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Subject: Re: AW: More aggressive Rockbox battery charging

Re: AW: More aggressive Rockbox battery charging

From: Jerry Van Baren <gerald.vanbaren_at_smiths-aerospace.com>
Date: 2005-02-09

Hi Jörg,

Thanks for taking the time to experiment. This is very helpful information.

[IDC]Dragon wrote:
>>Jens Arnold strongly recommends that we not exceed 0.05C for the trickle
>>charge rate (e.g. 75mA for 1500mAH batteries, 110mA for 2200mAH
>>batteries).
>
>
> You have to add the box' power consumption to that, in order to leave
> something for the cells. ;-)
> I guess you're aware of that, since you listed it so nicely below.

Yes I figured that was the case, but I was not 100% positive :-)

>>Estimating current based on the voltages we can read remains TBD,
>>although I'm fairly confident it can be done with sufficient accuracy
>>(we don't need a whole lot of accuracy). I sent an email yesterday
>>trying to understand what is read with ADC channel AN7 but nobody's
>>answered it yet.
>
>
> Trying it out with a lab supply and watching the port debug screen, it seems
> I was wrong. AN7 just tells us if the external voltage is above a threshold,
> about 6.0 V for my unit. It reads ~0 below and ~0x318 above. So it's good
> only for detection.
> Instead, AN1 does something useful, but only when charging, else it shows
> 0x3FF. My unit just stopped to do so, I can't investigate further atm
> without writing a software hack. It seems to be related to the charger
> voltage.
>
> Jörg

Bummer on AN7 but no surprise based on the schematic. Incidentally, I
believe R14 (listed at 10k but clearly wrong) is actually 1 ohm. If I
understood the data sheet properly, this would work out to what we
observe: ~350mA for the current limit.

AN1 is the output voltage of the charger/regulator before the diode that
feeds the battery. This may be suitable for our current estimation
since we will be able to measure the voltage drop across the diode. The
diode will have a non-linear current->voltage mapping, but hey, that's
what we have a processor for :-). Assuming the diode is fairly
consistent from unit to unit (pretty good bet plus we don't need high
accuracy), we should be able to generate a lookup table and get a
reasonable estimate of charge current based on the voltage difference.

gvb
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Received on Wed Feb 9 14:58:05 2005


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