Two Stupid Questions re Lithium Ion Battery

[synthrider]

Hi guys, I have this Lithium Ion battery: http://www.bmsbattery.com/36v/17-36v-lithium-ion-little-frog-abs-shell-ebike-battery-pack.html

It works great but I wasn't sure about the battery indicator (seems like it should read "low" more often than it does...).

I tested it fully charged with my voltmeter and it read 41.7 (which to me seemed really good).

I rode it out about 6mi on a little 650Watt scooter (25-30 minute ride estimate).

I tested it again and it read something like 37 volts. I had my charger with me and it was full again in only 1.5 hours.

Do the voltmeter readings mean it was still 89% charged, and I had only discharged it 11%? Or is it not linear like that?

Is it a bad idea or a good idea to be charging it so often? Would it be better to ride it home another 6mi and then charge it?

I know you're not supposed to discharge Li-Ion beyond 80%, so does that mean I could've safely discharged it down to like the last 9 or 10 volts? or at least to the halfway point at 20 volts?

Thx in advance cause I know this is pretty remedial.

 

[Ypedal]

first.. a " 36v " pack, is fully charged at 42.0v ..... and discharged 100% at 30.0v. not 0

Yes, charge as often as possible, and avoid discharging completely as much as possible to make the pack last as long as it can.

You need to calculate Amp Hours.. aka Ah

Voltage under load is more meaninfull than voltage at rest..

 

[Ypedal]

oh, and there is a big fat flat spot between 35~38v that accounts for most of the capacity in the voltage range... vs capacity.

andi'mdrunkasfuckrightnowsosorryifigotsomeofthesenumberslightlyoff... but i'm close.

 

[999zip999]

I would not go below 34v. maybe 35v. When your under load it's a lot lower. What scooter you talking about ? Really what motor and controller does it have ?

 

[dnmun]

wow, that is a small battery. 5Ah is hard to get a lot of juice from. you can charge as often as you wanna as long as you keep the cell voltage above 3V or so. the BMS will cut off when one of the cells reaches that level and will turn off the battery then.

but with a small pack like that you should be carrying a charger and opportunity charging when you can or you may be outa juice when you least expect it.

 

[dogman dan]

I think it's a 10 ah, but even so that's pretty small.

Charge as often as convenient, but you can wait till you get home if you aren't having it shut off itself along the way. A bms in the battery shoud shut off the current when it's about to get overdischarged. Do try to stop before that happens, once you get a feel for you maximum range.

My rule of thumb for 20-25 mph travel is to expect 1 mile per amp hour of battery. So at say 22 mph, you should have 10 miles for sure, including a small reserve good for another mile or even two. So try to plan on charging every 10 miles.

You can double that range though, if you ride much slower, like 12 mph.

The ideal way to track your usage is with a wattmeter. Fairly cheap ones are avaliable at online hobby shops like hobby king. The gold standard for bike wattmeters is a cycleanalyst, but a less expensive wattmeter will do you fine since you won't need to have one that is usable to 100v.

Once you have a wattmeter, you can carefully test your batteries real world capacity the way you ride. Then knowing you have say, 9.1 ah of real world capacity, you will be able to know when you are half way, and turn back for home in time to make it.

 

[synthrider]

Thx everyone for responses.

For those curious, the scooter is a little "schwinn 650" with 650 Watt Currie motor (got on craigslist for $200). It has a 24V 30 Amp Currie controller (http://www.monsterscooterparts.com/s500controller.html). The stock battery is an SLA pack made of two 12v12Ah batteries. I had this 36v10Ah Lithium battery sitting around from another project, slapped it on and its running great! (I've read some other good reports about over-volting this scooter) The speed, range, and acceleration are all much much improved. I clocked top speed at 21 mph, which combined with the almost instant acceleration makes this thing a lot of fun to ride... you have to turn the throttle very slowly at first or the front wheel will literally become airborne : )

dogman, thanks for your rule of thumb! that's really useful..

About wattmeters, I actually have a brand new cycle analyst that I'm not using at the moment, but I don't think I can hook it up... *** UPDATE, nevermind, all I need is this part (http://www.ebikes.ca/store/photos/Shunt-SA.jpg) which converts my "direct plug in" model into a stand alone and I should be all set. Prob solved : )

 

[o00scorpion00o]

I did a range test with my mac one day and got 21 miles from 8.5 ah 60 volt LiPo. @ 20-22 mph

Being gentle on the throttle and NO peddling and a few hills.

I was on 26 x 1.6 Continental sport contact tyres and pumped to 100 psi, these tyres make a big difference especially pumped so hard so 700c tyres should be better again.

If you want efficiency then skinny hard tyres pumped to 120 psi should get a great distance that and going easy with the throttle.

I would have said that you need a torque arm with a mac ? maybe not with no more than 20 amps ?

 

[dogman dan]

A 30 amps controller is pretty big for that frog battery.

Once you get your CA hooked up, try to learn to use the throttle in such a way as to minimize the big amps loads as you leave a stop sign, and try to avoid climbing hills that are too steep and start pulling more than 20 amps. More voltage will help, since you will get the same power as 24v 30 amps with less amps of 36v.

Idealy, try not to push that little frog much past 15 amps as much as possible. Cruising on the flat full throttle should be fine, it's the steep hills and full throttle starts that will flog the frog.

 

[dnmun]

don't dog that frog!

 

[999zip999]

30 amp controller can easly pop that frog battery