Im trying to use a 12V lead acid battery (the kind found in an alarm system) to charge a 3 cell 1600mAh Lipo (via a Lipo charger).
It seems to get stuck on the constant current phase with about 1.5Amps and never get to the constant current phase, but the current does seem to gradually drop off to about 20mA.
Given that a 3 cell lipo is 12V do I need to use a power source of more than 12V to charge it? ...or should the charger step up the voltage from the 12V battery?
After seeing the many many posts on LiPoly's and answering similar questions time after time I've decided to put up a guide for using LiPoly batteries.
Lithium cell technology, both lithium polymer (Li-poly or LiPo) and lithium ion (Li-ion), is quite different from the more commonly used NiCd and NiMH. There are many things to consider before using lithium cells for e-flight. But none is more important than safety. While all cells must be treated with respect due to the energy they contain when fully charged, lithium cells have the highest energy density. They also have some unique qualities that require special safety considerations. Before discussing other aspects of lithium cells, it's important to first understand these special safety considerations. (Thanks Dave Hendrich)
Charging/Saftey IMPORTANT! If you are new to flying DO NOT use lithium polymer batteries. NO EXCEPTIONS Read on to find out why.
Lithium cells must be charged very differently than NiCad or NiMH. They require a special charger specifically designed to charge lithium cells. In general any charger that can charge lithium ion can charge lithium Polymer assuming that the cell count is correct. You must NEVER charge lithium cells with a NiCad or NiMH only battery charger. This is dangerous. Charging cells is the most hazardous part of using lithium batteries. EXTREME care must be taken when charging them. It is important to set your charger to the correct voltage or cell count. Failure to do this can cause the battery to spew violent flames. There have been many fires directly caused by lithium batteries. PLEASE BE RESPONSIBLE when charging lithium batteries. Here are a few MANDATORY guidelines for charging/using LiPos.
Mr. Fred Marks made a good statement: "Saftey is a matter of discipline"...please don't forget that.
Use only a charger approved for lithium batteries. The charger may be designed for Li-Ion or Li-Poly. The both batteries are charged in exactly the same. Some older cell phone chargers may charge the batteries .1 volt to low (4.1 vs 4.2), but that will not harm the battery.
2.Make certain that the correct cell count is set on your charger. If you don't know how to do that, get a charger that you do know how or don't charge the batteries.
Before you charge a new Lithium pack, check the voltage of each cell individually. I do this after every tenth cycle there after. This is absolutely critical in that an unbalanced pack can explode while charging even if the correct cell count is chosen. If the cells are not within 0.1 volts of each other then charge each cell individually to 4.2 volts so that they are all equal. If after every discharge the pack is unbalanced you have a faulty cell and that pack must be replaced.
4.NEVER charge the batteries unattended. This is the number one reason for houses and cars being burned to a crisp by lithium fires.
5.Use a safe surface to charge your batteries on so that if they burst into flame no damage will occur. Vented fire safes, pyrex dishes with sand in the bottom, fireplaces are all good options.
6.DO NOT CHARGE AT MORE THAN 1C. I have personally had a fire in my home because of violating this rule.
If a cell balloons while charging DO NOT puncture the cell while it is still hot. Put the cell/pack in salt water and wait until the cell has cooled. Once it is cool gently puncture the pack's outer casing and put it back in the salt water. After this the cell is safe to throw in the garbage.
8.VERY IMPORTANT: If you crash with your lithium cells they may be damaged such that they are shorted inside. The cells may look just fine. If you crash in ANY way carefully remove the battery pack from the airplane and watch it carefully for at least the next 20 min. Several fires have been caused by damaged cells being thrown in the car and then the cells catch fire later and burns the car completly.
Charge your batteries in a open ventilated area. If a battery does rupture or explode hazardous fumes and material will spew from the battery.
Keep a bucket of sand nearby when you are flying or charging batteries. This is a cost effective way to extinguish fires. This is very cheap and absolutly necessary.
11.Realize that these batteries are dangerous, do not think to yourself that "it won't happen to me" as soon as you do that it will happen to you and you'll be trying to rescue your kids from your burning house or car. I'm very serious about this.
Now that we have covered that important topic let's move on to lighter matters:
Lithium What? Lithium Polymer batteries are used in many electronic devices. Cell Phone, Laptops, PDA's, Hearing Aids just to name a few. Most ,if not all, lithium polymer batteries are not designed for RC use, we use them in different applications than they were designed for. They are similar to Lithium Ion batteries in that they each have a nominal voltage of 3.6 volts, but dissimilar in that they do not have a hard metal casing but rather a flexible material encloses the chemicals inside. The "normal" lithium polymer batteries are thin rectangle shapes with two tabs on the top one positive one negative. The reason we use Lithium cells is that they are significantly lighter than comparable NiCad or NiMH batteries, which makes our planes fly longer and better.
Voltage and Cell Count: LiPolys act differently than NiCad or NiMH batteries do when charging and discharging. Lithium batteries are fully charged when each cell has a voltage of 4.2 volts. They are fully discharged when each cell has a voltage of 3.0 volts. It is important not to exceed both the high voltage of 4.2 volts and the low voltage of 3.0 resting volts or 2.5 during discharge. Exceeding these limits can harm the battery. The way to ensure that you do not go below 2.5 volts while flying is to set the low voltage cutoff (LVC) of your electronic speed control (ESC). It important to use a programmable ESC since the correct voltage cutoff is critical to the life of your batteries. Use the ESC's programming mode to set the LVC to 2.5 volts per cell with a hard cutoff, or 3.0 volts per cell with a soft cutoff. If your ESC does not have hard or soft cutoff, use 3.0 volts per cell. You will know when flying that it is time to land when you experience a sudden drop in power caused by the LVC. If you have previously been flying with NiCad or NiMH batteries, switching over to lithium polymer will result in a different number of cells being used. If you had 6 to 7 cells of round cells then 2 lithium polymer cells will correctly duplicate the voltage of those cells. If you had 10-11 cells then 3 lithium polymer cells would be right for you. There are a lot of 8 cell flyer's out there that are stuck between 2 and 3 cells. In my experience the best option is to determine how many watts you were using before and duplicate that with your Lithium Polymers, Motor, and Prop. For example. If you were running 8 cells (9.6volts) at 10 amps on a speed 400 airplane, then you have 9.6 x10, 96 watts. So if you went with 2 lithium polymer cells (7.2 volts nominal) then you'd need to change your prop such that you used 13 amps. If you went to 3 LiPoly's (10.8 volts nominal) then you'd need to reduce the amperage to 8.9 amps. These estimates are approximate, and some experimentation is required for best results but conserving Watts is a good way to start.
4.10C from 3S4P? How fast a battery can discharge is it's maximum current capacity. Current is generally rated in C's for the battery. C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour. All RC batteries are rated in milli Amp hours. If a battery is rated at 2000 mAh and you discharge it at 2000mA (or 2 amps, 1 amp = 1000mA) it will be completely discharged in one hour. The C rating of the battery is thus based on its capacity. A 2000mAh cell discharged a 2 amps is being discharged at 1C (2000mA x 1), a 2000mAh cell discharged at 6 amps is being discharged at 3C( 2000mA x 3). Currently LiPoly technology does not allow currents as high as NiCad or NiMH batteries do. Because of this many LiPoly batteries are put in parallel to increase the current capacity of the battery pack. When 2 batteries are wired positive to positive and negative to negative they become like one battery with double the capacity. If you have 2 2000mAh cells and you wire them in parallel then the result is the same as 1 4000mAh cell. This 4000mAh cell has the same C rating as the original 2000mAh cells did. Thus if the
2000mAh cells could discharge at a maximum of 5C, or 10 amps then the new
4000mAh cell can also discharge at 5C or (4000mA x 5) 20 amps. This method of battery pack building allows us to use LiPoly batteries at higher currents than single cells could produce. The naming convention that allows you to decipher how many cells are in parallel and how many are in series is the XSXP method. The number in front of the S represents the number of series cells in the pack so 3S means it's a 3 cell pack. The number in front of P means the number of cells in parallel. So a 3S4P pack of 2100mAh cells has a total of 12 cells inside. It will have the voltage of any other 3S pack since the number of cells in series determines the voltage. It will have the current handling of 4 times the maximum C rating of the 12 individual cells. So say our 3S4P pack had a maximum discharge of 6C. That means that it has a nominal voltage of 10.8 volts (3x3.6) and a maximum discharge rate of 50.4 amps (2100mAh x 6Cx4P ).
General usage tips.
1.Lithium batteries don't work well in cold air. If you are flying in the winter keep the batteries in your car for best performance.
2.Don't let the batteries overheat. Try and keep them under 140-160 degrees F. This will prolong your battery life. (60 C - 70 C)
3.Don't push the batteries past their rated maximum C rating. This will damage the battery and the apparent capacity of the batteries will drop. If when you recharge you are only putting ½ to ¾ of the rated capacity back into the batteries you are probably pushing them too hard.
4.If your building your own cells then put spacing between each cell in the pack to help cooling of the pack. This is most important when building packs larger than 2 cells.
5.Some LiPoly cells use aluminum tabs that you must solder to. Normal soldering procedures will not work on aluminum. You'll need to purchase aluminum soldering paste. The vendor where you purchased your aluminum tab cells should stock this paste.
If you have any suggestions for future sections or additions to the current document let me know and I'll add the information requested...if I know the answer!
The LiPo will be about 12.6v fully charged (4.2v/cell at cutoff). The lead-acid battery will be about 13v fully charged. When you try to draw a few amps from the alarm battery, the voltage can drop a bit which is why you probably can't charge it completely.
Find an old PC power supply and use that for charging. I have a few old ones around just for that. They can supply 12+V at 7-15A and are ideal for charger power supplies.
David - Can you elaborate on "seems to get stuck on the constant current phase?" If your 12V lead acid battery can deliver about 2A for the 1+ hour it will take to charge your LiPo pack, it should not be a problem. Some chargers will alarm if the supply voltage drops too low - my ThunderPower 425 does so at 11V. Yes, the charger does step up the voltage to whatever it takes to deliver the 1.5 A charge current you set. Something puzzling about the current dropping way off to 20 mA, yet you apparently have an indication that it is still in the CC phase. The current tapering off is as expected during the constant voltage phase, hence my Q in the first sentence.
| >Which charger do you use? | >Some can step up the voltage from 12V to whatever they need. ... | Name a charger that will step up the voltage. Never heard of one.
Every charger that can charge 4 or more LiPo cell packs, or 10 more more NiCd/NiMH cell packs, and is powered by 12 volt power supply can do this -- they have to.
To charge 4 LiPo cells, you need at least 16.8 volts. (Actually, even
3 cells requires 12.6 volts, which can be gotten out of a 12 volt lead acid battery, but only if it's mostly full.) To charge 10 NiMH/NiCd cell packs requires at least 14 volts, and more if you want the charge to actually be fast.
If you want specific examples, the Triton, the ICE charger, the Astroflight 110D, the Hitec CG-335, the Multiplex LN-2010, the Radio South Dual Pro Charger MKII and many many others.
Name a charger that will step up the voltage. Never heard of one.
Sure you have. They're called mobile 120V power supplies. You plug them into your 12VDC auto cigarette lighter and voila', you get 120VAC. However, you never get something for nothing. You sacrifice lots of amperage to get the voltage step-up. Check Ohm's Law.
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