I have a lithium battery pack that is supposed to be 12V.

Measuring it, it is 12V.

But it's only got 9 cells in it and a small circuit board, is this correct??

Anyhow, it's been fully charged for a week and with no use, the voltage has dropped to 11.5v. Does this seem right?

 

hi gray,
A li-ion cell say a 18650, has a charged voltage of approx 4V, so I would say the 9 cells are arranged as 3 * triple cells. IE: 3 in series and 3 of these series cells in parallel.

The small PCB is probably a BMS charger control, so it may draw a little current while the pack is not being used, hence the small drop in voltage.
What is the AHr rating of the pack.?
E

Added Graph:

 

It says on the pack 40Ahr

I've been trying to work on 3.7v, that's my naivety.

 

It says on the pack 40Ahr

Whoa, that's a lot. Way more than 9 x 18650 cells can deliver. Are you sure you're reading that right? What can you tell us about the individual cells? Are you sure they're lithium?

Self-discharge is a real thing and varies by chemistry and brand. Loss of half a volt in a week seems a little high but I'm not certain. It would not be surprising at all for a sealed-lead-acid or Ni-Cd cell.

 

HI - I am sorry its a 30Ahr pack

This is all I know. Its a bit iffy as the item is described as 50Ahr, but the option bought is 30Ahr. But all the text is for the 50Ahr one, so no spec for the 30Ahr.

 

hi gray,
The data states Single battery 3.7V @2 AHr/cell, my guess your pack is actually approx 20AHr.

You could discharge as a test at say 1Amp and check the duration.

E

 

Yes its a 3s3p (3 series x 3 parallel = 9 cells)

9 cells x 2 AHr/cell = 18AHr's which is way below the 30AHr's stated.

 

hi gray,
What is the battery powering, load etc.?
E

 

Hi - It will be a standalone pest control ultrasonic scarer (ultrasonic 20Khz and 25Khz, 8 different audio clips, flashing HIT LEDS). To be used for say 4 weeks with no battery change etc. Not too sure yet total power consumption. Might even think about solar charging.

 

Welcome to AAC.

Series connections stack voltage, parallel connections stack capacity.

So:

3S makes 3 12.6V (full charge) / 11.1V (typical) batteries each with 2Ah capapcity
3P makes 1 12.6V (full charge) / 11.1V (typical) battery with a 6Ah capacity.

This is assuming the 2Ah is not a lie. Modest a claim though it is, that battery is so dodgy and the capacity claims so absurd, it would not surprise me if the cells weren't even this very minimal capacity.

The 30, 40, and 50Ah claims are complete bunk on their face. I wouldn't trust that battery for anything that matters at all.

 

9 cells x 2 AHr/cell = 18AHr's which is way below the 30AHr's stated.

There's so much wrong with the stated specifications. It's not 50Ah or 30Ah.

In a 3S3P configuration, it would be 6Ah. That's assuming that the 2Ah rating can be trusted.

 

I have several Greenworks battery-powered yard tools. (They're great!) The battery packs consist of 20 x 18650 cells in series. They call it an 80V pack, 72V nominal so 3.6V nominal per cell, 4V max. They're rated at 2Ahr and 144Whr. These numbers are all quite believable.

A battery pack was sitting next to me when I made my comment in #4, that 40Ahr would be a BIG pack. That's the capacity of a typical car battery.

In other words, one more vote for that ad to be utter BS.

That's unfortunately not uncommon. I have some 18650 cells that were claimed to be 9,900mAh. (They came with a cheap flashlight.) They tested at about 10% of that claimed capacity. So I'd be worried to buy a battery pack from someone so inaccurate with the specifications. It might not even provide 6Ahr, it might be half that.

 

Everybody is perfectly right.

It's just buyer beware of buying from AliExpress and other similar vendors.

 

Hello there,

This is not the first time I have seen inflated ratings from sellers, and this is on many sites not just the one here.
I've regularly seen ratings for ONE 18650 cell as 10000mAh which is far above the max of around 3600mAh.
I've seen flashlights rated at 900,000 lumens (!) and in the same advertisement 90,000 lumens.
I've seen way over rated ratings like these all over the place even on Amazon. The funny thing is, when we see these ratings we also see a lack of education in how these ratings come about. In audio, we see watt ratings like 100 when the wall wart that comes with it is 10 watts (ha ha). You can't power anything that is 100 watts with a 10 watt power source. They don't realize this, and it's a good thing because it makes it easier for us to spot a fraudulent rating.

As to this battery pack and others like it, if we have one cell rated at N ampere hours and we put M number of cells in parallel, the resulting ampere hours is N*M in most cases. Thus, if we have a 2 ampere hour cell and we put 3 in parallel, the result is 6 ampere hours for those, and it does not matter how many we put in series this never changes. Several other posts here have graciously stated that already too. So the claim of 30Ahr for 3 cells in parallel that are 2Ahr each is just wayyyyyy overstated. Furthermore, they are not even sure if there are 9 cells in the pack or 15 cells. 15 cells would give us 5 in parallel which would give us 10 ampere hours, but it's uncertain if this is right or not.

Just to be clear, it does not matter what units we use. If we want to use 2000mAh per cell, then 3 in parallel is 6000mAh (3 times 2000mAh) which is the same as 6Ahr.

Probably the highest rating for 3 lithium-ion cells in parallel would be around 10000Ahr or a little bit more for cells made as of today 08/30/2025. This could change in the future, but more likely with a different chemistry or solid-state type battery. A pack like that could also have a lower maximum drain current rating which is partly responsible for the higher rating. The maximum drain current and maximum charge current is often an important factor for the higher power battery packs and so cannot be ignored.

 

It's just buyer beware of buying from AliExpress and other similar vendors.

I bought a number of different Li-ion batteries from AliExpress. The first 18650's I bought were advertised as 9900mAh (that was before I came to know that capacities above a few Ah were probably bogus). After that, I bought other capacities and tested them. Even some legitimate 1.6Ah 18500's I bought were fake on the second order (same brand and seller).

I even bought some of the 9V Li-ion batteries with an integral voltage regulator. They were terrible because they self-discharged in about a month and were noisy. Nice idea, but poorly implemented.

You can read about my AliExpress issues here.

 

Hi guys,
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