How to charge an electic bike batteris pac

[Bolek]

Ther is a public transportation strike in Paris , France.
I bought an electricaly assisted bike with a Ni Mh 24 V , 8Ah pac. The pac comprises 20 D size Ni MH batteries. The motor has 3 power levels the higher is 200W i.e. about 8.3A. I can not control the power level. For 3 times I completly charged and discharged the pac. After that is it better to compeletly discharge the pac before recharging- nomaly a round tipe to my office or to charge it at home and in the office (then I must take the charger with me).Thanks for your advice.

 

[SilverFox]

Hello Bolek,

I would go with charging at home only. On the weekends see if you can balance the pack by leaving it on charge for 6 - 8 hours after the charge has completed. Hopefully the charger has a trickle charge that will serve to balance the pack.

If you have access to a hobby charger, you can balance the pack by charging it at 800 mA for 6 - 8 hours after the pack has been charged, or starting with an empty pack you can charge at 800 mA for 16 hours.

Your pack will die when a cell gets out of balance and ends up over discharged which will then drive it to being reversed charged. With 20 cells, it is very important to pay attention to cell balance.

Tom

 

[Bolek]

1) Thanks you. I woried more about the memory effect, but you are right (as allways) the balance is the actual problem!
2)I have a power suply with the DC output current adjustable between 0 and 1.4A If my understanding is correct from time to time I can connecte it for a long time to the battery pac and set it to 0.8A. The standart chager is suposed to detect the charge end (Delta V ) or a high temperature. It charges at 1.8A. Is any owerheating risk with my power suply at 0.8A ? Would it be better at lower current for a longer time (e.g. 0.4A for 40h)?
How often do I have to balance the pac ?
Is it any nead to completly discharge the pac, and if yes is once a month OK? Is a complyte discharge at a high current (on the bike ) useful or have I to make a discharging device.

 

[rizky_p]

nice electric bike with D cell Ni-MH it is just easy to add more battery

 

[SilverFox]

Hello Bolek,

Your power supply set to 0.8 amps should do a good job of balancing your pack.

Once a month is a good goal to shoot for, but if you notice performance dropping off earlier, you may have to do it more frequently.

A complete discharge (down to 1.0 volts per cell under load) may help, but it depends on what the cut off voltage is for your bike. If your bike shuts off at 1.1 volts per cell, then it would be beneficial to do a deeper discharge every once in a while. Once every 3 - 6 months should do the job.

Be careful when doing this deep discharge. With a large pack consisting of many cells, it is easy to over discharge a cell and ruin it by having it reverse charge.

Tom

 

[Bolek]

Thanks SilverFox
The connector will be ready this weekend.


rizky_p :
I do not plan to build a new pac, but if one batterie fails in the pac it can be changed.

 

[Bolek]

3 months later :
It apears that it's not 20 but 24 D batteries in serie. It's 34V off the charger and 30.4V after 10km and 15 min reast. The bike is great for urban commuting in Paris. During cold days (about 0°C =water freezing temp) the batterie was not enough for a 10 km ride. The balancing woks great (thanks SilverFox).
Now it's maybe time to make a compleate discharge. I'll start with balancing to have all cells having about the same charge. Then run the bike until the bike shuts off. Then mesure the voltage.What is the best way to go down and to what voltage 26.4V, 24V or lower ? :thinking:

 

[SilverFox]

Hello Bolek,

I would suggest stopping when the voltage is at 1 volt per cell.

Tom

 

[Bolek]

Thanks, I'll try with a 220V bulb as external resistor.