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Solar Powered Low Power Embedded Device

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Kalcifer:
It’s been surprisingly difficult to find specific information on charging a NiMH. I want to SLOWLY (30uA or so, which is less than C/1000) trickle charge a NiMH to keep it charged. The idea is to keep it on a solar panel feeding it this approximate 30uA current indefinitely to power an embedded device.

What I don’t know is if this small current will damage the cell in some way. I’ve conducted some tests and at this current the cell does indeed charge, about 1mV/min increase in cell voltage. But what happens once the cell finally reaches its fully charged state?

When I’m trickle charging it, should I maintain a constant float voltage so that I don’t overcharge it? I’ve heard that NiMH don’t have a specific float voltage so maybe this isn’t possible? If not then how do you prevent the cell from overcharging? Will the cell be damaged even at this low charging current? I can’t imagine that 30uA will heat up the cell enough to damage it.

bdunham7:
It's actually way too low and probably won't keep the battery at full charge due to self-discharge. 

https://www.vencon.com/charging-nicd-nimh-batteries/

Kalcifer:

--- Quote from: bdunham7 on July 01, 2020, 03:25:57 am ---It's actually way too low and probably won't keep the battery at full charge due to self-discharge. 

https://www.vencon.com/charging-nicd-nimh-batteries/

--- End quote ---

Wouldn't that entirely depend on the quality of the battery though? I did a quick test like said with a NiMH battery connected to my solar charging circuit, and with a charge current of around 30uA, the voltage on the cell increased at a rate of roughly 1mV per minute which tells me that its charging just fine despite the self discharge rate of the battery.

bdunham7:

--- Quote from: Kalcifer on July 01, 2020, 03:33:07 am ---
--- Quote from: bdunham7 on July 01, 2020, 03:25:57 am ---It's actually way too low and probably won't keep the battery at full charge due to self-discharge. 

https://www.vencon.com/charging-nicd-nimh-batteries/

--- End quote ---

Wouldn't that entirely depend on the quality of the battery though? I did a quick test like said with a NiMH battery connected to my solar charging circuit, and with a charge current of around 30uA, the voltage on the cell increased at a rate of roughly 1mV per minute which tells me that its charging just fine despite the self discharge rate of the battery.

--- End quote ---

Yes, the self discharge will depend greatly on the characteristics of the battery as well as the state of charge.  Self-discharge is less at lower charge states, so you will charge the battery up to the point (eventually) where self-discharge equals your charge current.  What were the actual voltages involved?  And what type/size of battery do you have?

Kalcifer:

--- Quote from: bdunham7 on July 01, 2020, 04:31:41 am ---
--- Quote from: Kalcifer on July 01, 2020, 03:33:07 am ---
--- Quote from: bdunham7 on July 01, 2020, 03:25:57 am ---It's actually way too low and probably won't keep the battery at full charge due to self-discharge. 

https://www.vencon.com/charging-nicd-nimh-batteries/

--- End quote ---

Wouldn't that entirely depend on the quality of the battery though? I did a quick test like said with a NiMH battery connected to my solar charging circuit, and with a charge current of around 30uA, the voltage on the cell increased at a rate of roughly 1mV per minute which tells me that its charging just fine despite the self discharge rate of the battery.

--- End quote ---

Yes, the self discharge will depend greatly on the characteristics of the battery as well as the state of charge.  Self-discharge is less at lower charge states, so you will charge the battery up to the point (eventually) where self-discharge equals your charge current.  What were the actual voltages involved?  And what type/size of battery do you have?

--- End quote ---

Yeah I figured as much. It'd go until it reaches an equilibrium point. but for me that's honestly no big deal. For my application, the density of charge doesn't necessarily matter. I just want to be confident that I will reliably have some charge stored in the battery that will be able to account for off times with the solar panel (night). All I'm concerned about is the faint possibility that I DO generate enough power to be able to overcharge the battery. I know that the current will still only AT MOST be a few hundred microamps, but that could still offset the self discharge rate.

As for the voltages involved, the VOC of the solar panel is somewhere around 6-7V or so; but when the solar panel is connected to the battery for charging through a series diode, the voltage of the panel drops to that of the battery plus the voltage drop of the diode. As for the size of the battery I'm currently testing, its a 1000mAh.

P.S. Would I be better off using a NiCd? I've heard that they develop a memory over time and are supposed to be completely discharged. In my project the battery would never be discharged, so I wonder if this would end up being a problem.

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