MobileRead Forums - View Single Post - Fix for poor battery life on PRS-500?

InspectorGadget · 12-01-2008, 05:53 PM

Thanks for the documentation!

Well, it has a charging circuit blocked out as IC401 on pg. 14, which is spec'd out at SN412005RHLR (BQ24032) on pg. 27. I've worked with the TI BQ series of parts before (I've used their BQ24010 charger). Their selection guide specs out the BQ24030 series as linear chargers with an integrated power FET capable of handling max 1.5A at a max input voltage of 18V. The BQ24032 variant has a regulated Vout of 4.4V (I guess when A/C-adapter or USB power is applied) versus other regulated setpoints of other BQ2403x variants. The max current is the same across the family.

A glance at the data sheet shows these chargers have charging-current setpoints determined by pins ISET1 and ISET2. ISET2 just switches between max charge rate and min (USB) charge rate. ISET1 apparently controls the max charge rate, and the 2.2K resistor in their schematic would work out to a max charge rate of:

Ibat = Vset * Kset / Rset = 2.5V * 450 / 2.2K = about 500mA.

(Based on their datasheet: Vset = 2.5V nom., Kset = 450 nom. for high Ibat values.)

So regardless of the mAh capacity of the cell you put in, it'll charge at 500mA max under A/C power. No surprise there -- it's the max they can draw on a high-charging USB port.

The bottom line is, whatever you can fit in there will probably work, and I see no conceivable potential for overtaxing the electronics in the PRS-505; it's pre-regulated. However, if the mAh rating of the replacement cell is too low (<< 500mAh), you'll charge it at too-high a rate on A/C power and shorten its charge-cycle lifetime. If it's 500mAh or greater, it'll take longer to charge but will have more charge-cycles in its lifetime.

So far, it looks like my initial assumptions were held up. I don't really have the time to dig any deeper into it -- it's got a complicated switching arrangement for USB vs. A/C power and current demands of the load circuit, but from experience and a couple of lines in the datasheet, I believe Rset controls and limits the maximum charging current which is what concerns us.

If anyone wants to review my analysis you can find the BQ24032 datasheet and app notes on TI's product page (click here).

Please note: this advice is offered without warranty. I've made a good effort to consider the relevant issues and I've tried to outline the basis of my judgments. That notwithstanding, I must repeat: You must assume full risk for any replacements or modifications you perform and I cannot and do not assume any responsibility for damage or injury or any other negative outcomes resulting from reading my comments and/or advice.

12-01-2008, 05:53 PM	#15
InspectorGadget Enthusiast Posts: 47 Karma: 70 Join Date: Apr 2007 Device: Sony PRS-500	Thanks for the documentation! Well, it has a charging circuit blocked out as IC401 on pg. 14, which is spec'd out at SN412005RHLR (BQ24032) on pg. 27. I've worked with the TI BQ series of parts before (I've used their BQ24010 charger). Their selection guide specs out the BQ24030 series as linear chargers with an integrated power FET capable of handling max 1.5A at a max input voltage of 18V. The BQ24032 variant has a regulated Vout of 4.4V (I guess when A/C-adapter or USB power is applied) versus other regulated setpoints of other BQ2403x variants. The max current is the same across the family. A glance at the data sheet shows these chargers have charging-current setpoints determined by pins ISET1 and ISET2. ISET2 just switches between max charge rate and min (USB) charge rate. ISET1 apparently controls the max charge rate, and the 2.2K resistor in their schematic would work out to a max charge rate of: Ibat = Vset * Kset / Rset = 2.5V * 450 / 2.2K = about 500mA. (Based on their datasheet: Vset = 2.5V nom., Kset = 450 nom. for high Ibat values.) So regardless of the mAh capacity of the cell you put in, it'll charge at 500mA max under A/C power. No surprise there -- it's the max they can draw on a high-charging USB port. The bottom line is, whatever you can fit in there will probably work, and I see no conceivable potential for overtaxing the electronics in the PRS-505; it's pre-regulated. However, if the mAh rating of the replacement cell is too low (<< 500mAh), you'll charge it at too-high a rate on A/C power and shorten its charge-cycle lifetime. If it's 500mAh or greater, it'll take longer to charge but will have more charge-cycles in its lifetime. So far, it looks like my initial assumptions were held up. I don't really have the time to dig any deeper into it -- it's got a complicated switching arrangement for USB vs. A/C power and current demands of the load circuit, but from experience and a couple of lines in the datasheet, I believe Rset controls and limits the maximum charging current which is what concerns us. If anyone wants to review my analysis you can find the BQ24032 datasheet and app notes on TI's product page (click here). Please note: this advice is offered without warranty. I've made a good effort to consider the relevant issues and I've tried to outline the basis of my judgments. That notwithstanding, I must repeat: You must assume full risk for any replacements or modifications you perform and I cannot and do not assume any responsibility for damage or injury or any other negative outcomes resulting from reading my comments and/or advice.