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nerdmobile
New Member

USA
61 Posts

Posted - 02/24/2013 :  19:56:06  Show Profile  Reply with Quote
As I mentioned before, I live at the top of a hill, so no matter where I ride, I have to leave enough juice to get me back home, which includes mainly a 500 ft climb of about a mile back to the house. On level ground I can cruise at 16 mph on leg power alone, but like to pulse and glide to mantain about 20 mph. I took a longish ride and ran myself out of assist, so I had to pedal my 120 lb velo at 5 mph up the grade. so I installed a voltmeter to act as an electric fuel gauge. I have a 48 volt pack and was reading 58 volts fully charged on the start of a 20 mile ride today. when I got home, I was reading 48 volts and still felt like I had a full charge. Will the voltage drop be a straight line, or exponential as I use up my pack? Also, what will the voltage read when my batteries are exhausted. I would like to equate certain voltages to 3/4, 1/2. 1/4 etc. remaining charge. Any advice?

jeff garrett

jjackstone
recumbent enthusiast

USA
232 Posts

Posted - 02/24/2013 :  22:00:08  Show Profile  Reply with Quote
Jeff,
Lead acid is generally a linear voltage dropoff at steady power usage. This means that the voltage will drop in a linear slope over the period of usage. Lithium batteries will normally "burn off" the surface charge quickly and then remain at a steady voltage until almost all the power is removed from the battery. You would be better off with an amp-hour meter to determine the remaining charge. Also try to use no more than 80% of the maximum amp hours available. I used to run a 2300 mAhr pack on my bike and would quit using the motor after 1800 to 1900 mAhrs of use. It makes the batteries last longer.

JJ
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nerdmobile
New Member

USA
61 Posts

Posted - 02/25/2013 :  21:05:17  Show Profile  Reply with Quote
Thanks for the info, JJ. So let's say for the sake of argument my pack reads 56 volts on a full charge and 16 volts when the pack is dead. That would give me 40 volts of use. If I wanted to go no more than 80% depleted, I would try and charge when 80% of the 40 or 32 volts are used up. So would treat a 56 volt reading as full and 56-32=24 volts as empty. If I wanted to make sure I keep 10% reserve to get me back up my hill, I should stop using assist when I have 10% of my 40 or 4 volts remaining or a reading of 24 + 4 = 28 volts. Currently, my only indicater is a trio of lights, green, yellow and red. It stays green for a long time, and by the time it shows yellow, I have maybe 5 minutes of assist before red and a dead battery. This will be an interesting experiment. By the way, my 48 volt pack is made up of four 10.5 AH SLA batteries in series, which should give me 48 X 10.5 or roughly 500 watts of power. My motor is 48 volt 500 watts, so I should expect something like 1 hour of full assist, or 2 hours of 1/2 power, or 4 hours of 1/4 power etc.

jeff garrett
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graydog
recumbent enthusiast

United Kingdom
107 Posts

Posted - 02/26/2013 :  01:05:18  Show Profile  Reply with Quote
my second DIY recumbent build was a pedal electric, used daily for a 17mile each way ride. simple setup 450Watt motor two 12V,11Ah (24V pack). drive system allowed for 7speed shifting whilst running on electric only :-)

now rangeis always a problem, some days I would get there with power left and not feel knackered, some days the packwas dead atleast 3/4s the way.

I setup a charge station and recored each change,as a general rule the charge evened out within 10%. thre was the odd spick.
and with amp-hour meters started a 3 month test.

Wind was the number one natural killer. the invisable power drain. even a light breeze can suck you pack dry!!!
cold not fiind anything in temp as the test was not long enough

However, the biggest problem is 'accelerations', how much gas you give it off the lights or at the stop junction.
not to mention, the extra mph you take going up a hill.

now, Watt/speed: you generally will not get out the total power charge, but what you have to think about is what power is needed to move you bie at Velosity X. it is easy on you bike by setting up some meters. run at a number of velocities both dricetions on a wind free day.

note them down. this is a basic method to give you some idea how far you can go. BUT, this does not take into account hill climbs!!!
you might wish to run the same test (just the ups) or do some maths.

you might just find your 120lb velo uses more power than you think at 20mph....

regards
graydog




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jjackstone
recumbent enthusiast

USA
232 Posts

Posted - 02/26/2013 :  09:46:34  Show Profile  Reply with Quote
Jeff,

We are using a little bit of different terminology here. I'll try to clarify my post.

"So let's say for the sake of argument my pack reads 56 volts on a full charge and 16 volts when the pack is dead. That would give me 40 volts of use. If I wanted to go no more than 80% depleted, I would try and charge when 80% of the 40 or 32 volts are used up. So would treat a 56 volt reading as full and 56-32=24 volts as empty."

12 volt lead acid batteries normally have a maximum safe charge of 13.5 to 14 volts which is why you see 56 volts when your pack is fully charged. However, they do not like to be discharged below 10 to 11 volts per battery which means you would never want to go below about 40 volts. Very bad for their life. You can consider the battery as a tank that can hold watt hours instead of gallons. Watt hours can be calculated by multiplying the normal voltage(in your case 48 volts) times the amp hour rating(10.5 amp hours). As you calculated, that is about 500 watt hours of energy available. That is the number that really needs to be considered. You would want to use no more than 400 or so of those watt hours.

Graydog has given a number of scenarios that cost you watt hours--wind, hills, non-pedalling starts.

But the point is, you need to be more concerned with watt hour(or amp hour) usage than with the voltage drop unless one of your batteries is bad.
On my bike I used between 15 and 20 watt hours per mile. But I pedalled a lot and live in a very flat area. I would expect your values to be at least triple with the climbs and the additional weight of the velo.

Also, even though your motor is rating at 500 watts, unless the controller limits maximum power output it can pull much more than 500 watts.

JJ
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nerdmobile
New Member

USA
61 Posts

Posted - 02/26/2013 :  21:58:39  Show Profile  Reply with Quote
Ok, that all makes sense. I'm a hypermiler on my Honda Insight and know the effect acceleration and hills on power usage and fuel conservation. But an ammeter would only tell me current usage at the moment and not power remaining, and I need a relative measure of my reserve. 40 volts sounds like a good start for treating as empty. Also, on flat ground and full assist without pedaling, I can achieve nearly 30 mph, which is faster than I dare go. Which brings up another question. If I wired my 4 batteries as 2 pair in parallel, it would generate only 24 volts, but should last twice as long, right? So what would give me more range, running 48 volts at half speed, or running 24 volts at full speed. I realize for those really steep hills, I might need all 48 volts.

jeff garrett
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jjackstone
recumbent enthusiast

USA
232 Posts

Posted - 02/27/2013 :  08:57:24  Show Profile  Reply with Quote
While your motor might run using 24 volts it probably wouldn't be as efficient if it is designed to use 48 volts so the range would likely be better with 48 volts. The other problem is that to get the same usable power output the current would be doubled. P=IE, power=current times voltage. I would guess you would run the risk of overheating the motor coils more easily at the lower voltage.
Have you heard of the Cycle Analyst? They make a reasonably priced power meter that measures voltage , current, power, watt-hours, amp-hours,and about anything else you would need for around $125.00. I've dealt with them in the past and had very good service. Here's a link.
http://www.ebikes.ca/drainbrain.shtml

Are you on ecomodder?

JJ
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nerdmobile
New Member

USA
61 Posts

Posted - 02/28/2013 :  19:47:38  Show Profile  Reply with Quote
Neat item, but I try and do things on the cheap. The voltometer I am using is a freebie from Harbor Freight Tools. Using it as a voltmeter is very simple, just wire it across the batteries in parallel, but if I wanted to, how would I use it as as ammeter, it would have to be wired in series, right?, and in that case the wire would need to be at least 14 gauge maybe?

jeff garrett
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jjackstone
recumbent enthusiast

USA
232 Posts

Posted - 02/28/2013 :  20:25:00  Show Profile  Reply with Quote
Assuming there is an ammeter setting on it, yes you would wire it in series. But I would guess that you probably pull 20+ amps at certain times. Most of those inexpensive meters can't handle that high of a current and will blow the internal fuse(if there is one).

JJ
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nerdmobile
New Member

USA
61 Posts

Posted - 03/01/2013 :  20:49:01  Show Profile  Reply with Quote
I guess I'll need to make some practical experimentation. My goal is to be able to ride to work and back, a distance of 20 miles each way, including two 800 ft climbs on a canyon road. I'm worried about making it up, and also worried about making it down. I could hit 40 mph on my road bike, but don't dare go that fast in my velo, and don't want to overheat my brakes. At work, I can recharge the batteries for the ride home.

jeff garrett
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jjackstone
recumbent enthusiast

USA
232 Posts

Posted - 03/01/2013 :  23:30:57  Show Profile  Reply with Quote
What you could do is put a shunt in series with the batteries and measure the voltage across it to determine the current draw. A shunt is essentially a calibrated resistor which allows current to flow through it without a lot of resistance to current flow. As an example a shunt may be calibrated to some conversion like 50 milliVolt measured across the shunt equals 50 Amps going through the shunt. So every millivolt measured would be equivalent to 1 amp passing through. You can probably pick one of these up for under ten bucks on the Bay.

JJ
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nerdmobile
New Member

USA
61 Posts

Posted - 03/02/2013 :  19:22:01  Show Profile  Reply with Quote
Ok, since V=IR, the voltage measured across the shunt resistor would be whatever the amps are flowing x the resistance in ohms. Or, since the resistance is constant, the voltage divided by the resistance would give me a relative value of the amperage. What do you think a good value of the resistor be, and would this resistor give off any heat. Logically, the lower the resistance, the lower the heat being given off. It would kinda be like putting a light bulb in series in the circuit.

jeff garrett
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jjackstone
recumbent enthusiast

USA
232 Posts

Posted - 03/02/2013 :  19:43:19  Show Profile  Reply with Quote
Yes,
You are correct. It needs to be very low resistance. Shouldn't give off much heat with the low resistance.

Here is one that would probably work.
http://www.ebay.com/itm/50A-75mV-DC-AC-Current-Shunt-Resistor-For-Digital-Amp-meter-Analog-Meter-/170751319675?pt=LH_DefaultDomain_0&hash=item27c1925e7b

It is 50amps at 75mV. Resistance would be about 1.5 milli ohms. So every 1.5mV=1Amp


This one would be easier to calculate.
http://www.ebay.com/itm/50A-50MV-50-A-AMP-AMPS-AC-DC-CURRENT-SHUNT-PROBE-DVM-/380214040564?pt=LH_DefaultDomain_0&hash=item588685d7f4
Every 1mV=1A.

Obviously pulling these types of current requires some sort of non-conductive covering for safety.

JJ
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OpusthePoet
recumbent guru

USA
678 Posts

Posted - 03/02/2013 :  22:31:24  Show Profile  Visit OpusthePoet's Homepage  Reply with Quote
The easy way to do it is to buy a Cycle Analyst from ebikes.ca that records your use and charges and figures your range on the fly for you, and can also act as a limiter to keep you under the Federal (US) 20 MPH limit with power assist, the CAV3 also allows the use of pedal torque sensing (Thun sensing BB) or a encoder that only senses cadence to limit speed to 20 MPH without pedaling or as high as the posted limit when pedaling (TX law). The CAV2.3 can be used to limit speed, or act as a cruise control, or both, besides the collecting and logging power usage and range.

Also they have a e-assist simulator on the web site that lets you compare motors, controllers, and batteries to get the most bang for the buck, and if you know the specifics for your bike (CdA, Cr, weight) you can plug your bike into the simulator and get numbers that are close to what you would get in the real world. Just a note, they already have the range-reducing effects of high current on SLA built into the sim. A telling reminder is to put the same size battery in two simulations with one battery being SLA and the other a slightly smaller Ah rated LiPo of any of the common chemistries, then look at the range difference as the load increases (by increasing grade).

Opus

My gas is up to $0.99 a burrito, $5.99 for premium and I'm only getting 10 miles to the regular burrito. Dang $0.99 burritos are smaller now.
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nerdmobile
New Member

USA
61 Posts

Posted - 03/03/2013 :  20:15:50  Show Profile  Reply with Quote
Thanks for the info guys. I'd prefer to keep total control of my assist and do not want to limit my speed to 20 mph. That shunt that lets you read 1 amp = 1 mv sounds neat. But although it would give me an accurate reading of the current flowing, it would not let me know the remaining state of charge of my pack. However, it would give me a good idea of how much power would be used at full power compared with light assist. I like going about 20-25 mph on smooth level ground adding assist as needed. Today I took a 20 mile ride, purposely going up and down the steepest hills in my neighborhood until I could feel the assist starting to fade. I started with a reading of 56 volts, and finished with a reading of 45 volts. So roughly speaking for each 1 volt drop, I've used 10% of my battery. So when I reach a reading of 51 volts, I can assume I used half a tank. One thing I noticed is that when coming down hills, it gets a little scary around 30 mph if I use the brakes. Use of the front disc brakes tend to unload the rear end and make it light, and loose. If I was brave, and did not use the brakes, it's steady at higher downhill speeds, but then I've reached the point of no return and dare not use the brakes.

jeff garrett
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warren
human power expert

USA
6117 Posts

Posted - 03/04/2013 :  06:17:58  Show Profile  Visit warren's Homepage  Reply with Quote
The Cycle Analyst (CA) is great and I highly recommend it. You will want the "standalone" model with a bike speed pickup. You can use it as a speedometer, plus it will tell you how many amp hours you have used, which effectively tells you how much battery you have left. Also there is a bunch of other info that it will display.

There are other options though, that are much cheaper. The CA is cool because it is remote from the power wires. This means you don't have to route the power wires up to your handlebars where the CA is mounted. Other cheaper option have the power wires running in one end of the display unit and out the other. If that's not a problem, then check out:

$24
http://www.hobbyking.com/hobbyking/store/__10080__Turnigy_130A_Watt_Meter_and_Power_Analyzer.html

$50
http://www.hobbyking.com/hobbyking/store/__6380__Watt_Meter_Power_Analyzer_Watts_up_Ver_2_.html


When your lead-acid batteries die (in a couple months) you will want to get an lifepo4 battery to replace it. There are lots of options. They are more expensive but are half the weight and twice the power, plus they last for many years.

-Warren.

Edited by - warren on 03/04/2013 06:29:08
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warren
human power expert

USA
6117 Posts

Posted - 03/04/2013 :  06:19:20  Show Profile  Visit warren's Homepage  Reply with Quote
Also, have you looked to see if your motor / controller has a e-brake option? That way you can charge the batteries when you go down your big hill.

-Warren.
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nerdmobile
New Member

USA
61 Posts

Posted - 03/04/2013 :  19:39:45  Show Profile  Reply with Quote
I was hoping my SLA batteries would be good for 100+ charging cycles, which would be at least a couple of years for me. Lipo's are so expensive, roughly twice what I have invested in my body, motor, and batteries all together. I do not have any regenerative braking, though that would be neat. I may purchase one of those neat meters if what I have does not work out. I have four 10.5 ah SLA batteries giving me a tank of energy worth roughly 500 watts for an hour. My voltage drops approx 1 volt for every 10% of my pack usage according to my voltmeter. This means I can go 6 minutes at full power or 12 minutes at half power for every 1 volt drop. At least that's what I think will happen.

jeff garrett
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warren
human power expert

USA
6117 Posts

Posted - 03/05/2013 :  06:14:16  Show Profile  Visit warren's Homepage  Reply with Quote
You can also just pick up a small digital volt meter someplace cheap for about $5 if that works for you.
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nerdmobile
New Member

USA
61 Posts

Posted - 03/05/2013 :  06:56:33  Show Profile  Reply with Quote
Actually it was cheaper than $5, it was free at harbor Freight tools. They have coupons for something for free with every visit. I now have 3 of these voltmeters.

jeff garrett
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