Fuel gauge for LiFePO4 batts

[mythprod]

I'm sure this has been answered already but was having trouble finding a thread. I'd like to make some sort of rudimentary fuel gauge for my LiFePO4. To start I'd just like to check it when I get to work in the mornings or get home in the evenings and just using a multimeter would be a fine start. I can measure volts but due to the chemistry and its flat discharge curve I'm assuming measuring ah is better / more accurate? So far it's been a bit of a mystery how much is actually left in the battery and I'd like to get a better feel for how I'm doing efficiency-wise. Could someone walk me through a way of calculating how much battery I have left with a multimeter?

TIA for the help!

 

[dnmun]

you have to use a wattmeter to count coulombs.

 

[e-beach]

I'm sure this has been answered already but was having trouble finding a thread. I'd like to make some sort of rudimentary fuel gauge for my LiFePO4. To start I'd just like to check it when I get to work in the mornings or get home in the evenings and just using a multimeter would be a fine start. I can measure volts but due to the chemistry and its flat discharge curve I'm assuming measuring ah is better / more accurate? So far it's been a bit of a mystery how much is actually left in the battery and I'd like to get a better feel for how I'm doing efficiency-wise. Could someone walk me through a way of calculating how much battery I have left with a multimeter?

TIA for the help!

No doubt there are those who are better at this question then I am but, let me tell you what I did before I had a watt meter. (there are really inexpensive ones BTW..... :wink: )

I got a cheap multi meter and hooked it into the voltage-out connector on my controller and just rode the bike until the battery died. When I found the battery beginning to get soft, I kept an eye on the meter so I knew the voltage at LVC.

When I got home I got onto Google earth and calculated the actual distance I rode. I did this for several trips and I learned the miles I could ride before the pack was depleted to LVC.

After I knew my range, I would calculate the miles before I rode so I knew if I needed to take a charger with me to top-off my batteries somewhere. I rode without a watt meter for about a year and never had a problem with running out of juice because I knew ahead of time how many miles I was going to ride. In the end I got a $20.00 watt meter and it only confirmed what I already knew about my distances.

You can simulate here http://www.ebikes.ca/simulator/ But real world is better.

I would suggest going to LVC at least once, on an average ride for you. Then calculate the miles you traveled as a reference for future rides.

PS, you also need to know how long your charger takes to fully charge you pack (minus balancing time) so you can calculate the time you need to wait for you batteries to charge for the distance you still need to travel.

For my 12ah charger to charge my 15ah pack, it takes about 4.5 minutes per mile or 90 minutes to get 20 miles.

 

[izeman]

you CAN NOT say how much energy is left in a lifepo4 by using a voltmeter. it just ain't work.
there's only three things you can say: it's full at 3.5v/cell, empty is below 3v/cell and not empty/full if between 3v and 3.3v. these are rough guesses and you can discuss some tenth of a volt up and down, but more or less that's it.
just get a wattmeter, or even better one of the cycle analysts and you have an accurate reading of ampere hours used. once you really know how much Ah your battery can provide, you have a valid fuel gauge.

 

[sal_park]

I'd like to make some sort of rudimentary fuel gauge for my LiFePO4

So what do you mean by make ? Take a micro controller, read the voltage/current consumption and display the answer on a screen ? If so I might be able to help you - I've been working on a project (Bangxiety meter) for the last 18 months or so which will do that. I plan to share more details but want to get it functional enough so other people can build something that works pretty much out of the box*. I plan for the software and hardware to open source so you'll be able to build your own.

There are some pictures in this thread



sal_park



* If you're looking for the easy way to do this don't go down this route - but I suspect you already know this

 

[arkmundi]

I use the Watt's Up meter, readily available from RC shops.

 

[dogman dan]

I'll let you do the search, but there are several threads now on inexpensive watt meters. Using a voltmeter on lifepo4 is more like an " its empty now" gauge for sure.

I paid less than 20 bucks for my cheap watt meter just recently, so cost is no barrier to having a watt meter.

Before I got a CA, I went two years just using an odometer. It worked when I rode the same route every day. I knew I had a certain range, so a detour could be made if I kept the detour to a certain maximum distance. If I was not home at a certain point along the ride, I'd know to ride the last 5 miles slow in order to make it further.

But having a wattmeter is the way. Once you test your range, and know how many Ah or Wh you have, you can just look at the meter and tell if you are half way yet, and need to turn back for home. With a wattmeter, you can also measure how much charge you put back in, if you stop for a quick recharge along the way.

At less than 20 bucks for the cheapies on ebay, it's a no brainer to just get some kind of watt meter. If you have money, get the cycleanalyst. It's so much better than just a cheap knock off.

 

[Gregory]

Yes, the quality may be questionable but these $12 delivered power meters on ebay would be a cheap start for beginners.

If you wanted DIY you could look at TI chip which you could at a minimum set up with 4 LEDs as a fuel gauge, easier than programming a micro, but with a lot less features.

 

[Drunkskunk]

I'll start with telling you to get a Cycle Analyst, and then never worry about what you want to measure again. speed, mileage, wattage used, motor heat, voltage, amperage, percent of power from regen, watts per mile, cycles on the battery, ect, ect, ect.

But they do cost more than an average watt meter.

what you need for what you want is just a plain and simple watt meter. it won't tell you how much is left in the battery directly, it will tell you how much you have used. Different factors such as temperature and peukert effect from heavy load situations can cause the battery's capacity to be different from one ride to the next. But if you know how much power you have used, and your voltage, you'll have a better idea as to whats actually left.

 

[davec]

either a CA or you can get a DC GT power meter for 30 bucks that will give you the Ah you've used up.

 

[izeman]

not to forget to mention what voltage we are looking at: all those cheap wattmeters you find on hobbyking and ebay are limited to 65v. i haven't found any other solution that works for 65v+ but the CA.

 

[sal_park]

not to forget to mention what voltage we are looking at: all those cheap wattmeters you find on hobbyking and ebay are limited to 65v. i haven't found any other solution that works for 65v+ but the CA.

So what would be a good upper limit for the voltage ?

 

[Drunkskunk]

On Endless Sphere, or for the majority of the rest of the world?

65 volts would cover a 15s setup, and fill 95% of the real world's needs.
But on ES, 250 volts maybe?

IMHO, if you have more than a 48 volt setup, you should be looking at a lot more than just your wattage

 

[izeman]

not to forget to mention what voltage we are looking at: all those cheap wattmeters you find on hobbyking and ebay are limited to 65v. i haven't found any other solution that works for 65v+ but the CA.

So what would be a good upper limit for the voltage ?

the sky is the limit you can have a lot of fun with 48v. some prefer 100v+. some commercial ebike come with 24v or 36v.
i have 24s lifepo4 setup with is around 76v. it all depends on your needs, wishes and sometimes financial limits.

 

[dogman dan]

It's a fair assumption that if you measure the wh discharged from half of a 100v battery, the other half is discharged a pretty similar amount.

So if you are running 100v, monitor half of it, that is 50v, with the cheap wattmeter.

But by the time you are spending that much on your battery, the cost of a good CA is reasonable. Personally, I own two CA's, one cheapie wattmeter, and two cheapie voltmeters. In some cases, like running my lawnmower on 6s lico, just the cheapie voltmeter is enough. Lico has a less flat curve.

 

[Tommy L]

Also.......

Once you use a Cycle Analyst, or watt meter, you will really enjoy knowing your consumption for
different rides. Some rides are windy! And you will notice that your range that day will be much less than
calm days. And, if you hit a lot of hills on a ride, again, your watt meter will show you that you are drinking
a lot of energy

Personally, I know my commute ride (about 200-300wh one way), so if I can't charge at work I need around 600wh.
I only like to use about 50% of my pack, so I carry 1200 wh pack

You will get use to figuring all this out, but having a CA or Watt Meter is a great help.

You will find that most will recommend a CA or Watt Meter

Tommy L sends.......

 

[kfong]

I used one of these meters and a micro.

http://endless-sphere.com/forums/viewtopic.php?f=2&t=22675&hilit=fuel+gauge

 

[upward]

Does this mean that with a Cycle Analyst it will tell you how many watt-hrs went back into the battery? This would be useful like you mentioned when charging but need to unplug before it is fully charged. Will the watt-hrs figure on the CA keep track of the number of watt-hrs received from the charger with a negative figure?

 

[dogman dan]

Yes, you can hook up your CA "backwards" if you have the stand alone model. So the controller is unplugged, and the charger attached to the plug the controller was plugged into. If you have a bms, you then have to unplug the battery, and connect the charger plug to the CA.

So say you rode, and used 5 ah. Now as you charge, the AH used display will begin to count backwards to zero. Or if you have zeroed it out, you could have it count negative, or positive, depending on which end of the shunt you used as the input.

It will take a bit more wh to get fully charged, due to losses in efficiency when charging. (warming the battery) But once you know what that % is, you can then make a basic rule of thumb, like 10 ah from the charger yields 9.5 ah actually stored in the battery or whatever.

I'm not sure how you could do the same thing with a DP cycleanalyst, but I bet you could manage it somehow. For 20 bucks, its easier to just get a cheapo wattmeter to put on your charger. That's how I've been using my cheap one the most, putting it on a charger, so I know when it's getting closer to done.

 

[Tommy L]

I use a Kilowatt meter. I plug it in the wall, then I plug my charger into it. Most chargers are approx 80-85% efficient.
If you use 200wh from your pack, then you will most likely .24kwh recharging your pack.

Tommy L sends.....

 

[rojitor]

Honestly i don't think a watt meter is worth it, if you spend a few more coins you can get a cycle analyst and that's imho worth every penny.

 

[e-beach]

Run your battery pack to LVC, calculate your miles and you will know your range.

Plug a chepo volt meter into your controller voltage out and run your battery pack to LVC, caculate your miles and you will know your range plus how low your pack voltage can go just before LVC if you are watching your meter.

Install a cheapo watt meter and Run your battery pack to LVC you will have a very accurate idea of your range.

Get a Cycle Analyst and get all the information you need from your electric bike, at least until a thief steals it off your bike.

Around here the thief's are very-very fast. They will pull a CA off your bars, plus the wiring in less then a minute. Maybe your is safer for things like cycle computers, around here it is dicey.