Hi, i managed to work out how to send pics, I posted them on the forum yesterday.
18650 Batteries
- Thread starter Paul Marvin
- Start date
wormster
Active member
Well those cells in the photos look like low end HC or HCM Sanyo cells with quite a measley out put, my colleagues all have had a good laugh at this thread, considering what we do to poor 18650 cells!!
If you could get together and bulk buy direct from Moli Cell in Taiwan and get say 20,000 (about a morning’s worth of production) then you might be able to get something with a higher output and a better unit price to boot!
Sorry to bang on about this – but it's now my understanding that Ah is not a measure of the capacity of a battery/cell. To get that, you have to multiply Ah by potential difference, so in the case of the 4-Ah / 14.4-V battery mentioned above you obtain the capacity in Wh simply by multiplying 4 by 14.4, which gives 57.6 Wh. And in the case of each individual cell, its capacity is 2 x 3.6 or 7.2 Wh, and since there are 8 of them the total capacity is 57.6. OK?
aricooperdavis
Moderator
Strictly speaking an amp-hour (Ah) is a unit of charge and a watt-hour (Wh) is a unit of energy.
However, when most people are choosing a battery for their light the voltage is fixed; if your light takes 18650s then you're getting a nominal voltage of 3.7V. Since energy is charge x voltage, when the voltage is fixed the charge becomes proportional to how much energy the battery stores, which will give us an idea of runtime.
But yes, if you wanted to focus only on the energy stored by a battery you'd be looking at watt-hours. However you may end up with a lower runtime if you choose a battery that stores more energy but outputs it at the wrong voltage, as regulating that voltage to one appropriate for your electrical use-case has an efficiency cost.
royfellows
Well-known member
Look, the correct measure of a batteries capacity is watt hours, ampere hours only indicate capacity relative to the voltage. So a Sanyo cell as I use is 3500 mAh at a nominal 3.6V. Connect 2 in parallel and its still 3.6V nominal but we double the AH. Connect them in series and its 7.2V nominal but we are still at 3500mAh. Watt hours are the same, so we could accurately say that capacity wise, all things remain equal.
The nominal voltage could be regarded as the half way stage in the discharge cycle, 4.2V fresh off charge down to 2.5V manufacturers recommended disconnection voltage.
I wrote a paper on “Lithium Ion batteries, what cavers need to know”, wish there was some way to make it available on the site. You can download it from my www.ledcaplamps.com
The nominal voltage could be regarded as the half way stage in the discharge cycle, 4.2V fresh off charge down to 2.5V manufacturers recommended disconnection voltage.
I wrote a paper on “Lithium Ion batteries, what cavers need to know”, wish there was some way to make it available on the site. You can download it from my www.ledcaplamps.com
Chocolate fireguard
Active member
OK.
But don't expect everybody (or even most people?) always to use terms like this correctly.
Google "battery capacity" and you mostly get talk of Ah, unless it's a battery for an electric vehicle, then it's kWh.
That's why I put "capacity" in those inverted commas in an earlier post - one which you will have read avidly.
traff
Member
Just to confuse you more, Ah is a unit of electric charge which may be considered a measure of capacity, but only for a specific cell/battery potential.
A single 3500mAh 18650 cell should supply 3.5A for one hour at a nominal 3.7v
A single 3500mAh AA cell should supply 3.5A for one hour at a nominal 1.5v
As pointed out by others the actual energy delivered is calculated by (keeping it simple) Ah x V
What I'm getting at here is Ah is used as measure of capacity (as seen in industrial practice) when comparing cells/batteries of the same potential.
aricooperdavis
Moderator
Here's a direct link: http://www.ledcaplamps.com/Lithium Ion and Battery Safety.pdf
I've also attached it to this post.
royfellows
Well-known member
Thanks for that.
wormster
Active member
The photos posted are of low end (in terms of build quality and capacity) we don’t use Sanyo cells anymore, as we have switched to higher capacity cells for our power applications.
