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I am looking to purchase a remote water tank level monitoring system an Aquatel D110 Wireless Tank Level Monitor ( http://www.rainharvest.com/aquatel-d110-wireless-tank-level-monitor.asp ) - it uses 4 AA batteries.-

Can I use a 5v solar charger (power bank) for smart phones. Can I wire direct from the USB output of the 5v solar smart phone charger to the 4 AA battery pack of the water tank level monitoring system (minus the batteries)? such as a https://www.amazon.com/YOUNGFLY-20000mAh-Waterproof-Dust-Proof-Shock-Resistant/dp/B01IH304ZM/ref=sr_1_11?ie=UTF8&qid=1515518609&sr=8-11&keywords=solar+charger+for+cell+phone

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A 5 volt power bank cannot be used to charge a 6 volt battery (or four 1.5 volt cells forming a 6 volt battery). The charging source must have a higher voltage than the battery being charged - exact details of the charging procedure depends on the battery chemistry.

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A NimH batter requires about 1.45V/cell to fully charge at charge current of C/10 or less. For a 2500 mAh cell that's about 250 mA. As I charge rises Vfully-charged also rises BUT you then run the risk of overcharging.

4 cells thus need 4 x 1.45 = 5.8V.
With 5V available 4 cells get 5V/4 = 1.25V each
BUT 3 cells get 5V/3 = 1.66V each.
SO a 5V power bank WILL charge 3 cells but not 4.
A fully charged NimH cell at light loads (< to << say C/10) delivers about 1.3V initially dropping rapidly to 1.25V and then settling down to around 1.2V for a good part of the discharge.
So, 3 cells gives 3.9V initially dropping to 3.75V and then 3.6V.
Whether 3 cells will operate your equipment is "TBD".


The D110 operates from 4 AA cells. If it is well designed this means it should work when they are almost depleted = 4V or fully depleted =~ 3.6V.
When new Alkaline cells produce > 1.6V so 4 gives 6.4V SO 5V from a powerbank will operate the D110 IF available current is adequate.

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As peter has said, you won't be able to charge a 6v battery with your 5v supply. However, as you may be able to run the Aquatel D110 directly from the solar power bank(with no batteries). Its designed for 6v, but 5v may be sufficient. The Aquatel will not be damaged by too low a voltage, just make sure not to hook it up backwards.

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