Interested in NiMH scooter batteries?

[bmazin]

You've got the idea - the specs you need to compare are the discharge rates at 1C (1 hour dischage). Let me try to get the numbers more exact. At full throttle the Mako will burn through the "17.2 Ah" Yuasa Np18-12 in 50 minutes. The one hour capacity of this cell is actually 12 Ah - this means the Mako is drawing about 13-14 amps or so.

For a 26 Ah NiMH battery pack a 13 amp current means that the scooter will discharge the pack at .5C, and you get very close to the full 26 Ah at this discharge rate, so the burn time should be a full two hours. The numbers are about the same for the short body gavin ("18 Ah").

The "33 Ah" version of the Gavin would need a 52 Ah NiMH pack to get the bouyancy right, and this would cost a fortune, so it probably not going to be popular. However, with NiMH the short body gavin (66 lbs) would have the same life as the standard version (93 lbs), which is nice.

You are right that the upfront costs of the NiMH are greater, and the cost of a flood goes up. I still think some people will like to have the NiMH option available.

Ben

 

[Aviatrr]

I think what happened with your experiment was that your pack was rated at 16 Ah and you probably used the Mako/Gavin burn tester, right? This means that you dischaged the batteries at 1.3C or so (1C means you discharge the batteries fully in 1 hour).

Wait a second.....let me get this straight....you claim that a 26ah pack will be discharged at .5C, but a 16ah pack would be discharged at 1.3C? Is this that new math I've heard about? That would mean that the scooter draws approximately 20A.....not quite. 12-14A is more like it. Therefore, I was discharging at less than 1C(approximately 1.25C, if you want to get technical).

Also, if you're only paying $450 retail for 2 20 F cel packs, you may be getting some low quality batteries - there are some real differences between the different manufactures, especially at high discharge rates.

I would not be paying retail....the packs would be closer to $750-800 if I were. They are good cells, made by a well known high quality manufacturer, and are designed for high discharge rates.

Mike

 

[bmazin]

Aviatrr once bubbled...

Wait a second.....let me get this straight....you claim that a 26ah pack will be discharged at .5C, but a 16ah pack would be discharged at 1.3C? Is this that new math I've heard about? That would mean that the scooter draws approximately 20A.....not quite. 12-14A is more like it. Therefore, I was discharging at less than 1C(approximately 1.25C, if you want to get technical).
Mike

There are a lot of numbers floating around - sorry if there is confusion.

You discharged the batts in 42 minutes, so as you stated you were discharging at greater than 1C - the capacity was just less than you expected.

I'm not sure why it was so much lower than specs - what battery cutoff voltage did you use? What was the resistance of the burn tester? Even discharge rates of 2C only lower NiMH discharge capacities by 25% or so, making your burn data hard to understand. The capacity you observed

capacity = 42 minutes / 60 minutes/hour * 13 Amps = 9.1 Ah

seems to be about what you'd expect from just 1 D cell pack. Strange. In my experience the NiMH conform pretty well to the spec sheets.

Ben

 

[Aviatrr]

Therefore, I was discharging at less than 1C(approximately 1.25C, if you want to get technical).

Well, I'm sure you figured out that I screwed up the above....it should have read approximately .81C......just a good reason as to why you(or at least, I) shouldn't drink and post.

I'm not sure why it was so much lower than specs - what battery cutoff voltage did you use?

20.0v

What was the resistance of the burn tester?

1.5 Ohms. There are two 3 Ohm 300 Watt resistors wired in parallel.

seems to be about what you'd expect from just 1 D cell pack.

That's kinda what I thought....but I had a meter on each pack and the voltage dropped consistently between the two. Those same batteries, as two 12v 16ah packs, burn tested for 8.5 hours on an 18W HID until reaching 10v.....the packs came within 5 minutes of each other for burn time.

I'm not sure exactly why my results came out as they did....but it made me decide not to drop $450 on batteries to test my scooter theory. I'll wait a couple months for my buddy to get some new batteries for me to play with......

Mike