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Old 04-17-14, 10:04 AM   #1
whizzkid14
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noob needs help before buying battery..

hi every one, been looking through the site and I must say its very good indeed I would be great full if some one with more experience could give me some advice..

right then. I have a very basic understanding on bike battery's but im stumped on some things namely this..

I understand that on batteries volts x ah = watts im looking to purchase a 36v 800w ebike kit of ebay. now I understand that the wattage wont be 800 watts more like 20amps x 36v = 720w and then they add up the extra to make 800...

The battery iv been looking into is a 36v 10ah lithium battery. when working out the batteries watts I do my volts x ah and this makes 360watts and this is obviously half the wattage of the controller.

my concerns are this. if the battery can give out 10 amps but the controller wants 20amps will this burn out the battery or will it mean that the motor will take 20amps but drain the battery twice as fast as if I was running 10amps?

sorry if this seems confusing. but really do need some advice asap . I live in a very hilly area and will have to use full throttle to get me up the hills but didn't want to spend 250 on a battery that will get burnt out on the first hill because the controller wants more than the battery can give

. iv looked into the smaller wattage kits but they don't have the torque that the 800w kit has (40nm).. I'm aware I could just buy x3 12v 20 ah SLA batteries but would want to avoid that. sla's don't really belong on a modern ebike and the cheapest 36v 20ah lithium / lipo battery is near on 400 and I aint got the money for it.

I look forward to your replys

many thanks whizz-kid
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Old 04-17-14, 12:17 PM   #2
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Welcome to the Bike Forums whizzk!

Ok, first off, ah stands for "amp hours". So, a 36v 10ah battery will deliver 10 amps for 1 hour, or 20 amps for a 1/2 hour, or 5 amps for 2 hours, and so-forth, before reaching the battery's minimum discharge voltage. You can use this ah rating to determine the range or battery "run time" at various amperage draw rates as long as you don't exceed the battery's maximum discharge rate (which brings us to the issue below):

You'll also want to know both the battery's continuous and maximum amperage discharge rates. A battery's continuous discharge indicates what it can safely discharge continuously (i.e. in this case 10 amps continuously), while the maximum discharge rate indicates what it can safely discharge for only a few moments. A battery with a continuous discharge rate of 10 amps might be able to put out a maximum of 20 amps or more for 20-30 seconds or so.
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Old 04-18-14, 05:58 AM   #3
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Just sit right back and you'll hear a tale, a tale of a fateful first battery purchased off eBay, that started from these very boards. Take from it what you will, and make your own decision:

Seller advertised a 36v 10Ah LiFeP04 battery kit for a remarkably good price. I ordered. It arrived from Canada quickly. So far, so good. I did notice the packing material inside the box was newspaper over 3 years old, but, meh, packing materials don't necessarily equate to manufacture date.

Over the course of the next month, I built up the bike and took it for a test ride. According to my calculations, I should have gotten about 24 miles out of the battery on the 250W front hub motor set up I built. I barely got 12 before the battery cut out. This happened several times. I decided to take the battery apart and test the individual cells. The cells tested all over the place: from 3.97 volts to 4.23 volts, and every reading between. Several of the connectors between the cells were cracked, the wires connecting the cells to the balancer looked to have been soldered by blind drunken monkeys, and the internal date stamped on the battery showed it was indeed over 3 years old.

The best part, though, was the battery was clearly LiMnO2, not LiFePO4. Oh, wait, no, the best part was by the time I figured this out, 2 months down the road, the seller was long gone, no longer registered on eBay, and PayPal's response was "sucks for you, doesn't it?"

So this is the tale of a cheap castaway eBay battery. IIWY, I'd save myself time and aggravation and buy a quality battery from a reputable dealer who stands behind his/her products. Grin, Ping, BMS - they're all mentioned on this forum and on Endless-Sphere over and over again for good cause.

To finish my tale and be quit with it: I ended up spending an additional $250-ish and bought 4 x 24v 5Ah LiPos from HK to make a 48v 10Ah pack. With a quality charger from Progressive, it makes a nice set up with which I'm very satisfied. It was, however, an expensive lesson to learn.
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Old 04-30-14, 06:58 AM   #4
jimisox
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Great questioning! I'd go with a 48 instead of tgst 37. Pay the extra, it'll be to your benefit in the long run.
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Old 04-30-14, 06:34 PM   #5
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Quote:
Originally Posted by whizzkid14 View Post
hi every one, been looking through the site and I must say its very good indeed I would be great full if some one with more experience could give me some advice..

right then. I have a very basic understanding on bike battery's but im stumped on some things namely this..

I understand that on batteries volts x ah = watts im looking to purchase a 36v 800w ebike kit of ebay. now I understand that the wattage wont be 800 watts more like 20amps x 36v = 720w and then they add up the extra to make 800...

The battery iv been looking into is a 36v 10ah lithium battery.
I look forward to your replys

many thanks whizz-kid
36V 800w kit will require 1600w to output a peak 800w.
1600w / 36v = 44.4A
So a 10Ah battery would have to be ~ 5C capable
Continuous 800w output would drain battery completely in about 10-12 minutes.

Fortunately,

continuous :

15 mph cruising uses only 150w, ~80% efficiency = 187w.
36V x 10Ah / 187.5w = 1.92 hours ~28.8 miles
cruising at 15mph from a 10Ah battery.

20 mph cruising uses only 340w, ~80% efficiency = 425w.
36V x 10Ah / 425 = .84 hours ~50 minutes ~17 miles
cruising at 20mph from a 10Ah battery.

See - Motor Output Watts for MPH

You must determine your desired distance and speed, then you can figure your required battery size.

SLA batteries only output about 50-60% usable of "rated".

Last edited by DrkAngel; 04-30-14 at 07:14 PM.
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Old 05-01-14, 05:26 PM   #6
focusdesigns
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Quote:
Originally Posted by DrkAngel View Post
36V 800w kit will require 1600w to output a peak 800w.
1600w / 36v = 44.4A
So a 10Ah battery would have to be ~ 5C capable
Continuous 800w output would drain battery completely in about 10-12 minutes.

Fortunately,

continuous :

15 mph cruising uses only 150w, ~80% efficiency = 187w.
36V x 10Ah / 187.5w = 1.92 hours ~28.8 miles
cruising at 15mph from a 10Ah battery.

20 mph cruising uses only 340w, ~80% efficiency = 425w.
36V x 10Ah / 425 = .84 hours ~50 minutes ~17 miles
cruising at 20mph from a 10Ah battery.

See - Motor Output Watts for MPH

You must determine your desired distance and speed, then you can figure your required battery size.

SLA batteries only output about 50-60% usable of "rated".
Well said!
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