# Lithium Polymer Battery Capacity Lifetime

Discussion in 'Power Electronics' started by mrrrk, Apr 9, 2014.

1. ### mrrrk

2
0
Feb 10, 2014
Hi,

I would be grateful if anyone could help with calculating an estimation of a lithium polymer battery capacity over time?

I am using a 2000mAh lithium polymer battery with a device which averages a 21mA current draw whilst in operation; which is almost constantly, when not charging. I am looking to predict (to at least a reasonable accuracy) when I will need a new battery by plotting a battery capacity curve over time.

Thanks.

2. ### shumifan50

548
56
Jan 16, 2014
The lifespan of LiPo batteries are affected by many parameters:
1. How deeply they are discharged. If a cell voltage drops below 3.7V per cell without load, then the cell could/will be damaged. Below 3.3V per cell under load is also destructive.
2. As you draw very little current, the discharge rate should not affect you.
3. Different manufacturers specify different charge rates. If you exceed the recommended value, it will shorten battery life and could even damage it or cause a fire.
4. If it is a pack(multiple batteries in series) more than 1S then a balancing charger should be used to ensure a uniform charge across all cells.
5. Number of charge cycles before the battery needs replacement will vary by manufacturer (different qualities), quality/accuracy of the charger used.
6. If spare batteries are stored they should be charged to around 3.8V (not fully charged at 4.2V).

So I imagine you would have to monitor your specific setup over time to draw the graph.

3. ### jpanhalt

426
4
Nov 12, 2013
Apparently, it is not that easy to predict capacity. Although, your suggestion sounds reasonabe, if you always discharge to the same point. Can you also monitor your useage in maH to indicate the need to recharge?

It refers to a method called, "multi-model electrochemical impedance spectroscopy," or EIS for short.

John

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,360
2,756
Jan 21, 2010
One method is to place a 21mA load on the battery and time how long it takes t get to 3.7V.

Then you can do it over and over again to determine any aging effects. (Or use published figures for this)

Another option is to assume you'll get 80% of the power from the battery, 2000 * 0.8 / 21 = 76 hours

Sure there are complexities caused by temperature, initial charge, phase of moon, etc., but how close do you want to be? And do you want an estimate that will stay valid as the battery ages or one which is only valid with a new battery, or even one which takes into account aging?

Aah -- rereading, you want to know how long the battery will last in charge/discharge cycles, and therefore time.

You can use some rules of thumb. Assume that the battery can be recharged 500 times until the capacity hits some low limit. Assume the change in capacity is linear. (say 500 charges is to 60% capacity).

Total discharge time is 76 * (1 + 0.6) * (500/2) = 30,400 hours = 3.5 years. ou could also add the charge time to that calculation, but ow accurate do you want? BTW you need to get the life in full charge/discharge cycles from the manufacturer (or test to determine it!)