batteries

[mnplus1556]

hello, so right now i'm building an ebike with heavy sla batteries. it will run on 24v and go a max of 20 mph. i was wondering: if i upgrade my batteries, what type should i get? and why? i know that if i want lifepo4 i should go with ping. but where would i go to buy a lipo or li-ion pack? and what does it mean to have a max continuous discharge rate of 30C? i no what the continuous discharge rate is, but what does the 30C mean?? thanks everyone!

 

[Alan B]

C is the one hour capacity. So 30C means you can drain the battery in 2 minutes. If the battery is rated at 5 amp hours you can draw 30*5 or 150 amps. The rating may be optimistic, so add a safety factor of perhaps 2. So don't go over 75 amps.

 

[Alan B]

For Lipo most folks use HobbyKing.

 

[jimw1960]

Most people get their Lipo batteries from Hobby King. http://www.hobbyking.com/hobbyking/store/uh_index.asp
You'll want to read up about Lipo before jumping in the pool, but plenty of good information here on setting up a Lipo system.

The C number is an indication of how much current a battery can supply. You multiply the C number times the Amp-hour capacity of the battery and that tells you the maximum continuous current without ruining the battery. For examply a 20 amp-hour battery rated at 5C, regardless of the voltage, can safely put out 100 amps. Ping batteries are geneally rated at 2C continous, so a 15Ah battery can deliver a constant 30 amps, which is enough for many ebike applications. But if you want to run big motors at high amps, then you would want a higher C-rated battery, that uses cells like headway (5C) or A123 (10C), or Lipo (15C and up). Lipo can put out crazy high C-rates, but if you are only running a 25-amp controller or less, you don't really need it.

 

[mnplus1556]

ok thanks you guys! now, what do you suggest: lipo or lifepo4?? and why??

thank you!

 

[mnplus1556]

oh also, how many miles per wh could i expect from each type of battery? (lipo and lifepo4) my bike will cruise at 20 mph. i live in the suburbs of chicago so there aren't too many hills. also i am using a brushed motor. thanks!

 

[Alan B]

Energy consumption depends on your motor, not the battery. I use 25 watt hours per mile for level, 35 watt hours per mile for hilly as an estimator.

LiFePO4 is larger, heavier, safer, not as high current and longer lasting. Lipo is small, light, very high current and probably won't last as long.

There is also LiMn which lies in between LiFePO4 and LiPo. LiMn is use in most portable lithium tools (except DeWalt who uses LiFePO4).

I found LiFePO4 to be more difficult to mount on the bike due to the size and weight. Small blocks of LiPo are easier to package to fit. Ping batteries are popular because they are easy and complete. Going Lipo will require more fiddling with balancing chargers, cables, and low voltage monitoring. Ping has it all in the "big brick".

 

[mnplus1556]

ahh okay thank you. so if my bike will be going 20 mph, it will drawing about 330watts right? can lifepo4 batteries be completely discharged? ive heard that you should only discharge about 75%. so does that mean with a 24v 10ah ping battery i would only get about 11 miles per charge?? thanks

 

[Alan B]

ahh okay thank you. so if my bike will be going 20 mph, it will drawing about 330watts right? can lifepo4 batteries be completely discharged? ive heard that you should only discharge about 75%. so does that mean with a 24v 10ah ping battery i would only get about 11 miles per charge?? thanks

Depends on how hard you pedal.

20 mph times 25 watt hours/mile gives 500 watts.

24 volts times 10 amp hours gives 240 watt hours.

240 watt hours divided by 25 watt hours per mile gives 9.6 miles.

LiFePO4 batteries can be fully discharged, that occurs at about 2.5 volts. Cell voltage monitoring is advised for full discharges. Some folks get by with partial discharges and no cell monitoring. Overdischarge mistakes result in damage to the battery.

 

[dogman dan]

Here's where things can get really confusing for the new guy. Lithium batteries cannot be completely discharged without ruining them instantly. So when we talk about a 100% discharge, we mean discharging to the point where we can be relatively sure we didn't just kill em. 2.5v for lifepo4 for example. A battery like a ping will have a bms, that prevents us from overdischarging, and it may even be set higher. So for that particular battery, it may stop sooner. However, when we talk about a 100% discharge on a ping, we usually mean riding till the bms shuts the battery down.

Pingbatteries, and many others seem to stay balanced better when we discharge them even less. So you see 80% depth of discharge (DOD) talked about a lot. So a 24v 10 ah ping at 80% dod would be taking 8 ah out of it. Pings and other typical lifepo4 packs have a lower c rate. This means a 10 ah ping needs to be used on a bike with a controller with about 15 amps or less idealy. For this reason, I nearly never recomend using a pingbattery smaller than 15 ah. A 15 ah pack will supply 20 amps no problem, and can supply a 30 amp burst for starting the bike uphill.

My rule of thumb for 25 mph travel using 36v batteries is 1 ah per mile. This rule of thumb includes a bit of reserve, so 1 ah per mile is a good number for that speed and voltage. You will use 24v and a slower speed, so it may even out pretty close to the 36v rule of thumb.

For sure though, traveling at about 20 mph is going to average out at about 350 watts minimum. The slower you go the better the distance, and the more percentage the pedaling contributes. Pedaling = 100 watts or less for most of us.

Back to the battery, if you do choose a ping, you should consider at least 20 ah, simply for the range. 24v 20 ah is only going to weigh about 10-11 pounds and fit in a bag or frame mount easily. You'll be glad you bought more because a 20 mile range bike is so much more usefull than one where you always have to fret whether you can make it home. In fact, buy a 30 ah if you can afford it. it will still only weigh about 15-16 pounds.

 

[thenate1]

Dogman, thanks for that reply...I have been lurking in the forums for a while trying to get all this sorted out, but it is complicated for a newbie. So when you said this:

My rule of thumb for 25 mph travel using 36v batteries is 1 ah per mile. This rule of thumb includes a bit of reserve, so 1 ah per mile is a good number for that speed and voltage. You will use 24v and a slower speed, so it may even out pretty close to the 36v rule of thumb.

For sure though, traveling at about 20 mph is going to average out at about 350 watts minimum. The slower you go the better the distance, and the more percentage the pedaling contributes. Pedaling = 100 watts or less for most of us.

It really helped a newbie like me trying to put together my first build. I have been talking to Cell_Man and he gave me a quote for one of his packs. He said it was "a 52.8Vnom 18.4Ah A123 pack with 30A constant BMS" - I have been trying to research what kind of numbers that will mean with one of his 500 w DD motors. (how fast and how many miles I can hope to get out of it). From what I have read, I am thinking I might expect a top speed in the high 20's (possible low 30's?) and a distance of about 15 or 20 miles depending on how fast I want to get there?

Does that sound about right or in the right ball park? Anyone?

 

[jimw1960]

A 52.8volt 18Ah battery should carry you at least 30 miles no problem; it will work fine on a 48-volt controller. You will like that battery a lot more than the 24 volts you were considering. 24 volts would have pretty lackluster performance and would do poorly on hills. Do you know the amp limit on the controller you will buy? How fast it will go depends a lot on that. Cell man's batteries use A123, which can push a lot of current.

 

[thenate1]

I am going to get a controller upgrade to an Infineon 30A 9 fet - and I thinkI am going to get a smaller 36V 9.2Ah battery from him for my wife so we can go cruising together - she won't be needing so much power and won't commute on hers. We want to hit some of the MANY trails on the weekends around here.