What does it mean that NiMH batteries do not have a "float voltage"?

I am working on a project that requires a very tiny amount of power and will be used fairly infrequently for short periods. As such, I'd like to experiment with making it entirely self-contained by connecting rechargeable batteries to solar panels, allowing it to recharge from ambient light.

According to my research, sealed lead acid batteries can be subjected to a charge at a constant "float" voltage for extended periods, but are generally reserved for high-power applications due to their weight and bulk. LiPo and Li-ion batteries are extremely finicky.

According to this question, NiMH batteries "don't have a float voltage, so constant voltage charging doesn't work, as you've likely discovered". However, I'm having difficulty finding a resource explaining what "doesn't work" means, precisely. Other pages say that as long as a battery is subjected to a voltage greater than its current charge level, it will charge. I take this to mean that NiMH batteries can be charged at constant voltage, but in charging them to their full potential one runs the risk of overcharging them due to temperature-dependent fluctuations in their charge and discharge curves.

So my question is: can a NiMH battery be safely charged for an indefinite period of time using a "dumb" charger which supplies a constant voltage below the battery's maximum? i.e. if my application requires 3.6 volts, can I power it using a 4.8 volt NiMH battery that is deliberately charged to only 3.6 volts?

• When writing "constant voltage", there must be some source impedance or other limit on current. Commented Jan 15, 2023 at 18:06

As long as the charge voltage is below whatever the "fully charged" battery voltage is, presumably you should be ok.

The problems with NiMHs is that continuous charging can lead to a build up of hydrogen gas and a rapid temperature rise but this is when the charger keeps trying to pump energy into an already full battery.

Dropping the charge voltage below this threshold means you'll lose a little capacity (as it'll never quite fully charge), but now the battery can never be overcharged. The only thing I'm not sure about is any potential lifespan issues from having an incomplete charge cycle, you'll have to wait for someone with more in-depth knowledge of battery chemistry for that one.

• Trying to keep a NiMH "barely charged" is a nightmare, since battery voltage for a given SOC depends on temperature. The ONLY common battery chemistry which allows safe constant-voltage charging is lead-acid. All others, including NiMH need to charge via controlled current. Commented Jan 5, 2017 at 23:22
• @WhatRoughBeast Well then you pick a maximum voltage below the cut-off voltage at your highest operating temperature, sure you might only end up with 30% of the battery's design capacity but at least it'll work. Pb-acid is not the only abuse tolerant battery, Ni-H (not NiMH) cells are practically indestructible.
– Sam
Commented Jan 5, 2017 at 23:35
• @Sam -- NiFe is also rather good at surviving abuse, BTW Commented Jan 5, 2017 at 23:37
• @ThreePhaseEel I knew I was forgetting one (even good ol' NiCds are pretty abuse tolerant, with the exception of the memory effect), something about Nickel-based chemistries...
– Sam
Commented Jan 5, 2017 at 23:39