Utility Cycling Technology: How to Build a Battery for Bike Lights

Don't try this at home, it is a bitch and a waste of money to build your own removable cell holders. I just happened to have most of the parts so I wasted a lot of time putting them together. And I still need a bolt or two to clamp the top and bottom together, as well as a little electrical grease on the contacts.

It is much more sensible to buy a $35 Vruzend kit, especially if you want more than 8 cells. And an expensive BMS; the cheap ones from Amazon are a waste of money.
It will cost more to use protected cells, but you don't need a BMS, it is safer to do a small pack like this. That I can take th cells out of to recharge them. the cells with protection circuits on each cell, will keep them from dropping below 2.5v each.

But for an Ebike pack you need to use unprotected cells unless you are rich. Then you need a good BMS.

WARNING ! If you do not use a good BMS that can read the lowest voltage for each cell group and it should shut down the whole pack at that voltage. Otherwise you start killing cells. They are expensive at best. Disastrous at worst. These plastic trays are worthless crap, they worked at first but after being moist for a while the connections are bad [use electrical grease you say?] and the plastic started falling apart. So now I am just going to have to buy a 12v-15v pack with 8 cells. Unless I can do the welding or soldering of the wires myself and use only cells that are protected with good circuits in the tip of each cell.

New I-Max chargers will not charge through the protection circuits, but the old ones do. Don't buy a BMS through amazon !! they are all crap and there is no way to tell that was not your fault that it did not work.

charging in parallel

Never use two packs together always connect all the parallel cells together in bunches so that when one cell goes below the safe voltage the others in the bunch will balance it out.

When using old computer batteries or even new unprotected cells, it is much better to connect at least two cells in parallel so that one does not go below 2.5 volts [which can damage a lithium-ion battery] then the parallel buches in series to increase the voltage.

I had a new cell drop down to .7v then it would not hold all of the electrons I pumped into it. It went down to 1.744 after charging for about 30 seconds up to 2.5v on my so called smart charger. And every time I charged it up to 2.5v it went back down.

The strobe lights I put on my bicycle run on 12 to 16.8 volts and don't need a voltage regulator because they have that in the units. But some are so low powered that they can run on an even smaller voltage [three 18650 cells in series = 12v at 4v each] but to run a lot of them I needed 4 in series [= 16v] because the voltage lowers as the battery runs down.

The old computer batteries I used can't hold more than 3.9 volts each so I needed more in parallel to make up for the lack of capacity. It is best not to use cells that have vastly different capacity abilities because the lowest voltage will limit the total voltage you have available.

Charging 18650 Li-ion cells in series will leave them unbalanced [unless you have a balance charger]. Charging them in parallel helps balance the voltage. I leave them in the holder for 1-4 hours after charging to 4.2 volts [or less] until they all read the same. Then I put the cells in a different holder that lets them discharge in series starting at 15.6 volts. If your using small strobe lights you can use as little as 3 in series for a 12.6 volt supply [at 4.2v @]. Don't over charge any kind of battery, stress shortens battery life and can be very dangerous!! infact it is better to not try to charge to more than 4 volts each; you can make up for the lack of voltage with more capacity in parallel.

Look for a smart charger that has
an adjustable cut-off voltage setting.
Or at least one that shows
the voltage for each cell.

The cheapest way to charge 18650 cells is with a so called smart charger that will completely turn off the charge at 4.2 volts [or remove the batteries before going to bed so they can't overcharge.

If you don't want to use a discharge protection circuit board you must use cells that have the discharge protection circuit in the top. Even if they cost more money. If any cell goes below 2.5 volts it may not take a charge again. I set my discharge shut off circuit at 3 volts for each string of cells in series. [4 cells in series is 12 volts at 3v each]

these use 8.4 volts maximum

I have an old head light that needs two in series [6 to 8.4 volts]. So I built two trays that can hold 2s and 4p [“p”= parallel “s” = series] but I still need to remove the cells and put them in parallel to charge them. This is not suitable for an electric bike battery pack because there are way too many cells. So you must rely on an expensive BMS [battery management computer].

If you want to run your lights off your ebike battery, you will need to find a voltage regulator that will convert a high voltage input like 48 or more, to the lower voltage for the lights. I was not able to find an affordable regulator that would handle my 52volt battery.

Never use Tenergy batteries, they lie about the capacity. And UltraFire has been known to explode or burst into flames [and there are more]. If you want inexpensive cells buy from a whole seller.

Do not waste money on the battery trays that have springs! Your batteries will not make contact! These new trays are the only ones that will work, and you still may need a rubber band to hold them into the tray.

Dean's T-Plug Connectors

solder wire directly onto the tabs

or drill holes before soldering.

Old computer batteries are a waste of money, even if they are free! Most of them are damaged form age.

If charging at .3amps per cell is the ideal amperage, then multiply it by the amount of cells in parallel 4 = 1.2amps. Or if you have 2 or more in series .3 x 2 x 4 = 2.4 amps, but then you need a BMS with balance wires.

If a cell goes below 2 volts it is too low for even a smart charger to put more electrons into it. I tried recharging one that went down to .07volts while discharging in series with three other cells.. I put it in parallel with three other cells that were at about 3.5v each. Within about 60 seconds the dead cell started heating up to dangerous level, I should have had a heat sensor on it. Good thing I felt it with my hand.

It could have burst into flames or even exploded. Never walk away form from an experimental charge. And never use those cheap little transformer chargers that come with batteries. I believe that they are the prime cause of most battery fires.

Discharging used cells in series, even with an over discharge protection circuit can kill one. That is why all the cells in a pack should be fused. Even if they all tested out to be close to the same capacity. Or at lest use a good calibrated volt meter before recharging them.

I don't think makes a BMS computer that will read each cell in a large pack . So fusing each cell is the only safe-guard. And if you can make room for more cells of LiFePo4 use them even if they cost more per weight. For these small packs you need 5 instead of 4 cells.

But for a 20Ah pack you need to add up the amount series strings in parallel to find out if you have enough amps to cover your controller's worst draw. And convert that to watt hours.