Switching from Nicad battery to Lipo: Am I doing this right?

[Nohippychicks]

I sent a request direct to Yaesu...

THAT is very cool.  I would have thought they'd just laugh that off, but if you're in the industry I trust what you say!  Thank you for that.

I would start by replacing that charge/protection module...

Do you have a model I should look into that you trust?  I have one en route, but options are nice for down the line.

[thm_w]

That looks like a good 4AH cell now that I see it, had just assumed it was an 18650.

I would start by replacing that charge/protection module with a charge only version, your battery already has a protection circuit in place.

The protection circuit in the charge/protection module maaayyy be causing the issue as it isn't designed for your battery in particular.

Can bypass the circuit instead of replacing the module. Connect B- to Out- if the board is the same as mine, which will bypass the FET.
If you run into further issues may need to increase capacitance on input of boost converter.

Just plugging in  a vanilla guesstimate of 100R tops for the Rds of the output FET, and overcurrent voltage detector of 150mV, that puts the max current in the ballpark of 1.5A, min. Perhaps up to something like 10A realistically, on the high end (unless it has charge pumps for the gate driver). This is just guesswork from what I understand in there. I just can't figure out that datasheet. It's a doozy. I suppose there's some oscillation of this circuit if that is what is creating the 1.1V.  It 'posedly has some hysteresis/smarts, but damned if I can read that schoo.

You can see above I had calculated 3A.

[KL27x]

^Yeah, I didn't see that until later. You had it figured out. I was initially confused by OP's "schematic" and voltage readings. I was sure he had it hooked up incorrectly.

Anyhoo, this current limit of circa 3A is why this beautiful piece of kit for the $ is not very versatile. I think you need li ion or lipo battery smaller than matchbook before this is applicable. I want the low voltage cutout more than the max current, unless that max current is adjustable. If you want super small current limit to protect the circuit or battery, you can select the fuse with the exact rating you want.

What I would be inclined to do is to cut the voltage sense trace off the SOT part which measures the shunt current and ground that pin. This way, you retain the low voltage cutout, of course. It should be that simple. And it might be. But of course all sorts of funny things can happen due to the software/hardware algorithm in there doing all the weird stuff.

Addendum: You could maybe even use that pin as a RESET or on/off pin, for interface with any micro or whatnot. 150mV is above the threshhold for CMOS digital output low, anyway. Not sure about TTL or other stuff. But there is probably some criteria that must be met before it will turn back on, otherwise a short on the output side would make the output oscillate until the battery was drained below the line sense cutoff.... This would be weird, and it is why I still find it odd that OP measured 1.1V on the other side of the FET. The max current cutout on the circuits I have thus far encountered have to be reset with something like 4+ volts detected across the battery terminals (by plugging in the battery charger, temporarily, for instance). This could be the case for a simple hardware circuit, but this chip wears fancier pants than that. To get 1.1V out of a 40A battery, there's some high frequency oscillation going on. OTOH, his battery is so huge, maybe it resets the circuit just by being there?

[edavid]

The tiny boost converter has no chance of powering a tranceiver.  You should make a 3S battery pack and run it directly.

You will also need a 3S protection board and a 3S charger.

[TERRA Operative]

Do you have a model I should look into that you trust?  I have one en route, but options are nice for down the line.

I like these generic ones, same as what you used, but without the protection circuit.

https://www.ebay.com/sch/i.html?_from=R40&_nkw=tp4056

I usually stick a little heatsink on the chip if charging at higher currents (They will do up to 1A) as they do get a little toasty.

[Nohippychicks]

As an update, I got the Adafruit charge circuit installed and everything seems to be running just fine now.   I haven't put the case of the battery back together yet, so everything is delicately hooked together while I go through a discharge cycle of the battery, then a bunch of on/off cycles of the radio before I make things a little more permanent and start playing around with this thing to see if I can get a radio check with one of my other radios. 

I like these generic ones, same as what you used, but without the protection circuit.

https://www.ebay.com/sch/i.html?_from=R40&_nkw=tp4056

Great, thanks!!