Questions about building a NiMH battery charging circuit - Page 1
Forum section: electronics / beginners

author: Marmotta
author_rank: Contributor
timestamp: June 06, 2017, 09:58:44 pm
content: I wanted to design a system-integrated charging circuit in a Game Boy Pocket using Eneloop rechargeable batteries. I have one these fast-charge ICs (TI BQ2002/F) that seem to fit this purpose, but am having a few issues with it, namely what value resistors I need to populate the circuit - I don't know if I'm being slow, but I can't see any reference calculations on the datasheet:http://www.ti.com/lit/ds/symlink/bq2002f.pdfThe DC adapter I'd use is 5V, as specified in the datasheet. I've found a number of suitable ones on eBay (3.5mm x 1.35mm pin) ranging from 1A to 3A.The batteries are 2x Eneloop AAAs 1.2V 750mAh connected in series.Or if you think something else would be better suited, let me know!

author: KL27x
author_rank: Super Contributor
timestamp: June 07, 2017, 07:27:03 am
content: page 2QuoteSo... for a single cell, you have RB1/0 = RB2...

author: Marmotta
author_rank: Contributor
timestamp: June 07, 2017, 12:43:54 pm
content: That does seem odd for a single cell, although I was thinking more about R3, R4, RT and RST for temp monitoring. Although, just to be clear, my understanding is that I have RB1/RB2 = 1 (two cells), so any two resistors of the same value between 200K and 1M should be OK? Also, for the status LED, I'm assuming I should just connect it directly to the relevant pin with a resistor and have it go to VCC?Finally, and this is more of a simple general question relating to battery charging, I know the additional amperage won't harm anything, but will a 3A charger charge the batteries any faster if the circuit is only 750mAh? I've been reading up on C-rates, but I'm not sure and the datasheet specifies 5V as a typical power source, but doesn't mention amps.The datasheet doesn't seem too detailed. There is a reference sheet for a development board based on the BQ2002, but it just has far too many features I don't require, such as 4-10 cells and voltage regulation up to 25V.http://www.ti.com/lit/ug/sluu007b/sluu007b.pdf

author: KL27x
author_rank: Super Contributor
timestamp: June 07, 2017, 07:58:03 pm
content: Wish i could help. I have no clue how NiMh batteries work. And i won't figure it out from this datasheet. It is clear as mud.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 07, 2017, 11:26:00 pm
content: Resistors of 200k to 1Meg of the same value will work, as this will halve the cells voltage R1 = R2 (you have two cells, you provide the charger circuit with half of this to give one 'average' cells voltage).Also, using a high capacity power supply will make no difference. NiNH are charged with constant current, and terminated either by delta V (when the cell voltage starts to drop) or delta T (rise in temperature) or usually both. If you're charging at 1C, which is often the recommended maximum charging current for NiMH then that is in deed 750mA. Assuming the charge ilwl take a few mA quiestscent you of course want a power supply greater than this, ao minimum of 1A. A 3A power supply would make no difference.Yes, the datasheet appears to be a bit shit (odd for TI).The only way to decrease charging time is by increasing the charging current, but this will probably reduce the number of charge cycles of the batteries. Charging at 1C would take well over an hour, because there are of course losses (the battery will heat up) but shouldn't take more than 2 and 1C.