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I want to replace a battery to an old electronic but the company went out of business and I cannot find the same one and the one I have doesn't last long at all. I would love to replace it with a larger capacity battery but am worried about overpowering the circuitry. Can you help me determine what would be a workable substitute (preferably rechargeable, like lithium)?

The battery says 7.4V 1800mAh 13.32Wh

I used a multimeter and it gives off a solid charge of 4.25V when fully charged (as well as does the charging cord that says 'max 3.7V')

Why does the battery say 7.4V and give off 4.25?

Do I need a 7.4V battery or 4.25V?

I know that there are ways of manipulating voltage down but I would rather a straight battery than extra circuitry if possible

I have basic circuitry knowledge so soldering it wouldn't be an issue but when it comes to batteries I really am having a hard time knowing what to choose.

Thank you so much in advance for your help

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    \$\begingroup\$ (1) Could you edit your question to add a photo of the battery? Preferably with all the markings on it in focus. (2) Also, maybe if you tell us the make and model of the unit in question, someone might have an exact answer for it. \$\endgroup\$ Commented Apr 22, 2023 at 16:15

2 Answers 2

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Why does the battery say 7.4V …
That is twice the voltage of common Li cells ("NMC", for one).
… and give off 4.25?
Two easy explanations:
the battery (two Li cells in series) is deeply discharged (maybe due to a defective charger), or
something went wrong measuring battery voltage.

I don't know of 4.25 V batteries. That may be the voltage of a daring charger for 3.6 to 3.7 V Li cells (or a reasonable one for exotic cells such as LiCrO₂).

You most probably need a battery with not much less than 7.4 V nominal voltage, and useful life may be shorter with less than 13.3 Wh.
More often than not, there are mechanical constraints on usable battery size.
Possibly, you need to match placement of contacts on match connectors.

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    \$\begingroup\$ An older Lithium rechargeable battery cell is 3.7V when it is at the half charged selling and storage voltage. It is 4.2V when fully charged. You might have two cells in series producing 7.4V but each cell is at 3.7V. A Lithium battery MUST have a charger designed for it. \$\endgroup\$
    – Audioguru
    Commented Apr 22, 2023 at 20:24
  • \$\begingroup\$ It might be that one of the cells is shorted so it's working at half the originally specified voltage. And the remaining cell is probably damaged or severely depleted. Some chargers will show a lower voltage on an open circuit, or when a lower voltage battery is connected. Pictures by the OP would help tremendously. \$\endgroup\$
    – PStechPaul
    Commented Apr 22, 2023 at 22:14
  • \$\begingroup\$ Who makes the green battery that is "claimed" to be from LG but there is no manufacturer's name on it? Are its specs believable? \$\endgroup\$
    – Audioguru
    Commented Apr 23, 2023 at 1:59
  • \$\begingroup\$ (@Audioguru, @PStechPaul: Are these comments directed at me so I can improve this answer? (I don't have a 2S battery. If one of the cells was shorted, 4.25 V for the remaining cell is 1 % higher than what I remember as CV for NMC cells, both reducing life expectancy if held for extended time.) \$\endgroup\$
    – greybeard
    Commented Apr 23, 2023 at 5:03
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Ok here's how lithium ion works. Lithium Iron phosphate is a different voltage so don't mix them up.

Lithium Ion nominal voltage is 3.6 or 3.7 its the same thing...nominal meaning like middle or average.

If you have a battery pack that says 7.4V you have two cells in series. If it is at 4.25V that is 2.125 per cell. Minimum voltage per cell is typically 2.5V but sometimes 2.65, 2.75 or even 3.0. Most people don't like going below 3.0 because there is very little capacity down there and when you go below 2.5V you are damaging the battery

Wh=Amp Hours x Nominal Voltage OR mWh=mAh X nominal voltage

Using Ah can be deceiving without knowing voltage which is why we use watt hours or milliwatt hours

So you need a 2S or 2S1P pack (2 cells in series, 1 cell in parallel imagine two AA's on the table in front of you like this +[][][][]- +[][][][]-) negative to positive increases the voltage. It is likely arranged with the cells standing up, one will be upside down the other will be right side up and they will be attached with a piece of nickel on ONE end. The other end is your main pos and neg.

Lithium cells must be balanced. No two cells are the same and will discharge and charge unevenly over time by small amounts even if from the same batch. The solution for this is using a battery management system (BMS) that has cell balancing. Most BMS's do but a few don't. There are some boards out there that are JUST cell balancing but don't offer the other protections like low voltage cutoff that would have saved your pack from going that low (or that might be your BMS cut off level, sometimes they are a fair bit below the minimum voltage because you're not supposed to rely on them, they are a last ditch kind of effort) You can google what a BMS all does. Short circuit, overvoltage, undervoltage, blah blah blah protection.

So you need to figure out if your current pack has a BMS. You did not indicate what the device was.

So your current cells are 1800mah each. The capacity does not double when you put two cells in series. What concerns me is this is a low capacity and high discharge cells that put out a lot of current take the tradeoff of being lower capacity. So either you have some really really old cells that are your typical 1800mah 3.6A max discharge rate or you have some 5-10 year old cells that are 1800mah and 20A max discharge rate.

There is likely text on each individual cell under the shrink holding the pack together that will allow you (or me) to identify them. If they are generic Chinese it can be a bit hard but if they are name brand then there are huge databases that tell you all you need to know.

If this is for an electronic device like a laptop, bluetooth speaker or something else that is not super power hungry you probably don't need "power" cells that do 20A. You would be ok with just about any high capacity cell. The highest capacity 18650 that is easily accessible on the market are a few 3500mah cells from LG, Samsung, and Panasonic/Sanyo.

The LG MJ1 is the easiest to get, and arguably the best of the 3 although the Sanyo is a good cell. The Samsung is also a good cell but has a slightly lower discharge current so erroring on the side of caution I'm selecting the highest discharge one of the bunch.

https://www.18650batterystore.com/products/lg-mj1-18650-3500mah-10a-battery

And she's on sale for $3.95 each! Problem is you need them connected in series. It can be done with a soldering iron but...is not a good way to do it. You need to be fast, as little heat transfer as possible into the cell. Spot welding is the preferred way. You probably don't want to buy a cheap $30 spot welder for this but if you do we've finally hit the point where for $30 you get one that actually is halfway decent.

https://www.aliexpress.us/item/3256805232178924.html?spm=a2g0o.productlist.main.47.444112fcFNrvZS&algo_pvid=1fdd37cf-1e3d-47d3-8298-cc3bdeec5d77&algo_exp_id=1fdd37cf-1e3d-47d3-8298-cc3bdeec5d77-23&pdp_npi=3%40dis%21USD%2184.07%2127.74%21%21%21%21%21%402122457116821976992482290d0713%2112000032983477297%21sea%21US%213589144974&curPageLogUid=ORMdBJNoder7

that's a Bifrc clone (they're all the same probably same factory just different brands) at $28

I would strongly strongly advise NOT buying a 2S premade pack off ebay or Amazon. You will get ripped off. 99% of the cells on there are GARBAGE and the listings are full of lies.

You can solder the cells together with a piece of wire if you have to just use flux and make sure you have a decent soldering iron (60W or more) and don't spend more than 3 seconds on each solder joint before cooling it down and coming back to attach the wire.

Or I can make the pack for you, I have a new spot welder on the way right now and I actually have a bunch of very lightly used LG MJ1 cells or you're welcome to buy new ones.

No cost for me making the pack, you just pay for materials and shipping. My used cells have less than 100 cycles and most test around 3300mAh but I can find you two that tested at 3400mAh for $5.

If you aren't comfortable doing it yourself then hit me up, I'd be happy to help. Unfortunately I don't know of anywhere RELIABLE you can buy a 2S high capacity pack for a good price. 18650 Battery store, Li Ion Wholesale and Bulk Battery are the only 3 stores I trust for cells. Everyone else can alfjlafdjkjfda you know what I mean. My electronic mail address is eric h klein at hot mail dot com

I'm in the USA, West Coast...specifically a giant oven.

Edit: these clowns want $23 plus shipping for a pack that has no BMS, just 2 cells and some heat shrink and wire

https://www.getfpv.com/zohd-lionpack-18650-2s1p-3500mah-7-4v-li-ion-battery.html

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