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UC2909 - lead acid charger from renewable source - how to set max charge current

jeff hoogenboom
Prodigy 60 points
Other Parts Discussed in Thread: BQ24650, BQ25504

I have a small renewable energy source and I want to use a charger circuit to harness my produced energy and store it in a battery.  first of all does anyone have some good recommendations as to  what type of battery chemistry is best used for storage these days.  weather it be NI-mh,  NI-cd,  lithium Ion or lead acid etc etc. and second I have been looking at some of Ti's charger circuit chips for a lead acid battery mainly because I happen to have a few extra lead acid car batteries around the house.

I have been looking at the UC2909 chip and I have been having a lot of trouble trying to figure out how to design for the max charging current.  I believe that the specs say it has a max charge current  greater than 2 Amps.   can anyone help me out.  Rs = 350mv/max current load.  is the max current load what ever the device is that I have connected to the batteries output.  and can someone explain in a little more detail the iBulk equation and where the (setting of RG1 and RG2) comes into play.  I understand how to formulate them I just dont understand where they come into play with bulk current equation.  see attached pages of the UC2909 chip.

jhoogie75@gmail.com

uc2909.pdf
  • 0
    TI__Guru**** 150910 points

    First, the datasheet that you attached is for the -EP version of the IC.  This is the 'enhanced product' version which is targeted at military and other high reliability customers.  I do not believe you need this quality of IC for your application.  Here is the catalog version of the datasheet: http://www.ti.com/lit/ds/symlink/uc2909.pdf

    Second, for a renewable energy source which is typically only able to provide limited amounts of energy, I do recommend either lead acid or lithium chemistry batteries.  Nickel cells require a constant current to accurately terminate their fast charge and this is usually unachievable with a power limited source.  We do have a lot more development in the lithium area, so these batteries would be recommended, simply because we have a lot of products that can charge them--giving you more options.

    We have several IC's made for charging from renewable sources, such as the bq25504 and bq24650.  What type of renewable source do you have and would either of those ICs work for you?

    On the UC2909, this app note details an application circuit and can likely answer your questons: http://www.ti.com/lit/an/slua098/slua098.pdf

     

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    thank you for your answers.  my renewable source is wind so it would not always be charging the battery.  I am not sure that after reading the two documents that were posted on the uc3909 it would work.  I could be wrong though.  due to the nature of my source I am not sure what chip would work best.  I am ideally looking for an IC that has as high as possible charge rate in order to utilize small and short bursts of energy being produced.  I am also concerned about the wide range of Vout that my system produces, sometimes 3v and other times 10Vout.  how might that effect my charger circuit.  Ideally I would like to use a lead acid battery that is AGM or GEL and is some where around 300AH.  maybe that is to ambitious for what I can actually produce but thats why I am trying to correct for that with the charge controller.

    What are your thoughts on this.

  • 0 in reply to jeff hoogenboom
    TI__Guru**** 150910 points

    I would recommend using newer parts, such as the bq24650.  This app note describes how to use it in a boosting configuration: http://www.ti.com/lit/an/slyt424/slyt424.pdf  This looks like it would work for you.

    This app note explains how to use that IC to charge lead acid batteries: http://www.ti.com/lit/an/slva437a/slva437a.pdf

    I would recommend a large input capacitor to store the wind energy.  Then, you don't need to deliver huge peaks to the battery when the power is available, but instead you first harness the power when it is available, store it in the caps, and then charge your battery at a smoother rate.

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    I like your idea about using large input caps. on the input.  for every charger circuit that I have looked at I am not following what seems to be a simple concept.  this concept is CHARGE RATE.  What determines how fast my battery will charge up and how to set that with any charger circuit.  for example using the uc3909  do I need to set any current levels in my external circuitry to sufficiently operate this chip.  I have roughly 250ma produced from my turbine and anywhere from 3-10volts being produced.  my battery is lets say 10AH.  with this information how do I calculate my charge rate ie how long it will take to charge my 12V 10AH battery.  I have found that  battery charge time is =

           Battery capacity    .

    Charge Rate Current         @ 20% energy loss                  example 10AH/.25A @ 20% loss = 48hours  I just need to some how relate this with the charge controller circuit if my information is correct.

  • 0 in reply to jeff hoogenboom
    TI__Guru**** 150910 points

    Yes, in the UC3909 and most of our other chargers, you need to set the charge rate with a resistor, RS in figure 2 of the datasheet.  This is typically the fast charge current applied to the pack during fast charge.  Lead acid batteries have multiple stages of charging, pre-charge, bulk charge, and then some sort of top-off or holding charge.  It would be up to you to work with your battery manufacturer and understand how long your cells will take to charge based on their initial state of charge an your charging setup.  I believe most lead acid batteries should never be discharged more than around 70%, so this is one of many things you will need to consider.

     

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    I have been quite extensively looking into the bq24650 and the design methods that have been sent to me and I would like to know if you can tell me where the 40mv comes from on the design example for the sense resistor.  is the 40mv usually a typical voltage to use because thats how the chip is designed or it is it a design factor done by the user and the battery chosen.  I realize this is a very important component to calculate due to the fact that it will set the max charge current.

    The second question is such that does the max charge current get set based on again the battery chosen and its specifications or max charge current has nothing to do with the battery and its dependent upon the circuitry.

  • 0 in reply to jeff hoogenboom
    TI__Guru**** 150910 points

    Equation 3 and the paragraph above it on page 13 of the datasheet explains the 40 mV value.

    And yes, your choose this resistor to set the max charge current for your application.  The other power components in the circuit, FETs, inductor, etc. will need to be sized on what this current is.

    This device is a controller, so it is very flexible.