e-bird

There are many chargers that Cost $$ But you can build
your own Cheap The people that make IC's that the hobby
Buy chips from Why Not You. CU
http://www.national.com/catalog/AnalogRegulators.html

Steve Neu

My Feedback: (1)

Sure you can build your own charger--but there is a lot more to it than a couple of chips. Most of the better chargers have a DC-DC converter to boost the 12 volt car battery voltage to a level high enough to charge 20 or 30 Nicad cells. Power electronics is not cheap to make. Most chargers have a micro controller computer and a LCD display--again not cheap.To that add a few hundred hours to write the software to control everything--then add a box and switches with cables and connectors and soon the chargers that are offered for sale don't seem so expensive.

Steve

tailheavy

Depends on what the application is. If you just want to do li-po's you may be able to make a home made charger just for doing li-po's. I seen a thread on rcgroups that mentioned some one doing that successfully but I believe it was to allow him to charge up the li-po to ~90% only (too dangerous to go to 100%). He then used the expensive commercial charger to charge it the rest of the way. That's a good way to charge multiple packs with just one expensive charger to top off all your packs. I will eventually look in to it myself. Since I am very knowledgeable about electronics this sort of thing doesn't scare me, just be sure you know what you are doing and why. It also helps to have good test equipent too.

As far as making your own full featured charger, good luck. I agree with the previous poster. Better off buying a good quality commercial charger.

HappyHobit

I have to agree with the statements that it's better off buying a good quality commercial charger for NiCd and NiMh batteries.

But it’s really not hard, or dangerous, to build a charger for LiPoly batteries.

It can be done with 2-LM317 adjustable regulators, with heatsinks, a current limit power resister and a voltage reg pot. Total cost for a 1 or 2 cell, 1.5 amp charger is $8.00. The only thing you need to make it safe is an accurate voltmeter.

You will need a power source. You can use a car battery, a computer power supply or a wall wart AC to DC power supply. Anything that will supply 12 to 20 volts DC at the current you want.

This charger will charge a cell to “100%” safely. All it takes is time.

Here’s one I built for 145ma cells.

Jay

tailheavy

Did you design that circuit yourself or did you just copy someone else's schematic? Also are you saying that it can only charge batteries that are 145ma in capacity. If this truly does safely charge lipos to 100% I would be very interested in getting more info on it.

HappyHobit

I “designed” the circuit myself. I combined a current limiter and voltage regulator, as designed in the LM317 datasheets. (pg. 16) http://www.national.com/ds.cgi/LM/LM117.pdf

There are 3 rules for charging the LiPoly cells.

IF the battery is ‘Less Than 3 volts / cell’ charge at .1C until voltage greater than 3 volts / cell.
Limit the voltage to 4.2 volts per cell.
Limit the current to 1C.

An LM317 X 2 charger can be used to charge LiPoly batteries up to 1.5 amps and 3 or 4 cells, limited only by heatsink size and power source.

The picture I included in my origional post was for a dedicated 4.2 volt, 145 ma charger. It cost me about $7. If you wanted to add other current limit resisters and switches it could be used for other size calls. Just make sure you have it ‘programmed’ correctly before you plug the battery in. Mine is a ‘No Brainer’.

This thread in RC Groups is based on this charger design

http://www.rcgroups.com/forums/showt...5&pagenumber=1

I had built my charger before this thread started but the design is the same.

Jay

tailheavy

Wow that's some great info I need to make something for my 1200ma 3 cell Lipo's. I'll have to figure out how to do the programming though.

HappyHobit

The math for sizing the components is fairly simple. Well sort of.

You are going to need a power source of 18 or 20 volts and two good size heatsinks.

Each LM317 will ‘lose’ 2.5 volts. So for a 12.6 volt output you’ll need at least 17.6 volts input.

With a 20 volt supply you’ll have to dissipate 9.6 watts in the voltage reg. LM317 and 5.2 watts in the current limiter LM317. Each would be a couple watts less with an 18 volt supply.


Now for sizing the Current limiting resister. First you'll need a 2 watt or larger resister.

This resister must drop 1.25 volts at 1C. R = E / I = 1.25 / 1.20 = 1.042 ohms. Not easy to find.

A 1.0 ohm resister would limit the current to 1.250 amps or 4% over. A 1.1 ohm would be 1.14 amps. or about 9% under


The volt regulating Lm317 requires a minimum of 4ma to regulate properly. With a 12.6 volt output a 2k pot would produce 6.3 mili-amps.

That’s it. Nothing to it.

Jay

shenion

There's always mine at: http://shdesigns.org/lionchg.html

It is the first discussed in the posts.

I don't see and advantage to the dual LM-317 version. Mine needs only about 2.5 volts more in than out, and has a charge LED. It will work better on 2-cell Li-Ion batts with 12v input.

Apparantly hundreds of these chargers have been built. I get emails from users that once they built one, they need to build more for their friends. The schematics have been downloaded literally thousands of times. Web page has close to 8000 hits over the last 6 months.

tailheavy

That brings up a very important point. I cannot use my computer power supply since it only puts out 12.5 volts. So would need to get a different power supply right?

tailheavy

shenion, your circuit is for lithium Ion batteries. Aren't Li po's different?

HappyHobit

TailHeavy,

Unless the –12 volt supply has enough capacity you will. Try not to get a power source any larger than necessary. Any voltage you don’t need has to be disposed of as heat.

The same Lithium charger can be used for LiIon and LiPoly (but NOT LiMetal). Lithium Ion has liquid electrolyte that tends to boil and produce explosive quantities of gas if mishandled while the Lithium Poly has a polymer electrolyte which, when mishandled will expand but not explosively.

Have you been able to access the URL SHenion provided?


Jay

shenion

Not for charging.

My charger is used more for LiPolys than LiIon as I get more feedback from LiPoly users.

Mike Taylor

My Feedback: (1)

Are you guys POSITIVE the same charger can be used for all type of lithium batteries, or just making a guess???

The LiPolys have different voltages and charging profiles from the other types of litium cells. Chargers made for LiPolys say not to use them with other types of cells, and the literature with the cells says not to use chargers made for other types of cells (including LiIon cells).

If what the manufactures says is true, and my guess is that they know what they are saying, then any money you save by building a cheap charger will be wasted on ruined LiPolys. At ~$9 a pop for single cell, it won't take ruining many packs to have bought the best charger there is out there...

HappyHobit

Hi Mike,

I was just talking about Lithium-Ion and Lithium-Poly batteries

Check out this website for more info on Lithium Ion/Poly batteries. The last paragraph compares Lithium Ion/Poly batteries.

http://www.batteryuniversity.com/partone-12.htm


Jay

shenion

The data sheets I have for Li-Ion and Li-Poly show the same charge voltage.

The only difference was the charge rate, but that varies from cell to cell anyway.

Some Li_ions say 0.7C others say more. Some Lipoly's say 0.5c but I have seen some say 1.0C.

Basically you need to determine the current required for the particular type you are using.

Now Li-Metal (Duralites) are different. Basically the same algorithm, but different voltage thresholds.

e-bird

I have used many many LM317's they are very good but a newer
one on the the same idea is the LDO's (low drop out) with can
charge 12.4 volts from a 13 volt car battery just one is the
LM2931. CU

HappyHobit

Hi e-bird,

It’s a good idea, about the LDO regulators, if your power source is limited, but the LM2931 is only 100ma and you will need 2. So you’re going to need a 13.8 volt battery. The standard charger voltages will be 4.2, 8.4 and 12.6

How about,

LM1086IT-ADJ, 1.5 amp LDO http://www.national.com/pf/LM/LM1086.html
LM1085IT-ADJ, 3 amp LDO http://www.national.com/pf/LM/LM1085.html
LM1084IT-ADJ, 5 amp LDO http://www.national.com/pf/LM/LM1084.html

They have a Dropout voltage of 1.5 volts so you can save 2 volts using 2 of them. They are about $1.50 each. (LM317 are about $.70 each)

Jay

e-bird

Jay As I pointed out just one 2931 if you need more power then
the LM2941c .5 drop at 1a http://cache.national.com/ds/LM/LM2941.pdf

HappyHobit

Hi e-bird,

National certainly does make a lot of regulators.

The circuit I was proposing was an inexpensive, simple, “one size fits all”, and of course it doesn’t. As you pointed out there are many other adjustable regulators that will provide solutions for all of its shortcomings.

The one application where LDO regulators would be a big advantage for is for charging 8.4 volt (2SxP packs) on a car battery.

While .5 volt is typical for the LM2941c it would probably be better designing the circuit assuming a dropout of 1 volt.

Jay

shenion

Mine will work fine on 8.4v, that was what it was designed for.

The current limit stage drops about 0.65v at full current. The '317 will then have over 3v still accross it.

I could have used a 317 for the current limit stage, but that requires another 3v drop (1.75 minimun and 1.25 sense voltage.)

HappyHobit

I can’t discuss your charger without seeing it. The URL’s you provided don’t work for me.

Jay

e-bird

SHenion I use some lm2940 10v with zinners to get 12.4 and I charge from a car battery also. I use 1 ger and 1 sil zinner to comp for heat they go different directions so temp is pretty stable.
CU