I saw this first thing this morning on the Facebook from the Lonestar EV Racing page, with this caption:
Can you believe this? The newest 150C rated batteries. This is 1000 peak battery horsepower now in just two standard Lonestar 13″x11″x9″ cases. … .
Huh? “Battery horsepower”? Is this even a term, nevermind a good concept? Let’s take a look…
If it is a term, it’s not one that the Google recognizes. Nuthin. So let’s look at the numbers.
- Horsepower is Power, Power can be converted to Watts, which takes us to the Most Awesome conversion site, RapidTables.com.
- Watts = Volts x Amps. OK, simple enough.
Let’s say I know my max amps is 300, because I have measured it, or it’s what my controller maxes out at, or whatever. I have a 90V pack, I’m running 300A (peak, presumbably) and so I’m making 27,000W, or 27kW. If we convert that to horsepower we’re looking at about 36hp.
OK, sounds useful. You could keep everything in watts, I suppose, too. My pack is a 27kW pack, my motor is a 30kW motor, life is good… but it gets you into the gasser comparison using horsepower pretty well. My old RD350 had “true rear-wheel horsepower” of around 39hp, so running the R5, for example, with a 36hp rated electric motor isn’t far off. If I do that with a pack that can deliver 300A, I’m likely going to have similar performance – factoring out the substantial difference of the powerband of the two-stroke (very narrow) vs the powerband of the electric drivetrain (pretty much a straight line). The performance numbers bear that out too – similar top speed, similar acceleration.
Let’s assume we don’t know the maximum current we can deliver. Say we’re trying to plan a pack. Then we have to look at our C Rating for the battery, which is what they’re doing on LoneStar. To get a grip on that, see my post on C-rates and C-rating, here. You figure, based on the C-rating of your cells, what your pack can supply. That estimate will give you the rating you need for maximum amps from your controller.
For that matter, you could figure your controller on wattage too… right? I have a 300A 90V controller, it’s giving me 27kW. Can we call that 36 “Controller Horsepower”?
Watts or Horsepower, it’s not too important – but, to follow this stream of (un)consiousness to it’s logical extreme, it does look like a good idea to use either to figure your system design. Match up the peak ratings for the motor with the controller specs. Match your pack to that. Call it whatever you like, but it’s all Power to me.
OK, let’s bring this real-world. I have a bunch of weird leftover parts, including a strange and wonderful GoPet scooter. It looks something like this:
OK, work with me on this. Say I want to make it go up the Mt. Washington Auto Road. It’s sporting a 350W motor, which is probably way too lamesauce for that. I’d think I need at least something similar to the 750W hub that Black Sparrow is running – he made it partway up and ran out of battery, but the motor did fine. So what do I have laying around for controllers…
I happen to have this cute little Kelly 48V “mini” brushless controller. It’s rated at 30A. 30A at 48V = 1440W, or 1.44kW. So, bam. This will work great on even up to a 1kW hub motor from Kelly. Like this:
Then I need to figure the batteries. Reversing from my 1.4kW at 48V I get back to 30A. I need a pack that can deliver 48V at 30A. Pulling from the linked post above, “If you have a 3000 mAh pack – any kind – and it is rated at 10 C that means it can deliver 30 amps.” So, round numbers, ‘most any battery chemistry will do, since my capacity (intimately related to discharge rate) is going to need to be well over 3Ah to make it up the mountain. The pack I’m planning on is the Zero “molicel” pack shown here – roughly 2kWh and probably capable of 5C. For sure, not the weak link.
So that works great. Make everything “Power”, and you have a plan.