Working Out Battery aH Used
- Thread starter awol
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D
Deleted member 4366
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There's no way to work it out, but according to this graph, about 65%. Once they reach 3.6v, they fall off a cliff, so be careful if using a voltmeter to monitor. It's best to use a wattmeter because you'll soon see how many amp-hours they give before they reach the precipice. Then, from the amp-hours used, you can predict how much further you can go.
http://i.stack.imgur.com/LV91V.gif
http://i.stack.imgur.com/LV91V.gif
your post makes me think: lipo packs are attractive because they are cheaper than Li-ion. But if you can't use them at below 3.6V for fear of damaging them then the usable capacity needs to be topped and tailed between 4.1V to 3.6V, making a 15AH pack be like 10AH e-bike battery. Not an economical choice. So why would anyone bother with lipo?
But in your 10ah lion you don't have 10ah of actual usable capacity either?
Also you can charge to 4.2v (in fact hobbyking just brought new ones that go to 4.3v)
on standard Li ion, the manufacturers guarantee you can use 100% of the specified capacity, typically 30 miles for 10AH. You can ride your bike until LVC, about 3.1V per cell. According to d8veh post above, if you stick to 4.1V - 3.6V, you get 65% of nominal capacity. That would be around useable 20 powered miles for 10AH lipo pack. I understand you can stretch the range - but at greater risk of killing your pack.
D
Deleted member 4366
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That's not true. Most people charge them to 4.2v per cell. By the time you get to 3.6v they have little charge left, so 4.2v to 3.6v is about 95% of their useable capacity, i.e. instead of 8ah, you get 7.6ah.
Once charged past 4.1v, they rise rapidly, so you don't lose much if you only charge to 4.1v either.
The graph above shows 90% capacity between 4.1v and 3.6v.
Lipos are a very attractive option if you know how to use them. You get the capacity written on the label in normal use. They can give very high current for very little weight, and best of all, you can get a 12S (44v) battery which is the maximum that you can use with a 36v controller. The extra 2 cells is a nice way to boost the speed and torque of a 36v system, since you don't have to change anything other than the battery.
It was the lower weight to power ratio which made me choose the lipo's.
I originally wanted the lightest battery with just enough capacity to do my regular journey to work and back and 10aH was too much.
I think I may have even overdone it with 8aH too. Based on the above voltage readings do you think a 6000mah battery would have managed? which would have shaved a few extra grams of weight off,or is there no way of comparing?
Lets say I've used (rounded off) 0.35v of usable 0.5v 8000mah (about 70%?) and I have 0.15v left over (or about 30%) could it be as simple as saying I've used 70% of 8000mah=5600mah actual? or does it not wortk like that because the next battery would start with a smaller 6000mah?
I originally wanted the lightest battery with just enough capacity to do my regular journey to work and back and 10aH was too much.
I think I may have even overdone it with 8aH too. Based on the above voltage readings do you think a 6000mah battery would have managed? which would have shaved a few extra grams of weight off,or is there no way of comparing?
Lets say I've used (rounded off) 0.35v of usable 0.5v 8000mah (about 70%?) and I have 0.15v left over (or about 30%) could it be as simple as saying I've used 70% of 8000mah=5600mah actual? or does it not wortk like that because the next battery would start with a smaller 6000mah?
I did have a wattmeter on but guess who unplugged it forgetting to take the reading first? Doh!!
Also I just wanted to see if it could be worked out using the volts to compare and check the accuracy of the wattmeter for next time I use it.
D
Deleted member 4366
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It's dodgy using volts because they go up and down with current, and when yo get low, a small amount of discharge can make a big difference, plus cell balance has an effect, so best to use the wattmeter.