New NiMH 15min

Doctorbass

100 GW
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Apr 8, 2007
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Quebec, Canada East
At less than $1/cell, last I checked 2300mah AA NiMH was the most economical way to build a big pack, and lighter than larger size NiMH. I would have chosen AA NiMH over my lithium 18650s had it not been for the problem of charging in parallel. Does anybody know if any of these new NiMHs can be charged in parallel like lead and lithium?
 
There's 5 ways (that I know of) to charge an NiMH battery: trickle, timed, dV/dt, dT/dt and dP/dt (AKA I-C3). Trickle just charges endlessly at a very low rate. Timed charges at a medium rate for a fixed period of time. "dV/dt" refers to "Voltage delta vs. time delta" and basically charges at a medium fixed current and watches for a change in voltage in a short period of time - when the cell is charged, the voltage droops by a small but measureable amount. Then there is dT/dt or "temperature delta vs. time delta" which charges at a fairly high charge current and watches for a sharp increase in temperature in a short period of time. Lastly, there is dP/dt or I-C3 which charges at a very high rate of charge and watches for a sharp increase in pressure in a short period of time.

The Rayovac system has cells that have a pressure sensor in them. The sensor tells the charger to shut off when there's an increase in pressure. The cells are charged in parallel but the charger shuts off each cell individually.

It's hard to adapt dT/dt or dP/dt to a series charging system because you are relying on the ability to disconnect each cell as it reaches a certain temperature or pressure. So you wouldn't charge a 36V battery at 36V, but would charge all 30 cells individually in parallel and shut them each off as they reach full charge. This wouldn't be too hard to do, but it's not as cheap or as easy as the trickle, timed or even dV/dt systems. And it's nowhere near as easy as a basic SLA charging scheme.
 
And it's nowhere near as easy as a basic SLA charging scheme.

Exactly. This is a major drawback of NiMH packs IMO. I'm hoping somebody figures out how to modify the basic nickel cell so that they can be charged in parallel without complex charging schemes and routines.
 
It seems like it wouldn't be that hard to design a BMS that works for parallel strings of Ni batteries. A BMS is required for many Lithium chemistries, so the idea of needing a BMS is not unique.

Each string needs to be isolated during charge, then put together during discharge. If there was a big FET for each string that did the switching, the voltage drop during discharge would be minimal. During charge, each string would have it's own current and voltage monitor to disconnect (or reduce current through the FET) when they reach full charge.

With a purpose built BMS right the pack, it should be possible to use a relatively 'dumb' charger.
 
To charge something flat/totally paralleled would be a lot of current to charge a large pack at a high rate. It's the main reason why I didn't charge all 96 cells in my lithium ion pack in parallel as one flat pack: a 3.7V 211Ah will either take a really, really long time to charge, or the current will be very high. To charge at 0.3C, I'd need to route 63A@3.7V - which would be a lot of copper if I want my IR drop low. ...unless I'm missing something obvious here and Fetchers going to come back and gently chide me that I've forgotten something basic. :)
 
I didn't mean to charge the entire pack in parallel -- just the cells that make up each parallel subpack like in our lithium packs. If it could be done as simply as with lead and lithium, then it'd be soooo much easier and cheaper to use those super mass-produced AA Nimh cells to build big ebike packs rather than the beaucoup-expensive, large NiMH cells.
 
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