# I think I need a higher C rating, but how can I be sure?

1537 Views 13 Replies 8 Participants Last post by  Duncan
Working on my first EV conversion so I'm learning some expensive lessons. I got a 96v 100ah LiFePO4 battery that unfortunately can only put out 100 amps continuously. I have a 24kw DC motor that should be able to handle 500 amps. The car only goes about 25mph. I think it's because the battery can't supply more current. I can get a battery that will put out 300 amps, but before I drop another 3k on a new battery pack I'm wondering if there is a way to verify that increasing current will actually make the car go faster. If this were a smaller project I would connect up a wired Power Supply to verify but I don't have a 96vdc 300amp Power Supply laying around the house.
Any suggestions?
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I'm no expert but, according to "the math" (24,000W/96V = 250A)

So, it looks like your 24kW motor
...when ran @ 96V
...would/should require ~ 250A

Um, have you looked @ used automotive batt packs?
...they can easily dis-charge 100's of amps

FYI:
(2) 45V 45AH Chevy Volt batt pack sections (wired in series) would make a 90V (nominal) 45AH batt pack
...& (4) of them (2 in series & 2 in parallel) would/could make a 90V 90AH pack

Heck, you could probably pick up a whole batt pack (6+ modules) for (~\$2,000.00)
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Thanks
I should have mentioned that the motor is from a 48v forklift, and there was a 500 amp fuse. The kw rating wasn't actually marked anywhere on the motor. I figured it was about 24kw because of math magic.
I will look into used car packs. Is there a best way to find them? ebay? copart? craig's list?
Thanks again.
Have you checked the cell voltages of your pack? One bad cell could be the weak link in the chain that is limiting the current. A weak cell would lay down under load; maybe the whole pack is weak or worn out, but you need some data to verify that and determine why you are speed-limited. Are the controller settings limiting performance due to Under Voltage or ??

i would think that 100Ahr cells could provide 300A for 20 minutes or less. A bench test of a cell would be needed to verify performance.
If you have a 48 V motor, are you sure you can run it at 96 V? What controller are you pairing it with?

One thing you could consider is deconstructing the pack and running it in a 2p configuration (48 V @ 200 Ah). That should give you a 200 A continuous draw rating, but given it's DC, I'm not sure how the lower voltage would affect your system.

In terms of speed, is it possible that your gearing ratio isn't appropriate? What is your current reduction gear ratio?
Hi - your top speed is 25 mph - that is a bit low - I would expect 96 volts to get you to 40 mph!
You need more volts to drive the current at higher rpm

My 48 volt Hitachi forklift motor limited me to 60 mph when I had 120 volts

For a road car with a forklift motor you should be aiming for a minimum of 140 volts
+1, Duncan nailed it--he's the sharpest tack in the box. the rest of us totally overlooked the motor voltage rating.

The 500 Amp fuse is there because at low fork-lift speeds under load that motor can pull a bucket load of amps that could melt the wiring.

In your car at much higher-than-forklift speeds the back emf generated by the motor and which opposes the applied voltage, has reached the limits of your pack and controller. You will need a higher voltage pack in order to push current against the bemf to reach higher rpm.
What Cells you use directly? LiFePO4 Cells are often very powerfull (high kW/kg) and had a average Energy Density (kWh/kg). So 5C are often no problem, some Cells could manage 10-15C for short Times.
+1, Duncan nailed it--he's the sharpest tack in the box. the rest of us totally overlooked the motor voltage rating.

The 500 Amp fuse is there because at low fork-lift speeds under load that motor can pull a bucket load of amps that could melt the wiring.

In your car at much higher-than-forklift speeds the back emf generated by the motor and which opposes the applied voltage, has reached the limits of your pack and controller. You will need a higher voltage pack in order to push current against the bemf to reach higher rpm.
In electric vehicles voltage = speed. A 48 volt motor will never go fast enough for a car. It’s not battery voltage it’s motor voltage. You need a higher voltage motor and controller. 120 volts is what you need for highway speeds.
Thats wrong. You could build a 12v Motor that spins with 120000rpm...
In electric vehicles voltage = speed. A 48 volt motor will never go fast enough for a car. It’s not battery voltage it’s motor voltage. You need a higher voltage motor and controller. 120 volts is what you need for highway speeds.
It's not that simple. For the same motor, higher shaft speed requires higher voltage, but both motor design and gearing (and so shaft speed) are variable. For instance, a typical modern production EV can go 150 km/h with it's 360 volt battery, but since the motor and gearing are designed for that with 120 volts it would not be capable of highway speeds. At the same time, a 48 volt motor could certainly be designed to handle highway speed in the same car... but would be massive and would require a thousand amps of current.

If you're using a typical obsolete brushed DC forklift motor which was intended for operation on 48 volts, with that specific motor design and size then yes, 120 volts is likely required for highway speed.
With a 48 volt motor and 130 volts I was limited to 60 mph

You need at least 144 volts for such a motor to have a sensible road speed
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You cant just compare that! Its like "My green car only drive 90mph, but my red one drives 110mph. So i paint the green in red and i could drive 110..."
Yes I can compare them - and I just DID
A 48 volt forklift motor will be pretty damn similar to another 48 volt forklift motor - its the nature of the beast
And the old old old numbers for an EV with a DC motor were simple -