Float charging LiFePO4 batteries + analog BMS system?

[evalon]

Hi All,

I'm considering float charging a series of 6 LiFePO4 batteries (3.45 VDC/cell with balancing, vs 3.6 VDC full charge voltage) on a continuous basis, i.e. for about 10 days per period. Then the batteries will be disconnected from the charger, somewhat discharged and then again charged at the float voltage. Will this allow the batteries to maintain full specifications (no deterioration) or won't it work?

I may also be looking for a LiFePO4 charging circuit that is "analog", i.e. without switchmode circuits but with LVC & HVC and charge current setting including 1/10 (or 1/20C) when 3.6 VDC is reached. Any of you know if this exists?

Thanks for reading & hopefully replying

Jesper

 

[arkmundi]

Not sure, but you might look into Celllog's with appropriate alarm settings. Are you asking what's the alternative to a BMS, where shunt switching is programmed? There's no such thing as an appropriate analog solution that would be effective. Either you've got some digital or not.

 

[scoot]

Well I have been using that approach on several 8s Headway packs for a couple of years now. Using 24v SLA chargers with CC/CV capability (fairly intelleigent bulk charger) and a standalone balancer with high voltage cuttoff at 3.75 volts (Chargery db8). The cells always settle down at 3.45v in the end with the current typically bouncing around 10ma or less once they get there. I often leave it charging like that for many days in between use. Or, when I unplug for a few weeks I'll top off the same way if I think about it ahead of time before using it.

I really can't offer a reliable opinion as to whether it has any deterioraing effect. Haven't done any capacity testing on these packs since my initial testing. But if i did, and happened to discover any cpacity losses, I would be more inclined to believe it was due to my high current discharge practices rather than any charging related issues. :|

I know others out here believe the cells/packs should be partially discharged (like 80%) for long term storage, but I usually don't worry about it. But it seems to me that maintaing 3.45 versus 3.65 for extended periods could be less stressful, but I just don't know for sure :? I do lnow however, that 3.45 volts per cell is so close to full that any loss of capacity versus 3.65 is downright negligble (< 1%). I have actually measured that numerous occasions. :wink:

 

[dnmun]

yep, for the headways, the fully charged cell drops in voltage right down to 3.45 or so when the charger turns off. i don't think it is even 1% short of full. there is no such thing as trickle charging with lifepo4. unless the cell self discharges as fast as the charger adds current, in which case it is dead anyway.

 

[scoot]

I think with my setup it is mostly about the balancer (discharging) and charger achieving a quazi equillibrium at the 3.45v per cell level. I can observe the ultra low current toggling off and on via the cycle analyst once they are balanced at 3.45v. As far as the headways go, most of my cells self drain slowly over time even with them not hooked up to anything. I think that's just the way Headways behave when new and for quite some time/cycles beyond, and am not convinced it is necessarily an indicator of defectiveness per say.

The one exception is my original pack that has been extensively conditioned with ultra low current (100 ma balance charging from my icharger) for numerous and lengthy periods/cycles at the 3.9v level. I think of it as squeezing in all of the electrons into the far recesses of the previously unactivated substrate inside the cell... I know that sounds silly but it helps me wrap my head around the meaning of my experimental observations. Those cells will now hold 3.6 volts or more resting for days. I concluded it really wasn't worth the effort to do that level of conditioning with my other packs, so I just adapt to their mild self discharging behavior as needed.

 

[dnmun]

i have noticed that the tabbed, spot welded headway cells will retain their charged voltage a long time too.

the screw top cells seem to lose the free charge in the electrolyte faster and settle to around 3.45V within a day or two. not sure why, they both hold the same amount of charge when i do a capacity test.

 

[scoot]

All of mine have been the screw tops

EDIT: finally getting around to reading dogman's "never below voltage" thread... LOL :lol:

 

[evalon]

Hi all,

& thanks for your replies. Very informative to learn about your experiences and thoughts

Hmmm... In this context I do, however, need to be quite sure that the batteries are not damaged by float charging (need the low ESR for a long time) so I have decided to send a mail to Cadex (batteryuniversity.com) and hope they are capable of giving a very clear answer. When/if I get a reply from them I'll post it here FYI.

Regarding Li-ion batteries I got this feedback from one of the engineers at Linear Technology:

... "normal" LiIon batteries is not recommended to charge like that due to you get small charging/discharging going in and out due to temperature change. This does wear a little on the pack, as LiIon batteries gets worn by charging/disharging regardless how small the current is (kind of eats the battery) ...

Best regards,

Jesper

 

[arkmundi]

Hi all... "normal" LiIon batteries is not recommended to charge like that due to you get small charging/discharging going in and out due to temperature change. This does wear a little on the pack, as LiIon batteries gets worn by charging/disharging regardless how small the current is (kind of eats the battery) ...

Yes, that's our common understanding and experience. There is "wear & tear" on li-ion batteries due to charge/discharge. Understand the cycle life of your cells and try to get the most out of them. Staying in a "safe" range, as indicated by voltage, is how to prolong. But with time and age, expect things to go awry. Then its try replacing individual cells if you can, or the whole pack. If the pack is young, do the first thing. If its old, the second thing.

There are some who believe that smaller, more frequent charging is the best policy to prolong battery life. That rather then less frequent, deeper discharging & recharging. Keeping cells in balance is a sine-quo-non, however you do it. I charge after every use. I first simply test each of the 12 cells in my pack to see if there's an abnormally low one. If so, I use a single cell charger to bring it back into alignment. Then I bulk charge the whole pack. This approach is working for me right now. I'm tracking the individual cells, maintaining a log, and keeping an eye on balance. I have yet to either accept or reject use of a BMS, taking the "manual" approach for now.

 

[evalon]

Hey again,

I've now gotten a reply from battery university and apparently there's not much information about float (trickle) charging LiFePO4 batteries. However, it's an interest of the person I communicated with as well so if he finds more information he'll get back to me. As for now he would treat the LiFePO4 battery as a Li-ion battery, i.e. no continuous float charging.

Will get back if I hear more :wink:

Jesper