Question about batteries and solar charging

[LEDrock]

USB AA/AAA chargers can work, I have a couple of older 2-cell ones. One downside is efficiency; by the look of these two, 50-60% at best, probably lower. With small solar setup, wasting power needs to be minimized.

Another downside is adding another level of inefficiency. Running from 12v panel, down-converting to 5v you should manage 75-90%. Then there's the charger, and the "charge efficiency" of cells themselves. In the end, overall efficiency could be rather low.

The charger should not allow cells to discharge into it when its input (solar) power goes away. One 12v charger I was using about 15 years ago did this, lighting up some status LEDs in the dark. Sometimes the only thing to do is test it out.

I also solar-charge USB Li-ion battery packs and use them to run small fans, lights, and charge phone battery.


Dave

I just came across something on Amazon that looks perfectly good. It's a 90x90mm panel with a 2AA battery holder attached to the back of it, and there appears to be a diode there to keep discharging from happening. It's about as basic of a setup as one can get. Specs say the output is 250ma at 4 volts. What do ya think?
https://www.amazon.com/dp/B08YJRMS8D/?tag=cpf0b6-20

 

[Lynx_Arc]

I just came across something on Amazon that looks perfectly good. It's a 90x90mm panel with a 2AA battery holder attached to the back of it, and there appears to be a diode there to keep discharging from happening. It's about as basic of a setup as one can get. Specs say the output is 250ma at 4 volts. What do ya think?
https://www.amazon.com/dp/B08YJRMS8D/?tag=cpf0b6-20

I would say likely it is about half that output and probably ok for charging junk nimh with as without a smart charger you won't know when it is finished charging. Trickle charging nimh isn't a good thing and if you try and do so the current that is "safer" is well below 100ma so chances are you could cook your batteries unless you know the capacity they have left in them and calculate putting that much back in and shut down charging after a certain time.

 

[LEDrock]

I would say likely it is about half that output and probably ok for charging junk nimh with as without a smart charger you won't know when it is finished charging. Trickle charging nimh isn't a good thing and if you try and do so the current that is "safer" is well below 100ma so chances are you could cook your batteries unless you know the capacity they have left in them and calculate putting that much back in and shut down charging after a certain time.

Trickle charging isn't good for NiMH? I always thought charging batteries over a longer period of time made them last longer than more rapid charging. Or are you referring only to the part about how it could lead to overcharging?

 

[Lynx_Arc]

Trickle charging isn't good for NiMH? I always thought charging batteries over a longer period of time made them last longer than more rapid charging. Or are you referring only to the part about how it could lead to overcharging?

Overcharging happens with nimh when the charger cannot detect the voltage rise and drop and stop charging and timer based chargers don't take into account either partial discharge or lower capacity and the amount of charge put in a battery can be more than it can take and it overcharges it some. Chargers that put 2 nimh cells in series (2 per channel) can degrade cells that are mismatched in sets that is capacity differs either naturally or because the cells were disharged different amounts and one cell can be half full while the other is empty and it may overcharge the half full one trying to completely charge the empty one.
These problems is why I've moved on to lithium ion chemistry even though it is not as safe you don't have problems with cells in series with different capacities and internal resistance damaging cells. I've tossed a dozen nimh cells that were damaged in use in devices that drained them too low and weakened a cell or two.

Trickle charging by definition is an endless charge rate that is designed in mind to keep a battery full to counter self discharge and harmful to nimh cells and unneeded in LSD nimh cells that have low self discharge. it is advantageous with nicad and lead acid batteries but a no-no with nimh and lithium ion cells which usually need a high enough charge rate for them to be detected as full.

Typically solar cells that are nimh either are close or more than the capacity the charger can put out in a day and disposable anyway. If they are overcharging by trickle not a big deal as they are cheap and disposable anyway but overcharging expensive nimh like Eneloops can degrade them and they aren't as disposable.
In other words I would not recommend going super cheap on charging nimh if you aren't going to use super cheap nimh batteries in the charging setup.