4S LIPO battery Wh-Voltage-Capacity question - Page 1
Forum section: electronics / beginners

author: BinaryBits
author_rank: Contributor
timestamp: March 15, 2023, 10:59:52 am
content: So I have bought this portable no-name jump starter, which supposedly contains a 8000 mAh LIPO. I opened it up and its a 4S battery, so its at 16.8V when fully charged (I thought a LIPO jump starter would use a 3S battery?).Anyway, the specs on the jump starter says the following: "8000mAh (4.2V) 33.6Wh"So im wondering, when calculating mAh to Wh, the 33.6Wh is 8000 mAh only when calculating with a voltage of 4.2V, but shouldn't I calculate with the total voltage of the battery, 16.8V, or maybe with the nominal 14.8V? Because in that case it turns out to be only 2000mAh (when I use these online mAh to Wh calculators)?So I'm a little confused. Does the specs add up to 8000 mAh at 16.8V/14.8V. The "(4.2V)" in parenthesis is what makes me a bit confused. Why don't they just mention the Wh and capacity at the nominal voltage or full voltage of all the cells combined?Also, I have used a USB tester to measure the capacity taken from the battery when charging my phone. (it has built-in USB ports), and at around 5.14V I have only been able to draw about 3900 mAh, and the battery now shows 14.19V (3.54V per cell).Also, I think the size of each cell looks a bit small to be 8000 mAh cells, they are each around 78x50x5mm (3.07x1.96x0.19in) in size??So have I misunderstood this and the battery is in fact 8000 mAh...at 16.8V (and not only at 4.2V)? And in that case, what could explain the much lower capacity that I have only been able to draw from the battery?Or might each cell be smaller than 8000 mAh and that they also are connected in parallel (maybe 4S2P), in addition to being connected in series? Im just trying to figure out what capacity the battery has at the batteries full voltage/nominal voltage?Thanks in advance.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 15, 2023, 11:06:52 am
content: Sounds to me that it is 4 cells each of 2 Ah capacity.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 15, 2023, 11:08:25 am
content: That is just bullshit specification. The same as the PMPO power ratings of audio amplifiers in late 1990's. Completely made-up numbers.The internal logic of this scam is something like "if voltage was 4.2V instead of 14.8V then it would have this many mAh given the same energy". They even added this equivalency voltage in parenthesis. Except they calculated it incorrectly, it should read 3.7V or so.But it makes absolutely no sense to do so, except to confuse people. Which is kinda crappy situation, as people are confused about energy, power, current, charge and voltage units already.

author: MrAl
author_rank: Super Contributor
timestamp: March 15, 2023, 12:47:03 pm
content: Quote from: BinaryBits on March 15, 2023, 10:59:52 amHi,To find out for sure do a full charge then discharge test. That will tell you how it stands as is.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: March 15, 2023, 01:09:38 pm
content: Lithium battery charge capacity are a joke with most Chinese lithium cells.I have one 26650 cell that has 12,000 mAH stamped on it

author: jerryk
author_rank: Regular Contributor
timestamp: March 17, 2023, 06:25:39 am
content: Quote from: BinaryBits on March 15, 2023, 10:59:52 amThat is interesting. NOCO is using 4S packs in all of their Boost X or GBX series jump packs. They do not disclose this and still call them 12V jump packs like their standard 3S packs. It looks like a trend and they use more magic math on the higher pack voltage to concoct more exaggerated claims about the capacity and output amps. The best thing you can do is test it with some variant of a load tester till the battery discharge limit triggers. I do it a couple of times on each pack I check. It never matches their claims.Jerry

author: Nusa
author_rank: Super Contributor
timestamp: March 17, 2023, 07:19:30 am
content: As for why it's 4S instead of 3S, remember that a car charging system normally operates at over 14V when the vehicle is running, not the nominal 12V number that everyone refers to. Also, in this application, it's not actually going to be at the full 16.8V once you hook it to a poorly charged car battery, which will be a heavy load that will suck energy as fast as the pack is willing to give it.

author: MrAl
author_rank: Super Contributor
timestamp: March 17, 2023, 09:36:41 am
content: Quote from: BILLPOD on March 15, 2023, 01:09:38 pmHi,Yes but that's because they made a mistake, isnt it obvious?They meant to leave off the little 'm' so it would read 12,000AH.See, the Chinese have a different numbering system then we do. They multiply everything by 10 to 10000 depending on what kind of product it is. <big chuckle>I have to admit though i get a lot of Chi stuff that i like.

author: BinaryBits
author_rank: Contributor
timestamp: March 17, 2023, 06:48:49 pm
content: Thanks for your answers. Yep, I was suspecting that it means something along those lines...that if the voltage is 4.2V then it would have 8000 mAh, but since its 14.8V then its lower, or something like that??So would it mean that at 14.8V It would only have like 2000 mAh?Anyway, I have returned it and ordered a 3S LIPO battery, since I wasn't going to use it as a jump starter, I wanted to use it as a spot welder battery. It was actually cheaper than buying a proper LIPO battery locally. A jump starter LIPO should have enough peak amps for a spot welder was my thought?Yeah thats true about the cars higher voltage, I just thought that because the Lead Acid batteries are around 13V fully charged (even the lead acid jump starters), so that would be more similar to a 3S LIPO, but maybe the different battery technologies makes them use different voltages?

author: Nusa
author_rank: Super Contributor
timestamp: March 17, 2023, 07:38:04 pm
content: Quote from: BinaryBits on March 17, 2023, 06:48:49 pmNo, their math is right (8 Ah * 4.2V = 33.6 Wh), but their facts are wrong. First off, the useful charge of the battery is not going to be at 4.2V the entire time, so the math is usually done with the nominal voltage of the battery (3.6 or 3.7V). Secondly, the cell is almost certainly not really 8 Ah, this is where Chinese vendors exaggerate to the moon. Bottom line, unless you've got a quality cell with a datasheet, you're only going to get the real number empirically, by testing it. Thirdly, 4.2V clearly isn't the output voltage, so even that premise is wrong. When you put identical cells in series, voltage goes up and wattage goes up. Current capacity remains the same. So, 8 Ah * 14.8V = 118 Wh, if you actually had 8 Ah batteries. (I believe ~3.5 Ah is the actual limit for high-end 18650's at the moment.)