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Feb 22, 2020, 06:59 PM
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Bookstar75's Avatar
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Lithium Ion Battery Question


Hello so I am wondering how low a lithium ion battery (not lithium polymer battery) is able to be discharged?. I was hearing that ion batteries can be discharged much more than polymer.
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Feb 22, 2020, 07:09 PM
stegl
Not all lithium-ion batteries are created equal. Make and model of cell would help. The voltages ar3 sort of between aA123 and a lipo.
Feb 23, 2020, 12:17 AM
If it flies, I can crash it.
rocketsled666's Avatar
Yeah, it's highly dependent on the specific battery chemistry. For Lithium Ion and Lithium Polymer a safe rule is no lower than 3.0V (but some chemistries are good to 2.85V). Regardless of the voltage, discharging the battery this low significantly accelerates the battery's wearout. If you don't care about that, provided you're drawing very low current (mA), you can safely discharge a Lithium battery pretty much all the way down to zero (after which you'd throw it away).

Battery capacities as specified by the LiPo manufacturer (not the reseller. All the batteries we buy come from a few suppliers and are getting "branded" by whoever sells them) are often measured at a C/10 rate from 4.2V to 3.0V.

The mechanical construction of an Ion and Poly cell are different, but chemically they're pretty much the same thing. In terms of charge and discharge voltage, they're the same. They differ primarily in the rates they can support. Polys support higher charge and discharge rates. But Ion batteries are generally cheaper than Poly. Which is why they tend to get used most in consumer electronics like phones and tablets, and we don't use them much in RC models.

Lithium Iron and other derivatives with meaningfully different chemistries have different max and min cell voltages. Because of course it's the chemistry that defines the voltage.
Feb 23, 2020, 03:01 AM
Registered User
Really you want to get or build a mapping of voltage vs DoD%, ideally at least 5% resolution, 2% would be better.

Voltages at rest, at least 20min isolated, a few hours better, depends on the discharge C-rate, lower is better 0.1-0.2C.

From that curve you can see the "drop off" shoulder, maybe around 3.55V or 3.50V at rest, likely within 10-15% above the SoC of the mfg LVC rating.

Then you look at the under-load voltage that gets you to that point, will greatly vary depending on the C-rate at the time.

A higher rate means more sag, so a lower LVC to get the same SoC% point.

Set it too high, at peak discharge will cut out too early.

But, the higher the SoC% you cut out - shallower the DoD% - the longer the lifespan of your pack in cycles, huge impact stopping at 70% DoD compared to say 85 or dog forbid 90%.

But bottom line, make sure you don't (habitually) let the pack go lower than that drop-off shoulder, more than the occasional exigency situation.
Feb 23, 2020, 08:54 PM
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Bookstar75's Avatar
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Thanks!
I have a Titan Ion and the website says they can safely be discharged down to 2.5 per cell. But I will probably keep it around 3 to increase the life expectancy of the batteries since they are so expensive. Thanks for the info
Feb 23, 2020, 09:46 PM
If it flies, I can crash it.
rocketsled666's Avatar
What are you using them in?
Feb 23, 2020, 10:23 PM
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Bookstar75's Avatar
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A long distance flying wing...I fly out 10 miles with (2) 4s 3500 Ion pack in parallel. Works great
Feb 23, 2020, 10:35 PM
If it flies, I can crash it.
rocketsled666's Avatar
OK. I asked for a reason... packs are made of individual cells which are not all exactly the same. Some will have a little more or less capacity than others. So even though they all start at the same voltage, at the end of the discharge they can be at different voltages. The voltage you measure in flight is the pack voltage, which you're averaging to get a single cell voltage. If you're only discharging to 3.6V, a few 100mV difference between cells at the end of the charge doesn't matter. One cell at 3.8V and one cell at 3.4V makes a pack at 3.6V. But the lower you discharge the pack, the more it starts to matter. If you discharge to 3.3V/cell, in the same example a cell at 3.5 and 3.1 makes 3.3 average. But that 3.1V cell is dangerously close to its minimum. The problem gets worse the more cells you have in the pack and the lower you discharge. So you should get in the habit of checking the pack balance when you land and if there are any cells way out of whack compared to the rest, adjust your minimum discharge to add some margin for those cells...

Note, I have a large hex copter I fly with a 10Ah 4S pack, it stays in the air for 30 minutes if I discharge to 3.2V/Cell. But as my packs have aged the cell deltas have increased and I've had to bump my minimum up to a higher level.
Feb 24, 2020, 08:31 PM
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Bookstar75's Avatar
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O.k. makes sense! Thanks for the info! Will do!!


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