Battery not charging after rundown

[dr2chase]

2 WiPy, two shields, two new 1200maH lithium batteries from Adafruit (Lithium Ion Polymer Battery - 3.7v 1200mAh[ID:258]) . Plug Wipy, batteries, microusb into shield, charging commences, WiPy is happy blinking its little LED. Charge overnight.

Unplug USB in the morning, leave WiPy running on battery to see how long they go. A day passes, batteries are discharged.
Reattach microusb, but this time, the batteries do not charge.

I assume either the batteries or the charger (I am guessing the charger) thinks that the batteries have are below the level at which they can be used and is refusing to charge. Is there a way to get the charger to try a little harder, or is the future remedy "do not let the Wipy+shield discharge your LiP battery all the way down"?

[danicampora]

What voltage do you measure on the discharged batteries?

Cheers,
Daniel

[dr2chase]

It measures 0. I tried to make the contact 3 different ways and double-checked against a known-good battery, it is zero.
However, the product:

The included protection circuitry keeps the battery voltage from going too high (over-charging) or low (over-use) which means that the battery will cut-out when completely dead at 3.0V.

So I guess I took it down to the bottom, and it decided it had had enough.
I could experimentally put 350mA into it for a little while (constant-current LED supply) to see if that would revive it, but is there a way to get the Wipy shield charger to do this for me (a Python program or something?) It's a bit of a pain to undo the plug, since it seems to latch.

[dhylands]

It measures 0. I tried to make the contact 3 different ways and double-checked against a known-good battery, it is zero.
However, the product:

The included protection circuitry keeps the battery voltage from going too high (over-charging) or low (over-use) which means that the battery will cut-out when completely dead at 3.0V.

So I guess I took it down to the bottom, and it decided it had had enough.
I could experimentally put 350mA into it for a little while (constant-current LED supply) to see if that would revive it, but is there a way to get the Wipy shield charger to do this for me (a Python program or something?) It's a bit of a pain to undo the plug, since it seems to latch.

You may have killed the batteries. Normally, the lithium cells don't like going below about 3v. You normally need a low-voltage protection circuit to prevent them from discharging too low or they'll be permanently damaged.
http://www.electricrcaircraftguy.com/20 ... LiPos.html

[dr2chase]

"You normally need a low-voltage protection circuit to prevent them from discharging too low or they'll be permanently damaged"

That would be the low-voltage protection circuit mentioned in the text that I quoted from the product page, yes. I quoted that in hopes that someone would read it.

I attached a 350mA CC supply powered by a 9v battery, it is happily charging and 3.4 volts, and in the time it took me to check that the 9V battery was healthy (it wasn't) and replace it with something more robust and rechargeable, it had risen to 3.6 volts.

For grins, I then removed the charger and plugged the battery in to a usb-connected WiPy, and the charging light came on, so I am good to go, and the problem is some sort of an interaction between the protection circuit on these batteries and the charging circuit on the shield. I'm not certain if this is a limitation of the charger or how it is configured, but it would be nice if there were a way to get it to do this for me -- not everyone has a constant-current supply of the right size lying around spare.

[dhylands]

Looking at the datahseet for the charger chip, it looks like it will pre-charge the battery if its low. But until the voltage rises above the forward voltage of the RED LED, the LED won't light (because the RED LED is wired between Vout and ground, not Vin and ground).

[dr2chase]

I can check, but I don't think that's it. I had left them plugged in for a while expecting them to charge, unplugged the USB, and no go.

And also, being fed 350mA (using a BuckToot, see ledsupply.com for details) it perked up fast, meaning maybe 10 minutes, and it was back to making the LED light when charged. And further, can you even find a red LED nowadays with a forward voltage as high as 3V? I mostly mess with power LEDs, but the red/orange/amber family has been below 3V for years. I just searched all the red LEDs at Digikey (620-740 nm) and the highest Vf was 2.4V. Same results when I add everything including "yellow" to the search.

[dhylands]

Yeah - most red leds are 1.4-1.7v. I was just trying to think of reasons why the RED LED wouldn't come on, but it might still be charging.

With the PRETERM resistor set to 2K, the pre-charging current will be 10% of the full charge current.

The schematic says that the full-charge charge current would be 450 mA with JP3G installed. So only 45mA while the voltage is below 3.3v (UVLO)