Trickle Charging / Slow Charging Headway Cells

[ncwa]

Does anyone have experience trickle charging / slow charging Headway cells? I have a 72V 40ah pack consisting of 96 H-38120S. When I charge with a 20 amp LiFePO4 charger, all of the cells reach approximately 3.65 volts in 1-3 hours depending on depth of discharge. However when I charge with my onboard 6 amp charger, only a few will reach 3.65 and the rest will stay around 3.50 to 3.54 volts even after many hours of charging. Some of the cells even seem to reach around 3.540 and then "rest" down to 3.520 while still being charged. While watching the BMS display, it almost looks like there are not enough amps to push all the cells to the full 3.65V. I have set my top end BMS to balance above 3.635 volts so I don’t think it is doing much with the 6 amp charger, but it is really working hard (hot) with the 20 amp charger.

Thank you for your help.

 

[Ypedal]

As long as you reach 3.5xx volts, consider the cells fully charged.. gettin them all the way to 3.65 is only netting 1 or 2 percent more energy and not needed .

the slower charger is kinder to the cells, should increase life somewhat.

 

[Pure]

As long as you reach 3.5xx volts, consider the cells fully charged.. gettin them all the way to 3.65 is only netting 1 or 2 percent more energy and not needed .

the slower charger is kinder to the cells, should increase life somewhat.

What he said, and not pushing the cells to the max 3.65 volts, will also help prolong their life.

Personally, given the choice of having both chargers, I would only use the 20 Amp for those times that you need a quick charge.

On a side note, that's a huge pack. Pics and build details would be awesome.

 

[Ypedal]

to be a bit more specific..

The charger will have a fixed voltage at wich it reduces power and maintains this voltage .. refered to as " CV " constant voltge phase.. all the charger cares about is the sum of all the cells combined voltage and amps go up or down accordingly to maintain this voltage..

at 20 amps the charging voltage will overshoot resting voltage by quite a bit.. but at the end if you disconect the charger all cells should maintain 3.5xx volts for a while and then over time ( minutes to hours, to weeks depending on the cells ) they will , all on their own, drop down to a resting voltage of 3.3xx volts or so, they are still fully charged ( or almost ) but most cells will not retain 3.6 volts once removed from the charger.

 

[Tommy L]

Hi Guys!

I've been testing the new Headway 38120HP cells. Ypedal is absolutely correct, as he has done many testings as well.
These are my findings:

(4s x 4p 32amp/hour pack)
3.5x to 3.6x hot off the charger (per cell)
3.45 resting per cell (13.8v as a pack)
3.28 working per cell (13.12v as a pack) @ 1C 400 watt Resistance Load (4x 100watt lights)
3.16 working per cell (12.64v as a pack) @ 2C 800 watt Resistance Load (8x 100watt lights)

Tests were performed with a Xantrex HF1000 watt inverter powered from 4s x 4p (16 Headway 38120HP 8amp/hour cells)
and home built Light Board for resistance load.

I have a couple of Xcel files with data and graphs.
I'd like to post them, is there a good file share where I can link my files here?

I've also taken temps. Interesting that as the cells get close to being depleted, their temps start to climb rapidly.
From what I've read, this is normal, due to the internal resistance rise. The voltage really starts to drop and the
amperage rises near the end. Hence, temp rise So far at 1 and 2C, this pack has given me it's rated Amp/hours.

 

[circuit]

If your onboard 6A charger is unable to rise all cells to 3,65V, there is a problem with:
1. crappy BMS
2. some cells leakage current is >6A at higher voltages (doubt that, but still..)
3. charger voltage lower than cell_count*3,65V.

For proper balancing and equal aging of the cells, all of them should hit 3.65 at the end of charge. Otherwise weaker cells will die sooner than they could.

 

[ncwa]

Hey Everyone - thank you for the posts. Attached is an image showing the voltage (green bars) at each of the 24 series locations on my 72V 40ah pack. The blue lines indicate history. What is interesting is that the cells that don't reach 3.65 V actually start dropping when connected to the 6 amp charger. :? (for this charge, the top end BMS was set to start shunting at 3.635V)

At the bottom of the image, the details on the forth bar are shown. The max voltage (Vma) was 3.548 but is has dropped to 3.510. There are a few that have dropped.

My 6 amp charger and relays are mounted in the car so I tend to use it more often and then manually shut it off if one of the batteries start to loose voltage before the relay shuts the system down automatically. Then every couple of months or so I'll hook up the 20 amp charger and hit ~3.650 - 3.6555 on all cells. Hopefully this is acceptable to the batteries :|

 

[ncwa]

I only know how to post one photo at a time - so here is one photo of the batteries for you Pure! It is around a 70 pound pack. I wired it up so that in order to short the full 72 volts, one would have to touch the two most distance points - however as this was by far the largest pack I have ever build, I was still rather nervous about working on it

 

[ncwa]

Tommy - where were you measuring temperatures? I am going to have to watch for my temps at the times you mentioned.

 

[Tommy L]

Tommy - where were you measuring temperatures? I am going to have to watch for my temps at the times you mentioned.

So sorry! I lost track of this post!

I use a laser temp gun and I aim it middle/top/bottom of cells as well as the bus bars.

I use either one or two 0.25 T6 6061 aluminum bus bars or one 0.25 T6 6061 and one Headway bus bar stacked.
I pull 3000 watts from my pack and I want as much as possible to the rear wheel. Some might call it over kill, but
I want nearly zero loss to "C" rise.

I'm running 89.6v nominal as a pack now. 28S1P

Tommy L sends....