Knocking a battery pack together on the cheap

Batteries, Chargers, and Battery Management Systems.
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salfter
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Knocking a battery pack together on the cheap

Post by salfter » Dec 29, 2017 3:05 am

Actually, I'm looking at doing the entire ebike conversion on the cheap: about $120 to convert from rim brakes to disc brakes and replace the twist-grip shifters with "rapid-fire shifters," $325 for a 1-kW geared rear hub kit, and then whatever the battery and charger end up costing.

On that front, prices were looking depressingly high (like starting around $500 for 650 Wh assembled, or $5-$6 each for cells to assemble) until I stumbled across some links here and elsewhere pointing to sources of cheap cells. Here's a summary of the options I'm considering:
  • 14s3p made of Orbtronic 5.75-Ah 26650s...894 Wh for $419.58 (most expensive option here, but the lowest cell count...cells support 15A discharge)
  • 14s5p made of Samsung INR18650-30Q cells...777 Wh for $349.30
  • 14s10p made of Lishen LR1865LA cells...1036 Wh for $174.99
  • 14s7p made of Sanyo UR18650A cells...798 Wh for $132.30
  • 16s3p made of Fullriver LiFePO4 32700 cells...829 Wh for $71.99 (cheapest option here; cells support 9.9A discharge)
The first two still represent some savings over buying a preassembled battery pack, but the other three are deep discounts built around new-old-stock cells. Is there any reason I should go for the more spendy options? More cells would obviously weigh more, but these cells seem light enough that it's not likely to be a huge difference (about 2.8 lbs. less for the Samsung pack than the Sanyo pack, for instance).

Also, with the LiFePO4 pack: while I have found a 16s BMS labeled as supporting this chemistry, is there anything different about the charger? I've used CR123 Li-ion and LiFePO4 cells in flashlights before, and they needed separate chargers (presumably to handle the voltage difference). 16s LiFePO4 puts out 51.2V nominal, which is just a little bit shy of the 51.8V you'd get from the usual 14s string. That difference, divided across 16 cells, is only 37.5 mV. Would a standard 52V charger be safe to use with a 16s LiFePO4 pack?

Addendum: After posting, I found this, which suggests that the little bit of overvoltage involved here likely wouldn't bring harm to a LiFePO4 battery pack. A "52V" charger would put out 58.8V; divided across 16 cells, you get 3.675V each, which is only 25 mV above LiFePO4's 3.65V full-charge level. Additionally, LiFePO4 appears to be much more tolerant of overcharging (700 mV per cell vs. 100 mV for other Li-ion chemistries).

All that said, this is my first time out with this stuff, so I just want to make sure I'm not missing something glaringly obvious to the more experienced.

999zip999
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Re: Knocking a battery pack together on the cheap

Post by 999zip999 » Dec 29, 2017 2:23 pm

The battery is the heart of an electric bike and the most important part. A battery should be overmatched for the situation of your controller and motor .

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dogman dan
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Re: Knocking a battery pack together on the cheap

Post by dogman dan » Dec 30, 2017 9:13 am

Lifepo4 would be the safe one. Bear in mind, whatever they say about the cell, cut it in half.

Oh sure, it will do what they say,, they just don't say how much that cell will hate it. That cycle life test they spec,, it was not done at 9.5 amps. That is for sure. At best, half that. Call that lifepo4 a five amps cell is what I mean.

The more you cut the cost on battery, the more it might cost you. So charge it outside. Inside once charged not so dangerous, its just while charging I'm saying.

16s lifepo4 can charge to 58v no problem, and the 5 amps size on up chargers can usually be adjusted up or down a few volts.

ldmvcd
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Re: Knocking a battery pack together on the cheap

Post by ldmvcd » Jan 01, 2018 8:39 pm

Hi Salfter,

Where are you planning to purchase your cells from?

salfter
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Re: Knocking a battery pack together on the cheap

Post by salfter » Jan 04, 2018 6:02 pm

ldmvcd wrote:
Jan 01, 2018 8:39 pm
Where are you planning to purchase your cells from?
https://www.amazon.com/gp/product/B01M9AD7NN

Price has gone up by a third since I last checked, but the per-watt-hour cost is still the lowest.

So far, I've only bought parts to convert my bike over to disc brakes and most of the components for a capacitive-discharge welder (see https://gitlab.com/salfter/cdweld for details on that).

ldmvcd
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Re: Knocking a battery pack together on the cheap

Post by ldmvcd » Jan 04, 2018 6:13 pm

Thanks :)

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