SparkFun Forums

[mikeB1]

Hi,

I have a sparkfun PRT-13856 Lipo battery pack ( https://www.digikey.com/product-detail/ ... ND/6605200 ) and am attempting to build a charging circuit with the LT3650 charger IC ( https://www.digikey.com/product-detail/ ... ND/2138773 ). My circuit is based on the sample application outlined on page 18 of the IC datasheet, with the exception that I've eliminated the suggested UVLO circuitry (SHDN and CLP are wired to VIN, TIMER is wired to GND and RNG/SS is .1uf to GND). According to the datasheet, this configuration should charge at a max rate of 2A up to C/10.

For prototyping, I have the IC soldered to a SMD breakout board (MSOP-12), and everything wired up on a breadboard with VIN as the 12V out from an old ATX computer power supply (measures ~11.3V). Everything appears to power up properly- the LED which I have connected to the CHG pin lights up, and FAULT does not- and I can measure 4.2V being sent to the BAT pin with the battery disconnected. However, after running the battery down until it shuts off (reads 0V), and then connecting it to the charge circuit in series with an ammeter, I am seeing a current of only ~20-30ma- a far cry from the expected 2A. I had expected to see *some * inefficiencies due to the breadboard wires, less than ideal layout, etc., but not a factor of 100?? The battery does eventually charge up, but being that it is a 6000ma pack, it literally takes SEVERAL DAYS at this 20-30ma rate.

This being my first attempt to build a Lipo charger circuit, I'm sure I'm missing something obvious and/or simple. I double checked the recommended specs. for the SENSE inductor/resistor, and replaced the initial inductor that I was using with another one that better matched the recommended specs.- but still no material difference in the observed charge rate. I also realize that this IC says that it is designed as a "single cell" charger, however, as I understand it, this pack is supposed to be "balanced", and therefore a "single cell" charger should still work, correct? Also, I see from the battery datasheet that it also contains a "protection IC" (DW01+), so perhaps that is somehow limiting the charger current? However, the battery datasheet also recommends a charge rate of 1200ma, so it seems unlikely that it would limit it all the way down to 20ma?

Any ideas would be appreciated- if I can't figure it out, I plan on contacting the IC manufacturer for help next.

Does anyone else have any experience with this particular charging IC and/or chraging circuitry for this battery pack? Any other charger IC/circuits that anyone can recommend that I can use to charge this pack at about a 2A rate with a 12V input?

Thanks in advance!

[jremington]

after running the battery down until it shuts off (reads 0V)

That one act did serious damage to the LiPo battery, so it can't possibly be charged properly.

LiPo batteries should NEVER be discharged below about 2.8 V.

Of course, there may be other problems with your circuit.

[mikeB1]

Thanks for the thought- but I don't think it actually discharged al the way to 0V- it simply just reads 0V on the meter.

When discharging, once it gets down to below 3V, the reading on the meter suddenly drops to 0 and the battery shuts off. I suspect this is a feature of the "protection" IC included in the pack. Putting it back on the charger, it immediately comes up to about 3.3V and starts charging again (albeit as I said very slowly- max 20-30ma).

[jremington]

Sorry, I didn't notice the bit about the protection circuit being built in.

The battery data sheet posted on the Sparkfun product page is not all that informative, but the protection circuit could be limiting the battery charge current if an overvoltage situation is detected. What voltage do you measure across the battery terminals while it is supposed to be charging?

You might try operating the charger circuit with a 2 ohm 5 watt resistor as the load, to see if the expected current is being delivered.

If that works as expected perhaps the battery pack is defective, in which case you should contact Sparkfun Technical Support.

[jremington]

Make that 2 ohm, 10 watt.

[mikeB1]

...but the protection circuit could be limiting the battery charge current if an overvoltage situation is detected.

a little googling and I found a datasheet for the protection IC- DW01+ ( http://www.ic-fortune.com/upload/Downlo ... -17_EN.pdf ). However, nothing jumps out at me that would cause a limited charge current. Seems like it's designed to simply shut down in over/under voltage situations?

What voltage do you measure across the battery terminals while it is supposed to be charging?

It starts out around 3.3V, and then climbs at a literal snail's pace- 2+ days to climb up to approx 3.8 - 3.9. Haven't had the patience to let it go longer than that.

You might try operating the charger circuit with a 2 ohm 10 watt resistor as the load, to see if the expected current is being delivered.

That's a good idea....I'll try that later and see what happens.

Thanks again for your help!