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Power-User

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Powering 36v 800w Electric Bike on 10s LiPo

08/07/2013 9:31 AM

I will be using a 10s lipo to power a 36v ebike. The nominal voltage of the battery is 37v and its fully charged voltage is 42v. Would this be ok if I don't accelerate too hard and be gentler with it? I will be using enough batteries to make a 50ah pack

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#1

Re: Powering 36v 800w electric bike on 10s lipo

08/07/2013 10:05 AM

I don't see any problem, but the proof is in the pudding....I might check the motor temp closely and amp draw of max acceleration....I would definitely do some testing before I would be comfortable going out on the road....

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#2

Re: Powering 36v 800w electric bike on 10s lipo

08/07/2013 10:06 AM

800W full power at say 40V is 20A. 50Ah at 20A is 2.5h minimum between rechargings. Is that long enough?

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#3
In reply to #2

Re: Powering 36v 800w electric bike on 10s lipo

08/07/2013 2:04 PM

Well, not really. I would have 80Ah if I could but I am not spending too much on batteries. The charger I will be using is a 400w Turnigy charger from Hobbyking.

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#4

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/07/2013 3:21 PM

If the nominal battery voltage is anywhere close to the motors rated input voltage you will be fine.

One volt over won't make a bit of difference and to be honest even at 6 volts over initially it probably won't make any difference either.

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#5

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/07/2013 4:31 PM

The battery voltage will also sag a certain amount when the motor is under load so that will help bring the voltage down a bit too.

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#8
In reply to #5

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/08/2013 6:39 AM

These batteries are very powerful lipos so their voltage will not sag at all with an 800w load. I will be using many of them in parallel. 2 groups of 8 in parallel. The two groups will then be linked in series. The maximum current that could be safely drawn from this battery bank will be 800 amps. That is 28000 watts at 20c as the batteries are rated 20 - 30c discharge. You could make a pretty fast bike out of one.

If I were to use nano tech a-spec batteries which are rated 65 to 130c discharge, the maximum power at 65c would be 93600 watts. At 130c for 10 seconds the maximum power would be 187,200 watts. If I could put a motor that powerful in my citroen c1 we discussed in another thread, I could leave most stuff on the road for dead. The battery would have to be 20 Kwh for the car though.

If the 20kwh battery was to be constructed out of these nano tech a-spec batteries, I came out with 1.4 megawatts as the power output at 65c.

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#6

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/07/2013 11:24 PM

Maybe. Most eBike controllers use MOSFETs. They generally use the lowest voltage FETs that they can. Power MOSFETs come in 50V and 100V and higher - maybe some in between. It totally depends on the specifications for the controller. If you don't have a maximum input voltage in the specs for the controller, try and get a figure from the seller.

The motor is a pretty secondary concern.

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#7

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/08/2013 6:35 AM

I don't have any difficulty with the maximum voltage, but I do with 2 other aspects, namely the maximum discharge and the charging system. A deep discharge is the best way to ruin a LiPo battery, so your battery management system needs to cut off before the battery gets too low. The second worry is that your battery is going to be a series-parallel combination, which complicates the charging process considerably. Ideally each cell is individually charged in a balanced charger (and ideally the battery management system monitors each individual cell during discharge as well). I manage to escape several such problems with my relatively tiny model aircraft batteries by routinely charging at less than maximum rates.
If you are building the pack yourself then consider spacers to allow better cooling.

http://www.electricbike.com/lithium-battery/

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#9
In reply to #7

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/08/2013 7:04 AM

My whole battery will consist of 16 of these:

http://www.hobbyking.com/hobbyking/store/__26674__Turnigy_5000mAh_5S_20C_Lipo_Pack_UK_Warehouse_.html

I will make two batteries. 18.5v 40ah The discharge and balance plugs will all be connected in parallel. When charging both batteries will be connected in parallel. When I put them on the bike, one will go in each pannier and they will be linked in series. Charging lipos in parallel is done frequently. A voltmeter on the handlebars will be used to determine battery state.

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#10
In reply to #9

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/08/2013 11:57 AM

It would be interesting to know the power-to-weight ratio and compare it with other forms of motive unit on other vehicles.

Energy-stored-to-vehicle-weight would be interesting for comparison too.

Together these figures will give sign posts towards the maximum range. Range anxiety is an important psychological factor in most vehicles.

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#11
In reply to #10

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/08/2013 2:18 PM

The power to weight ratio of lithium polymer batteries namely Turnigy A-spec batteries is just phenomenal. They far outperform lithium ion in terms of power density. This battery here weighs 840g, is 5000mah and has 6 cells. it is 65 to 130 c so that's 325 to 650 amps that can be drawn.

7215 to 14,430 watts

http://www.hobbyking.com/hobbyking/store/__24302__Turnigy_nano_tech_A_SPEC_5000mah_6S_65_130C_Lipo_Pack.html

Just think that if a powerful enough motor and esc was to be created and enough of these linked together to give enough voltage and capacity and the whole thing put into a stripped out racing car, no petrol powered thing would be able to come close to keeping up with it.

I see all these attempts at very fast electric cars and if they chose better batteries, then they would really be breaking records.

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#12
In reply to #11

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/08/2013 7:42 PM

There are two problems with LiPo being "better batteries" for EV racing and general EV use.

One is that they are dangerous in large quantities. Tesla use LiPo and there is a huge amount of battery thermal and current managament in the vehicle to ensure there are no problems.

The second issue is the low number of charge/discharge cycles. There are other chemistries (LiFePO4) that have many more cycles. I understand that the Turnigy LiPos are great for RC and light EVs but take care. Youtube is full of LiPo RC and eBike fire videos.

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#15
In reply to #11

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/09/2013 11:21 AM

Erm, a full 65C discharge rate implies emptying the battery in less than 1 minute. Is this a useful capability?

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#14
In reply to #9

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/09/2013 11:14 AM

The balance plug on the battery is to ensure that each individual cell is individually charged. In effect you have an individual charger for each cell. If you connect two balance plugs in parallel you will lose this protective effect, and if one cell is weaker than the other trouble will follow if you charge too fast. Furthermore, if you parallel tthe balance plugs of batteries connected in series you will have a fabulous short circuit.

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#13

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/08/2013 8:47 PM

The voltage should be fine (provided the controller is called a 36V controller). The actual voltage of a charged, resting "36v" volt lead acid battery bank is 38.1V. Even very low end controllers will take a 30% (or more) overvoltage from nominal. The motor will not be damaged, because the controller prevents that -- there should be no need to go easy on it. That's what the controller is there for.

However, a very cheap controller will leave out some protections, so with less battery sag, if the original arrangement was putting the motor near its heat limit (and if there is not thermistor on the motor sending temp data to the controller) then you could overheat the motor from a continuous overcurrent. But you can check that by riding around and frequently checking motor temp.

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#16
In reply to #13

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/12/2013 4:07 PM

ahh reminds me of the whopping 8mph, single 35 ah 12 volt minibike-trike...

Wall-20a switch cooked in a few starts, replaced by a cermic 3/8" by 1/16" x 4" Cu Knife switch.

1972. Powered by a 4-brush generator from the scrappers., geared 8:1 on a usual looking minibike tire, but a trike.

0-8 mph in 2 seconds (a top-droll take off) !

After arriving at the gas station job:

On the charger for the trip home. -Excellent cost of fuel.

Thanks so very much 4 your report!

(yes, later traded to 12:1 ration 3 hp, and sold as gas powered~ 22 -25 mph, maybe, but could really climb well)

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#17

Re: Powering 36v 800w Electric Bike on 10s LiPo

08/14/2013 11:14 AM

I have owned and used a currie tech E3 Metro (500W 36V Li Ion 10Ah) for more than a year now and feel that the 500W is enough power for climbing grades of Approx. 4% without much loss in capacity or range (while simultaneously pedaling with the motor engaged). However, the Li Ion is now running at half the capacity after one year of use (900 charging cycles before dropping to 80%) Now, I have to be light with the torque to maintain my range.

For traveling to Seattle and hitting the hills from downtown I think the 15 Ah is enough for most uses without adding too much weight or taking up too much room. Usually you can find somewhere to park and charge the bike (parking garages) while watching a movie or doing your grocery shopping, etc...

The 10Ah battery was fine in getting around town if I didn't try to travel to the U-District from the Seattle Ferry Terminal and back (I could do it if I was light on the throttle but would have to be careful on which hills I hit and how much I throttle it).

The 80Ah seems like overkill but will give you plenty of range at the cost of efficiency. Your 800W motor is probably just fine for this though. However, you need to consider how heavy your bike is as this will also affect your efficiency as well. My bike stock is 54 Lb but I got similar performance on a 45 Lb bike with 250W motor w/o pannier rack on back for added storage capacity.

A little bit off track but very important, I use my panniers to carry groceries and sometimes load up to 40-60 Lb added weight on top of my 200 Lb body weight. This required me to upgrade my spokes on the rear wheel to 12GA stainless from the stock 14GA due to spoke breakage. The bike was rated to handle 240 Lb but that is static weight, not dynamic loads. Hence the need for beefier spokes.

I also recommend Schauble Tires with green slime added to avoid flats and fenders for riding during the wet season. There is a new superhydrophobic/oleophobic treatment that can protect certain areas i'd recommend as well but use sparingly for environmental reasons.

Back to electrical performance aspects of EB, I didn't go with regenerative braking because my dealer said the recapture is very low (a few percent) but I felt if it may help reduce brake pad wear to make it worthwhile (semi-metallic disc brake pads are worth their added cost)

.I believe more braking energy can be captured and acceleration utilized if a super cap of sufficient size is added and thus lower the wear and tear on the battery while simultaneously lowering my weight and required Ah needed for my uses. I just haven't had the time to think on a design that would take advantage of a super-cap added to the bike.

My initial thoughts is to use the regenerative charging for the super cap only and have a separate throttle function to discharge the cap during selected acceleration. The braking circuit may need modification or you build your own to match the super cap's performance window.

I think this option would yield more braking capture than relying on the braking circuit provided by the vendors for recharging batteries. Though not 100% sure, I believe the resistor limiting current during regenerative braking will need to be be a higher capacity (wattage) due to the capacitor's ruggedness. Another question I have on using a super cap is, would it be more beneficial to charge the cap at a higher voltage to gain more braking energy but limit the discharge to protect the motor if the cap can handle it? (I need to do some more reading on this but someone here may already be familiar with this) The ampacity of your wiring might be fine if you decide to go with the hybrid approach but I'd double check that as well.

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