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LiPo Battery Lifetime – Internal Resistance Growth

Battery life can usually only be extended by preventing or reducing the cause of the unwanted parasitic chemical effects which occur in the cells. Ways of improving battery life and hence reliability are also considered below.

Battery Cycle Life is defined as the number of complete charge - discharge cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity. Key factors affecting cycle life are time t and the number N of charge-discharge cycles completed. An obvious example is the Depth of Discharge which is a simple reciprocal mathematical relationship, but there are many more complex factors which can also influence performance.

lipo battery life time

Lifetimes of 500 to 1200 cycles are typical. The actual ageing process results in a gradual reduction in capacity over time. When a cell reaches its specified lifetime it does not stop working suddenly. The ageing process continues at the same rate as before so that a cell whose capacity had fallen to 80% after 1000 cycles will probably continue working to perhaps 2000 cycles when its effective capacity will have fallen to 60% of its original capacity. There is therefore no need to fear a sudden death when a cell reaches the end of its specified life.

My F-22 uses 25C lipo 7.4 v 5000mah batteries as the norm. Due to the way I fly the plane I can't rely on the burst rate of other batteries so I can't move down much unless I plan on going to very light batteries (1000-1300mAh 20C).

I paid $38 each for them and between the helicopter they were used in previously and the F-22 V1 they are at well over 200 cycles and still going strong. One is showing the early signs of getting tired (has been since a particularly bad crash that required the battery being resoldered) but it's still got a lot of life left in it yet.

Some of my cheaper 2s lipo batteries (I paid about $4 each) have been looked after just as well. ! is going strong, one is dead and the other 4 are beginning to show signs of tiredness after about 80 cycles. These are used in my F-18 V1 that doesn't have thrust vectoring so they tend not to get run hard either.

All of the above occasionally get charged at the field.

My Gens Ace 1600mAh 20C from my T-50 V1 (get run hard) are at about 40 cycles and going strong but I never charge them at the field. There is another guy at the field using the same batteries being run about as hard and they are beginning to get tired after about 40 cycles.

There are some things you can do to extend your battery investment life:

• Store your batteries at the proper storage voltage of roughly 3.7 vpc. Don’t get hung up on exact voltage as a range of 3.6 to 3.8 vpc is good enough. On the higher side for long term, on the lower side for short term. Point is never ever store them fully charged/ discharged or sit more than a day fully charged up. That will cause swelling and damage.

• Store them in a cool dry space. A good place is in a zip lock bag in the door of your refrigerator, but not freezer. Cool batteries last longer, warm batteries age faster for any battery type. Just don’t confuse that with operation. You want your batteries warmed up before using. Ri decreases significantly as the battery warms up, and Ri rises significantly when cold. Put a cold battery in your plane and you will notice sluggish performance until it warms up. By warm I mean 80ish, not hot above 90.

• Just because you can charge faster than 1C or up to 100% SOC does not mean you should. Current flowing through resistance creates heat and heat is your batteries enemy and causes the swelling. Try to use discipline and charge at no more than 1C. You battery will stay cooler and significantly lessen the chance of thermal runaway where your battery can explode into flames. No reason to take your battery to 100% SOC. You can sacrifice a minute of run time by only charging to 90% SOC or 4.13 volts per cell. Doing so will double your cycle life. Likewise don’t push the discharge to 3 volts. Set LVC to 3.2 vpc. Following this one tip alone will double your battery cycle life.

• Use the right C-Rate for the application and leave a little head room. High C-Rate batteries are expensive. Just make sure you are not pushing the upper limit. If you have a 100 amp ESC and a 3000 mah 35C battery is cutting it to close. Give yourself some breathing room and spend a few extra dollars for that 40C battery. Again this is a heat issue, and heat is the enemy.
What else can Ri tell you? Well for one it has everything to do with C-Rates and heat. If you take two 3000 mah batteries and one has a 20C rating and the other has a 40C rating, the 40 C rating is going to have much lower Ri. For example I have such batteries although they are 3S 2600 mah one group is 25C and the other is 40C. On average the 25C group has an average Ri of 15 mo, and the 40C at 10 mo. The 25C group battery when fully charged up and supply 65 amps are sagging voltage down from 4.2 vpc to 3.2 vpc and burning 65 watts as heat on the battery. The 40C battery at 100 amps voltage sags the same 1 volt but drops 100 watts as heat on the battery. Now if I were to force the 20C battery to 100 amps I would be dropping 150 watts of heat on the battery. That sucker is going to get red hot. However if I limit current to 65 amps on the 40C battery I am only dropping 42 watts and sagging only .65 vpc and that makes for a much cooler higher voltage battery.

What if you race RC cars competitively for prize money and it is the final race. You have 10 batteries to choose from in your box of tricks. Which 1 of the 10 do you choose. Easy, the one with the lowest Ri because it is the strongest healthiest battery you got.