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I am planning to use BQ25703A or BQ24780s for charging system of 6 series Ni-MH(1.5V charging voltage per cell) cells. But datasheets says “Charge 1- to 4-Cell Battery from Wide Range of Input Sources and Battery voltage regulation 1.024 - 19.2 V” in BQ25703A and "Charge 1- to 4-Cell Battery Pack from 4.5- to 24-V Adapter" in BQ24780s .

  1. Would it be nice to use these parts for my product(for 6-cell ni-mh)? Which is better?

  2. If is it available, why usually datasheets says only "1 to 4 cells" ?

  3. Would I have a disadvantage if I use these IC ?

IC Options

BQ25703A: http://www.ti.com/lit/gpn/bq25703a

BQ24780S: http://www.ti.com/lit/ds/symlink/bq24780s.pdf

Thanks.

Best regards.

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  • \$\begingroup\$ Is 6 between 1 and 4? \$\endgroup\$ – Brian Drummond Dec 19 '17 at 14:45
  • \$\begingroup\$ If using the negative delta V method of charge termination, the number of cells matters. You are looking for a drop of 5mV in charge voltage per cell while charging with constant current. So, about 30mV for your six cell stack - except that the cells may not (probably will not) all reach the drop off at the same time. So, your charging circuit has to be able to tell when you've dropped enough or count the drops, or something. The more cells, the trickier. Or just trickle charge like all the cheap chargers do. \$\endgroup\$ – JRE Dec 19 '17 at 16:04
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What's confusing about TI stating this charger is for 1-4 cells? It is not a 6 cell charger. Just looking at the parametric data on their website for the BQ25703A, TI states its max battery voltage is 19.2V. It shows it right in that chart, and probably warns you about it where you scribbled it out. If you had six cells at 3.8V you're looking at 22.8V you would clearly be over the maximum battery voltage. Worse if they float up when fully charged. If you want to charge 6 cells look for a 6s charger.

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    \$\begingroup\$ What's not clear from the datasheet or your answer is why the number of cells matters more than the battery voltage. The IC is designed to handle NiMH among other chemistries, and NiMH has a lower chargin voltage than the Li your answer seems to assume. Everything would appear to be in spec except the cell count, and the IC doesn't have any way of counting cells. Perhaps the OP's question would have been better phrased "why does it say I can't charge 6 NiMH cells?" \$\endgroup\$ – Chris H Dec 19 '17 at 14:15
  • \$\begingroup\$ Fair point I missed the NiMH requirement. \$\endgroup\$ – Some Hardware Guy Dec 19 '17 at 14:16
  • \$\begingroup\$ I agree with you but I have to say, datasheets say "...up to 4 Li-Ion cells". But, I am going to use 6 Ni-MH cell. \$\endgroup\$ – Ugur Baki Dec 19 '17 at 14:16
  • \$\begingroup\$ If I were you I would go to the TI page for this. Click on the support tab, then click the "Battery Management - Chargers Forum" link under engage in community. Post your question there and a TI employee will answer you. It usually takes a day for me to get an answer. That's how they're doing support now and they're pretty responsive. \$\endgroup\$ – Some Hardware Guy Dec 19 '17 at 14:19
  • \$\begingroup\$ @SomeHardwareGuy I did it. Someone said to me"you can change CELL_BATPRES pin to set for 2s configuration. You can use SYSOVP pin." By the way, nobody answered to me yet. If all ICs are available for me, which one is better for me, what is your advice? Did you checked the screenshot? \$\endgroup\$ – Ugur Baki Dec 19 '17 at 14:26
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These ICs are charge controllers, not stand-alone battery chargers. They are intended to be configured and monitored by a host system. The BQ25703A is described as 'multi-chemistry' (including Nicad and NiMH) but the datasheet provides no information on how to use it with anything other than Li-ion. The BQ24780s datasheet does not mention NiMH at all.

To charge a 6 cell MiMH properly the host would have to monitor battery voltage and adjust charge current to match the desired profile (delta-peak, fast charge or trickle charge). The BQ25703A is probably more suited to this, though the BQ24780s might also work if set up for 2 cells (1 Li-ion cell is roughly equivalent to 3 NiMH cells).

Charging a three-cell nickel-based battery pack with a Li-Ion charger

it is possible to charge a 3S NiMH pack safely and to nearly full capacity with a single- cell Li-Ion charger. The Li-Ion “nickel charger” can be classified as a hybrid fast/trickle charger, getting 70% of the bulk charge in 5 hours. The charge current tapers toward 0 Amps near the end of the charge, which reduces the chance of any thermal issues and possibly provides longer cell life. Most noteworthy is that the CC-CV method can be used to charge battery packs with either nickel-based or Li-Ion chemistry with no changes in hardware or firmware.

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  • \$\begingroup\$ Thanks a lot for your answer. I have to say something. Firstly, yes I am going to use host for control of charging system. Secondly, if you will use ti.com/product/bq24780s you can see something about NiMH for bq24780s. Thirdly, are you sure about CC-CV mode for NiMH? \$\endgroup\$ – Ugur Baki Dec 20 '17 at 5:13

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