Charging a LIPO battary

[kirill578]

Hey, I bought a 7.4V 2 cells lipo on ebay with a proper charger, however the charger I got is defected and I'm reluctant to wait another week or two just to play with my RC chopper. So I thought maybe I build a charger until I get a replacement. I couldn't find any charging circuit, so I came up with my own, (see attached picture)
I want to build it with LM317 as a constant current source and an arduino to monitor the charging process.

Is it going to work? do you have any suggestions?

the code is going to be something like

loop{


OUTPUT1 < HIGH
OUTPUT2 < HIGH

OUTPUT3 < LOW //CC OFF
if ( ADC1 - ADC2 > 3.7 ) OUTPUT2 < LOW
if( ADC2 > 3.7 ) OUTPUT1 < LOW
if( ADC1 - ADC2 > 3.7 && ADC2 > 3.7 ) break LOOP
OUTPUT3 < HIGH //CC ON

delay 10 sec

}

[yanir]

You have to be real careful charging lipos, it's best to get an IC specifically designed to do the job or your trial an error will result with at best damaged batteries and at worst a house fire.

[amyk]

I'd strongly recommend against designing your own battery charger based on microcontroller control. A somewhat safer and simpler design would be a 4.2+/- 0.05v regulated voltage source and a series resistor to limit the current. When not full the resistor will limit the charging current to what you need and when close to full the 4.2v provides the CV part of the charge curve. This is enough to charge a single cell. With 2 cells if you want a simple solution it would be easier to charge them independently to the same voltage rather than doing a series charge and worrying about balancing.

[metalphreak]

I've charged single cells with a bench power supply before. Multiple cells would require individual cell charging or a balance charge controller.

Step 1: Set target voltage on powersupply (ie 4.2V)
Step 2: Set current limiter to lowest setting. Short PSU outputs with banana lead or a resistor. Set the current limiter to an appropriate current (0.5 times the "C" rating is usually safe). Remove lead/resistor.
Step 3: Connect the battery to the output terminals.

This essentially gives you constant current mode, followed by constant voltage charging. A proper charger is best though since a lab power supply has no protection for charging batteries.

[M. András]

i would get some li-fe cells those wont blow up on your face like the li-po can

[typeglob]

I'm reluctant to wait another week or two just to play with my RC chopper.

Get used to it, waiting is definately part of flying RC helicopters. Waiting for parts after a crash, waiting for batteries to charge, waiting for the wind to die down, etc.

What kind of heli did you get? Something small and twitchy like a Trex 250 (clone)?

Anyway, I recommend against some quick-and-dirty solution.

[typeglob]

i would get some li-fe cells those wont blow up on your face like the li-po can

Too heavy for the really small helicopters, especially with low cell-count packs.

I accidentally short-circuited a 5000 mAh 6S LiPo pack (rated to deliver 100+ amp continuously) by plugging in the balance connectors incorrectly. There was a flash and a loud bang and I nearly shit my pants. The spot on the balance board where the mating connector used to be had been replaced with a blackened splotch. It certainly taught me more respect for LiPo packs, because they have really low internal resistance and can blow up (but that's mostly while overcharging them).

LiFe packs are a lot safer, charge quicker, last more cycles (don't fade as much as LiPos) and  the performance/weight ratio is not that bad compared to LiPo packs.

[M. András]

yes they are heavy. but i wouldnt trade lightweight battery for safety, but for heli and some area of rc sports you dont have a choice. but as far i know there arent so many life cells which are rated with 1-50c discharge as the lipo cells