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Lithium Ion Battery bottom balancing


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I recently bought an electric bicycle. It has a 0.5kWh battery (Panasonic 36V LCM). According to the owners manual (to improve lifespan) I am supposed to fully drain the battery after every 15 charge cycles. Can anybody explain why this will be beneficial for the battery? Does it have anything to do with bottom balancing? I remember @plonkstermentioned in another post, that some BMS's have a bottom balancing feature. 

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6 minutes ago, Fuenkli said:

Does it have anything to do with bottom balancing?

I kinda doubt it...

Bottom balancing has more to do with getting more energy out of the battery. Let's say you have 4 cells in series (a "12V" battery) and 3 of the cells are 100Ah while the 4th is just 95Ah. The battery is then "empty" after you extracted 95Ah from it. Three of the cells still have 5Ah left in them, but that cannot be extracted without damaging the low cell. A bottom balancer will allow charge to be transferred from the other 3 cells to pull up the low cell, thereby allowing more charge to be extracted from the higher cells.

Most BMSes only does top-end balancing. When the cells are almost full, charge is transferred from a high cell to a low cell to make sure all of them is fully charged. Top-end balancing can also be done passively (by bypassing a cell, pretty much like lead acid balancers do). This is usually done through a resistor, which causes some energy to be converted to heat, making this sort of balancing less efficient, but significantly less complex (and cheaper!) to implement. On the other side: Bottom-end balancing can only be done by an active balancer, which is essentially a DC/DC converter (a bit like a mini MPPT that charges one cell from the one next to it).

Long story short, I'm not quite sure what you'd achieve by taking the cells down to 100% DoD. Balancing can be done just as well at the top, and draining batteries to >90%DoD shortens their life quite a bit.

Could it be that whoever wrote the manual (or was tasked with it) wrote this in because they still remember NiCd batteries that had a memory effect?

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20 minutes ago, plonkster said:

I'm not quite sure what you'd achieve by taking the cells down to 100% DoD

exactly my thinking. I will not do it. On top of it, fully draining the battery on an e bicycle literally means getting stuck without power. Not something my old legs would enjoy at the bottom of the steep uphill to my house 😱.

Edited by Fuenkli
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Just now, plonkster said:

Does it have a range-indicator?

No. It only has a charge status indicator (5 LED lights). You make a very good point though. I do however not care much about the remaining range since I never get anywhere near the distance the battery is capable of delivering. I do on average 40 km per trip and usually have still more than 50% of charge left.

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Using 20700 cells - "Introduced by Sanyo/Panasonic for use in portable power tools as higher-power and higher-capacity successor for 18650 cells". Nominal cell voltage is 3.6V / NMC chemistry so should not need any other conditioning besides the normal charge (CC/CV) 


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21 hours ago, Richard Mackay said:

Tell us more about the e-bike.

as much as I hate to admit it, they work very well. The doctor gave me the choice between regular exercises or high blood pressure medication ☹️So I ended up getting an e-bike because it was the least painful option for me to become more active. Now I actually enjoy going for my daily drive. With an e-bike you can go much faster and further than we a normal bike. Another attraction is that I can charge it for free with my solar PV system


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