Batter capacity - what does it really mean?

[solarnoobie]

If I had a battery that was rated 14 ah @ 3.2v, does that mean:

A) 14ah will be discharged from its maximum voltage (3.7) to 3.2v (Nominal)
B) 14 ah will be discharged from 3.7v until it's dead (deep/complete discharge).
C) 14 ah is the most that can be discharged from 3.2v?

Thanks for helping.

[bstedh]

A 14AH battery will deliver 14 amps for one hour.
Plan on only pulling 2.8A for one hour to keep yourself inside the 20% discharge level to extend the battery life. Or 1.4A for two hours.

[Sunking]

Amp Hours takes 20 years of college to fully understand in the USA or 5th grade anywhere else.

Amp Hours = Amps x Hours

From that formula we can also immediately say

Amps = Amp Hours / Hours

Hours = Amp Hours / Amps

So the answer to your question is in the formulas.

For example 14 AH / 2 A = 7 hours.

[Wy_White_Wolf]

Another useful formula would be:

AmpHours * Volts = WattHours

14AH * 3.2V = 44.8WH

WWW

[solarnoobie]

Thanks Sunking.

I understand the nuiances of Amp-Hour....

However, I was more interested the actual capacity.

bstedh has mentioned that 14 AH at 3.2 V means that 14 amps can be discharged over one hour at 3.2V.

However, the battery can be charged to 3.7V. Does that mean that the battery has more than a 14 AH capacity? Or does the 14 AH at 3.2 V really mean that 14 AH can be obtained when discharging from a fully charged batter (at 3.7V) to 3.2V.?

- Thanks!

[Sunking]

Based on the voltage I assume Lithium Ion battery?

The rules of Lead Acid battery does not apply to lithium Ion batteries. You can discharge them 80% DOD with no cycle penalty or adverse Peukert Law affect. So to figure out how long a fully charged battery LFP battery will supply a given load use an Adjustment factor of 80%.

So the formula would be

H = [AH/A] *.8

For example a 1 amp load on a 14 AH LFP will be [14AH/1A ] * .8 = 11.2 hours with a ending SOC voltage of 2.8 volts

[bstedh]

Lithium Ion batteries should not be run below 3.5 to 3.2 depending on the manufacture and sit at 4.2 at full charge. Also, unless you have high quality Japanese or US made cells their AH rating will be over stated. I am currently using several Panasonic 18650 batteries that actually are exceeding their rating and can discharge at up to 10A without damage. You also want to look up your specific batteries C rating to see how much current draw you can safely put on it. Too much draw equals flaming bottle rocket....

I got these cells at around $8 per 2.2AH through a co-op buy that was an awesome deal.

The nominal 3.2V would probably be Lithium Polymer and I am not familiar with their charge characteristics as I mostly use AGM and LI-IO.

Ed.

[Sunking]

Lithium Ion batteries should not be run below 3.5 to 3.2

The nominal 3.2V would probably be Lithium Polymer and I am not familiar with their charge characteristics as I mostly use AGM and LI-IO.

Ed.

Ed he mentioned 3.2 volts nominal which indicates LFP aka Lithium Iron Phosphate (LiFePO₄₎. LFP electrical characteristics are:

Cell voltage = min. discharge voltage = 2.8 V.
Working voltage = 3.0 V – 3.3 V.
Max. charge voltage = 3.6 V.

Since you mentioned a 18650 cell standard is Lithium Cobalt (LiCoO₂) type used in the Tesla Roadstar, Laptop Computers, and Flashlights. Very good energy density but very prone to over heating and must be constantly monitored during charging.

[bstedh]

The 18650's I am using are a new line from Panasonic. CGR18650CH They are a hybrid IMR and one of the safest on the market. You can use standard LI-IO chargers and their life and capacity are outstanding. I abuse mine fairly good and they are ticking along very nicely.

Here is a fun little stress test that shows how they react to a dead short. Try this with most other Li-Io batteries and you would get fireworks.

http://www.youtube.com/watch?v=haMDZOfjqsE&feature=plcp

[bstedh]

Thanks Sunking.

However, the battery can be charged to 3.7V. Does that mean that the battery has more than a 14 AH capacity? Or does the 14 AH at 3.2 V really mean that 14 AH can be obtained when discharging from a fully charged batter (at 3.7V) to 3.2V.?

- Thanks!

No it doesn't have more than 14AH.

All batteries will have a voltage curve from high fully charged level to optimal level to the lowest you should run them down to. The capacity measurement covers this entire range. This is why most devices are rated to operate between a voltage range. Even a AA battery will only be at 1.5V for a short period before the voltage starts to drop. As the voltage goes down your load will actually start drawing more amperage. (edit: that last is usually seen with a reactive load like electronic devices, a light bulb would just get dimmer.)

The output voltage on my 48V AGM battery strings actually will start at 54V and dip down to a high 48V than go back up to 49V for about 1/2 an hour before starting a downward slide again.

[billvon]

If I had a battery that was rated 14 ah @ 3.2v, does that mean:

It means that you can get 14ah out of it max, and it has a nominal voltage rating (average discharge voltage) of 3.2 volts.