Any rechargeable battery experts out there???

[bcla]

We designed our cells using NiMH exclusively (thermally stable and doesn't have the exothermic potential of Li-ion), but in large format (45Ah to 85Ah). As a domestic manufacturer, we had to go through Asia for sourcing some of the base materials for cell construction. There was also licensing from Ovonics who most major NiMH battery manufacturers needed to pay a usage fee. Needless to say, Ovonics/BASF knew me very well and wanted to know what markets I was in.

With that being said, cell improvement often comes via materials development. Electrolyte, separator and even carbon nano-tubes are some areas that can increase energy and/or power densities.

Of course, we're talking here about a low-end economical, commodity battery that can be purchased easily online. It's apples and oranges.

I believe the consumer grade AA NiMH battery has probably been maxed out in terms of performance. And the Eneloops (or relabelled from Sanyo/FDK production) I've had over the years have been great. I've had one here or there that failed, but for the most part I've got ones I've been using for 6-10 years that are still fine. I also generally don't use up their capacity, so that's probably helped. I have about a half dozen wireless computer mice that use a single AA, and I'll usually put a charged one in after a week or two. I've heard that some prefer the low capacity ones for that purpose, but I've never really seen them.

 

[hertamaniac]

I believe the consumer grade AA NiMH battery has probably been maxed out in terms of performance. And the Eneloops (or relabelled from Sanyo/FDK production) I've had over the years have been great. I've had one here or there that failed, but for the most part I've got ones I've been using for 6-10 years that are still fine. I also generally don't use up their capacity, so that's probably helped. I have about a half dozen wireless computer mice that use a single AA, and I'll usually put a charged one in after a week or two. I've heard that some prefer the low capacity ones for that purpose, but I've never really seen them.

Consumer NiMH cells are designed for very low rate charge and discharge applications. That is why you are seeing solid life as it doesn't tax the C-rate capability. Additionally, the BMS plays a pivotal role into cycle life.

You/consumer aren't driving your cells anywhere near a voltage reversal as the application isn't very taxing to the capacity. Also, NiMH generally doesn't experience memory effects that was seen in the early days of Ni-Cd.

The best cycle life for NiMH comes from an operating temperature that isn't extreme, a BMS that monitors overcharge/overdischarge conditions and an Ah throughput that doesn't come close to those limits. For example, the early Toyota Prius NiMH battery pack barely extracted 20% of the usable capacity (later increased the shallow cycling keeping the SOC between 40% and 80%). Through monitoring and opening up the SOC limits, they were able to extract more from the cells (and reduce the voltage string from the 1st generation). We dissected those 6.5Ah NiMH batteries and the design was solid.

I tend to agree that consumer grade NiMH batteries in a wound cell (<5Ah) don't have much more room in power/energy advances.

Apologize to the OP as this is probably way more information than you asked about.

 

[bcla]

Consumer NiMH cells are designed for very low rate charge and discharge applications. That is why you are seeing solid life as it doesn't tax the C-rate capability. Additionally, the BMS plays a pivotal role into cycle life.

You/consumer aren't driving your cells anywhere near a voltage reversal as the application isn't very taxing to the capacity. Also, NiMH generally doesn't experience memory effects that was seen in the early days of Ni-Cd.

The best cycle life for NiMH comes from an operating temperature that isn't extreme, a BMS that monitors overcharge/overdischarge conditions and an Ah throughput that doesn't come close to those limits. For example, the early Toyota Prius NiMH battery pack barely extracted 20% of the usable capacity (later increased the shallow cycling keeping the SOC between 40% and 80%). Through monitoring and opening up the SOC limits, they were able to extract more from the cells (and reduce the voltage string from the 1st generation). We dissected those 6.5Ah NiMH batteries and the design was solid.

I tend to agree that consumer grade NiMH batteries in a wound cell (<5Ah) don't have much more room in power/energy advances.

Apologize to the OP as this is probably way more information than you asked about.

There can be cell reversal any time there’s multiple cells used together and they’re unbalanced. It’s not likely to be a problem in many electronics that cut off when the current/voltage is inadequate, but there are dumb devices like flashlights where the user might forget to turn it off.

The 2000 mAh Eneloop is pretty good. It gives up maximum capacity for longevity. That’s ideal for my use where I’m really just going through a small portion of the capacity before I recharge.

 

[hertamaniac]

There can be cell reversal any time there’s multiple cells used together and they’re unbalanced. It’s not likely to be a problem in many electronics that cut off when the current/voltage is inadequate, but there are dumb devices like flashlights where the user might forget to turn it off.

The 2000 mAh Eneloop is pretty good. It gives up maximum capacity for longevity. That’s ideal for my use where I’m really just going through a small portion of the capacity before I recharge.

We often drove single cell designs into reversal by manipulating the amperage and VCO (it doesn't have to be a string).

The BMS in a string is what monitors the coulomb counting and Ah throughputs with SOC. That is what is used to perform load/cell balancing at low rates (you can't balance effectively at high rates). By using the correct BMS and measurement equipment, it was nearly impossible to drive a balanced series string into reverse. If the VCO didn't catch it, our TC would, if the TC failed, our pressure monitors would terminate the string. In effect, most high power/energy batteries have redundant terminations to mitigate the risk of overcharge/over-discharge.

 

[bcla]

We often drove single cell designs into reversal by manipulating the amperage and VCO (it doesn't have to be a string).

The BMS in a string is what monitors the coulomb counting and Ah throughputs with SOC. That is what is used to perform load/cell balancing at low rates (you can't balance effectively at high rates). By using the correct BMS and measurement equipment, it was nearly impossible to drive a balanced series string into reverse. If the VCO didn't catch it, our TC would, if the TC failed, our pressure monitors would terminate the string. In effect, most high power/energy batteries have redundant terminations to mitigate the risk of overcharge/over-discharge.

That's not happening with simple batteries without much of a management system other than checking voltage and current.

I rather marvel about the 5-cell battery packs used in older hybrid cars, and how they manage to avoid reversal and/or keep them balanced.

 

[hertamaniac]

That's not happening with simple batteries without much of a management system other than checking voltage and current.

I rather marvel about the 5-cell battery packs used in older hybrid cars, and how they manage to avoid reversal and/or keep them balanced.

Yes, simple monitoring of A and V is probably sufficient for consumer applications. The cost for a BMS can be staggering. Ours had to be developed custom due to the C-rates we could achieve in NiMH (world leading).

Speaking of hybrid cars, I was fortunate to work with several Formula 1 teams on their initial hybrid batteries (KERS program). Balancing and maintaining our test stack (5-cell, 6Ah) at the rates we were given was not an issue since we kept our SOC window pretty narrow. Our testing was done with the power map of the Monaco Grand Prix given to us from one of the top Formula 1 teams.

 

[bcla]

Yes, simple monitoring of A and V is probably sufficient for consumer applications. The cost for a BMS can be staggering. Ours had to be developed custom due to the C-rates we could achieve in NiMH (world leading).

Speaking of hybrid cars, I was fortunate to work with several Formula 1 teams on their initial hybrid batteries (KERS program). Balancing and maintaining our test stack (5-cell, 6Ah) at the rates we were given was not an issue since we kept our SOC window pretty narrow. Our testing was done with the power map of the Monaco Grand Prix given to us from one of the top Formula 1 teams.

Li-ion batteries routinely have some sort of onboard BMS. Even a cell phone battery with a wholesale cost of $8 has one. And they absolutely need them because of the risk of battery fires.

I was thinking of the 5-cell Panasonic prismatic units used in earlier versions of the Toyota Prius. I don't think divesting that division was a requirement for Panasonic when they bought Sanyo.

 

[hertamaniac]

Li-ion batteries routinely have some sort of onboard BMS. Even a cell phone battery with a wholesale cost of $8 has one. And they absolutely need them because of the risk of battery fires.

I was thinking of the 5-cell Panasonic prismatic units used in earlier versions of the Toyota Prius. I don't think divesting that division was a requirement for Panasonic when they bought Sanyo.

The BMS is not the main concern with Li-ion fires. It's the risk of a manufacturing defect in the cell construction that can lead to a thermal runaway that a BMS cannot attenuate. Once oxygen is introduced into a pierced Li-ion cell, it experiences an exothermic reaction which is violent. When compounded in a series of cells, it is the cascading effect that can cause catastrophic failures. One only has to look at the GS Yuasa Li-ion batteries on the Dreamliner 787 and their solution of compartmentalizing the modules.

You can vent cells via manifolds and check valves in NiMH which would not be able to be done safely in Li-ion. We ran a UL 24 hour C-rate overcharge test in our labs. The results, below, show the destructive nature in a Li-ion pouch cell.

So the lesson here is that cell architecture plays a key role when married to the cell chemistry for a given application. This is why there are pouch, wound, bi-polar and various other cell architectures. Couple that with different energy potentials in chemistry (even just in Li-ion beyond LiFePO4) that requires cell designers to meet an engineering requirement.