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I recently came across a device (a headlamp) with lithium-polymer battery, which is marked 3.8 V nominal voltage, instead of usual 3.6-3.7 V. It's charging circuit is based on ME4057D chip, which is a 1 A lithium battery charger. The suffix -D in the chip's name indicates a variant which, according to the datashaeet, charges the battery to 4.34 V, instead of normal 4.2 V.

I would like to reuse this battery along with it's charging circuit in some of my projects, but I'm concerned about safety, because I always heard charging lithium batteries to voltage higher than 4.3 V can be dangerous.

Complete marking of the battery is: WT 902554 3.8V 1600mAh, but I didn't manage to get a datasheet.

My questions are:

  1. Is the 3.8 V nominal voltage something really unusual, or there are some special types of lithium batteries, where such nominal voltage is normal?
  2. Can be charging to 4.34 V dangerous or significantely decrease lifetime of the battery?
simonov
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The chip from the original circuit charges to 4.34V as stated. This would imply that the cell is a high-capacity Li-ion (a type popular in cellphones), so that would also imply you could safely charge it to that level.

That's a lot of weasel-words. The scary part is relying on the good faith of an offshore manufacturer who made the lamp to have always procured the appropriate cell for 4.34V end-voltage. Since the cell is a commodity item, compared to an engineered Li-po pack in a phone, you can't really be certain, can you?

What to do then?

Limiting the charge to 4.2V will avoid the uncertainty, while getting more charge cycles out of the battery. You'll be trading off ultimate capacity, but it is a prudent choice out of an abundance of caution, given the apparent confusion over the battery's actual characteristic.

Here's a relevant discussion. Why are 3.8V lithium-ion batteries used in mobile devices, rather than 3.6V or 3.7V batteries?

hacktastical
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    The battery listed is has a max voltage of 4.2V, if you use a 4.34V charger on a 4.2V battery that would be bad. – Voltage Spike Aug 19 '19 at 16:51
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    I agree, and updated my answer to be more unequivocal about using 4.2V. – hacktastical Aug 19 '19 at 17:24
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    The reason for the higher voltage is that they surge charge the battery to 4.2V. Of course to get a higher current into the battery the voltage would have to be above the 4.2V aiming point. If you had a maximum charging voltage of 4.2V it would take a looong time to get to full charge. Most phones, tablets and laptops use this higher voltage strategy to shorten the charge time. – Jack Creasey Aug 20 '19 at 15:09
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    Maybe. But you're relying on some not-well-known vendor's promise that they do exactly that. Too many variables in the supply chain to know, really. – hacktastical Aug 20 '19 at 20:04
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The maximum a battery can be charged is determined by the chemistry of the battery. For a lithium-polymer battery the charging curve looks like this:

enter image description here
Source: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

You first give the battery a constant current determined by the cell, then a constant voltage of 4.2 volts.

Can be charging to 4.34 V dangerous or significantly decrease lifetime of the battery?

If you go to a higher voltage, it will reduce the battery lifetime or cause a failure, don't do it.

If you need a voltage high than 4.2 volts use a DC DC boost converter.

Overcharging Lithium-ion

Lithium-ion operates safely within the designated operating voltages; however, the battery becomes unstable if inadvertently charged to a higher than specified voltage. Prolonged charging above 4.30V on a Li-ion designed for 4.20V/cell will plate metallic lithium on the anode. The cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO2). The cell pressure rises and if the charge is allowed to continue, the current interrupt device (CID) responsible for cell safety disconnects at 1,000–1,380kPa (145–200psi). Should the pressure rise further, the safety membrane on some Li-ion bursts open at about 3,450kPa (500psi) and the cell might eventually vent with flame.

Source: https://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

Voltage Spike
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    LiFePO₄ cells have an end-of-charge voltage of 4.35 V, don't they? – Hearth Aug 19 '19 at 16:40
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    It looks like the cells you listed WT 902554 are regular 4.2V batteries: https://www.alibaba.com/product-detail/3-7V-Cylindrical-18650-Rechargeable-Li_60291039870.html?spm=a2700.9099375.35.8.d81b689fEWLLlc – Voltage Spike Aug 19 '19 at 16:50
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    See this table for info on voltages: https://batteryuniversity.com/learn/article/confusion_with_voltages – Voltage Spike Aug 19 '19 at 16:50
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    Sure, I didn't bother looking up the part number they listed, I just thought it worth mentioning that some cells _do_ charge to that voltage. – Hearth Aug 19 '19 at 16:51
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    Oh, yeah for sure, It looks like LiFePO4 go to 3.6V. Regular LiPOs go to 4.2 and LiPOs with surface coating and additive go 4.35V – Voltage Spike Aug 19 '19 at 16:55
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    The point missed by the answers is that the charging voltage is only loosely related to the fully charged terminal voltage. – Jack Creasey Aug 20 '19 at 15:11
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    @Jack Creasey, can you tell why on the chart at 20% capacity cell voltage is 1V? In a phone I've seen like 3.6V at 20% remaining capacity. Chart is just qualitative, not quantitatively correct? TIA BTW clicking the link produces "page not found". – Martian2020 Oct 02 '21 at 05:58
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    The Internet does change, links do expire. The chart does not say 20%=1V. If the cell voltage graph is near vertical for capacities less than 40% @1C then it can no longer support 1C. You'd have to reduce the current to chart the cell voltage extremities'. Think of the internal resistance of the cell. If you measure the cell voltage at very low currents it would measure perhaps 3-3.2V but as soon as you increase the current the cell output voltage would collapse. – Jack Creasey Oct 02 '21 at 16:33
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Different LiPo cell designs have different end-of-charge voltages and end-of-discharge voltages. There are "high voltage" cells that are designed to be charged to higher voltages, and have higher capacities as a consequence.

Even for a given cell design, a system designer can make tradeoffs between cell life (the number of times that the cell can be charged before it dies) and the voltage limits.

For example, if you have a cell that's rated by the manufacturer for 4.2V charge and 3.2V discharge, you can get more life (I can't remember how much, IIRC 10 or 20%) by limiting it to 4.1V and 3.3V -- but you get lower effective capacity.

As mentioned, you're trusting the light manufacturer to have done the right thing rather than just tossing a bunch of parts together and selling them quick.

TimWescott
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    Planned obsolesetance. Manufacturers never include an easy to toggle option in phones (in modern laptops its at least sometimes present) to limit maximum charge to 70-80%. They are interested in battery wearing out faster, so user buys new phones more. Only way to enforce this is rooting the phone, and might not work. I got idea to make kind of middle-man device that will monitor charging current and then cut of charging when it senses current increase/decrease in the end of charging cycle. This can also work for devices like shavers/electric brushes etc. that don't even have screens. – ScienceDiscoverer Jul 24 '22 at 15:20