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DIY - NI-Mh battery charger, no delta-v and no delta-t.

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Avelino Sampaio:
Hello everyone.

I'm starting a project for a charger, with characteristics very different from traditional chargers. I want the charge to be defined by the battery itself. Observe the circuit in image 1 - see through the calculations presented, that the charge decreases as the battery voltage rises. See image 2, the behavior of the voltage and current I want to obtain. As the final charge decreases, there won't be the possibility to measure the delta V or the delta T. My real intention is that the battery doesn't heat up as it approaches the final charge. I intend to build a circuit in which the cut will be determined, when the battery voltage reaches 1.495v. I am aware that the final charge may not reach 100% but I want to reach at least 80%. My question: what kind of problem might occur with "non-linear" loading?

Image 3: Reply #22 - Final project with excellent result!

TheMG:
In practice this is really not much different than charging the battery through a simple resistor from a regulated 1.5v supply. No need for the opamp transistors etc.

ogden:
Never ever charge batteries w/o current limiting/regulation

Avelino Sampaio:

--- Quote from: TheMG on November 19, 2021, 02:35:51 pm ---In practice this is really not much different than charging the battery through a simple resistor from a regulated 1.5v supply. No need for the opamp transistors etc.

--- End quote ---

Exactly! The idea applied here is very simple. I will use either 5v or 3.3v from an ATX power supply. I will keep the transistors and the opamp, as I intend to carry out other functions with them (cut and protection).

--- Quote from: ogden on November 19, 2021, 02:54:39 pm ---Never ever charge batteries w/o current limiting/regulation

--- End quote ---

The circuit is still pretty incomplete. Current limiting will either be built in with a transistor on the output, cutting off Q1 and I will use a simple fuse on the input.

floobydust:
I found it did not work. With Eneloops or Ikea Ladda LSD NiMH, they need the "top up" charge or else their capacity drops. This is the peak delta-V phase. Then you must lower charging voltage, then stop charging completely.

Steady voltage (after bulk charge) will kill NiMH either from overheating or lack of top-up. I have a cordless phone with constant-current 1.50V (it was NiCd) and NiMH don't do well, they don't last. I just pulled Ladda batteries that had dried out from staying in the charger too much, because they have low capacity without the top-up phase. They do behave differently from Eneloops I have used the same way.

You cannot "trickle charge" NiMH, this is not recommended. Max. is 0.033-0.05C which is way less than NiCd, so they dry out if charged too long or in a CMOS memory-backup use.

Graph taken from https://lygte-info.dk/info/batteryChargingNiMH UK.html