# Ways to tell you should start to think about going home (voltage monitoring)

Hi there! I was thinking today…

how do you tell when you should start thinking about heading home and how do you tell when batteries are getting low?

I think evolve had a good idea that it shows kilometers made or kilometers left on their new remote…

Thinking in similar fashion, I thought there could at least be some sort of beeper, similar like lipo alarms has, but for different voltage levels… let’s say, one beep is when you reach 50% battery left, then 30% left has 2 beeps, then 15% left has 3 beeps…

I know u guys use these ‘‘coulometers’’ where the voltage / battery is shown as percent and you probably judge your distance from that… but with this topic I wanted to come up, perhaps, with a few innovative / unique ideas, which could be used or implemented…

I think a good way is also to put voltmeter in front of the board or just make a simple 4-5 led indicator, which also shows the voltage / battery left with how many leds are light (or in which color…)

Anyways, Im using wattmeter myself (with total voltage shown), so basically when voltage is around 21v (in my case) I know the cells are around 3.5v and that’s when the ride should be soon ended… but it is a bit hard to tell how much juice I got left when im in the middle range… so a display with percentage would be better suited for this…

Lipo alarms (cheap ones) suck in this application, they are quite loud and it is not easy to turn them off once they start buzzing… plus, for me they are inaccurate and show a big wrong voltage for a few cell groups.

figure out wh/km consumption in your system by testing with voltage meters, so that you can learn the maximum distance your board takes you. Track your route using GPS to know how far you’ve travelled and use that to see how much farther you could go…

Yeah I’ve already done this … but the best way would still be direct kilometers/ miles, like the evolve does…

I know that my consumtion (emtb) is around 900mah per km or about 20wh/km (should try harder tires) but to make a calculation in real time I still need to figure out my batteries parameters (at what voltage how much is left etc)

Would be great to know how much km I can do with 1volt of drop in total battery voltage…

So yeah, thanks @anorak234 for leading me onto this path… I might actually consider just wiring up my wattmeter, so that it stays on the whole ride and then just see the total ah consumed and from that I can calculate how much is left… this should give me a somewhat accurate result… the only donwside is that ah consumtion is not shown constantly on the screen.

@Okami here’s my low-tech solution: I do a range test on my build from a full charge. Then I use the distance I’ve travelled to estimate available charge at any given point. It’s not a great system because unpredictable factors influence the use of energy (temperature, terrain, detours from the planned route, etc). However, I’ve never been stranded using this system.

I carry a skate tool with me and my backup plan is to disconnect the belt and push home if needed.

A better solution would be a programmable power meter that could use range from full-charge as an input to calibrate available energy.

Yes voltmeters suck because as we all know there is a long period during use where voltage is the same and then it rapidly drops!

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When my acceleration is noticeably lower

@treenutter, yeah this is something I am aiming for… Im not quite sure yet how to make it happen but this ‘‘arduino code / device’’ thing should be able to measure spent energy and available energy…

I imagine something like cars have as ‘‘bort computer’’ (not sure how it is entirely called in usa) Basically it shows consumtion and other stats since the engine was started or sometimes from a longer period of time also…

So yeah, it should be able to see the average wh consumtion over a period of time or best over distance and from that give some sort of estimate how much range is left… not sure how practical this might manifest in real life as the terrain and riding conditions might be quite diverse… to make accurate prediction / calculation but there should be at least some guidelines to follow…similar like with this full/empty battery thing and the total distance you can do with it…

I imagine it could be way easier to manifest this ‘‘prediction of distance left’’ if someone is having a vesc… that vesc monitor app looks quite handy and I believe it could do real precise measurements and suggestions too…

Unfortunately im not a vesc user yet and there’s still ppl riding with non-vesc esc’s… so this ‘‘product’’ / device might be handy for more than a dozen ppl, if someone comes up with it…

@Mark mh that is also a good indicator but unfortunately it might be too late to find out you dont have much battery left… If I can do 10km / 6miles with full battery and im on ‘‘budget’’ with left battery capacity, it might be hard to tell whenever I can do these extra 3 - 4 km (~2miles) or not…

Will think about doing some more tests… especially at the lower end of voltage, since right now I have usually rode very conservatively… only once I’ve ridden the batteries so empty that I could not get any speed at all from them…

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I think you’d need to gather a significant amount of data in a variety of different situations in order to make an accurate prediction of leftover range. Even something like windows can’t properly predict the amount time a laptop battery has left.

A simple \$6.00 voltage meter is all you need to know excactly how much is left!

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These 2 videos could be interesting for you. there you can see how the wh consumption changes with the way we ride. In the first video i tried to keep a constant speed at around 25km/h and in the second i did the same round and tried to be quick. And you can see that the consumption goes up by roughly 84%.

What really saves energy is when you don’t accelerate and just push to your speed and only use the motor to keep the speed. This way you can safe a lot of energy.

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The % battery display on my DIY Electric battery pack is nice to have but when I hop off the board I can always count on it going up at least 5% to 10% within ten minutes of not riding. With this in mind my go home point is around 42% or so and I know I’ll be fine

Most accurate way: -Get discharge curves for your pack -Get a voltmeter on the board -Figure out how much power you use in several conditions: slow cruise home, average way of riding, regular climbs, max climb, top speed.

Based on discharge curves at a power condition, you can figure out what your voltage should be at 50% You could also step off for a minute and let the voltage rise to its no load and then you only need the no load curve. As long as you have a .01v resolution, you should be able to measure capacity within a few percentage points.

The easier way is to buy a coulomb counter and have it on your board. You know the capacity of your battery, you know how many ah you’ve spent, you know how much you have left. If you do it this way, you need to leave some buffer on the lower end depending on how hard you’ve been riding.

Interesting comparison videos. Watched them both a few times. I really need to get my VESC set up and pick up one of those BT modules. They are just too cool.

What battery/motor/gearing were you using in that video? I assume it’s 12s LiIon, but I was surprised to see your voltage sag from 49 to 35 volts under heavy acceleration. Is that normal?

the evolve is only a guestimate. Personally I ignore it’s % reading and go by the cell voltage readout.

@Mobutusan Those batterys are from harvested broken Laptop battery packs. So they are absolutely not made for that purpose. But they work. So the voltage sag is huge with them. But i still can get around 2000W of power. Even 2500W is possible when they are fully charged. And gearing is 14/36 with 2 190kv Enertion R-SPEC.

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Sorry, I’m volunteering today to ask the dummy question in this post: I thought the ESC/VESC had a cut-off limit where the whole thing would stop to avoid going too low in voltage? Are we just here talking of not being cut-off 5Km from home in the middle of nowhere and not actually avoiding to damage the battery?

@jga No, Im referring here to outside methods of reading your total battery voltage… and being able to tell midway when battery is reached 50% of usable capacity… to head back home (if it is a ‘‘joy ride’’ where you basically just go in one direction and then head back…)

@PXSS

You know, I was reading your first part of text and you just made me: woah… wont probably do this in the future

Though I would agree that if there was a specifically designed coulometer with some advanced functions, like predicting the distance left based on your existing speed and power consumtion, then yes, it might work…

Otherwise I would probably believe my ‘‘gut feeling’’ and look at simple capacity display and tell from that how much further can I go

So thank you too @devin for the tip, as I will probably just improve my current wattmeter (coulometer) and will make it stay on the whole time… it just happens that if you dont have outside power attached to it, it loses all its values once it is reset.

This way it is not easy to keep track of the energy spent, if it is reset each time you turn your board off…

I just use a coulometer. Check my percentage before my ride and make sure I have at least half that to get back home. How far you can go on how much depends a lot on how you ride, the temperature, grades, terrain… I’ve gotten as much as 30 miles on a charge and as little as 15. If I’m running low and home is still a ways off, I push more to start off and up hills.

No meter is going to be anything more than a rough guestimate of how many miles you have left. There are too many variables. One thing for sure is that having any sort of meter helps a lot. My Gen2 Evolve didn’t have any indicators so you had to guess. Had to push myself home a couple times. Especially a pain since you can’t tell how much charge your battery has when you first start out. Once I did a DIY build with the LCD display, I haven’t been stranded once. I just know that when I get down to 35%, I have about 8 miles left. 10 if I push a little.

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Tell me how much power you use on average on your way home and what type of battery you have. I’ll tell you at what voltage at that power and no load you have 50% left.