I'm doing some testing of the battery charging that my bluetooth intercom does to the small LiPO cell within (400mAh).

I am using a hobby charger linked to my PC which worked great for the discharge as you can watch the graph build.
When I came to recharge it I realised that this hobby charger does a terrible job of recharging LiPo's. I thought the charger was fully programmable but it seems it is not. You have a small window of voltage adjustment for each battery type and that's it.
So now my only option seems to be to use my bench PSU and DVM to fully charge this battery manually.

The desired charging scheme is 0.2C constant current until 4.2 volts is reach and then switch to constant voltage until the charge current drops to 0.05C.
So that's 80mA CC untill 4.2v is reached and then hold CV until current drain drops below 20mA.

Nearest thing I can find would be an adjustable TP4056 Charger Module but it'd be great is there was an actual product that could do this which recorded the mAh charge and maybe interfaced with a PC to draw nice graphs or at least store measurements.

Is there anything on the market or is it time to learn Arduino?

Many thanks for reading, Rich.

 

Analog will do just fine.
First off, don't charge your LiPos to 4.2 Volts if you expect them to last.
~3.9 to ~4.0 will double the number of Charge/Discharge Cycles that you will get, (~6-700 cycles),
the same goes for the lowest acceptable Voltage, don't go below ~3.6 Volts to extend life.
But, of course, you will only get ~80 to ~90% of its rated Amp/Hr. Capacity,
you decide which compromise works for you.
Life expectancy is also shortened by drawing extreme levels of Current

There is no need to turn off the charger if you go with the lower suggested Voltage Range.
Check out the attached Schematic, you'll need to adjust Resistor Values to suit your needs.
Bypass Capacitors are not shown, make sure you use them to prevent possible oscillations.
Download the PDF for the LM-317 for more application tips.

 

Thanks for the circuit diagrams.

First off, don't charge your LiPos to 4.2 Volts if you expect them to last.
~3.9 to ~4.0 will double the number of Charge/Discharge Cycles that you will get, (~6-700 cycles),

That might explain why my hobby charger stopped at just 3.97v, in which case there's no need to make an alternative.
From looking at the graphs I've made I can certainly see what your saying. When discharging the voltage drop is a straight line until around 3.5 volts at which point it starts to drop away faster.
When charging (on the hobby charger) the line is straight until the cut off, so I'm guessing that is in the safe zone for longer battery cycle life.

The reason I'm doing this is to understand why its' run time has suddenly reduced substantially.
Before starting this investigation I allowed it to charge fully whilst still wired in the bluetooth device and I monitored the battery voltage. At 4.06 volts the screen showed full for a few moments and then turned off. What I then noticed was that it carried on taking charge even though it looked to have finished. Around 30 mins later it had reached 4.23 volts and then the battery had stopped gaining voltage - (Maybe this is a design oversight?).

I then snipped the battery wires and connected it to my hobby charger to run a discharge cycle. It slowly ran down until 3.00 volts at which point it stopped and had recorded 329 mAh, which I'm happy with.

Following this I did the recharge program but it appeared to stop short at just 3.97v (and recording just 179 mAh??).
So it seems that if I want to get near the full capacity out of this cell I need to go all the way up to 4.2 volts.

The values that I originally posted were taken from a datasheet of a similar battery btw. The one in my device is unbranded so I can't be sure but I think these are all pretty similar. It's just a little foil slab but does have a protection circuit on it.

So anyway..., earlier this evening I topped off the charge of this cell on my bench PSU - 4.2 volts until the current had dropped to below 20 mA. Then I started another discharge measurement but this time the cell came up short - just 289 mAh.
So I'm not sure what's going on now.

Maybe this battery is just weak or damaged now so the only option is to replace it.
I think I'll cycle it a few more times on the hobby charger to see what it does and decide from there.

I have found replacements on ebay (from China) so I'll probably get one or two ordered. I can get what looks like a direct replacement and also one that is the next size up (which I think might just fit). If so it'll give a 50% capacity boost which would be nice.

 

Are you working with a single Cell ??, or 2 or 3 Cells ??
If you have multiple Cells, there's a strange thing about LiPos in that
once an individual Cell becomes fully Charged,
it will seriously reduce the current feeding through to the other Cells,
so eventually one, or some of the other Cells, will never receive a Full Charge,
unless you do a Cell Balance virtually every time you Charge the Battery.

That's what the second Circuit is for.
It's a somewhat unconventional way of insuring that all the Cells are Balanced all the time.
It simply places a fine tune-able "Shunt-Style" Voltage Regulator across each Cell.
These work in conjunction with the Current Limiter to force any low Cells to the Full set Charging Voltage,
rather than the weak-kneed ~10ma balancing you get from most Hobby Chargers.
With this setup,
the weakest Cell will still get the maximum Current Limit allowed,
until it is completely Charged, every time.
The main Voltage Regulator can be safely set higher than the maximum safe Voltage,
You could even eliminate the main Voltage Regulator, and just rely on the Shunts,
then the Current Limiter will always be supplying its maximum allowed Current,
even when the Battery is Completely Charged,
because the Shunt Regulators will simply dissipate that Current as Heat after the Cells attain Full Voltage.

With Hobby LiPos, ( extremely high Current Rated ), you get what you pay for ........... some times .......
I'll stick with a company called "MaxAmps", they're not cheap, but their quality is very high.
I really don't know about the garden variety Cells for Electronics, but I do know they're made differently.
You probably don't really need a high Current Battery.
.
.

 

This one is just a small single cell, it's a 502535 (which is the package size I think).
Looks like this..


Another thing I'm planning to do is power the intercom via a PSU and DMM so I can see how much current it actually takes when playing music. That way I'll know what capacity I actually need to get 8 hours run time.
When it was new it could do this but it is over a year old now, and god knows how old the cell is.

Another plan is just wire it to run from an external battery, then package a 14500 or 18500 in a small box so it can fit to my crash helmet and power this intercom for days at a time, which would also make swapping the battery easier too.

 

From your tiny picture, it looks like you have a single cell because it is marked as being "3.7 V".
It also appears that it has built-in Charge/Dis-Charge protection circuitry,
so it's hard to say what might really be going on inside the bare Cell,
and being ~2 years old, the capacity is probably down to ~70 or ~80%,
which will depend upon whether or not it's been accidentally abused,
like being run dead, and left in a discharged condition for more than a few days.
LiPos don't like being out of their "Comfort Zone" of ~3.6 to ~3.8V ,
this will gradually reduce their capacity as the number of hours they spend outside of this range accumulates.

And then, there is the question of manufacturing quality ...........

It has also occurred to me that that "Protection Circuitry" could also contain proper charging control circuitry
which is designed to be supplied with ~5 Volts from a USB Connection,
but unless you can access the actual Cell connections for measurement,
I would not risk trying to charge it at 5V.

If you do want to try 5 Volts, do it OUTSIDE ON A CONCRETE SURFACE.

Remember ...... LiPos can catch on fire when abused,
and it's a chemical-reaction fire,
you can't put it out,
you can only cover it with Sand and wait for it to burn out.
.
.

 

I'm pretty sure the circuit is just protection and not a charge controller. I did monitor the terminals whilst it was being charged by the intercom itself (from a USB phone charger) and the voltage was not 5 volts, it rose steadily to just 4.10 volts and then indicated full.