I have a compact digital photo camera that will be on for very long periods(for a time-lapse video). It is powered by a Lithium-Ion 3,7V, 720mAh battery

I want to supply 4V to the battery terminals, while the battery is in the digital camera. This will power the camera and occasionally charge the battery. When the power drops, the camera will still be powered by the battery.

Is it safe to have 4V supplied to the battery for long periods(months, years)?

I'm thinking that at 4V the battery will be kept at around 80% charge(since a fully charged battery reaches 4,2V). The 4V will be supplied by a LM317 regulator.

  • \$\begingroup\$ The headline asks "Is it dangerous?" and the body asks "Is it safe?" ... to which I dare not answer either yes, or no, for fear of being misunderstood... \$\endgroup\$
    – user16324
    Commented Sep 14, 2013 at 17:07
  • \$\begingroup\$ Edited as you commented :) \$\endgroup\$
    – Twinsen
    Commented Sep 14, 2013 at 17:09
  • \$\begingroup\$ No option for an AC adaptor? \$\endgroup\$
    – user6972
    Commented Sep 15, 2013 at 6:05

4 Answers 4


Updated October 2022: This applies to Lithium Ion cells (LiIon or LiPo) rated at a nominal 3.6V or 3.7V.

Floating at 4.0 volt is safe.
Floating at 4.2V is fatal (for the battery and possibly for others as the battery may catch fire.)

If floating at 4.0V (or below) ENSURE that the voltage is well regulated and never rises above this level.

I am interested in the answer to this question and, while I have read much on LiIon batteries, I have not seen it answered with certainty anywhere or addressed directly.

I agree with the intuitive logic of Ignacio's answer, but I think that he is probably incorrect in practice.

  1. Practical information supplied to me by an experienced manufacturer of battery powered products is that the general experience of Chinese manufacturers of LiFePO4 based products is that LiFePO4 cells, which are similar but not identical in general chemistry to Lion, will degrade and die if constant float voltage is applied to them. LiFePO4 cells would if anything be expected to be more robust and resistant to adverse treatment than LiIon (due to the Olivine internal matrix which resists the mechanical degradation mechanisms which LiIon suffers from.)

  2. It is generally advised that use of a lower terminal constant voltage will increase LiIon cycle life at the expense of lower absolute capacity per charge-discharge cycle. However, ALL LiIon charging algorithms and chargers that I have seen terminate the charge cycle at some point and remove the charge voltage. ie they never "float" the battery. Less aggressive chargers terminate charging when Ichg is say 50% or 25% of Imax, and more aggressive chargers charge until Ichg dropsto say Imzx/10, but none ever let Ichg trickle off to zero.

As allowing Ichg to drop to some very low value would maximise capacity and simplify charging it would seem logical that manufacturers would do this if it was acceptable. None do, that I have ever seen.

LiIon battery manufacturers all advise a minimum end of charge current.

Charger IC manufacturers typically offer several end of charge currents but none ever offer "float" as an option.

So, it seems highly likely based on the above, that floating a LiIon battery will cause premature degradation.

  • 1
    \$\begingroup\$ "Li-ion cannot absorb overcharge, and when fully charged the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium, and this could compromise safety. To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible." (via batteryuniversity.com) \$\endgroup\$
    – Twinsen
    Commented Sep 15, 2013 at 6:33

Yes, since the Li-Ion battery will only absorb as much current as required from the supply. Note that this assumes that the battery isn't "smart" in that it requires signaling from the device in order to begin charging.

Also note that this is not true of NiCad or NiMH unless the current is throttled to very low values (0.04C or lower).


The alternative is to take the battery out and connect a 3.7V supply instead for this application. The maximum current needed may need to be measured unless there is some documentation saying what the peak current is.

  • \$\begingroup\$ Some devices won't power up if they can't verify the battery type/vendor on the battery pack's safety circuit. In this case you have to open the battery pack, unsolder the cells and connect the external supply there using the battery pack housing and safety circuit. \$\endgroup\$
    – user6972
    Commented Sep 15, 2013 at 6:04
  • \$\begingroup\$ Yes, I can see that does make it a harder task. But from a safety aspect if there is no battery to get damaged there is a lower risk of fire. \$\endgroup\$
    – Spoon
    Commented Sep 15, 2013 at 11:10
  • \$\begingroup\$ This also means I will not have the benefit of a back-up, if the mains power goes out for a few hours. \$\endgroup\$
    – Twinsen
    Commented Sep 16, 2013 at 8:51
  • \$\begingroup\$ This is true, but it depends on how important it is to have that backup. If it took 24 hours to repeat an experiment then I would not bother here in the UK. (The chances of 2 power cuts in 48 hours is very small) Adding a Sealed Lead Acid battery as backup is not that hard and it is a more stable technology. But if you only have one chance to get it right.. go with a backup power of some sort, which ever solution you go with. \$\endgroup\$
    – Spoon
    Commented Sep 16, 2013 at 22:19

Based on what I have found.

Float charging a Lithium ion cell at 4V is safe. The charging current will eventually taper off to the internal leakage level of the battery.

Float charging lithium ion at 4.2V (or above) is not safe. Extended retention of the cell at that voltage causes plating.

It seems as the golden value is 3.92V: Lithium Ion prolonged lifespan


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