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Old 07-12-12, 10:00 PM   #1
Apache Thunder
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Battery meter accuracy...

I have googled around a bit not found anything specific to my question. My question pertains to if the battery meter I originally bought for the SLA batteries is going to report similar readings with the new battery or if it's going to be noticeably more inaccurate due to the different chemistrys and discharge curves.

I upgraded my ebike to a Lithium battery (LifePo4 to be specific). Originally I had 3 12v 12AH Sealed Lead Acid batteries (which had to be at least 30 pounds and the single Lithium battery unit I got feels like it weighs in a 8 pounds which is close to what just one of the 12v SLA batteries weigh in!).

I bought a battery meter off eBay (from a US seller, but would not be surprised if his stock was made in China though) and it appeared to be accurate since it would go down in step with the discharging of the batteries and worked great with my SLA batteries since once it goes past halfway on the meter I begin to notice a slowdown in the motor. The exact item I got was this:

http://www.ebay.com/itm/250957472770

(I had to dig through my email archive to find it though since I bought this late last year and eBay only shows purchase history going back 60 days, which is kinda lame)


As I recall with the SLA batteries, I could ride up to Wal-Mart (where I worked) and the battery meter would lose one to two bars. After leaving it powered off for a while and turning it back on, the battery meter would return to full. Then after the second trip, the battery drops 3 bars or more which then even after power off and back on, it doesn't rebound anymore and will continue to go down with further discharge.

I think it's close to or slightly 1 mile to my house and Wal-Mart so it's a close indicator of the range I was getting. So it appears I was getting roughly 10 or so miles out of the SLAs had I chose to run them to the red. But knowing the weaknesses of SLAs, I would recharge them once they go past half way on the meter so I really only was going 5 to 6 miles before I take the battery off the bike and recharge it. After going past half way I would notice a drop in the top speed and the torque would be noticeably less then full.



Now fast forward to today when I got the new Lithium battery. The motor has noticeably higher torque then even the SLAs had at full charge and I got 2+ extra MPH on my highest gear (since I now have a mid drive system).

So far I've ridden at least 5 miles on the new battery and the battery meter hasn't even dropped one bar yet. So this makes me think that perhaps the meter isn't accurate for this battery chemistry? Or maybe the battery chemistry when compared to SLAs is just that good?

Also despite going at least 5-8 miles on it thus far it has not slowed down or felt less powerful. It's still got the same kick to it that it had when I installed the batteries. (I did charge them before installing them, though it took the charger only 30 or so minutes to charge it so it looks like the seller charged it before shipping it to me)

So for the moment I'm sure the battery is still in the green. But longer term I am thinking of replacing the battery meter if it turns out it's not reading the charge correctly. As with the SLAs, I don't want to over discharge the Lithium batteries. I've heard you can discharge them lower then SLAs without damage, but I want to keep the battery with at least 45% to 50% at all times so I need to determine what the range is for the system. Which so far is proving to already exceed the SLAs, the distance I covered in one day on these batteries would have sent the battery meter to red already if I rode on the SLAs. The weight of the batteries may also be factoring in to the increased range as well. (I am also a relatively light rider only coming in at 130-140 pounds on average!)

It's a 350 watt system and I've had the new mid-drive system using the SLAs for at least a few weeks before I got the LifePo4 so I do have something to compare to.


Knowing how long it took the charger to top off the battery when I first got it (around 20-30 minutes I think) I will ride another 5 miles tomorrow and then put it on the charger and see how long it takes to charge. The charger is stated to have a 6amp charging current so someone with some decent math can chime in with some numbers on what charge times I should expect from it. Then I can gauge how much I've really discharged the battery and what my range will start to look like.

Any suggestions or info on this matter would be appreciated. I'm on a tight budget so I can't invest in some expensive super computer device devoted to super analyzing battery usage with tons of statistics. I am trying to keep it simple.

Oh and my bike does have an odometer on it so I can see the speeds and miles my bike has traveled. I will pay close attention to those numbers to see what I'm getting.


EDIT:

Ok after riding another 2-3 miles on it, the battery meter finally dropped a grand total of one bar.

I put the battery on the charger and the charger finished charging it in about 1 hour and 20 minutes (perhaps minus 5 to 10 minutes since I was not in the room when it finished, so it may have have finished a bit quicker. After some googling it is supposed to take up to 3 hours to charge this type of battery if it it was charged on a "rapid" charger and was anywhere close to being fully discharged. So my guess is perhaps I only used one third or so the rated capacity of this thing if the charger is a bit weaker then what it's rated for. I don't think it actually puts out the full 6amps advertised since the price I got the battery for said "free upgrade to rapid charger". Thus it's probably something higher then the standard charge current of 2a but not the full 6amps the battery can be charged on. So the charger is probably more on par with putting out 5 or so amps.

So if that's the case then about 14-16 miles of riding only used a 3rd of the capacity of the battery in a worst case scenerio....wow....just wow. I will NEVER, EVER use SLAs again!

Last edited by Apache Thunder; 07-13-12 at 12:34 PM.
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Old 07-16-12, 08:44 AM   #2
Apache Thunder
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Really? Nobody?
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Old 07-16-12, 09:45 AM   #3
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Your question as i understand it is "can a battery meter meant for SLA give an accurate reading on Lithium batteries?"

My answer is, probably not. But in reality, battery meters are fairly inaccurate, period.

Lifepo4 batteries aren't too far from SLA batteries in terms of voltage. Lead Acid '36V' rated batteries are actually 39V when fully charged. LiFePO4 36V batteries are fairly close -- a full charge might be 40 volts (mostly depends on the design of the charger!).

Likewise, when you are NOT running a current, your LIFEPO4 should be about 36 volts when it gets to low voltage cutoff (that is to say if you keep using them at that point you will substantially reduce the life of your batteries). The same is true for your lead acid batteries.

If you want your batteries to last as long as possible, try to keep them at 'all bars' or 'all but one bar'. If that doesn't work for you, don't worry about it. USE your batteries rather than treating them like they should be kept in a display case. (they will degrade over time, whether you use them or not).

It's not at all bad for your batteries to be discharged to low voltage cut-off ONCE. Check how many miles you get (use your normal riding style as far as how fast you go, how hilly is the terrain you choose, and how much you pedal). Then you'll have a better idea what percentage of your battery life you use up to go 15 miles.
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Old 07-16-12, 03:35 PM   #4
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Yeah as I understand it, the battery meter I have reads what the voltage is to determine the capacity of the battery. If the Lithium battery is one or two volts higher then a SLA, then I'm guessing it would only be one or two bars off from before. I'll keep that in mind, thanks.
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Old 07-19-12, 10:57 AM   #5
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Alright so I finally rode it until the battery cut off. So it looks like the absolute range is 13 miles (+/- 1 mile). So that means I was only getting about 5-6 "useful" miles out of the SLAs, I could go a few more miles after that but I would have significantly less speed and torque due to the SLA's voltage sag.

The new batteries went all the way to 13 miles and the last mile or two was when it had "slight" drop in torque. Which means I would have only noticed it if I had tried to go on sixth gear (which is the gear I need to use to get to 25MPH). So even after near empty battery I could still get around 22MPH out of it. The SLAs would have me stuck on 17-18 MPH after half the battery was used up.

As for the battery meter. It's completely inaccurate. It never drops past the second bar and every time I turn power off and come back to it later, it would rebound back to full and would drop one bar after a few more miles of riding. It wouldn't drop one bar until I was roughly 75% through the battery capacity. So at this point I will probably just get rid of it once I get a new battery case. I'll stop it at 10 miles and put it on recharge anytime I hit 10 miles. And this is all on 100% electric by the way since my bike does not yet have the pedals connected to the chain. Thus I did no pedaling on it.
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Old 07-19-12, 11:37 AM   #6
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I am curious, what kind of bike do you have? Is it a conversion or factory made ebike. Is your commute hilly or fairly level? I was looking at various makes and what their true range is. Thanks...
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Old 07-19-12, 02:58 PM   #7
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My bike is this one:

http://www.huffy.com/Products/Product.aspx?pid=383|4|8

I converted it to a ebike late last year. Then about a month ago, I converted it to a mid-drive system by getting a new mounting bracket for the motor. After I did that it's been pushing along with little down time. The original side mounted method was unreliable since the bracket eventually began flexing under the torque causing the chain to derail.

Now that it's a mid drive system, the chain is further away from the wheel and is going through the derailer. Now I'm able to change gears and keep proper tension on the chain. Once I get the bottom bracket spacers, I will align the bracket even better and then install the motor freewheel cog and run it through the pedal chainwheel (which is now on a freewheel as well). That way I can pedal the bike when I want to. But for now it's all electric and can't be pedaled yet.

It will do about 25MPH on level ground with no resistance after hitting sixth gear. However it will average about 21-22MPH if there is a slight incline or wind resistance.

My town is reletively flat. The "hilliest" areas in my area would be the intersection with the rail rode tracks which appear to be roughly a 10-15 degree incline. My bike has no trouble going over it. With my setup, I'm pretty sure I can handle most hills that I could theoretically run into. For a place like San Fransico which has very steep inclines. I suspect it could still work there too provided that I get a new rear freewheel with a bigger first gear. Since it's mid-drive, I can use the same gearing that the pedals originally used, so I can just use bigger gears on the back to get more torque.

My system is a 36v 350 watt with a brushed motor. The battery is a 36v 12ah LifePo4 (Lithium battery and weighs in at 13 pounds almost) I ran it through 3 or so cycles thus far and it is holding up great. The last cycle I ran it all the way till it cut off since I needed to figure out the range. So even after doing that, it charged up to full charge is under 2 hours and still works great. So looks like the battery has no defects what so ever. Compare that to my old 30+ pound SLA batteries. They went less then half the distance my new battery takes me (and they were 12ah capacity too!) and were heavy as ****.

The motor does have a gear box by the way so there is usually enough inertia in the motor so that when I throttle down it will take a bit of time before it stops spinning. So because of that, I can still change gears if I let go of the throttle.

Last edited by Apache Thunder; 07-19-12 at 03:09 PM.
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Old 07-27-12, 09:07 AM   #8
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hiya apache thunder, just read your post as i am fairly new on here & am in the papamotor section of the ebikes.you might consider going EBike.ca & check out there cycle analyst.that might help you out.good luck. Mike
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Old 07-27-12, 09:45 AM   #9
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That would be great, but $120+ for a battery meter (even if it can do other things like odometer and amp read out and stuff) is way off target for my budget.
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Old 07-27-12, 05:45 PM   #10
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HELLO "AT": I use a Cycle Analyst ($95.)with the Crystalyte 500Whub laced into the 16"wheel on my BOB trailer, the CA and throttle are mounted on my ride. The CA records miles\Kms, watts, amps, speed etc. I use the hub mostly for hill assist and have achieved over 100Kms on a single charge (36V 10ah). With the CA I'm able to monitor the amount of watts I'm consuming, if I keep the watts around 100w-150w, I've been able to extend my range.
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Old 07-30-12, 03:17 PM   #11
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Lifepo4 battery packs do not work the dame as SLA batteries. As an SLA is used it lowers in voltage. This allows it to be easily read on a meter. However, if you allow the battery to rest for a short while you will likely notice the voltage will rise again. But as soon as you throttle up again down goes the voltage again.

Lifepo4 has a much more stable voltage. While it is generally charged to 3.65v the normal voltage is considered to be 3.2v per cell. You have a 12 cell pack. Or about 38.4v. Depending on the quality of the cells this voltage will be much more stable. Therefore, difficult to read a change on a voltage meter. The proper meter for tracking your packs remaining capasity is an amphour meter. But you will find it works in reverse of what you are used to.

An ah meter reads how many ah were used not what is remaining. Therefore, you need to make certain assumptions. For example; If you have a 10ah pack and it is fully charged you would ride your bike and look at how many ah you have used. Lets say you used 4ah. Therefore, you have 6ah remaining till the pack is gasping for breath. You NEVER want to run a pack lower then 80 to 90%. In fact if you want maximum life don't go below 50% if possible.

BTW, you need to rezero the ah before each ride. Let me know if you need infor on ah meters for your bike. Cheap ones go for about $60 and good one twice that and more.

Bob
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