Alittle advice with charging final phase with lifepo4

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glyndwr1998
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Joined: Wed Mar 06, 2013 12:34 pm

Alittle advice with charging final phase with lifepo4

Postby glyndwr1998 » Thu Jul 31, 2014 8:39 am

Hi,

I have got a 76 cell lifepo4 pack that i have been quietly tinkering with for optimal set up.

I have modified the zivan charger with help from Dave at electrofit zapi which required a 20k turnpot to be fitted in place of a 2k on the voltage set point which gave more voltage adjustment, plus they supplied a reprogrammed eeprom with a better setting for lifepo4 cells.

Previously the charger would never shut off at voltage set target, my cell loggers would end the charge when any cell reached 3.5v, the charger would continue delivereing 15 amps until the any cell hit 3.5v.

Now i have had some time to tinker and lowered the charger voltage set point, when the charger has charged the pack closer to the set voltage, tha amps of the charger reduce from 15a to about 5 amps then to 3 amps so the cells are slowly approaching the set voltage point (instead of climbing rapidly after 3.4v).

Is this ok to keep this set up.

The charger isnt in a floating stage just low current, i have read somewhere that lifepo4 cells must not be floating, just wanted some advice as i havent ammassed great experience in battery management.

Many thanks for the advice with this.

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skooler
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Re: Alittle advice with charging final phase with lifepo4

Postby skooler » Thu Jul 31, 2014 9:09 am

glyndwr1998 wrote:Hi,

I have got a 76 cell lifepo4 pack that i have been quietly tinkering with for optimal set up.

I have modified the zivan charger with help from Dave at electrofit zapi which required a 20k turnpot to be fitted in place of a 2k on the voltage set point which gave more voltage adjustment, plus they supplied a reprogrammed eeprom with a better setting for lifepo4 cells.

Previously the charger would never shut off at voltage set target, my cell loggers would end the charge when any cell reached 3.5v, the charger would continue delivereing 15 amps until the any cell hit 3.5v.

Now i have had some time to tinker and lowered the charger voltage set point, when the charger has charged the pack closer to the set voltage, tha amps of the charger reduce from 15a to about 5 amps then to 3 amps so the cells are slowly approaching the set voltage point (instead of climbing rapidly after 3.4v).

Is this ok to keep this set up.

The charger isnt in a floating stage just low current, i have read somewhere that lifepo4 cells must not be floating, just wanted some advice as i havent ammassed great experience in battery management.

Many thanks for the advice with this.


Interesting problem!

Firstly, do not float charge - they will swell and eventually fail. I deliberately killed four cells to find out!

LiFePO4 charging consists of two phases.

Constant current (CC) phase is where the current remains at the same level and the voltage rises as the cells SOC increases.

Constant voltage (CV) begins where a set voltage is reached. At this point, the voltage remains the same and the current tapers down to keep the pack voltage at the same level. When the current drops to a certain level (normally a 20th of the cells capacity) the charge stops.

The way I am setup is to run the CC phase until the first (weakest) cell hits 3.5v and then switch to CV until the current drops to 5amps (C20 on a 100AH cell).

I find CC gets me to just over 80%SOC and CV gets me into the low 90%. this is based on the initial cell testing i did with the powerlab before installing.

It's not 100% relevant but I use the bottom balanced method which is why I never reach 100% SOC as I am undercharging the cells slightly.

I'd say that your CV phase is starting to early?
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glyndwr1998
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Re: Alittle advice with charging final phase with lifepo4

Postby glyndwr1998 » Thu Jul 31, 2014 10:20 am

Hi Skooler,

I have 33ah cells made up of 11 3Ah cells in parallel (k2 type 26650)

Please may i ask how long in time should the cv stage be.

In my set up as it is at the moment, (for the last 2 charges since set up alteration) the cells reach about 3.4v then the charger current drops from 15a to about 9a for about 5 mins then current drops to about 5a, most cells at this point are about 3.41v and my weakest cells (about 4 cells around the same level of weakness) go to 3.5v within about 10 mins.

After charge and resting most cells are resting around 3.4v with a few at 3.3v. all 76 cells are around 10mV or less in balance.

Is it your opinion i should increase the charger voltage set point 1 or 2v.

I have found that even a small turn on the turnpot makes a big difference, and sometimes taking the pot back or forward 1 revolution doesnt equate to a stable point, ie, i can be at a set voltage, turn the pot 1/4 of a turn adjustment, then think maybe gone too far, turn back a 1/4 of a turn from start point and find it not the same as when i started.

Thats why i have stayed at this point, as it is consistent.

I shall if advised obviously spen another hour or so tweeking further to reduce the cv stage.

Many thanks skooler

Grumpy-b
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Re: Alittle advice with charging final phase with lifepo4

Postby Grumpy-b » Sat Aug 02, 2014 6:42 pm

I now regularly change the Zivan chargers in the C1s back to their original control, and the EPROM I buy from ELectrofit also has a time limit of either 6 hrs CC and 6 CV or 9 Cc and 9 CV.
The CV stage is just that, when it drops in charge rate to around 5amps it will drop lower over time maintaing the CV stage. Individual cells can still keep rising and its helpful if you have a BMS with individual cell readings to use this to cut the charger off if any one cell goes above a set threshold. But the overall pack voltage will be maintained and the current reduced accordingly. It does not stay at 5amp (or whatever its set at) for ever.
You could also use an external programmable Voltage controller to turn the charger off completely once it hits a preset voltage above the CC limit. On the C1s the final voltage is two volts above the reset point for 100% on the pack (As originally built) and I now cut the finish of the CC stage off nearly 10v (25cell pack) lowere than the original BMS control of the charger did. In practice this difference only takes about 5mins to reach and so is really insignificant in terms of overall pack travel distance. But does help to ensure the cells dont go high voltage.
I do keep stock of the Zivan chips. As well as used chargers for 25 or 32 cells.

Grumpy-b

glyndwr1998
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Re: Alittle advice with charging final phase with lifepo4

Postby glyndwr1998 » Sun Aug 03, 2014 12:09 pm

Hi guys,

As this is my first project ( although I have had the pack built and running ok for about 7 months now) and I have never had experience in charging any chemistry of battery in an ev application, the terminologies are new to me.

So, if I am reading into this correctly, when my charger has dropped to 5 amps I am now in the constant voltage phase, and this could continue indefinitely ( or timed) until there is no current flow and the battery pack is the same voltage as the charger is delivering.

So, with this in mind, and my weakest cells do climb to 3.5v and activate a relay that cuts the charger ac power, that I am Not floating the cells, as I do not want to float at all.

Is the terminology floating mean that all cells are held indefinitely at a set voltage because there is now now current flowing....

If this is correct then I ham relatively happy with my set up as thcharger is still delivering 5 amps and the cells are climbing in voltage But now at a very very slow rate.

Many thanks again, just want to clear up the floating meaning.
Thank you

Grumpy-b
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Re: Alittle advice with charging final phase with lifepo4

Postby Grumpy-b » Sun Aug 03, 2014 4:13 pm

The Zivan chargers already have an open contact cease charge facility. So you can just open a relay/switch and the charger will stop, low voltage so much easier than any relay in the mains lead. In the Constant Voltage phase the charger does just that, maintains a constant voltage by lowering the charge current. But you could still have an individual cell go high in this phase, but at a vety low current. Which setting have you got the rotary switch on the Zivan set to? You can select 6 or 9 hrs for the Cc/ CV phase.

Float voltage is where a charger will maintain a specific charge voltage until you turn it off, a minute or a year or whatever. It just keeps the battery at that voltage indefinitely.

CV ist that, its a stage in the charge, but actually offers very limited extra storage in the lifepo4 cell, unless you have the cut off voltage for CC set really too low.


Grumpy-b

Beemer
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Re: Alittle advice with charging final phase with lifepo4

Postby Beemer » Thu Aug 28, 2014 10:37 am

Is it worth the teensy weenie little bit at the end which is so close to hurting your pack?

I'd just bung in current to 3.4V max on the known first cell to hit maxV then stop.

But that's me.

Grumpy-b
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Re: Alittle advice with charging final phase with lifepo4

Postby Grumpy-b » Thu Aug 28, 2014 10:48 am

But what if the cells need to go to over 3.6 as per thunderskys, charging to 3.4 will be quite undercharged, probably by 10% .
The voltage is a bit of a misguidance, as the voltage is read during charge and a higher rate of charge will give higher voltages at the cell, but turn the charger off and the cells will drop a lot more than if charged at a lower current. So you have to understand the specific cells, the charger, the rate of charge and the drop after charge.
It doesnt help to give explicit voltages in general discussion, the end of the CC stage has to be based upon the specific cell requirements and they vary a lot between makers, even between cell size.

What you get into the cells above CC is in my opinion of limited value to range, but can increase the likelyhood of other cells in the pack going high voltage. The only saving grace is that at CV stage the current is usually very low, so limiting damage.

Unlessyou have an excessively large pack in AH terms, most packs need to be fully charged to get the range and performance required. If its way over AH then kts not such a deal, but most people dont spend the cash required to achieve this.

Grumpy-b

Beemer
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Re: Alittle advice with charging final phase with lifepo4

Postby Beemer » Thu Aug 28, 2014 11:17 am

Then add another 10%!

The Leaf has a 24KWH pack but is allowed to only 'see' about 20KWH from new. The useful range is within that! It hits the red line at 30%. Often my pack does not charge over 90%, sometimes 93% of the allowed limits due to common variances in the cells. The Leaf pack is not balance or never see's any balance until right at the very bottom edge -- maybe. I've never seen it elsewhere in the charge profile.

My foot rule is 12.23" long... You get my drift? :(

Grumpy-b
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Re: Alittle advice with charging final phase with lifepo4

Postby Grumpy-b » Thu Aug 28, 2014 11:51 am

Adding 10% voltage does not ingrease the charge ah by 10%.
The leaf was designed to have over capacity and not to fully charge the cells to theri ultimate level. That way they could extend life. Thay was my earlier point. Many with lower capacity packs need the cells to be fully charged to get the range the pack is potentially capable of.

Its simple charge the cells under the appropriate CC rate until they reach the required voltage. Shift to CV then turn off according to the makers requirements.

If you dont wish to fully charge the pack, then dont. And adopt whatever regime to not fully charge the pack suits you. But of course your 200ah pack wont perform as a 200ah pack could. But thats your choice. Just be aware of that and drive accordingly, not expecting the ultimate performance the pack could deliver.

But it is not someting that you can be highy prescriptive with as it depends on the Cell make up, the charge rate, temperature etc etc.
Grumpy-b


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