looking for build-in reserve battery charger schematic

[001]

Hi!

It is possible wellknown theme
But not for me

I`m gonna build mains-powered unit with reserve NiMH (4*AA cells, serial wiring)
Any standard circuits?

Thanx!

[David Hess]

There are lots of circuits but the problem is implementing charge termination with a NiMH battery.  Rechargeable lithium cells are much easier to deal with.

[001]

Rechargeable lithium cells are much easier to deal with.

Thanx!

Li-Ions like 18650 looks surplus for RESERVE battery in the 20mA unit
Any ideas?

[David Hess]

Lithium rechargeable batteries can just rely on constant voltage charging with a current limit so there are lots of simple options.  Any voltage regulator that includes current limiting can work and there are many intended specifically to charge lithium batteries.

If you can live with a single cell or cells in parallel, then charge balancing is not an issue either although charge balancing for two cells in series is easy enough.

Some of the charge controllers also support limiting current from the power source if that is necessary so they can both charge the battery and power the device without exceeding any power adapter's current rating.

[001]

Charging for a years is not good for LiIon isnt it? It is RESERVE battery it charges everytime

[Zero999]

Li-Ion batteries can be float charged indefinitely without any problems. Once the battery's voltage is equal to the open circuit voltage of the charger, no current flows apart from what's necessary to replenish the loss through internal leakage. The problem is the charge current needs to be lowered, if the cell voltage is below a certain level, but that's a non-issue if it doesn't need to be charged quickly: a low charge current can be provided whatever the voltage. A dead cell detection circuit should be included, so it doesn't attempt to charge the battery, if it's fallen below a certain voltage, as it can be dangerous.  The only issue I can see is that Li-Ion batteries store better at 50% charge.

If the battery doesn't need to be charge quickly, there's no need to worry about termination of NiMh batteries, which can safely be trickle charged at low currents. A current of ¹/₁₀C is acceptable for 10 to 20 hours and much lower currents, ¹/₃₀C to ¹/₃₀₀C, depending on the manufacturer, can be applied indefinitely. A timer controlling a current source all that's needed to switch from trickle to maintenance charge.
https://en.wikipedia.org/wiki/Nickel%E2%80%93metal_hydride_battery#Trickle_charging

[David Hess]

Charging for a years is not good for LiIon isnt it? It is RESERVE battery it charges every time

That is a larger problem with NiMH rather than lithium.

Lithium batteries degrade in relationship to their fully charged voltage so if you want longer life, the float charging voltage can be lowered sacrificing some capacity.  Whether they (or NiMH) cells will provide the service life you need under any conditions is a different matter and will depend on the specific battery.