1
\$\begingroup\$

On my application I have four 200 mAh LiFePo4 cells, configures as sereies of two parallels, thus achieveing a 6.4V battery.

I understand this cells in series need balancing, I would like to leave this mattar aside for now, only keeping in mind that the cells are being balanced.

I'm drawing 130mA from the batteries for 1h, recharging them afterwards with ~10mA.

At some point they get stable at 3.45V, not the 3.6V it suppose to achieve. After this, with the same load, they discharge in minutes.

Any ideas of what could be happening for the batteries not achieving maximum charge voltage and discharging with such speed afterwards?

\$\endgroup\$
  • \$\begingroup\$ Sounds like a cut-rate charger. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 15 '15 at 16:19
  • 1
    \$\begingroup\$ What does "at some point" mean - Minutes? Hours? Days? \$\endgroup\$ – brhans Apr 15 '15 at 16:31
  • \$\begingroup\$ @brhans a couple of hours. \$\endgroup\$ – Sergio Apr 15 '15 at 16:33
  • 1
    \$\begingroup\$ I'm not familiar with the details of a LiFePo4 charge curve, but a 200mAh battery being charged at 10mA is going to need over 20 hours to recharge. A couple of hours is't going to get you all that far, and an apparent plateau isn't all that uncommon somewhere along the way. \$\endgroup\$ – brhans Apr 15 '15 at 16:38
  • \$\begingroup\$ @brhans yes, it's suppose to go steady around 3.4V. I tried some longer charges, like 8h. Didn't really noticed great changes. I'll try some longer charges and come back with some more data. By the way, thanks for the input. \$\endgroup\$ – Sergio Apr 15 '15 at 16:45
3
\$\begingroup\$

LiFePO4 cells have a very flat charging voltage curve that doesn't peak until just before full charge. When charging a 400mA pack at 10mA it may stay below 3.5V for 30 hours or more.

As a result of the sharp voltage peak at end of charge, a small imbalance can cause large voltage variations between cells in series. An aggressive balancing circuit will fix this problem, but may consume some current itself (which could be significant at low charge rates).

How to charge Lithium Iron Phosphate

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Let's say I would be charging with the same 10mA, but now I would have the four cells in series, thus having a 14.4V battery with 200mAh. Would the charging time drop to aprox. half, by the expense of more energy being used by the charger, because of the greater voltage to maintain the same current? \$\endgroup\$ – Sergio Apr 16 '15 at 8:53
  • 1
    \$\begingroup\$ Yes.The capacity would be halved and voltage doubled, so the battery would charge twice as fast but the charger would draw twice the power to deliver the higher voltage. \$\endgroup\$ – Bruce Abbott Apr 16 '15 at 18:44
0
\$\begingroup\$

You should charge at 3.45vpc, 3.50 max. Ideally at .3-.5 of C. When current drops to C/50, stop, don't float.

No load, rest for a couple hours, are they around 3.40vpc?

That is "full" if you want longevity. And within .1% the SoC you'd get pushing a higher more harmful voltage.

If you actually try to get 3.6v resting, you will only get 10% the lifetime cycles treating them gently will get you.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.