NiMH Charger - Critique my Schematic

Thread Starter

Travm

Joined Aug 16, 2016
331
I'm designing a little NiMH Charger, based on a Ti chip BQ2002.
The schematic below is from the DV2002L2/TL2 Datasheet.

Can anyone tell me what the purpose of R15 in this schematic is? To me it looks like if the charger is left connected to the power, even after the IC has ended charging current will continue to flow into the batteries through R15. Even if it is only 5-10 mA.

Also, if it isnt obviously implied by the answer to question above, what happens if I omit R15?

Thanks
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Thread Starter

Travm

Joined Aug 16, 2016
331
Likely just a trickle-charge current to keep the batteries topped off.
Probably nothing significant if you remove the batteries right after they are charged.
Was planning to keep this as a permanently connected circuit in a portable device. The batteries shouldn't discharge anymore than the leakage current through the diode. I'm considering a couple MOSFETs to isolate the power and batteries controlled by a small pic, but trying to keep the cost and complexity down.

But thanks that is more or less what I figured it was.
 

Thread Starter

Travm

Joined Aug 16, 2016
331
Surely you are looking at a 'trickle charge current' of considerably less than 0.5mA rather than 5 - 10mA? 5V, 10K...
That schematic DC can be as high as 25v. It's relative to the number of cells you intend to charge. Realistically for my circuit the current will be very small because I'm using a fixed pack of four cells and DC will be 8v.
Currently I'm onto the next concern which is limiting the voltage on my load circuit to 5v during charging. I think I'll need a zener and resistor but I don't like the idea of a parasitic resistor in my battery powered load. I'll post my schematic later.
 

Thread Starter

Travm

Joined Aug 16, 2016
331
I am planning to use this to power a small audio amp and speaker, maybe more if it works well.

MCU is connected to Bat+ and to TS, both to read voltages. Bat+ to prevent overdischarge, and TS to prevent charging below 0.
Bat + undervoltage causes the Mosfet on output to turn off disconnecting the battery from the circuit (will require reconnected to charge power to re-enable). TS when out of spec, causes the charge inhibit pin to go high, disabling charging.

Big ask but i'm planning on building on this, wondering if anyone is willing to take a look at this and let me know if they see anything blatantly wrong.
 

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