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I would like to build a battery charger with my UC3906 IC. I found a lot of data-sheets, application note, etc. but nothing that comprehensively explain how to choose the values of the resistors. I would expect a "few lines guide" of the form "for a n cells 12V battery, choose R1, R2, etc. such and such." But I found nothing like this. Can someone help? In my case, I have a 6 cells 12V lead acid battery (cells in series apparently, capacity not specified), but it would be nice if the answer be general and valid for any type of lead acid battery.

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  • \$\begingroup\$ Have you read this ti.com/lit/ds/symlink/uc3906.pdf and figure 2 \$\endgroup\$ – G36 Mar 9 '17 at 14:04
  • \$\begingroup\$ Yes, of course, this is the standard data-sheet. But they do not explain how to choose I_D, V_F, V_OC, V_T, I_max and I_T. \$\endgroup\$ – MikeTeX Mar 9 '17 at 16:47
  • \$\begingroup\$ Obviously you did not read Fig 2,5 , all batteries are not the same. Pretending so, may invalidate some warranties. \$\endgroup\$ – Sunnyskyguy EE75 Mar 9 '17 at 16:53
  • \$\begingroup\$ But this is the object of the question: how to choose these parameters according to the battery (number of cells, capacitance etc.). At least, I would expect some typical recommendations. \$\endgroup\$ – MikeTeX Mar 9 '17 at 17:04
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    \$\begingroup\$ Lead-acid cells are nominally 2 Volts, so a 12 Volt battery would have 6 cells in series. \$\endgroup\$ – Peter Bennett Mar 9 '17 at 17:28
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Look at the spec for the BQ24450. It's the exact same IC, same die - just changed the name for some reason - much better explanations. Also, if you have a problem initially getting the IC into the right state when first powering on (i.e bulk charge ), try putting a 10uf cap or so across the sense resistor that goes to pin 13 (the bottom resistor in the output voltage divider). Or you can put a reset switch across that same resistor - just press the switch after powering on to start the the bulk charge mode. Took me a while to figure that out - the circuit was going directly into the float state - skipping over bulk charge and over voltage. Hope this helps someone!

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  • \$\begingroup\$ For me, this post is old now, but yes, I'm sure this will help someone. \$\endgroup\$ – MikeTeX Nov 20 '17 at 8:18
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Thanks a lot for your suggestions! I had the same issue : when power on, the BQ24450 goes to float mode (13.xV). Adding 1uF (I had only such value) across the Rd resistor (from Fig.8) resolve the problem. But adding a switch across the Rd resistor and pressing it, stops the bulk charge mode.

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  • \$\begingroup\$ Hello. As a new contributor, I just want to inform you that if you are satisfied with some answer, don't forget to upvote it. Also, upvote the question if it helped you. \$\endgroup\$ – MikeTeX Apr 15 at 5:50

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