Suggestion on headway pack / BMS

[johndjmix]

Ill soon be needing a battery for my ATV tire ebike prototype. I need a battery with a very high amp draw ability.

Im thinking Headway cells, seems like they are the only way to get the amp draw I will need. Here is what im thinking:

Two - 60V or 72V 15AH headway pack(s), ill build it. Ill have both on the bike, so when one runs out, I can switch to the other. Or possibly parallel the 2 packs for more amps, will i need to? I want around 120 amp peak ability, but i cant see pulling that for more than 5 seconds....
At 72V the watage output should be sufficient. (72vx120A=8640 Watts). Lets see how my first 2 motors test with this.

Im open for recommendations on a BMS that can handle this as most seem to have limits around 30-60A....and a good charger.

--John

 

[dogman dan]

IMO, at 120 amps you will not want a bms on the discharge. Just monitor the pack, and practice a maximum 80% discharge. You can build the pack with some 8s balance taps and be able to stop and check individual cell voltage at a glance with some cellog 8's on the dash. Then have a wattmeter such as a CA monitoring capacity discharged and pack voltage.

I'd also be choosing RC Lipo batteries as well, for the much higher c rate of lipo, but understand the appeal of the headway cells. 120 amps from 30 ah would be 4c, so still in the capability of headways. I think you should build your pack by paralelling the headway cells, then series connecting them so you have a big 30 ah pack. If you want to, you can still charge and balance with a regular bms. Bypassing it to discharge, you won't need something huge to get the charging job done if you only charge at 5 amps or so.

 

[dnmun]

the headway BMS will handle 120A with no problem on discharge. if you use a pack without a BMS, you run the risk of losing the pack to overdischarge. you should always wire up all the cells you have piled up on the bike, the idea of having a second pack to go to is silly since it increases the stress on the cells by raising the Crate on discharge and charge and you will be pulling one pack down to LVC each time, which means longer times on the charger to counteract the increased imbalance, just generally bad thinking all around.

 

[JimmieD]

Hi John, there are some PCM's/BMS's available that can handle that amount of current for the 5 seconds or so that you are looking for and dnmun has given a good recommendation!

For my two cents, I recommend using a PCM/BMS even for discharging at higher currents, just be sure to get a quality PCM or BMS that can fit your needs and your wallet!

Anyways, just remember when doing the calculations for estimates,

At 72V the watage output should be sufficient. (72vx120A=8640 Watts).

there will be voltage sag when drawing 120A from the 40152SE 15Ah Headway cells which in turn will drop the actual usable voltage for your estimates for watt output. For a 120A discharge (8C) you will see a voltage sag to about 2.6~2.7V or so per cell. So to play it safe, let's call it 2.5V per cell * 24 cells in series which will be around 7.2kW in lieu of the 8.6kW you had originally anticipated.

I agree with dogman, if going with the Headways, it would be good to put into parallel so it will be a 4C discharge at 30Ah in lieu of an 8C for the 15Ah, this will dramatically lessen the voltage sag giving around 3.0~3.1V per cell resulting in the 8.6kW that you were anticipating for the 5 seconds.

I apologize I do not have any current (no pun intended!) discharge tests for the 40152SE 15Ah cells, but will be performing some within the month or so, been busy with the 38120HP tests.

Although not using a 40152SE 15Ah cell, here are test results of using the 38120HP cells and you can see the voltage sag I was speaking of, and then how much it increases by simply doubling the C rated discharge. (Tests completed using 4S1P 38120HP cells using CellLog8S for data logging)

 

[dnmun]

i just dug out the new 16S headway BMS that jimmyD sells with his headway packs.

it has 14, not 10 or 12, but 14 of the RUI chip 190A 80V mosfets in the output. each mosfet has an on resistance of 3.4mohm so the total equivalent resistance is less the .3mohm.

at 120A of current, that is 36mW of power lost in the mosfets. not watts, milli watts!

there is no reason to use a powerful battery pack without protection from over discharge. it makes no sense to build two packs and you should use all the cells available to build one pack.

 

[johndjmix]

Thanks guys! Suggestion on where to buy?

Will the headway BMS handle the amount of cells required for 72V?

--John