Prius traction batteries

[bwilson4web]

Ahhh, now it makes sense. Let me see if I can explain why the NiMH battery is not a one-for-one substitution for a lead-acid battery.

This is the typical charge curve for lead-acid battery:

Source:
Charging the lead-acid battery
The lead-acid battery nicely develops a back-EMF, an anti-charging voltage, so it stops taking current. But that is not the case with the NiMH batteries.

This is what an NiMH battery does:

Souce:
Battery Chargers and Charging Methods
It just keeps taking a charge and transitions from charging to gas generation and heating mode ... until it does something bad.

The two battery chemistries have distinct charging characteristics and are not a one-for-one substitute. The charging circuit has to be matched to the battery chemistry. The lead-acid battery having a longer history is really pretty easy to deal with. NiMH dates from the early 1990s and takes an entirely different approach.

We can design a circuit to let the NiMH battery replace the lead-acid original. However, it won't be trivial. The question is what advantage does the NiMH battery and specialized charging circuit bring over the easier to deal with lead-acid battery. This is not a question I can answer.

Bob Wilson

[Christ]

Bob, I can answer that question for you. There is not enough of an advantage to warrant the experimentation and expenditure that would be necessary to properly re-purpose the charging system for the battery module.

If you'd like to chime in in the other thread, I'd appreciate any thoughts about cap usage, as well.

I'll continue to input whatever ideas/thoughts I can contribute to this thread, whenever it's valid, as well. I want to learn about it, just not at my own expense.

[bwilson4web]

The capacitor solution has risks too. One of the things a lead-acid battery brings is a voltage somewhat independent of the load. As such, it helps keep the voltages constant versus wandering. A capacitor is going to show an immediate voltage drop under a current load ... it is how they work. In contrast, battery chemistry just increases its activity to sustain the voltage while current varies.

It is an interesting challenge, trying to put together a capacitor system substitute for a lead-acid battery. I'll have to think about it.

Bob Wilson

[Christ]

Bob -

In the essence of simplicity, would it be better to just get another lead acid battery to replace the old one? I still have the old one, I was considering cleaning it out and trying to bring it back to life with a new sulfuric solution, or converting it to alum.

(Roundabout way of saying "I give up!!!" or in the words of Charlie Brown "AAAARRRRGGGGHHHHH!")

[bwilson4web]

Originally Posted by Christ

. . .
In the essence of simplicity, would it be better to just get another lead acid battery to replace the old one? I still have the old one, I was considering cleaning it out and trying to bring it back to life with a new sulfuric solution, or converting it to alum.

(Roundabout way of saying "I give up!!!" or in the words of Charlie Brown "AAAARRRRGGGGHHHHH!")

First I appreciate the effort! Too many are unwilling to do the test in the first place. But I have been thinking about the problem and may have a solution. We have a similar problem with the Prius in folks would like to replace the heavy, lead-acid, aux battery with a lighter weight, NiMH assembly.

The solution is a buck-boost, switching power supply designed to let the NiMH function like the lead-acid battery. It handles charging the NiMH battery when power is available but instantly switches to a high-efficiency, high current, step-down circuit when there is a load:

• high current magnetics - needs to be sized for maximum current
• high current Schottky diode - alternative is sync. MOSFET
• high current discharge MOSFETs - to handle starter loads
• low current charging MOSFETs - to recharge NiMH battery
• microprocessor controlled - handles health and status of NiMH

In theory, almost any light-weight, battery technology could be used with this proposed circuit. The race is to find out if the weight of the magnetics, power MOSFETS and heat sinks is less than just using a lead-acid battery. Definitely an interesting problem.

Bob Wilson

[Christ]

I'm going to guess in my case it probably won't be. The 12Ah 6VDC battery that came on the bike only weighs a couple lbs as it is, and I'm guessing by the time I mount the module and build the circuit you propose, I'll either be at the same weight, close to it, or just too complex/expensive.

What is a good estimated cost for the proposed circuit? (Scratch built, of course.)

[dcb]

300 man hours

[bwilson4web]

Originally Posted by Christ

. . .
What is a good estimated cost for the proposed circuit? (Scratch built, of course.)

Compared to the design, integration and test labor, the parts, ~$100, are free.

Bob Wilson

[Christ]

Yep, it's outside my ability right now.

What direction should this thread go in to be helpful to other (potential) EV'ers?

Still debating on whether to use the batteries for something, or just sell them to someone who will use them.

[bwilson4web]

Originally Posted by Christ

. . .
Still debating on whether to use the batteries for something, or just sell them to someone who will use them.

Hold on to them for a week. I have a kludge, a somewhat inefficient circuit, that I could test one of mine with. It would NOT be pretty but it might work. I think I've got the parts in my junk box.

You want to be able to start the bike with the NiMH modules, right?

For those interested, the kludge uses a forward bias Si rectifier to drop 1.2 V from the nominal 7.2 VDC B+. But in reverse, an adjustable LM317L is biased to provide no more than 7.9 VDC charge voltage. Empirically, I've seen a little over 8 VDC has the peak voltage these units should ever see. Meanwhile, the rectified alternator is tied to the vehicle 6 VDC supply to supply bulk power for lights and horn.

The theory of operation is:

• normal operation, engine off - whatever load flows through the rectifier runs the lights at ~6 VDC while the NiMH battery (hopefully) floats up in the 7.2 VDC range. About 20% of the power is wasted heating the rectifier. This mode should be as brief as possible.
• normal load, engine running - the vehicle B+ load is provided by the directly attached alternator, thus helping the NiMH stay unloaded. The rectifier prevents unregulated flow back into the NiMH that can generate excess gas.
• starting - the starter B+ side lead is directly connected to the NiMH battery. This avoids having to size the rectifier to handle starter current load.
• reverse bias LM317-type regulator - biased to provide no more than 7.9 VDC to the NiMH module, this will keep the NiMH modules under the 8.0 VDC that seems to be the peak voltage for these modules. This can be tweaked down to 7.8 for a little more safety margin. It won't be a 'fast charge.'

In practice, you'll want a higher power, adjustable voltage regulator but these parts are available from Radio Shack (not the best source.) I've got them in my junk box and a 6 V battery charger. I can at least test the theory of operation.

You'll need to figure out the maximum current draw by the lights, turn signal and horn. Does your bike have one or more fuses identified for specific loads? Whatever the fuse current rating is will be your rectifier current load limit.

Understand this is a KLUDGE as in a McGyver type circuit. It is just as likely to fail due to things we don't know about your bike alternator and loads. Also, it is not terribly efficient, the rectifier and regulator will need a heat sink.

Bob Wilson