Can I drop in a 1s Li-Ion for a 3s Nimh?

[harrisonpatm]

My wife's baby monitor's battery no longer holds a charge, so she told me to fix it. It takes a small 3s 900mah Nimh pack, so nominal 3.6V. I have a box full (100+ea) of small Li-ion prismatic cells. They're basically the same size, even a bit smaller. So I could easily hotswap it into the monitor's compartment. I already tested externally, the "new" voltage works fine. So, to sum up, it works perfectly in the space, works perfectly for voltage. Problem is charging. If I swap it in, the battery monitor will attempt to charge the lithium cell according to a Nimh profile, and I don't know what that is, even after attempting to google it .

Can someone advise whether this will work at all? I should state I just need it to charge the new lithium battery a little bit. I don't care if it's optimal charging, or whether it's bad for the battery's longevity. Only concern is that I don't want it to overcharge and explode. That's it. If it works fine for a few dozen cycles before the new battery is toast, I really don't care, I'll put in another one, I literally have a box of 100 sitting around not doing anything, and the kids will grow up eventually anyway.

I have already thought about working in a TP4056, but that won't fit in the existing case and my wife doesn't want anything external sticking out of the monitor.

It's killing me that I might have to fix the issue by PURCHASING an 8$ battery online when I have hundreds of pounds of free salvage energy storage sitting around. The pain is real.

 

[Korishan]

Yeah, that'll work just fine. adding the TP unit (tho, a TP5100 would be better) then you could just cut a hole in and hot glue the board so you can plug it in with a usb charger when needed

the circuitry in the baby monitor will probably handle the 4.2V. However, if you're worried about that, you could also get a small buck regulator (as small as the TP unit, smaller even for some) you can put that between the battery and the monitor and have it set to 3.2V. No worries of over-volting the monitor.

 

[harrisonpatm]

I'm not worried about overvolting the monitor. I'm worried that when I plug the monitor in to charge, it'll overcharge the battery and cause it to explode. Isn't there any chance of that happening?

What I can try to check is, plug the monitor into charge (USB5v) and check the leads to the battery compartment. If the voltage being delivered to the battery is anything under 4.2, I'm safe, right?

 

[Korishan]

If you put the TP charger unit in between the cable and the battery, then no. It'll charge up to 4.2V, +/-1%, and stop.

If the monitor currently uses 3 NiMH batteries, max voltage is only going to 3.6V anyways. 1.2V is the max for NiMH battery. This is why I said about the overvolting the monitor. As the circuitry is probably 3.3V logic. 3.6V it would be tolerant of. If they don't have a regulator in the monitor to make sure 3.3V, then a fully charged 4.2V cell could overload the monitor and burn out chip(s). They may not explode, but they could overheat and kill the device in short period of time.
That's why I said to use a regulator/buckconverter on the output as well to make sure of the proper output voltage.

 

[harrisonpatm]

Ah, ok, I understand what you meant. Might work even better than: if I use just the baby monitor's charging circuit, which should top out at 3.6V, then the battery I put in there should also not rise about 3.6V, and then not worry about overcharging the circuit.

In either case, I realized I have some of those cheap 1s single-cell protection boards from Aliexpress, which I can add onto the cell with enough space. That should give the battery an extra layer of protection. I'm also going to do a disassembled charge-discharge-charge test using the monitor's charging circuit, and monitor it to see what's actually happening to the battery. Then I'll have peace of mind. Thanks.

 

[Korishan]

baby monitor's charging circuit, which should top out at 3.6V, then the battery I put in there should also not rise about 3.6V, and then not worry about overcharging the circuit.

Only problem here is that this circuitry doesn't have a high enough low voltage cutoff. 1V is the bottom for NiMH is 1V, which in standard configuration 3V. If the monitor disconnects at this voltage, fine. But if it continues to draw until the battery just won't output anymore, it could pull that lithium cell down <2.5V.

I would recommend testing using a variable output voltage connected to the monitor and see where it cuts out as you lower the voltage. If it dies at >2.8V, then that's perfectly fine. But if it keeps pulling until the battery is dead and can't deliver output, then you need additional protection on the low end.

This is where a generic 1s bms would come in handy. It should disconnect when the cell drops below 2.8V usually.

lol just read the rest of your previous:

I realized I have some of those cheap 1s single-cell protection boards from Aliexpress

 

[Hairyloon]

Ah, ok, I understand what you meant. Might work even better than: if I use just the baby monitor's charging circuit, which should top out at 3.6V, then the battery I put in there should also not rise about 3.6V, and then not worry about overcharging the circuit.

I wouldn't count on that. Nickel batteries like constant current chargers and it may turn up the voltage as high as it can in order to maintain that current. What's the voltage of the power supply?

In either case, I realized I have some of those cheap 1s single-cell protection boards from Aliexpress, which I can add onto the cell with enough space...

I should've read that first: it renders my comment largely redundant.

 

[harrisonpatm]

I wouldn't count on that. Nickel batteries like constant current chargers and it may turn up the voltage as high as it can in order to maintain that current..

I forgot I had this thread up,so thank you for your comment. I manually monitored its charging for a day, and it behaved exactly as you said: slowly raised voltage to maintain current, all the way to 4.2c when the bms kicked it off.

With that info, I just decided to give up and..
Ugh... purchase a Nimh replacement. I didn't want to have to rely on a teeny Chinese mass produced circuit to correctly cut off charging every time, while I was sleeping. Not worth it.