Charging 8 NiMH 2600 mAh in series. Am I on the right track?

[Ronn1954]

Hi all,

I have an old flat, portable cassette recorder (Philips D6350) that I would like to use with batteries, not AC.
But I would like to avoid taking out the 6 C cells to charge them.

On the shematic of the cassette player I see that, when powered with AC the trasformer puts out 12V.
So I was thinking to replace the 6 C cells with 8 2600mAh NiMH AA cells (brand is VARTA).

1. use a 12Vdc 1A power supply from an old electronic device (a switch or router or something like that).
2. use 16 ohm, 2 watt resister in series with the 8 AA cells.
3. use a timer at the electrical outlet to cut the power to the 12V supply after a couple of hours.

My calculations are (maybe wrong???):
I calculated for 3 different voltages on the AA cells
1.47 Volts - full charge        ***     1,2Volts     *****     0.8Volts (very discharged)

Voltage drop
@ 1.47 per cell      =  12 - 11.76      = 0.24V
@ 1.2   per cell      =  12 -   9.6        = 2.4V
@ 0.8   per cell      =  12 -   6;4        = 5.6V

Current through the circuit
@ 1.47 per cell     = 0,25V / 16Ohm   =   0,015A
@ 1.2   per cell     = 2.4V / 16ohm      =   0,15A
@ 0.8   per cell     = 5,6V / 16ohm      =   0,35A

Power over the resistor = 2W max.

So, if I am right the current at which the batteries will charge will be 0.35A when discharged.
And the current will decrease while the batteries are charged untill a mere trickle (0.015A) when charged to 1.47V.

I read somewhere that the newer NiMH cells prefer to be charged with a minimum of 0.5A. Is this true? Would my 0.35A be bad for the life of the batteries?

Thanks a lot

Ron

[mariush]

Rechargeable AA batteries will be 1.35v when full, 1.2v nominal.   

Fresh non-rechargeable batteries will be 1.65v, 1.5v nominal.

NiMH cells don't like being charged when they're not fully discharged - if you want them to last longer it's a good practice to fully discharge them and then charge them. No, 0.5A is a bit much. I would say 100-200mA would be safe charging current for modern AA cells.


You have the service manual here : https://archive.org/details/manual_D6350_SM_PHILIPS_EN/page/478/mode/2up

You can see on 2nd page (pages 478-479) in the bottom corner that the transformer output is rectified with diodes and passed to some capacitors and when you put the AC cable in socket, a switch is pressed disconnecting the batteries.

So as far as the cassette player knows, it doesn't differentiate between ac input and batteries, so there's no point going with 8 cells to get 12v DC, you won't get better performance or anything.

You can also see that they use a time delay 630mA fuse on the secondary of the transformer so the current is probably less than 4-500mA , making it 12v x 0.5A = 6 watts maybe, max.




I wouldn't use NiMH, I would use lithium cells. Two lithium cells in series would give you 8.4v peak voltage, 2 x 3.7v = 7.4v nominal.  6 rechargeable C batteries x 1.2v = 7.2v ... if the player works now with 6 rechargeable cells, it will work with 2 lithium cells in series.

So that leaves you with finding a small charger board / module that can charge two lithium cells in series. Here's one : https://www.ebay.com/itm/364722753475

This particular charger board is powered with 5v from USB. There's other models that need a voltage slightly higher than the voltage needed to charge the 2 cells (for example 9v, because the peak voltage for 2 cells in series is 8.4v)

Here's a link for chargers that accept up to around 22v and can charge 1, 2 or 3 in series (select from the dropdown list that you want the module for 2 cells in series ) : https://www.ebay.com/itm/134024292844


If you want , you can add a step up regulator to boost 6v ... 8.4v to 9-12v, but that step-up regulator may introduce some noise in the audio, from the high frequency switching. 

Here's an example .. it takes any voltage up to 24v and outputs 12v : https://www.ebay.com/itm/274714390574

[Phil1977]

I´d also implement 2 lithium cells as mariush has proposed.

The problem with NiMH is they need a certain end-of-charge detection method that practically requires a microcontroller. As long as you use such a smart charging IC, especially the "Low-Self-Discharge"-NiMH-cells are great cells with superb longevity and safety. But to trickle charge them with a resistor doesn't do the job.

For Li-Ion you find infinitely more charging boards and cell types. Just stay away from the charging boards that have no connection to the "middle"-voltage-levels between cells in series. Especially when using cells from dubious sources it´s important to do reliable overvoltage control on all voltage levels in the battery pack.

Disclaimer: These cells can be dangerous. There have been cases of fire, injury and poisoning by Li-Ion misuse. Just to have it said 

[iMo]

FYI - I've been trickle charging my 6 NiMH eneloops 2000mAh in my TRX for at least 15y already, no problem, batteries are still ok. With trickle charge (max 5% of 1C current -> 2000*5%=100mA) you do not care on the max cell voltage. My current is around 75mA afaik (there is a constant current source inside the radio).

[Phil1977]

Eneloops are crazily robust. I could recover a few of them after being reverse charged and now the capacities of all cess in the pack are again in a +/- 2.5% window. My oldest eneloops in use are from 2009.

I would not recommend to do it anyhow.

[Ronn1954]

Thanks for your replies.
I will look into the Li-ion option and charger boards. Maybe I can find them in a local online store.
I also have an old transistor radio (6 x C cells) somewhere lying around, I will put that one on Li-Ion.

(My idea with those old machines is to put a bluetooth module in them, or maybe one of those newer mp3 players (they come with wifi, buetooth, etc. etc...)

But before dumping the NiMH option for the cassette player:
I think that, since the cassette is powerd by 12V when on AC and 9V when on alkaline batteries, I can use whatever voltage in between those two for powering the device. That is why I did choose 8 X Nimh in series. They are close to the 12V power supply that I already have and I don't need a big resistor for the voltage drop.

As far as I see it, with only a resister to limit the current, when the batteries are empty, more current flows, when the batteries are nearly full much less current flows. Isn't that a good thing? Or is constant current a better option and more gentle om the batteries?

On the data sheet of a NiMH AA batt I found on the internet,  I see:
max charge voltage 1.45V
standard charge  270 mA
trickle charge 25 ... 90 mA

And since I will use a wall outlet timer, overcharging wil not be an issue.

[Phil1977]

And since I will use a wall outlet timer, overcharging wil not be an issue.

Look at the following page:

https://www.researchgate.net/figure/Charge-curves-of-NiMH-cells-at-02C-rates-and-different-temperatures_fig4_223211274



The charge voltage of NiMH only indicates a full cell if it´s not too warm. If cell temperature rises for any reason a resistor- or even CC-charger can not recognize when the cell is full, so it continues delivering power and the cell heats up even more.

So the resistor system you proposed may work if the radio never gets warm.

The usual way to solve this problem is to limit the charging current with a constant current source to 0.1C or less. That way the cells won't get to hot and together with a 10h-timer it´s quite a standard charging procedure.

[iMo]

DIY a simple current source (like a resistor+LED+transistor, some caps around), set it to say 50mA, and that's it.
The wall charger's output DC voltage should be by 3-4V higher than the max battery voltage (8*1.4V in your case) in order the current source gets some headroom..

PS: something like this

[Phil1977]

To charge 2000mAh with 50mA takes 40h! But it'd be safe.

I assume with a smaller R2 you could increase the current.

[iMo]

It is not for "charging", that is trickle charging (like 24x365.25)

[iMo]

Also you may try this - with a 12V DC wall adapter it creates an interesting curve (simulation only, try it in situ) at around 11V on the batteries (8*1.4V=11.2V) the trickle charging 50mA current starts to drop down.

So a combination of a charger and trickle-charger, or, I would call it a "safer trickle-charger" (patent pending)

[Ronn1954]

Thank you iMo for your schematic. Looks very good as trickle charger.
I will try it as soon as possible. I will need to order the components online, all electronics shops in and around town are gone now. Modern times I suppose.
Once upon a time I had all these components at home but I gave everyting away when I thought I was done with this hobby...

Just for the heck of it, could someone confirm that my calculations I mentionned before, are valid for the charging with only one resistor of 16 ohm, 8 NiMH cells and power supply of 12Vdc?

I calculated for 3 different voltages on the AA cells
1.47 Volts - full charge        ***     1,2Volts     *****     0.8Volts (very discharged)

Voltage drop
@ 1.47 per cell      =  12 - 11.76      = 0.24V
@ 1.2   per cell      =  12 -   9.6        = 2.4V
@ 0.8   per cell      =  12 -   6;4        = 5.6V

Current through the circuit
@ 1.47 per cell     = 0,25V / 16Ohm   =   0,015A
@ 1.2   per cell     = 2.4V / 16ohm      =   0,15A
@ 0.8   per cell     = 5,6V / 16ohm      =   0,35A

Power over the resistor = 2W max.

Thanks again
Ron

[Andreas]

Hello,

there is nothing wrong with your calculation.
So just some considerations.

- 16 Ohm "high load" resistors are not main stream (E12-Values) so you might consider to use 2 * 33 Ohms in parallel. (or 3 * 47 ohms if it is easier to get).
- the resistors may get hot (wire wound resistors usually have up to 350 deg C at nominal load) so do not place them near your NiMH cells
- The larger the resistor the lower the surface temperature. And if you also want to cover a accidently short cirquit you might consider a 5W type.
- does your 12V supply discharge the battery (or even gets destroyed) when being back powered by the battery without mains supply?
- How are the tolerances of your supply (over temperature)? +/- 5% or even more?
- In any case the charging current for a AA cell has to stay below ~50 mA when being full to avoid cooking the cells (C/50-C/100 with modern NiMH cells) 0.1C is way too much for continous charging.
- I typically calculate with 1.45V/cell at 20 deg C. And -3 mV/cell derating above 20 deg C for charging at a low ohmic supply.

So at 40 deg C you have to calculate with 1.39V / cell and maximum tolerance of your wall adapter to stay below the 50 mA in full state.

- it is essential that during discharging none of the cells gets reverse charged (in this case the cells get destroyed). So I usually discharge them not below 1.15V/cell and switch off to avoid destroying.

For 12 cells I use the attached cirquit as battery protection.

with best regards

Andreas

[iMo]

Basically - when OP wants to charge the batteries via a resistor (btw I do it that way with all my NiMH, Li-IOn, Li-poly batteries since ever) he must use a stabilized constant voltage source, set to a voltage which represents the max total allowed voltage at the battery when fully charged, call it Vcharged.
Like 8*1.39V for example in his case.
So he would need a LM317 set to 11.12V mounted on a small heatsink, for example.

The value of the resistor shall be such the current at the beginning, when the batteries are "discharged", will be lower than the max allowed for "charging".
Like, say, 1000mA (an example only) in his case, therefore the resistor's value will be aprox (Vcharged-Vdischarged)/1A.

You do not need any timer in this case as the charging current at the end, when the battery gets fully charged to the Vcharged, will be close to Zero, because (Vcharged - Vcharged)/Resistor_Value is Zero current.

But, in case you do not have the constant voltage source set to Vcharged, you have to use a different more complex approach.

With the "trickle charging" (I do it with NiMH batteries only) with those <0.05C currents you simply wire the battery via a constant current source set to the 0.02C=50mA in his case for example, and leave it there forever. You do not need a constant voltage power source then.

When you are using the cassette recorder from time to time the trickle charging will sloowly charge the battery up during the time the recorder is off. Of course, when using the recorder every day for 8 hours, the trickle charging will not help much as the battery will never get fully charged..

Anyhow, when I leave my home for a longer time I disconnect the trickle charging wall adapter, to be on the safe side..

PS: below a scenario where the Wall adapter gives 12-20V DC output, and also added a schottky diode D3 before the battery (any 0.3-1A one will do). You may see the battery trickle current even with this primitive current source stays around 50mA with wall adapter's DC voltages from 12V to 20V..

PPS: for a wall adapters with higher output DC voltages, like 15-20V for example, you would need a beefier transistor there, like BD136-138 PNP or similar, as the power loss at the transistor will be a bit higher, like (Ve-Vc)*50mA..