24V 20Ah vs 48V 10Ah, or something similar

[fitek]

Just curious if anyone has tried running their bike with the same batteries at a lower voltage but with a higher Ah rating.

I'm guessing due to the Peukert effect, especially with SLAs, the higher voltage, lower capacity would be better. But that's just a thought a real test would answer this quickly.

All my batteries are 12v 38ah (bit heavy to mount 4 at once to a bike) and I don't have a 48v controller anyway.

 

[steveo]

This would all depend on the chemestry

say a123

even if you used 11.5ah worth of batteries .. say 30 mini 5p packs in series.. @ 100v

you would have a great top speed ? & low end power would be better!

say you do 23ah worth of batteries.. say 30 10p packs in series say 66v .. your top speed still would be good and range will double & with a123 batteries they will not suffer from that overdischarging effect.

-steveo

 

[safe]

My bike is made of three 38 Ah SLA batteries and it does pretty well.

(40 mph top speed when all things are working right)

Actually I was pondering the idea of getting five 12 volt 18 ah batteries to replace what I have so that I would have a 60 volt system. I'd have to change my gearing, but it would definitely give some nice peak performance for a while...

 

[fechter]

Assuming you're geared for the same top speed in either condition (therefore the same power consumption), there won't be much difference in the configurations. The I2R losses in the battery wires will be less for the higher voltage condition, but that loss will be fairly low anyway if the wire is properly sized.

If the gearing stays fixed, you'll go half as fast at the lower voltage, consume less power and discharge at a lower C rate, which will give you much more range.

 

[safe]

If the gearing stays fixed, you'll go half as fast at the lower voltage, consume less power and discharge at a lower C rate, which will give you much more range.

That's the biggest problem... the "C" rate... when I look at ideas involving more batteries with less Ah's it translates into higher and higher "C" rates. With SLA's that means you get worse and worse range and lifetime, but for a short time you get higher peak performance. SLA's can pull 5C when they are new... but within a year you can pull about an equivalent of 4C, then a year later more like 2C or 1C. Things seem to get much worse as time goes on. (the decay is "progressive")

 

[fitek]

Thanks guys. Sounds like practically there isn't too much difference. I totally forgot about gearing, which does bring one thing up-- if the motor spins faster, I need bigger gear reduction (since I'm one of the few here who doesn't seem to be a top speed junky ). For me, I'm already maxed out in whats commonly available. Adding a bigger sprocket will add slightly more weight too and some cost besides that of the controller.

 

[fechter]

If the motor is spinning faster, it will waste a bit more power due to windage and core losses also.

 

[safe]

There are times when I go in circles with my designs because I try to maximize one thing and then it cuts into something else. You just have to arrive at a configuration that matches what you want to do and go with it. SLA's have their limits... the Peukert Effect tends to discourage the higher "C" rate options so you are often better off from the battery standpoint with something that is worse for the motor.

One or the other is unhappy...

These newer chemistries (NiMh or LiFePO4) allow for much more flexibility in design, but at a higher introductory price. Having the ability to pull 5C or 10C can completely change your design specifications.

 

[Drunkskunk]

I have a dual tapped motor running on a dual voltage system. My choices are 37 or 74 volts, and a 4012 or 408 motor.

What I've found is efficancy is based on speed, and the voltage is almost irrelevent. the higher the windings, the lower the peak speed so the higher the efficancy at lower speeds, but it doesn't matter what voltage i'm at in my testing so far.

But the preformance diffrence is astounding. its not just about speed, at doubled voltage, you get higher wattage out of the motor at the same amperage. and since the base coil resistance stays fixed, your initial current can be higher with the higher voltage, so your wattage available fromt he motor increases there, too. what you end up with is about twice the peak power (due to back EMF resistance increasing) but more than twice the power for lower rpm acceleration.

 

[safe]

My choices are 37 or 74 volts.

Judging by the numbers that looks like LiFePO4 which doesn't have the Peukert's Effect losses when you switch to higher "C" rates. You end up being far more restrained with SLA because the higher "C" rates mean a drop in total Watt Hours delivered. It sort of forces you into having big batteries and low current rates in order to get anything out of the battery. This means more weight. But on the positive side if you can set things up so that you can manage the weight (and my batteries on the #001 bike weigh 86 lbs) then your range stretches out a great deal. When the batteries were new I used to get about 20 miles of hard riding out of them.

What you say is true for LiFePO4 or NiMh, but SLA is a special case with it's own special rules...