Slow charging Li-ion's

Discussion in 'General Electronics Chat' started by gbar, Feb 16, 2011.

  1. gbar

    Thread Starter New Member

    Feb 16, 2011
    I'm having trouble finding information on how slowly I can charge Li-ion batteries.
    Background: I building a device that will need 10-12 Amp current spikes lasting 1.5 seconds every 15 seconds (10% DC), six times in a row, once or twice a day from a single cell. I'm considering a 18650 cell (~2.2k mAh) because they are available and relatively cheap (I would also consider a pouch style) and it could go 2-4 weeks without recharging.

    If I use a plug-in charging approach, I can re-charge it at practically any rate I choose; easy to do. However, if it must have an inductive charging design, the charging current will be limited to about 200 mA, or about C/10. Recharging time available is 24 hours and there'll be the proper CC/CV charge circuitry on-board, so this isn't a long-term trickle charge situation.
    Can I go that low?

  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    Sounds interesting, what's the purpose of this device?
  3. Markd77

    Senior Member

    Sep 7, 2009
    I don't see any problem charging slowly as long as you stop at the fully charged voltage. I've only seen maximum rates on Li-ion datasheets.

    btw, 2.2k mAh is better written 2.2 Ah
  4. gbar

    Thread Starter New Member

    Feb 16, 2011
    Got it, thanks.
    I appreciate the feedback on the charge rate. Is this based on experience, intuition, data sheets or that no one says not to? What do you think the lower limit would be, C/15, C/20, lower?

  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
  6. JMac3108

    Active Member

    Aug 16, 2010
    I hope you did not actually mean that you would use an 18650 cell ... and you really meant you were using a "battery pack" that had a single 18650 cell in it. As I'm sure you know, LiIon batteries are extremely volitile and can be dangerous unless a proper safety circuit is built into the battery pack :)

    I would not suggest anyone work with raw LiIon cells. As a matter of fact, the major cell suppliers (Moli, Panasonic, Sanyo) will not sell you the cells for your battery until they approve your safety circuit.

    I've designed LiIon chargers at relatively low charge rates, around C/10 and had no problem. Charging at lower rates will get you a slightly reduced capacity compared to charging at the manufacturers recommended rate. But it is not dangerous or destructive, and it works fine.