppers Drug Mart today.
The 7.2V one is at least 5 years old.

 

[hollowman]

Yeah, they are different -- especially WRT quality (= reliability and longevity). Maha Energy (maker or Powerex and Imedion NiMH) makes absolute junk -- 9v, AA, AAA, even their chargers! This is a major Japanese company which make their poor quality all the more surprising.

9v batts. are comprised of small in-series cells (1.2v) that are not that reliable. AA and AAA are, for all manufs., a much more reliable size.

I like Sanyo's Eneloop, but that's not avail. in 9v (batt. manufs don't like the 9v size for some reason).

I also have an Energizer "9v" (7.2v actual) 'Made in Germany' that seems to be holding up well after many years. Dunno why, tho'? Maybe because it has FEWER of those troublesome miniature 1.2v comprising cells (if one of them goes bad in the batt., the whole batt. is shot!).

 

[KJ6EAD]

An 8-pack of AA or AAA cells is more economical and reliable if more capacity than a single 9V battery is needed.

 

[hollowman]

I have a headphone amp -- similar to the one that takes 9v batt., noted above -- that incorporates 8 AAA. It has a built-in charger (they charge in SERIES, I think) and gets quite warm when doing so -- that doesn't inspire a lot of confidence (i.e, the audio electronics getting baked in there). I have a stand-alone charger, too, but it's limited to 4 AA(A) cells at a time.

Anyway, we're getting OT! Anyone have some thoughts on the orig. query?

 

[KJ6EAD]

Anyway, we're getting OT! Anyone have some thoughts on the orig. query?

What was the question?

 

[bountyhunter]

Paralelling batteries works as long as they are healthy and well matched. As they age, they no longer are.

 

[hollowman]

Paralelling batteries works as long as they are healthy and well matched. As they age, they no longer are.

This seems to be the same UN-referenced "answer" Tangent provided in quote from my OP.
Two problems:
(a) Science-based proof not provided (more than a sentence response -- links, refs, work too, etc.)
(b) My experience in paralleling 9v batts: using two UNmatched 9v batts (an Eveready [7.2v] and Hi-Watt (China) [8.4v]). I've used this same pair for FOUR years, recharging 100s of time. Even the overly-sensitive Maha chargers (which fault out even for their OWN brand of batts [parallel use or not; it's due to poor QC or engineering]) accept and successfully re(chg) my dynamic duo.

 

[bountyhunter]

This seems to be the same UN-referenced "answer" Tangent provided in quote from my OP.
Two problems:
(a) Science-based proof not provided (more than a sentence response -- links, refs, work too, etc.)
(b) My experience in paralleling 9v batts: using two UNmatched 9v batts (an Eveready [7.2v] and Hi-Watt (China) [8.4v]). I've used this same pair for FOUR years, recharging 100s of time. Even the overly-sensitive Maha chargers (which fault out even for their OWN brand of batts [parallel use or not; it's due to poor QC or engineering]) accept and successfully re(chg) my dynamic duo.

Well, this is the internet, I just post opinions like everybody else. However, my opinions carry a total of 45 years in electronics behind them and about 30 years as a specialist in power management and the resident scientist at National Semi regarding rechargeable battery technology.

As I said, paralleling batteries works only in a perfect world and relies on near perfect matching. It's a stupid and dangerous design practice to use especially for the inexperienced but like I said: just my opinion.

If you are asserting that it is OK to parallel a six-cell 7.2V battery and a seven-cell 8.4V battery, I think that's sufficient. Go ahead. One cell in the 8.4V has probably been killed (shorted) in the process if they function in parallel, but whatever works for you.

 

[hollowman]

As I said, paralleling batteries works only in a perfect world and relies on near perfect matching. It's a stupid and dangerous design practice to use especially for the inexperienced but like I said: just my opinion.

If you are asserting that it is OK to parallel a six-cell 7.2V battery and a seven-cell 8.4V battery, I think that's sufficient. Go ahead. One cell in the 8.4V has probably been killed (shorted) in the process if they function in parallel, but whatever works for you.

Couple of things ...
The danger aspect you allude to makes no sense. The rapid chargers used for NiMH often make the batts VERY warm -- this never happens with running respective NiMH batts in parallel (they don't even feel mildly warmer in parallel (than single)). Since heat is an enemy of batt. physical chemistry, the chargers themselves pose a much greater relative "threat" if one is concerned with explosion/leakage/fire hazard as well as cell longevity.

The 9v species of NiMH -- as I noted earlier -- seems to be troublesome to engineer. Many major manufs of NiMH -- Sanyo, Duracell, etc. -- tend to stay clear of offering it in NiMH.

I don't understand your comment " One cell in the 8.4V has probably been killed (shorted) in the process if they function in parallel...". I have quite few brands/models of 9v batts and chargers in my "collection". When one of the 1.2v in-series cells does go bad ("cell reversal", etc.), the internal cell/batt. impedance messes up and the whole batt. either faults out the charger and/or it does not maintain (hold) correct charge (voltage) over time. And yes, those mini 1.2v internal cells DO go bad JUST AS OFTEN in single (intend, i.e., non-parallel) use, depending on manuf. (e.g., Maha is v. bad; see Amazon reviews).
**broken link removed**

I've researched this topic, fairly extensively I guess -- including direct communication with manuf. engr depts, and referenced various datasheets and topical web sites/blogs, and recent-pub'd battery-technology textbooks, as well as engaged in myriad forum discussions (such as THIS!). Bottom line: it's a gray area ... not a whole lot written about or extensively discussed. Maybe, this is all due to market demand: Li-Po seem to be the hot trend. IAC,and FWIW, Maha Energy says it's okay to parallel.

 

[WTP Pepper]

There are no issues if you simply want to parallel them as long as you diode 'OR' them and don't want to charge them in parallel. If you do not OR them then they will each discharge each other. The weakest in the pack will dominate the discharge.

 

[audioguru]

I paralled six series AA Ni-Cad cells with six others for many years in my Beach Sound System. The cells were not paralleled, the strings of six cells in series were paralleled. The cells lasted for many years like me. Nobody had a ghetto blaster that sounded as good as my Beach Sound System. It was a chic magnet.

 

[hollowman]

There are no issues if you simply want to parallel them as long as you diode 'OR' them and don't want to charge them in parallel. If you do not OR them then they will each discharge each other. The weakest in the pack will dominate the discharge.

There is a diode placed immediately following the POS (+) terminal of each battery. This is an anti-rev-chg-protect feature. But that does not prevent them from charging EACH OTHER. IAC, I usually disconnect them from the device after use as they are ready for recharging anyway...On that note: I never chg multiple 9v in parallel. I always use an external commercial (Maha) dedicated 9V charger that is UL listed and microprocessor-controlled. Not sure how it's set up internally. The batts do get warm during initial charge. And when they start to "go" (end of useful lifespan), the charger will fault out and never commence charge -- I think this is an impedance-based check the charger performs.

 

[bountyhunter]

Couple of things ...
The danger aspect you allude to makes no sense.////

I don't understand your comment " One cell in the 8.4V has probably been killed (shorted) in the process if they function in parallel...".

My last post on the subject: if you parallel a six cell (7.2V) and a seven cell (8.4V) it forces them to be the same voltage which means the higher voltage battery is pumping current into the lower voltage battery and driving it like a load. At some point one of the cells in the 8.4V battery gets depleted and it will have current forced through it in the wrong direction by the remaining cells which usually destroys the cell which has been reversed.

Bottom line: it's a gray area ... not a whole lot written about or extensively discussed. Maybe, this is all due to market demand: Li-Po seem to be the hot trend. IAC,and FWIW, Maha Energy says it's okay to parallel.

It's not a gray area. I worked with battery makers many times and yes, in some cases battery packs use parallel cells. They also select cells for this to get good matching and they design in safety circuitry into the pack to monitor the cells and "balance" them to keep them healthy. They also have thermal fuses and FETs which can open circuit a string of cells in case the monitor IC sees a serious problem. No manufacturer I know of has ever sent out a battery pack with cells hard wired in parallel with no monitor/protection hardware to protect the battery from catastrophic failure.

 

[ericgibbs]

Hi bh,
Many marine buoys do have directly connected 'D' type Alkaline batteries connected in series/parallel configurations.

https://www.google.com/search?sourc...0l14074l29l29l1l13l0l0l352l1931l3j10j1j1l15l0.

 

[bountyhunter]

Hi bh,
Many marine buoys do have directly connected 'D' type Alkaline batteries connected in series/parallel configurations.

https://www.google.com/search?sourc...0l14074l29l29l1l13l0l0l352l1931l3j10j1j1l15l0.

The thread is not about alkalines, it's about rechargeable batteries. Alkalines are RELATIVELY safe to parallel because their initial capacities are fairly well matched and they have a very high internal impedance which makes it unlikely they will cook anything in case one cell "goes down" and the other one pumps current into it.

My point is that rechargeables are only safe to parallel if the pack has enough "smarts" built in to monitor the cells and disconnect them in case of failure. Especially in the case of Li Poly, which are known to catch fire with only minimal encouragement.

 

[bountyhunter]

There are no issues if you simply want to parallel them as long as you diode 'OR' them and don't want to charge them in parallel. If you do not OR them then they will each discharge each other. The weakest in the pack will dominate the discharge.

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.

 

[hollowman]

My last post on the subject: if you parallel a six cell (7.2V) and a seven cell (8.4V) it forces them to be the same voltage which means the higher voltage battery is pumping current into the lower voltage battery and driving it like a load. At some point one of the cells in the 8.4V battery gets depleted and it will have current forced through it in the wrong direction by the remaining cells which usually destroys the cell which has been reversed.


It's not a gray area. I worked with battery makers many times and yes, in some cases battery packs use parallel cells. They also select cells for this to get good matching and they design in safety circuitry into the pack to monitor the cells and "balance" them to keep them healthy. They also have thermal fuses and FETs which can open circuit a string of cells in case the monitor IC sees a serious problem. No manufacturer I know of has ever sent out a battery pack with cells hard wired in parallel with no monitor/protection hardware to protect the battery from catastrophic failure.

Just to be clear: I'm talking about PERSONALLY (ad hoc) using two/more 9v NiMH batts connected in parallel. This is totally independent of manufs' suggestions (and there are no specifics -- hence gray area) for NiMH powering devices with their cells/batts in this. It's not in their Application Manual, datasheets, etc.) I emailed Maha about this and they said it's okay as long as both are the SAME model.
IAC ...
The leakage and cell-reversal danger exists even for normal IN-SERIES use. E.g., see the image on the last page of the 9.6v Maha batt with the cover removed. NO: there is NO built-in protect circuit in this batt. I've only seen those on Li-Ion or Li-Po batts, which are MUCH more dangerous.

Here's another thought (and a possible reason to parallel): I noted the high heat radiated by the batts in their "approved" charger. Heat kills batts faster than anything I can think of (other than deliberate abuse or purposely-destructive safety/UL tests!). Paralleling (e.g. with two 9v batts) effectively doubles their mAH. This means -- all else being equal -- the batts have to endure the charger half as much compared to single-batt use.

 

[audioguru]

There is a diode placed immediately following the POS (+) terminal of each battery. This is an anti-rev-chg-protect feature.

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

 

[audioguru]

Maha Ni-MH battery cells are advertised to be Canadian. But no store sells them.
Oh, their website is 6 years old from Taiwan. Maybe their batteries were sold six years ago in The Dollar Store.