Prius traction batteries

[Christ]

Bob, I wanted the NiMH modules as direct replacement for the SLA that came on the bike hoping for the benefits of reduced weight/size.

Based on our discussions here, I don't think that I'll be able to achieve those goals without added complexity, but provided the circuit is inexpensive and relatively simple for a novice, I'll humor it.

I'm also thinking that since the bike's electrical system is rated at 6VDC, the battery module could remain somewhere well below the 8V threshold that you point to and still be able to perform it's duties acceptably. I'd be guessing here, but I'd think that 6.5VDC is probably the low-end of the safe SOC spectrum. If you've tested this, care to enlighten?

I know (not know, but have understood from reading) that the NHW20 modules are charged in such a way that they maintain a tight threshold between 50% and 80% SOC, so as not to reduce the life of the pack. Toyota claims that the pack should last the usable life of the vehicle, and I've seen nothing to indicate otherwise using this method.

Is there any way that I can clamp the rectifier's output to under 8V using an inline circuit, or is that what we're working on now?

[bwilson4web]

Originally Posted by Christ

. . . I wanted the NiMH modules as direct replacement for the SLA that came on the bike hoping for the benefits of reduced weight/size.

Hopefully the earlier discussion of relative battery chemistries explains why that is improbable.

Originally Posted by Christ

. . . Based on our discussions here, I don't think that I'll be able to achieve those goals without added complexity, but provided the circuit is inexpensive and relatively simple for a novice, I'll humor it.

Let me suggest searching for the parts needed and posting the list. I can give comments about the strengths and weaknesses. More importantly, you will have identified your source and know the price.

Originally Posted by Christ

. . . I'm also thinking that since the bike's electrical system is rated at 6VDC, the battery module could remain somewhere well below the 8V threshold that you point to and still be able to perform it's duties acceptably. I'd be guessing here, but I'd think that 6.5VDC is probably the low-end of the safe SOC spectrum. If you've tested this, care to enlighten?

Tested, theory verified (see web page,) and the answer is no. Each cell has a nominal voltage of 1.2 VDC and there are six of them giving 7.2 VDC for the module. Once the NiMH cell falls below 1.2 VDC, it is exhausted. One other 'death' is reverse charging a cell and the 1.2 VDC rectifier is insufficient protection. If you ever run the battery down, it is a coin toss whether it will ever come back.

Originally Posted by Christ

. . . I know (not know, but have understood from reading) that the NHW20 modules are charged in such a way that they maintain a tight threshold between 50% and 80% SOC, so as not to reduce the life of the pack. Toyota claims that the pack should last the usable life of the vehicle, and I've seen nothing to indicate otherwise using this method.

They use a dedicated, battery control module who handles everything. I'm proposing something very cheap, less robust but probably usable.

Originally Posted by Christ

. . . Is there any way that I can clamp the rectifier's output to under 8V using an inline circuit, or is that what we're working on now?

That is the circuit I described. The linear regulator is adjusted to limit the maximum voltage to the NiMH battery. That is why it is in parallel but reversed to the protection rectifier:


Bob Wilson

[Christ]

Bob -

I appreciate your efforts, and I'm sincerely hoping that if you build and test this thing it will prove fruitful to your studies of the technology, but I just don't think it's going to be something feasible for me to do right now.

I guess I was kinda looking for an "easy way out", if you know what I mean. I'd like to be supportive and provide any assistance I can, but I'm going to have to say that this is outside my current level of capability. I'm working with another member to rectify my mistake (buying the modules with improper intent for them) in a satisfactory way.

Please, if I can be of any assistance with measurements or whatnot when they do finally get here (shouldn't be too much longer) let me know!

[bwilson4web]

Originally Posted by Christ

. . . I just don't think it's going to be something feasible for me to do right now. . . .

No problem. If you change your mind, let me know.

BTW, I found an interesting converter circuit last night called a Split-Pi. A UK patented design, it provides symmetrical, buck/boost with bi-directional power flow. The designers claim exceptionally high efficiency, 98%.

Bob Wilson

[dcb]

Hey Bob, I'm actually playing with some NHW20 modules now, and my tiny rc smart charger rig is struggling just to do one at a time, at 200milliaps (it's a problem w/my setup).

Is there a "trickle" charge rate that you would consider safe for these? Can I just slap a 200ma source together and do them all at once and not worry about it do you think?

[bwilson4web]

Originally Posted by dcb

Hey Bob, I'm actually playing with some NHW20 modules now, and my tiny rc smart charger rig is struggling just to do one at a time, at 200milliaps (it's a problem w/my setup).

Is there a "trickle" charge rate that you would consider safe for these? Can I just slap a 200ma source together and do them all at once and not worry about it do you think?

Owch!

I would want the tickle charge to be a lot less, closer to 2 ma. Even then, I'd want an over-voltage and over pressure safety.

Bob Wilson

[dcb]

you take the term "trickle" very seriously ok, I'll think of something else.

FYI, .2 amps hasn't phased them heat and buldging wise (knock on wood) , and given they can stop a car (or at least slow it down a little) I didn't expect it too.

[bwilson4web]

This is what happened when someone left their solar array charging NHW20 traction battery on a 'cloudy day:'


The thing about NiMH chemistry is they have this relatively flat voltage vs charge curve until the very end of the charge. Then there is a bump up and they become 'wicked.'

I believe Radio Shack carries something like an LM317L and if you bias it for 7.8-8.0 V. max, you should be safe. But anything over 8.0 V. and my experience is the NHW11 modules (and I suspect the NHW20s) become 'gassy' and bad things can happen. You'll need ~1.2 to 1.5 V. head room above the output adjusted voltage. A 9-12 V. 'wall wart' would be sufficient.

Of course the other approach is to detect the temperature rise but that tends to lag gas generation.

Bob Wilson

[dcb]

ah, I can probably improve on a constant voltage source. Will probably pick up one of these and a mosfet and crank up the charge rate:
BQ2002PN Texas Instruments Battery Management it does fast charge/peak detection/thermal option/pulse trickle (supposedly better than constant trickle)

[bwilson4web]

That looks like a nice part. I didn't see in the datasheet how the MOSFET works to handle a six-cell, module but I eventually found another application note that did.

Bob Wilson