OK so i have a 36v 700w 2500 rpm electric motor and i dont want PHAT EXPENSIVE batteries
...also do you think the batteries may explode for any reason?
Thank all of you!
First, I fully understand and respect the fact you're not looking for maximum preformance, I too love to tinker just to see what I can get myself into & I am
not recommending you go out and spend a ton on some fancy-pants batteries that cost eleventy-billion dollars each, in fact I'm not even recommending against anything you've asked about... but I did wish to mention the absolute number one reason folks find tinkerin' with electric vehicles disappointing is the choice made to scrimp on batteries.
Unlike most vehicles where if you're looking for preformance, you concentrate on the engine - electrics are a bit different in that while the motor isn't unimportant (and yours is quite respectable), the batteries are the deal breaker... much like a Chevy V8 won't run very well or for very long if fed by a teaspoon sized tank through a pinhole.
Cordless power tool batteries are actually a bit of a rip-off, unless you've "free" ones kicking around, I wouldn't recommend buying any for this project. While they seem far less expensive than the lithium or LiFePO4 battery packs usually chosen for electric vehicles, it's only because you're getting a fraction of the the capabilities & paying a lot more for that fraction...
As an example, the following is based on two of the least expensive no-name 18v replacement tool batteries I could find (in a quick search);
18V NI-CD Replacement Battery for DEWALT (cloned/generic) $39.89ea x2 = $79.78 (S&H not inc)
18v 2000mah x2 = 36v 4000mah = 4ah
http://www.rapidtables.com/convert/charge/mah-to-ah.htm
36v @ 700w = 19.4a
http://www.rapidtables.com/calc/electric/Watt_to_Amp_Calculator.htm
time of operation @ max = 0.2hr
...but you
will not get that result as you won't be able to achieve maximum operation as cordless power tool batteries are unable to supply the required amperage, a typical DeWalt drill needs 2.6 amps no-load so you'll get a much,
much lower achievable RPM under load/speed & there's the slight possibility of damage to the cells as you're essentially "shorting" them w/such an amp demand.
Compared to;
36V 10AH LiFePO4 Battery Pack $318.00
time of operation @ max = 0.5hr
....only this time you will be able to achieve maximum operation @ full amperage for the duration.
Which granted, seems like a lot but to get a similar amperage
and capacity from the above tool batteries, you'd need at least five of them for a total of $199.45...
So, $199.45 to be able to use your motor to it's potential with cordless tool batteries, or $318.00 for a LiFePO4 pack... but I'll mention Ni–Cd is only good for 500 full recharge cycles, whereas LiFePO4 can exceed 2000 charge cycles - so if there were long term usage goals, to meet or exceed the $318 LiFePO4 pack with these Ni–Cd cordless tool batteries, it would cost something like $797.80...
Yea... and that's the problem with electric vehicles lol, much as I love 'em any way you slice it that's a lot of pennies to cough up for a whopping 0.93hp for maybe a 1/2hr of use (10ah/20a);
http://www.rapidtables.com/convert/power/watt-to-hp.htm
BTW, LiFePO4 is significantly more expensive than Lithium, but I didn't use Lithium packs for the example as they're chemically & thermally unstable - the potential for fire isn't worth making the math seem more impressive, they're good and all but some experience is defo recommended.
While they're comparatively heavy & don't have the cycle life - for what you seem to want from this project Id' reco something like these basic SLA scooter batteries (or larger) as they're really inexpensive, still pretty small, safe, very easy to use, charge and can supply the amperage you need;
12V 5Ah SLA $14.98ea x3 = 36v 15ah for $44.94 total (free S&H) - all that for the cost of just
one cordless tool battery
