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Enough juice to make it?

Posted: Dec 30 2013 1:23pm
by g1nko
I'm running lipo batteries in a 12s2p config. Each pack is 5Ah so I should be getting 10Ah total. My commute is 16 mi round trip, so I took an 8.5 mi test ride to see the numbers before committing to getting to work and back on a new bike.

Because the batteries are also new, I ran them through a couple of charge/discharge cycles and they seem to be balancing and discharging pretty evenly; all the cells are within 0.02 tolerance. Two of the packs are dead on at 4.20 4.20 4.20 4.20 4.20 4.20. The other two read 4.20 4.19 4.20 4.20 4.19 4.20.

I'm new to lipo and spent a few weeks reading and learning on these boards because I really don't want to be a statistic. One of the suggestions I read here said to be very conservative with the low-voltage cutoff around 3.6v per cell, so I set my LVC to 43.2v.

The batteries read 50.2 hot off the charger. At the end of the 8.50 mi ride, I had 46.0v and had used 3.13Ah. The fuel gage read about 1/3 depleted. FWIW, watt hours were 146.36, watt-hours/mile were 17.0 and human watt-hours were 19.01.

From what I see from these numbers, I should be good to go for the full 16 mile round trip. I'm a little concerned about the LVC considering the drop from 50.2v to 46v. How critical is it to be this conservative?

Also, when I went to charge the batteries after the ride, three of the cells looked almost identical 3.81 3.83, etc, but one looked like it hadn't discharged at all as the cells read 4.19 4.18 etc. Any idea what I'm looking at here?

Re: Enough juice to make it?

Posted: Dec 30 2013 3:00pm
by Drunkskunk
Having one battery stay at 4.19 sounds like that pack wasn't discharging at all. You need to check it's connectors, they may not be making a good connection.

Its fairly normal for Lipo to drop down to just under 4.0 volts after the first ~10% discharge. After the initial drop, the discharge curve is pretty much a flat slope down to 3.7v at around 80-90%, with the last 10% or so falling off on another sharp curve.

Re: Enough juice to make it?

Posted: Dec 31 2013 3:40pm
by heavymetalthunder
Your lipos are new but after 50 or so cycles performance will start to drop off. You will have to be travelling at a fairly low speed and do quite a bit of peddling to be able to travel 16 miles.

Re: Enough juice to make it?

Posted: Dec 31 2013 4:07pm
by wesnewell
Sounds like you were running off just half the pack. If 12cells were still over 4.1V, I'm certain of it. This is how to wire a 12s2p pack.
12s2p.jpeg (11.8 KiB) Viewed 2274 times
BTW, nothing wrong with lvc at 43V, but to get the most range out of the pack, set it to 42V. That's 3.5V per cell under load. Resting voltage will be 3.6V. That will get you another mile or two in case you need it.

Re: Enough juice to make it?

Posted: Dec 31 2013 4:33pm
by g1nko
Just FYI, it was 6 cells: one pack in a parallel wired pair. Three of the 6s packs looked as I would have expected, one 6s pack had high voltage. I can't figure out how that happened. All the voltages read correctly at each part of the wiring. It makes no sense.

I switched the harnesses and take another run tomorrow to see if it happens again.

Re: Enough juice to make it?

Posted: Dec 31 2013 4:50pm
by wesnewell
The only way this could happen is if one of the Y cables came off one of the packs on one or both ends, and the parallel connection between 2 packs was not connected. This is assuming you have them paralleled at the balance plugs. What harness are you talking about? You don't need a harness for 12s2p other than a Y cable for the positive and a Y cable for the negative end. If you want parallel at cell level,you just need a couple of 2X 6s parallel balance cables.

Re: Enough juice to make it?

Posted: Dec 31 2013 6:30pm
by g1nko
Harness, y-cable; potato, po-tah-to...I meant the y-cable. Maybe it came loose, it didn't seem like it did but that is the only logical explanation.

Re: Enough juice to make it?

Posted: Jan 01 2014 10:37am
by dogman dan
I assure you it did come loose. You were absolutely, definitely running on 5 ah. You are very lucky, you could have done what I have done twice, and rode till you ruined half your battery. I did this riding with no voltmeter, and rode till the pack was ready to blow.

Now I have enough voltmeters to have one on every bike. And I put tape on connectors to prevent accidental unplugs. Check that your connectors aren't having the contact back out of the housings too.

But, at the speed you are riding, 8 miles from less than 4 ah, your pack is big enough to make it under the conditions you rode in the other day. Someday, you will have a headwind and need more. On that day, ride even slower. I like to have a cellog 8 in my pocket, in case I need to ride the pack down to 3.4 v. Once you feel that you are off the cliff at 3.6v or so, check to be sure you aren't too low on one cell to continue.

Cheap and easy to turn it into a 12 ah pack, and be able to fogettaboutit.

Re: Enough juice to make it?

Posted: Jan 01 2014 3:16pm
by Racer_X
You never mentioned what kind of hubmotor you are using. My geared motors are more economical than my Direct drive motors.
15 miles is a bit of a stretch. If you could get a 6 amp bulk charger and give it a quick 60 minute charge at work it will prolong the life of your batteries.

Re: Enough juice to make it?

Posted: Jan 01 2014 4:34pm
by g1nko
It's a Q85 36V 250W geared hub motor. I'm over-volting it to 48v.

Re: Enough juice to make it?

Posted: Jan 01 2014 5:42pm
by Racer_X
a Q85 36V 250W will draw about 5ah or less for 15 miles. my DD will draw about 8 - 9 ah for the 15 miles. If you pedal and keep the speed low can get even better economy on those batteries.

I could be wrong with the estimate. You should get a $25 wattmeter and test yourself.

Re: Enough juice to make it?

Posted: Jan 01 2014 6:11pm
by teklektik
Here's a quick estimate of ranges for your pack. At your reported 17Wh/mi, you have more than enough range at 22+mi, although a headwind will eat into your surplus pretty quickly.
12SLiPo10Ah_rangeEst.png (89.85 KiB) Viewed 2185 times
I frankly don't run LiPo and so am unfamiliar with operating voltages - I've found a range of opinion/values there... I hacked what appeared to be reasonable voltages into an existing range estimator I had on hand. The math is trivial so if the mean cell voltage under load and DoD appear okay, the results are valid.

EDIT - based on a better discharge rate of around 1C (still too high... for the roughly 300W being used at 17 Wh/mi) the mean discharge voltage was adjusted to 3.8v yielding these revised estimates:
12SLiPo10Ah_rangeEst_Mean=3-8v.png (90.17 KiB) Viewed 2089 times
The 17Wh/mi rate of 24.1 from this table is very close to the more direct estimate using Ah instead of Wh for 90% discharge at the identical 17Wh/mi rate:
  • 10Ah / 3.13Ah * 8.5mi * 0.90 = 24.4mi
The Ah calculation is more accurate since it doesn't introduce additional estimates related to discharge curves, but the Wh/mi Excel table is nice because it offers up a bunch of 'What If' answers for other consumption rates....

Re: Enough juice to make it?

Posted: Jan 02 2014 2:09am
by wesnewell
A 10ah 12s pack is 440wh. So at 17wh per mile, that's 25.8 miles if you run the pack empty. At 90% discharge that would be 23.3 miles. 17 miles shouldn't be a problem unless you get throttle heavy.

Re: Enough juice to make it?

Posted: Jan 02 2014 5:29am
by teklektik
wesnewell wrote:A 10ah 12s pack is 440wh.
Ahh - so you are using 440Wh/10Ah/12s = 3.66v as a mean cell voltage. Actually, in this case the OP may do a bit better since the bike is drawing little current - under 1C, so the discharge curve will be flatter. My estimate above is too low - I used a curve for discharge rate that was too high so the mean discharge voltage was too low - 3.8v would be more reasonable based on the a somewhat typical set of curves (below) - hard to find them for low discharge rates:
LiPoDischargeSample_1.png (99.69 KiB) Viewed 2147 times
(Added revised estimates in post above based on 3.8v....)

Re: Enough juice to make it?

Posted: Jan 02 2014 7:09am
by wesnewell
Sorry, went brain dead for a while. 10*44.4=444Wh.

Re: Enough juice to make it?

Posted: Jan 02 2014 7:28am
by dogman dan
For sure, if he rides slow he's got plenty. I just wanted to say that adding 2 ah more is cheap if he really wants to make it in any kind of weather, including all 16 miles in a 30 mph headwind. And at speeds that use more wh/mi.

It's happened to me enough times, with a front going through, stiff head wind to work, then after a change in weather, same thing going home. He won't get 17 wh/mi on that day, no way. But with a wattmeter, he can adjust to a speed where he is getting the wh/mi that gets him all the way.

If it's really cold, you also might need more because you just don't have 440 wh that day.

Re: Enough juice to make it?

Posted: Jan 02 2014 8:50am
by dogman dan
I think of 12 s 10 ah as 400wh, at most. I thought of my pings as being only 700w, much the same. On a cool day, they were less. But I would get closer to 800wh, if it was 100F out that day.

Whatever the calcs come out to, you don't have it every day of the year. Much less if you let the pack get very out of balance. That's part of why I tend to recommend 700wh packs over 400wh.

Re: Enough juice to make it?

Posted: Jan 02 2014 9:21am
by g1nko
I have a '73 Peugeot retrofitted with a Bionx SL350 HT RR 48v system with an 8.8Ah Li-Ion battery. Bionx claims 423Wh. I can make my full 16mi round-trip commute on this battery, but one thing I have definitely noticed is the battery is temperature sensitive. If temps are above freezing, I easily have 10-15% remaining capacity at the end of the ride. In the winter, though, when temp are below freezing, I have to keep a careful eye on battery use as several times I've taken it down to 0.

The system I'm asking about here is a front-wheel mounted Q85 250w motor on a 2011 Workcycles Secret Service. I went with this motor because it was one of the only motors I could find that was roller-brake compatible.

I'm looking to commute at 15-20mph. I had a bad experience with a 36v battery LiFePO battery I bought on eBay that turned out to be poorly made LiMn junk from a disappearing seller. After wasting money on that fiasco, I got 4x 5Ah 6s 25C Zippy packs from HK. Over-volting the Q85 modestly from 36v to 48v has definitely made the motor a bit zippier. With my pedaling contribution, 22-24mph is no problem.

I know the chemistry is different from the Bionx, but I think based on past experience, what I'm hearing here, and the smaller draw of the motor, I should be fine. Now if it would just stop snowing....

Thanks for all the info.

Re: Enough juice to make it?

Posted: Jan 02 2014 9:33am
by dogman dan
I found lico to be pretty sensitive to cold too, like the pings were. They won't self heat themselves at 17 wh/mi either. at 50 wh/mi, they will toast themselves warm, but you just lost that capacity into doing that.

But you can take them from a warm place, and if your battery enclosure has some minimal insulation, they will stay warm enough to run normal. My winter performance has really improved since I switched from a metal box, to a padded EM3ev bag.

You do have enough, but not a big reserve. The days that got me, were riding home from work in 35 F, with the battery at 35F having been parked at work. And then getting a 30 mph headwind. Day like that, I would ride slow and still run out half a mile from home. 700wh would not take me 14.5 miles, even at 18 mph. Normal for that speed is 30 miles.

Re: Enough juice to make it?

Posted: Jan 04 2014 2:33am
by 1KW
this info has helped even me out.

I still think higher voltage gives you lower wh/mi?

lets say he ran 2 more 6s in series…that extra voltage gives you lower wh/mi at a faster speed…?

Ive been told that my 8ah zippys CANNOT net me 11 miles on a commute, however even at 30mph with hills. it will get me there. 20% on em is 7ah. will up the voltage to 72v and see what happens.

Re: Enough juice to make it?

Posted: Jan 04 2014 4:13am
by friendly1uk
We have comparable goals. I have to do 7.5 miles carrying 5Ah. 44.4v through an unlimited 6 fet (so lets say 15amp) but my motor is a 36v 500w geared hubby (it's a bafang cst).
I use about 3.6Ah with lazy pedaling. I come the long way home and use less. 8 miles needing about 3.2Ah.

There is a map somewhere(or even two)that has a route planner that gives pretty accurate ebike performance projections.

I run them from 4.2v down to 3v which is within manufacturers specifications. At best I have had 4.76Ah from the pack, and at worse 4Ah. My worst consumption over this 7.5 miles is 4.3Ah getting a free ride(no pedaling). There is an overlap there where this 5Ah pack won't take me 8 miles. If I do nothing to help over a particularly troublesome 8 miles I could conk out.

As your peugeot can make it, this bike really should do too. What you need is that ebike route planner. I don't have the link though.

Reading between the lines you can't have a bms and you have already run in to a wiring fault that tells me this. I don't want you to be a statistic either.
Personally, I fitted the $20 bms and use a $40 charger. Monitoring my consumption with a $10 watt meter. What more do i need.
I'm a qualified auto electrician, installation electrician, electrical test and inspection engineer, portable appliance tester and I hold associated electronics certificates. You get the picture. The above is the very best I could do. The only time I will take a voltmeter to my pack is if there is a fault. Don't fanny around with avoidable and life threatening stuff, It is illogical.

Re: Enough juice to make it?

Posted: Jan 04 2014 6:47am
by dogman dan
Range can vary greatly for many reasons, but 2 miles out of 1 ah of 48v is easy to do, if you just slow down. But you can use a lot more depending on the route, or the weather. My rule of thumb for sizing a battery big enough to make the commute every day, is .75 ah of 48v per mile, at 20-25 mph. .8 ah should work for 12s, 44v. 14s is two cells, about 40 wh larger. ( for 5 ah packs)

Two days ago, I rode 47 miles on 48v (14s lico) 19.5 ah. I climbed and descended about 1500 vertical feet on the way. But yesterday I rode in the mountains, Climbing and descending about 8,000 vertical feet. Range of the same pack, using 19 ah was 30 miles.

Both rides, at 15-18 mph except the descents, on a 150 pound longtail bike. So no extra efficiency for being light or aero. Pedaling all the way. The weight of the bike costs you in the mountains.

But a headwind, it will easily cut your range in half if it's a strong one. Side wind can still cut you by 30%.

Re: Enough juice to make it?

Posted: Jan 13 2014 8:48pm
by g1nko
Thanks to everybody who contributed info on this thread and gave me confidence I could make it on my new bike. The weather hasn't been cooperative with temps in the single digits and snow. With temps today near 50, today was my first opportunity to take the bike back and forth to work and the bike performed with flying colors.

16.3 miles total, round-trip. Hot off the charger, I had 50.1 volts; ending voltage was 45.4. Each cell in the pack discharged evenly and read 3.79 volts at the end of the ride. I used 4.9 Ah; 229.25 watt-hours, for an average of 14.1 watt-hours per mile. My contribution to the ride in human watt-hours was 82.61. At an average speed of 15.7 mph, I made it to work and back in just over an hour (62 minutes). I took it easy and used the PAS on the way in, so I was only 7.8 wh/mi on the way in. On the way back I hit the throttle hard to see if I really needed to be careful, especially since I was also fighting the typical Connecticut River afternoon headwind. I'm pretty happy with how things worked out.
My WorkCycles Secret Service
20140113_160824.jpg (134.01 KiB) Viewed 1966 times
The Q85 motor with the roller brake.
20140113_160840.jpg (122.95 KiB) Viewed 1966 times
Battery handiwork.
20140113_192004.jpg (129.52 KiB) Viewed 1966 times

Re: Enough juice to make it?

Posted: Jan 16 2014 5:20am
by kiwipete
BTW, nice bike.

As we would say in the Netherlands: a "proper" bike..

Re: Enough juice to make it?

Posted: Jan 16 2014 4:56pm
by teklektik
Ya - very nice indeed.
Which wind/brush guards are those? Link?