Voltage drop vs AH

[PRW]

So... can you calculate amp hours used from the starting and ending voltage on a trip?
For example from yesterday, my wife did 34.3km (21.3 miles) with a 14S7P 30Q battery, BBSHD. Starting voltage was 57.4, ending 53.8.
How do you turn this info into ah used?

 

[fechter]

53.8V / 14= 3.84V

Assuming the voltage measurement were taken at rest. So from the graph, using the top line, it looks like about 1.1Ahr per parallel cell x 7p = 7.7Ahr used.

This is going to be a very crude estimate as the voltage depends on discharge current, temperature, etc.

 

[markz]

Nice info here
@9:47
https://youtu.be/IxB2j-egWcQ?t=553

Indepth on Grinspector
https://www.youtube.com/watch?v=qFSfx5Pf6GE

Cheesy asfuck but the calculator is coolass
@1:50 https://www.youtube.com/watch?v=7_EpW6kODTI
That is how I do it.
I measured the current while charging and the charger was going full blast.
I measured 16A
Battery was 48V
Charger set at 54.00V on a 13S is 4.15V for each string.

Charger is CC/CV - Its a constant current of 16A with a slowly rising voltage until it reaches or gets close to 54V, then the charger slows its current in stages until the final where it has a steady constant voltage of 54V and has smaller current going into the battery.

The chargers fans slow down in 2 or 3 stages, you can tell by the sound of the chargers fan.
Time the charge
If it takes one hour then it is 16Ah into the pack, now it might be off a little bit, how much I have no clue without a GRINSPECTOR!
No idea because the 16A is not 100% until the end, and you dont want that.

I have read of people buying a Coulomb Counter.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=48V+Coulomb+Counter&_sacat=0

 

[docw009]

Inexpensive Tenergy wattmeter. The display cycyles thru AH used, WH used, Vmax, Vmin, and Ipeak. I used to take readings during a ride, but after a few runs, I figured my usage habits. Now I just use it check the AH going back in on a recharge.

In my opinion, based on several of these $14 meters, the WH number is not reproducible or accurate, but the AH is. In the picture, the battery has been balancing overnite, and still taking 30 ma of current

 

[PRW]

Samsung INR18650-30Q Graph.png

53.8V / 14= 3.84V

Assuming the voltage measurement were taken at rest. So from the graph, using the top line, it looks like about 1.1Ahr per parallel cell x 7p = 7.7Ahr used.

This is going to be a very crude estimate as the voltage depends on discharge current, temperature, etc.

that's a great graph, thanks very much

 

[PRW]

thanks, docw009 and markz.

 

[markz]

Justin mentions that in a video.
How much you put back into the battery.

Now I just use it check the AH going back in on a recharge.

 

[wturber]

Inexpensive Tenergy wattmeter. The display cycyles thru AH used, WH used, Vmax, Vmin, and Ipeak. I used to take readings during a ride, but after a few runs, I figured my usage habits. Now I just use it check the AH going back in on a recharge.

In my opinion, based on several of these $14 meters, the WH number is not reproducible or accurate, but the AH is. In the picture, the battery has been balancing overnite, and still taking 30 ma of current

My Tenergy wattmeter was faulty with tallying accumulated watt-hours. It gave values that were way too low.

It tallied amp-hours seemingly correctly, but to get a good estimate of watt-hours you would need to multiply the amp-hours by the average of your start and stop voltages.

 

[MadRhino]

I like this one. 0-100A, 6.5-100v

https://www.amazon.com/gp/aw/d/B07FN3VXH4/ref=sspa_mw_detail_0?ie=UTF8&psc=1

 

[markz]

Spoiler: I like this one

In a rich and ideal world..... www.ebikes.ca/product-info/cycle-analyst-3.html

 

[dustNbone]

Agreed on the meter MadRhino listed. I use 2 of them and as far as I can tell by comparing with my (expensive) Fluke meters, they're more than accurate enough for measuring ebike consumption. They're not particularly durable though, so package them accordingly.

 

[fechter]

Here's another one that I use. Around $30usd.


https://www.ebay.com/itm/TK15-50A-H...hash=item1c724326f6:m:miXPQcDCpTFz2kYMsHWguLg

The display is pretty small but easy to read. Displays amps, volts and amp-hours. You program it for your pack's capacity and it gives you a bar graph and percent remaining. If you install the shunt in the pack, you can also track charging. The shunt resistor is really nice.

 

[wturber]

The display on the one I use receives the data via radio wave. All it needs is a voltage supply (12v works but I think it can be up to 28v or so). This lets me mount the shunt near the battery and I can avoid one extra pair of wires at the handlebars since I already have a 12v feed there. The particular display I'm using has been replace with newer variants like this one. Though this one is probably on the low side for amps. The radio connection has been perfectly stable and reliable. No observed glitches.

https://www.amazon.com/KNACRO-Multifunctional-Wireless-Bi-directional-Voltmeter/dp/B06XKTNT9X/ref=sr_1_14?ie=UTF8&qid=1537901803&sr=8-14&keywords=wireless+watt+meter

 

[dogman dan]

Watt meter of any kind works best. The CA is wonderful to have even in stand alone mode use. But the cheap wattmeters work fine, and give you the real data of your battery capacity on a ride till its empty test, and how much you use on later rides.

before I had any meters on bikes, I used to time the recharge, and then figure it out based on the stated watts ( amps x the voltage) of the charger. 20 watts times an hour is 20 watt hours in. 5 amps x an hour is 5 amp hours, but thinking in watts and watt hours works best.

This of course included all waste in the charger, but multiple times measuring it did give me numbers that could relate to each other. Ride a, I rode till it was completely empty, and got a number. Ride b I could compare that number to ride a, and know it was half, or three quarters, or whatever of ride a. This is fine method for just understanding things like how weather affects a repeated ride of a set distance. Or other variables that affect range, like you pedaled hard, or did not. Or hills.