Much better than the last similar looking unit, and this one is easily hackable to your chosen current by swapping resistors. In this video I did a simple hack for a 24/7 trickle charge.
This unit is worth buying just for the case alone for your own custom charger project.
I get the feeling this is another clone of a clone - where a circuit has been modified without fully understanding the purpose of some of the components. In the case of the strange pair of differently sized parallel resistors, I think it may be based loosely on the previous unit which had a similar arrangement.
As supplied, the unit works as a very simple charger, but the current is high enough to require that you remember to take the cells out of it after a suitable length of charging time. It's too easy to forget that cells are charging and a prolonged period of high current overcharging can be detrimental to cell lifespan.
NiMH (Nickel Metal Hydride) and NiCd (Nickel Cadmium) cells are not like lithium cells, where there is a very decisive end of charge state (usually 4.2V). Instead the smart chargers have to detect a slight change in voltage when a cell is fully charged and bubbles start to form on its electrodes, or they may detect a rise in temperature or stationary voltage. When a NiMH or NiCd cell is fully charged it has a chemical reaction internally that converts the gas bubbles back into liquid, and during this there is heat generated and higher internal pressure.
For fixed current charging at higher currents you do have to avoid overcharging as the chemical reaction that recombines gas to liquid is only capable of handling a low level of charge current.
At very low trickle charge currents, the cells can be left charging continually without affecting their lifespan too much. However, the trade-off is the time taken to charge the cells. Not an issue if they're just being topped up or being kept on standby.
The other advantage of a simple charger like this is that it will handle older cells or ones that have been accidentally over-discharged, where a smart charger will either not recognise their presence, misinterpret them as being non rechargeables and reject them, or terminate the charge far too early.
In the case of accidental over-discharging, a seemingly dead cell can be brought back gently in this type of charger before being charged quickly in a smart charger.
Because the charger doesn't do any tests it will also work with difficult cells like the low capacity ones used in solar lights.
Aside from the hack/mod that I show in the video, you could also just fine tune the current by swapping the 18 ohm resistors for a higher value.
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