Eneloop C & D Cells Exposed

[gSPIN]

the danger with parallel charging NiMH or NiCd happens when u charge past the peak.
no matter how closely matched, one of the cells out of the parallel group will always reach peak first.
it's after peaking where its voltage drops that's the big problem which prevents the others from reaching their peak all the while continually diverting all the current that is supposed to be shared equally.
until it finally overheats & vents while endlessly waiting for the remaining cells to fill up.

to parallel charge safely one simple way is to charge reely, reely slo.
so slo that if one cell does hog the current meant for all of them it still doesn't exceed the max trickle charge of a single cell.
that would make for something like a 50 hour rate so for a fast charge u need to terminate well b4 any cell can reach its peak.

this means -dv/dt can't be used at all which is really what all the dire warnings against parallel charging NiMH/NiCd are targeting.
to allow for fast and safe parallel charging it's likely Sanyo will opt for dT/dt (having them thermally equalized housed within the same small container certainly helps) altho dv²/dt might work too.
a slight reduction in capacity by not filling to the brim is the only trade-off for the improved reliability & longevity.
i think negative deltaV charging is generally damaging to NiMH cells anyways, parallel or not.

The capacity of a cell is roughly proportional to its internal volume.

Therefore, we can estimate:

Volume(AA) = π/4 · 14.5² · 50 = 8300 mm³
Capacity(AA) = 1900 mAh

Volume(D) = π/4 · 34² · 61 = 55,000 mm³

Capacity(D) = 550/83 · 1900 = 13,000 mAh

We find D size NiMH cells with capacities up to 12,000 mAh, so this estimate stacks up.

now plug in 2500 mAh for the Eneloop XX.
everyone raise their hand, who can use a 17 Ah LSD or 20 Ah super lattice D cell?
then try & tell me there's no market for hi cap D cells.
i hoping when the Chevron patents expire in a couple more years that prevent anyone from manufacturing large high density NiMH cells that Sanyo will still be interested in producing a true D size Eneloop.

 

[LuxLuthor]

Looks familiar, except I put them in series.

​

 

[niktu]

Hmm, lets look at problem scientifically ... *evil grin*

What really separates bigger cell from two smaller cells connected together in parallel?

What makes it work properly in single cell, even if end part of Nimh charging process seems to be tolerating any imbalance poorly?


1. All parts of single cell have same use history - parameters are similar at start and change in similar conditions.

2. single cell equalize all pressure changes during charge (and that pressure changes quite significantly, and gets to quite high levels at end of charge). Same goes for any little gas venting that happens when conditions are not ideal.

3. single cell equalizes temperature well - cylindrical metal electrodes wrapped in steel can, then wrapped in plastic wrap - temperature change will equalize internally much faster than any outside cooling asymmetry might try to tip it off

4. single cell equalizes electrolyte saturation over time (slower than pressure and temperature, but over time it still happens ... think concentration change after gas venting)

5. resistance between parts of electrodes inside single cell is probably much lower than you can connect whole cells together ... and for sure will be more consistant between those parts and external electrodes - less voltage and current fluctuation


So, why it might work in Sanyo case?
1. and 3. are taken care of by mating cells during manufacturing (and they stay like that for their life), while their steel cans are pressed together, while whole assembly is wrapped in plastic case (working temp equalization, same service history),
5. can be taken care of by properly engineered connections made at factory (so even if resistance might not be as low as in case of single cell, it will be reliably consistent across cells, during life of battery)

Lack of 2. and 4. might be mitigated by amazing consistency of eneloops (while other quality cells might keep their parameters close while cycled often, many of them will veer off wildly only after few months of storage and require few cycles to get them back in line between themselves)

Thats why I think that Sanyo execution might work well enough.
And why putting off-the-shelf cells into off-the-shelf battery holder is doomed to fail during charging, sooner than later (no consistency of contact resistance, and no proper temperature equalization, and it would require vigilance on part of user to use same set of cells as set for their entire life).

Avid dyi'er might get some success by imitating Sanyo build:

get fresh cells from one batch, strip thir plastic casings leaving only steel cans, affix them together making sure there is very good thermal and electrical contact, wrap them together with something with giving partial themal isolation (so single cells equalize temperature between each other much, much faster than between surroundings)

Would that work? it should, if done well ... i doubt that Sanyo uses any additional tricks in their product.



Ughh ... it turned out much lengtier than planned, my apologies

I guess i will have to leave "everybody knows NiMHs can't be float charged!" and "with some caveats you CAN float charge even a parallel string of NiMHs!" rant for some other time then

 

[Chrontius]

So far, one vote for Accupower Evolution 10 Ah cells. Anyone else used LSD Ds, or are they just that niche?

 

[Wangstang]

So did these ever come to market in the US? If so where can you find them & how are they priced?

Thanks
Wes

 

[Lynx_Arc]

So did these ever come to market in the US? If so where can you find them & how are they priced?

Thanks
Wes

I haven't heard of them being made anywhere on CF, I would expect a review of them if they were available.

 

[Illum]

Looks familiar, except I put them in series.

​

I wonder if Sanyo arrived at the ability to test cell resistances and combined those with similar resistances to avoid unbalanced cells charged in parallel

 

[Lynx_Arc]

I wonder if Sanyo arrived at the ability to test cell resistances and combined those with similar resistances to avoid unbalanced cells charged in parallel

I don't think it is much of an issue with nimh cells in parallel, but rather in series. Once they reach voltage balance in parallel the weaker cell should be supported by the stronger one unless under severe loads that would require two cells to support the load.

 

[bcscheps]

Too bad they (eneloop) couldn't have just made some 3AAA>C and 3AA>D adapters and sold them instead of this. I would like to see the D cell version taken apart to see if you can remove the cells or if they are soldered/welded in place. I am guessing it is easier to retrofit AA/AAA into D/C cells instead of starting up another plant to make C/D cells and deal with designing and testing cells. C/D cells with greater capacity than these would either sell and cut into the AA/AAA profits or not sell because they cost too much. I am mostly interested in subC LSD cells myself for two battery soldering irons I use once every 6 months that are always dead (nicads).

I purchase all my "LSD" batteries from "ALL-BATTERY.COM." Their "LSD"-Battery-line are called "CENTURA" and are VERY reasonably priced. (Their "C" and "D" "Centura" cells usually cost no more than $16.00 plus postage.

 

[Shadowww]

I don't think it is much of an issue with nimh cells in parallel, but rather in series. Once they reach voltage balance in parallel the weaker cell should be supported by the stronger one unless under severe loads that would require two cells to support the load.

NiMH cell charging is terminated by delta -V signal, which can't be properly detected with multiple cells in parallel.

 

[Lynx_Arc]

NiMH cell charging is terminated by delta -V signal, which can't be properly detected with multiple cells in parallel.

I'm not so sure of that as eneloops tend to be high quality so the variance of cells is minimal and the internal resistance of the cells in parallel will actually be reduced vs one cell thus making it easier to detect a signal. What makes it harder is cells with high internal resistance which many chargers will even refuse to charge. I've not heard of a lower internal resistance cell being rejected by a charger even though it is possible.

 

[alpg88]

NiMH cell charging is terminated by delta -V signal, which can't be properly detected with multiple cells in parallel.

yes. i agree.

i used parallel nimh cells in my packs, but i never used smart charger for it, low current timed "dumb" charger, and it works just fine.

 

[VidPro]

NiMH cell charging is terminated by delta -V signal, which can't be properly detected with multiple cells in parallel.

Agrees also Shadoww & Gspin. i would far prefer that the cells are put into a parellel adapter, and charged seperate. Dont matter how well matched (gspin) "it's after peaking where its voltage drops that's the big problem"

Charging with a nice slow rate for the cells that are internal to it like say <C/5 for single AAA or AAs in it, or stopping at a voltage max (some maha chargers) would work just fine Trying to get a v-drop going for termination would not go well , reducing the life of the thing each charge.
I dont care how well they are tied together, or matched, there aint no special curcuitry in there to fix the parellel ni-?? thing (when used in v-drop terminating chargers)

Some of the sanyos own chargers use "different methods" than relying on v-drop so they may have it covered in some of thier own stuff.
If you know about how parellel charging fails, and know the drop current (single cell) or control voltage high, on the internal cells then you can pull it off.

I would put these on my "must be a better way" list and forgedabout it. sanyo made some of the best D-cell rechargables available in ni-mh that were not LSD special, they worked wonderfull and for long and hard and had good capacity. These are an embarassment compared to them

darn-it sanyo, I want a Real eneloop Dcell, saving money just for it, this aint it At All.

Side notes: "slow rates" would be actual slow rates, and not "averaged" slow rates that are higher current rates pulsed to average lower. So pummeling it with a high current with a "pulse averaged" low rate is not the same thing.

 

[mccririck]

C and D cells do not provide near the profit margin that AA and AAA cells do. Not only are manufacturing volumes for AA and AAA cells are much larger, but the amount of raw materials in C and D cells are obviously much higher as well. A 10Ah NiMH cell should cost 5X more than a 2Ah NiMH cell, but that is typically not the case.

Eh, that's because there's only one casing. btw have you ever noticed how AA and AAA tend to cost the same even though AA uses about 2.5 times as much inside stuff?

 

[sedstar]

someone mentioned up early in this thread... and skimming down thru i didnt see it mentioned again...

"would this mean its a battery and not a cell?"

=====================================

Humm. Actually, i dont THINK so.

we call a car BATTERY a "battery" because there are actually 6 individual CELLS built together in a series connection... to up the voltage.

i think all of us know that a 9v BATTERY is actually a battery, as there are several little AAA or AAAA sized cells inside in series...

...on this logic train, i *think* but am not sure... this is still a CELL as they are in parallel.

 

[SilverFox]

Hello Sedstar,

Technically, a cell is a single unit and a battery is made up of 2 or more cells. In actual discussion both battery and cell are used interchangeably. A battery can be looked at as stored energy and a special case is the one cell battery.

Tom

 

[mccririck]

In the UK people tend to just use the word battery for any type of cell/battery.

 

[bobrip]

In the UK people tend to just use the word battery for any type of cell/battery.

I visited a Sanyo factory in Japan about 7 years ago. They showed us a Nimh D cell. It was used in the battery of a Hybrid car. If memory serves me right it was 10 ampere hour. Of course they will have improved them by now. Another comment made by them is that they grade cells into two groups. One was a lower capacity. They they build a battery out of the low or high capacity cell, not both. I worked with Sanyo for about 10 years and I developed a lot of respect for Sanyo's battery manufacturing. Of course the Eneloop helps justify that respect.

 

[mojo-chan]

So can someone explain why they wouldn't just sell 3xAA or 4xAAA with an adapter? It seems like people could then use their existing chargers and all concerns about unequal charging would be taken care of.

The only thing I can think of is that consumers just want to buy D and C cells and can't understand this complex adapter thing.

 

[Lynx_Arc]

So can someone explain why they wouldn't just sell 3xAA or 4xAAA with an adapter? It seems like people could then use their existing chargers and all concerns about unequal charging would be taken care of.

The only thing I can think of is that consumers just want to buy D and C cells and can't understand this complex adapter thing.

One problem with adapters is putting unbalanced cells in them imagine an almost dead cell with 2 full cells in parallel the dead cell could experience being charged at a higher rate than the adapter can handle melting your adapter or worse. Single cell adapters can be found at times along with series adapters but parallel ones are a rarity in the stores.