question about NIMH in Toyota Prius (or any other hybrid car)

[viorel00]

PRIUS has "Sealed 168-cell nickel metal hydride (NiMH) battery providing 201.6 volts" (wikipedia). So the cells are in SERIES. at least for discharge. I just wonder if they are also charged in series or not, because it is common wisdom on this forum that repeatedly charging/discharging cells in series (i.e. flashlight and then dumb charger) is not good for the cells.

If the cells in Prius are NOT charged in series, I wonder how they do that, is there an bank of relays that switch ON and now the cells are in series (for discharge) and then switch OFF and the cells are now individually charged???

anyone?

 

[chimo]

This is what Google turned up.

 

[evan9162]

The prius cells are charged/discharged in series. The charge level of the pack is maintained between 40% and 80%. The batteries are specifically designed for extremely high charge and discharge currents so charging them in series is not a concern.

Mass produced commodity batteries are constructed in a completely different manner (i.e. cheap), and thus, many of the concerns with consumer NiMH batteries don't apply to the Prius pack - one of which is significant performance differences between individual cells that can cause a large charge imbalance within the pack.

 

[bob_ninja]

Can anyone explain in detail the logic for 40%-80% SOC limits?

 

[SilverFox]

Hello Bob,

Primary reason is longer cycle life. Also, when at 80% they can still handle a regenerative braking charge surge without problems.

Tom

 

[bob_ninja]

Ah, right. Never 100% to leave some room for regen breaking. Forgot that one.

What about 40% min limit. Sounds rather high?

I am still puzzled by how they manage to maintain so many cells balanced when we have problems with 12-16 cells in power tool packs???
Assuming excellent quality control that ensures uniform cell performance from factory, over their lifetime they would degrade at a different rate, no? How does charge current get distributed to maintain a similar SOC between so many cells?
Conversely, is there something in the design of this pack that can be applied to power tool packs to make them more robust and durable, besides simply using higher quality cells?

 

[SilverFox]

Hello Bob,

I believe that 40% allows for voltage retention during hard acceleration before the generator kicks in.

I am not sure what the Prius uses, but there have been several studies involving active cell balancing during charging. Each cell is monitored individually and the balancing program makes sure the cells stay in balance.

The DeWalt A123 battery packs have this incorporated in their charging system.

Tom

 

[wptski]

I remember somebody stating that "one" of these car battery packs was $4,000, so your comparing that to a DeWalt? They may have individual taps for balancing but that's just a guess.

 

[65535]

Depending on how their wired (doubt you will find this out) it is highly possible their are built with balancing taps into each prismatic pack can be charged in series and balanced via the computer similar to what happens to thunder tiger rc packs (lithiu polymer) cells are connected normally and have a special (and simple) wiring system that uses weak leads to each batter that balances them. check out www.rcgroups.com and search for balancing cells.

 

[bob_ninja]

Not making any comparisons. Just trying to figure out the differences. So the majority of battery packs for power tools are "dumb" and only higher end ones like (Tom said) DeWalt A123 have the balancing circuits. Got it.

thanks for info

 

[wptski]

Way back somebody, somewhere posted a link to, I think it was Panasonic who made a battery pack, not sure if it was the Prius or what. Pictures showed a almost like clean room atmosphere!