I have a 3.7v, 90mAh Lipo battery pack that I wish to charge but do not really know where to start.
I obtained a charger IC from RS:
Now from my basic Googling, I thought I determined my charge current by dividing 90mAh/1000.
But, I end up with a tiny charge current. I am charging with a 5v supply.
From the current resistor selection graph, it looked like I needed a 67k resistor.
This circuit charged the battery correctly, but it took an entire day.
I dropped the programming resistor to a 10k, and it didn't seem to make a lot of difference.
I don't know anything about that chip and I don't know much about charging LiPo battereies.
But I took a quick-look at the [u]datasheet[/u]. Figure 2-4 does show a fairly-flat line with very little difference until the programming resistor gets below about 7K.
Now from my basic Googling, I thought I determined my charge current by dividing 90mAh/1000.
Well... If the battery-charging was 100% efficient it would take 1000 hours to "put" 90mAh into the battery. Of course it's not 100% efficient so you have to put more than 90mAh in before you can get 90mAh out. And, 6.1.1.1 of the datasheet says you should charge a 500mAh battery at 500mA.
Lipos are charged using a CC/CV method, generally at 1C so in your case 90mA. So 10K is near enough. CC mode chages at a constant set current until voltage reaches 4.2V then the CV mode maintains the voltage at 4.2V while the current tapers down to some percentage of the initial charge current often around 10%.
Unfortunately that chip doesn't seem to allow you to change the pre-conditioning threshold, current or the charge termination percentage and both of those can have a major effect on the length of time a completely full charge takes. So if you're running the battery down too low and waiting for the end of charge then it can take a very long time.
If you want a faster charge but at the expense of losing a few percentage of charge then monitor the voltage and take it off charge as soon as it hits 4.2V instead of waiting for the full CV mode termination.
The battery arrived (new from RS) holding 4.2v. I was surprised at that , seeing as it is a 3.7v battery.
However, dropped to 3.7v upon adding a small load.
I ran it down to 2.7v and then put it on charge.
It reached 4.2v (slowly) and switched off. But took a whole day to do it.
Battery with no load on it now showing 4.1v.
Unusual, though 3.7V is only a nominal voltage, you won't see at level very often. Lipos are normally delivered with about a 50% charge, so roughly 3.85V. Exactly what battery is it that you have?
Either way 2.7V unloaded is far too low for a lipo. Damage tends to start at around 3V. Most people don't run them down below about 3.3-3.5V before charging. If you want it only for low current use you might get away with it.
Also at 2.7V you're probably running into that chip's "pre-conditioning" trickle charge mode as it tries to avoid doing any further damage. Only when the voltage gets up to a more sensible level will the charger start to run at anywhere near the current you've chosen (what resistor did you end up with?).
That was 2.7v under load. Bounced back up to 3.2v off load.
It was a decent battery from RS (will have to find the part number)... not a dodgy Ebay one.
Wow.. it can't go below 3v? Doesn't give it much of a run time.
Think I may look at 3.6v rechargable cells of a different type then if Lipo's are going to be hard work.
This project doesn't need a long run time (if I ever start it - still working out the PSU), but I don't want a temperamental charging regime either.
Without knowing anything about your project I've no idea what your real requirements are.
So good luck with your search. I don't know of any commercially available battery technology that allows you to use a greater percentage of its notional capacity than a Lipo. How much you get always depends on the discharge rate you use. And of course lipos are relatively small and light so the usable energy density is excellent.
All current reasonably useful batteries are lithium-based so they will have a similar charging regime but there are plenty of other charge controllers with different characteristics.
I think your running your battery down too low. When you do that like others have said the chip will trickle charge the battery until it gets to 3 volt then constant current charge at the rate based on your prog resistor. 90ma-hr is a really small battery. A lot of people are using the 18650 li-ion cells that are 4.2 volt max and about 2500 ma-hrs. So with that capacity you will have a much longer run time. I don't know what your load wants so hard to say.
When the chip charges your battery it will charge it up to 4.2 volts, the current at that time will be decreasing until it reaches a small percentage of the charging current, again based on the prog resistor, then it will stop charging the battery until it discharges to some voltage.
I totally have no idea about charging batteries (as you probably noticed!)
Still trying to get my head around it.
The project was a small rechargeable toy that had leds in it for a friends son who has severe learning issues.
It is very small, and basically runs 12x SMD leds for about 10 minutes max on a charge - so the load is negligible really.
The whole project is housed in a very small ball, so the battery needed to be a very small footprint.
I had no idea you can't run a lipo below 3v. Learnt something there then.
It's only dropped to 2.7v once, and it seemed to recover. Hopefully not permanently damaged then.
Check out Andrea Spriess channel on youtube. He has some excellent videos on charging lipo batteries and explains the whole deal. He has some videos using the charger ic your using or something simular.
Yes, after further investigation... Lipo's are the only real answer.
I thought I got this Lipo from RS, as that is where the rest of the parts came from.
However, can't find the part now.
But, found the same battery on Ebay with protection circuitry onboard to stop over/under charging.
For £3.99, its probably worth a try (except for the long China delivery time)
IanStMartin:
But, found the same battery on Ebay with protection circuitry onboard to stop over/under charging.
For £3.99, its probably worth a try (except for the long China delivery time)
Unfortunatly some of the inbuilt 'protection' circuits have a cut off of 2.4V, a lttle on the low side really.
