Trickle charge C-type NiMh?

[Terry Pinnell]

I have some inexpensive 220 mAH C-type Ni-Mh batteries,
https://www.amazon.co.uk/gp/product/B07BJ85NNZ
The equivalent 3300 mAh C-Types I bought a couple of years ago specified
charging with 330 mA for 16 hours, but these new ones have no charging
data. I have three soldered together and obviously charge in series.

I've been using a trickle charge of 25 mA in the circuit of my curtain
controller, which typically gets used for 4-5 seconds four times a day,
but that doesn't appear to maintain the batteries' power for more than a
month or so.

Q1: Can I safely use that same 330 mA current for these new 2200 mA
types? F0r say 10 hours?

Q2: Is there a safe maximum trickle charge I can use for each type, for
long periods (months)?

Terry, East Grinstead, UK

 

[ehsjr]

On 5/7/2018 4:57 PM, Terry Pinnell wrote:

I have some inexpensive 220 mAH C-type Ni-Mh batteries,
https://www.amazon.co.uk/gp/product/B07BJ85NNZ
The equivalent 3300 mAh C-Types I bought a couple of years ago specified
charging with 330 mA for 16 hours, but these new ones have no charging
data. I have three soldered together and obviously charge in series.

I've been using a trickle charge of 25 mA in the circuit of my curtain
controller, which typically gets used for 4-5 seconds four times a day,
but that doesn't appear to maintain the batteries' power for more than a
month or so.

Q1: Can I safely use that same 330 mA current for these new 2200 mA
types? F0r say 10 hours?

Q2: Is there a safe maximum trickle charge I can use for each type, for
long periods (months)?

Terry, East Grinstead, UK

Q1: Yes you can charge them at 330 mA _IF_ you disconnect when they
reach full charge, or lower the charge rate when they reach full charge.

Q2: "Safe" is a variable. C/10 is generally thought of as a "safe"
trickle rate for "16 hour chargers". So the 16 hour safe rate for
your 2200mAh cells is 2200/10 or 220 mA. You won't instantly kill an
NiMh if you go 1 mA too high, or 1 minute too long, but to be safe you
want to reduce the charge rate, or disconnect, when full charge voltage
is reached. (You can also charge at a far higher rate than C/10, but
that charge _MUST_ be terminated or reduced long before 16 hours. See
the answer to Q1.)

All batteries will go bad in time. Over charging and over discharging
shortens their life. For the over charging issue, I use a lower
charge rate after the battery has been charged at C/10 to full charge
voltage. I use 1.43 volts per cell as the full charge voltage, and
a comparator to switch to a lower rate - say C/50 - when that voltage
is reached. That would fit both of your questions in practical terms.

Ed

P.S. I remember your curtain controller from years ago - quite
impressive!

 

[Terry Pinnell]

ehsjr <ehsjr@verizon.net> wrote:

On 5/7/2018 4:57 PM, Terry Pinnell wrote:
I have some inexpensive 220 mAH C-type Ni-Mh batteries,
https://www.amazon.co.uk/gp/product/B07BJ85NNZ
The equivalent 3300 mAh C-Types I bought a couple of years ago specified
charging with 330 mA for 16 hours, but these new ones have no charging
data. I have three soldered together and obviously charge in series.

I've been using a trickle charge of 25 mA in the circuit of my curtain
controller, which typically gets used for 4-5 seconds four times a day,
but that doesn't appear to maintain the batteries' power for more than a
month or so.

Q1: Can I safely use that same 330 mA current for these new 2200 mA
types? F0r say 10 hours?

Q2: Is there a safe maximum trickle charge I can use for each type, for
long periods (months)?

Terry, East Grinstead, UK


Q1: Yes you can charge them at 330 mA _IF_ you disconnect when they
reach full charge, or lower the charge rate when they reach full charge.

Q2: "Safe" is a variable. C/10 is generally thought of as a "safe"
trickle rate for "16 hour chargers". So the 16 hour safe rate for
your 2200mAh cells is 2200/10 or 220 mA. You won't instantly kill an
NiMh if you go 1 mA too high, or 1 minute too long, but to be safe you
want to reduce the charge rate, or disconnect, when full charge voltage
is reached. (You can also charge at a far higher rate than C/10, but
that charge _MUST_ be terminated or reduced long before 16 hours. See
the answer to Q1.)

All batteries will go bad in time. Over charging and over discharging
shortens their life. For the over charging issue, I use a lower
charge rate after the battery has been charged at C/10 to full charge
voltage. I use 1.43 volts per cell as the full charge voltage, and
a comparator to switch to a lower rate - say C/50 - when that voltage
is reached. That would fit both of your questions in practical terms.

Ed

P.S. I remember your curtain controller from years ago - quite
impressive!

Thanks, Ed, much appreciate your very thorough answer.

Poor performance prompted me to buy more batteries, but I couldn't find
the original 3300 mAH types. (Others were far more expensive; maybe I'll
have to expect replacing these cheaper ones regularly.)

The circuit for my venerable curtain controller (2004 completion)
includes a constant current source charger adjustable with a preset.
After charging and installing the 2200 mAH batteries I found the trickle
current was down at 25 mA instead of the 35 mA I'd originally set. I
reckon it may have been inadvertently altered and that contributed to
the sluggishness of the action I was seeing. And as the circuit relies
on microswitches at the extreme open/closed positions to switch the
battery power to the motor, it only takes one unnoticed failure to drain
them completely.

Anyway, shortly after my first post I upped the trickle to 40 mA and
form a day or so after that I've been getting sweet operation again.
https://www.dropbox.com/s/wiv9tzmjr6hqsd1/20180509-CurtainController-2.mp4?dl=0

The three series 3300 mAH are now on the shed bench. I'm undecided
whether to:
- wait until I need them, when the 2200s fail, and then just fully
recharge, meanwhile operating curtains manually.
- or dust off my stuff and make a timer circuit to apply constant
current 330 mA for 16 hours (from my variable power supply) via a relay
every few weeks.

Downside of the latter is that it risks tempting me back into
electronics stuff and away from my current long-standing obsession with
video/DVD making ;-)

Terry, East Grinstead, UK

 

[ehsjr]

On 5/9/2018 5:34 AM, Terry Pinnell wrote:

ehsjr <ehsjr@verizon.net> wrote:

On 5/7/2018 4:57 PM, Terry Pinnell wrote:
I have some inexpensive 220 mAH C-type Ni-Mh batteries,
https://www.amazon.co.uk/gp/product/B07BJ85NNZ
The equivalent 3300 mAh C-Types I bought a couple of years ago specified
charging with 330 mA for 16 hours, but these new ones have no charging
data. I have three soldered together and obviously charge in series.

I've been using a trickle charge of 25 mA in the circuit of my curtain
controller, which typically gets used for 4-5 seconds four times a day,
but that doesn't appear to maintain the batteries' power for more than a
month or so.

Q1: Can I safely use that same 330 mA current for these new 2200 mA
types? F0r say 10 hours?

Q2: Is there a safe maximum trickle charge I can use for each type, for
long periods (months)?

Terry, East Grinstead, UK


Q1: Yes you can charge them at 330 mA _IF_ you disconnect when they
reach full charge, or lower the charge rate when they reach full charge.

Q2: "Safe" is a variable. C/10 is generally thought of as a "safe"
trickle rate for "16 hour chargers". So the 16 hour safe rate for
your 2200mAh cells is 2200/10 or 220 mA. You won't instantly kill an
NiMh if you go 1 mA too high, or 1 minute too long, but to be safe you
want to reduce the charge rate, or disconnect, when full charge voltage
is reached. (You can also charge at a far higher rate than C/10, but
that charge _MUST_ be terminated or reduced long before 16 hours. See
the answer to Q1.)

All batteries will go bad in time. Over charging and over discharging
shortens their life. For the over charging issue, I use a lower
charge rate after the battery has been charged at C/10 to full charge
voltage. I use 1.43 volts per cell as the full charge voltage, and
a comparator to switch to a lower rate - say C/50 - when that voltage
is reached. That would fit both of your questions in practical terms.

Ed

P.S. I remember your curtain controller from years ago - quite
impressive!

Thanks, Ed, much appreciate your very thorough answer.

Poor performance prompted me to buy more batteries, but I couldn't find
the original 3300 mAH types. (Others were far more expensive; maybe I'll
have to expect replacing these cheaper ones regularly.)

The circuit for my venerable curtain controller (2004 completion)
includes a constant current source charger adjustable with a preset.
After charging and installing the 2200 mAH batteries I found the trickle
current was down at 25 mA instead of the 35 mA I'd originally set. I
reckon it may have been inadvertently altered and that contributed to
the sluggishness of the action I was seeing. And as the circuit relies
on microswitches at the extreme open/closed positions to switch the
battery power to the motor, it only takes one unnoticed failure to drain
them completely.

Anyway, shortly after my first post I upped the trickle to 40 mA and
form a day or so after that I've been getting sweet operation again.
https://www.dropbox.com/s/wiv9tzmjr6hqsd1/20180509-CurtainController-2.mp4?dl=0

The three series 3300 mAH are now on the shed bench. I'm undecided
whether to:
- wait until I need them, when the 2200s fail, and then just fully
recharge, meanwhile operating curtains manually.
- or dust off my stuff and make a timer circuit to apply constant
current 330 mA for 16 hours (from my variable power supply) via a relay
every few weeks.

Downside of the latter is that it risks tempting me back into
electronics stuff and away from my current long-standing obsession with
video/DVD making ;-)

Terry, East Grinstead, UK

Terry,

At 40 mA you're applying a float charge - not a trickle charge.
(I mention that in case you decide to read up on battery charging
and encounter those terms.) Anyway, you can charge them more
aggressively than 40 mA. 40 mA on a 2200 mAh battery is a charge
rate of 40/2200 ~ .018 or roughly 1/50. That's a good float rate.
It won't overcharge, but will be slow to charge.

If you want (or need) to charge the cells a little quicker, you
can add a simple circuit that would allow you to charge them at
over double the 40 mA until they reach full charge, and then
automatically reduce the rate back down to ~40 mA. If you
want, I'll post an ascii schematic here, or I can send a .jpg
to your email.

Ed

 

Thanks Ed. Yes please, if you have a circuit to hand I’d appreciate seeing it. However I’m optimistic about the, er, current setup using that float charge of around 40 mA. After all, the batteries are typically supplying full power for only a few seconds daily.

 

[ehsjr]

On 5/11/2018 4:06 AM, terrypingm@gmail.com wrote:

Thanks Ed. Yes please, if you have a circuit to hand I’d appreciate seeing it. However I’m optimistic about the, er, current setup using that float charge of around 40 mA. After all, the batteries are typically supplying full power for only a few seconds daily.

Terry,

If it's working reliably now there may be no advantage to adding
the circuit I was talking about. I'll post it anyway, in case
you find you need it at some point in the future.

Ed

(View in courier font)

The circuit is a simple shunt regulator, designed to draw
roughly 65 mA when the voltage across the battery rises to
a level determined by the ratio of R1 to (R2+R3).
Your supply provides a constant current of 110 mA. About
108 mA of it charges your battery, until it charges to
about 3.92 volts. Then the shunt regulator begins to
conduct. Any current conducted through the shunt regulator
is subtracted from the 110 mA from the supply, and only the
remainder goes to the battery. So when the battery nears
the full charge voltage the regulator conducts and the
result is 108mA - 65mA = 43 mA.



+ --+-----+----+------+
| | | |
| R4 R5 |
| | | |
R1 +----+ |
| | |
| __|_K_/ | All resistors 1/4 watt
| / / \ | R1 = 1000 ohm
| R / \ | + R2 = 1000 ohm
+--/TL431\ Battery R3 = 330 ohm
| ----- | R4 = 120 ohm
| |A | R5 = 120 ohm
R2 | |
| | |
R3 | |
| | |
- --+-----+-----------+


Set your charger to provide 110 mA current. The
battery will charge at that rate until it reaches
about 3.92 volts. When the voltage rises to 3.92 volts,
the TL431 will conduct, pulling about 65 mA through
the 60 ohms of R4 in parallel with R5. That means
about 43 mA will flow into the battery. As the battery
continues to charge, its voltage rises. At 4.29
volts, the charge current to the battery will drop to
about 37 mA.

I picked standard easy to get resistor values. Precision
is not important, but use the values I posted as they determine
the voltage at which the TL431 will conduct. The parallel
combination of R4 and R5 provide 60 ohms resistance in
series with the TL431, protecting it. The TL431 is available
in a 3 lead TO92 package so it's easy to work with. The
maximum allowable current through it is 100 mA, so that 60 ohms
protects it.

 

Ed,

That’s great, thanks a lot. It may be a while before I build it, but that will be my first project for a few years!

Terry, East Grinstead, UK