Trickle charger advice

[Alexander Avtanski]

Hello,

I'm working on a hobby project (something like a really fancy timer).
The thing will be powered mostly from the grid, but when necessary I
should be able to unplug it and use it on internal power. My plan is
to use a rechargeable NiMH 9V battery, that is being kept topped off
by a trickle charger when the thingy is plugged in (that would be,
like, over 95% of the time).

I don't have any practical experience with building battery chargers.
So, if I'm making wrong assumptions somewhere, please let me know.
Here is my plan and line of reasoning:

Let say I use a 12V DC external power supply, and charge the battery
through the simplest possible trickle charger I could come up with - a
1 diode and 1 resistor circuit (no need for diagram here). A NiMH
battery, according to Wikipedia should have about 30%/month self-
discharge rate. For an arithmetically-challenged person like me this
means that a 150mAh battery (seems pretty typical for a 9V NiMH) will
have about 0.07mA self discharge current. Leaving some margin,
something like 0.3 to 0.5 mAh trickle charging current should keep the
battery nicely topped off. With 12V and a fully charged 9V battery
and taking off a 0.7V voltage drop over the diode I'll have 2.3V
voltage difference. This means that the resistor should be in the
5-10k range. That is ignoring the internal resistance of the battery
(which I don't have a clue about, but it should be insignificant
compared to the resistor).

Do you see any problems with this? Will that thing be safe, or it
will explode in a spectacular fireball after couple of hours? Do you
have a better idea for building the charger, that is not too complex?

Thanks,

- Alex

 

[dalai lamah]

Un bel giorno Alexander Avtanski digitò:

Let say I use a 12V DC external power supply, and charge the battery
through the simplest possible trickle charger I could come up with - a
1 diode and 1 resistor circuit (no need for diagram here). A NiMH
battery, according to Wikipedia should have about 30%/month self-
discharge rate. For an arithmetically-challenged person like me this
means that a 150mAh battery (seems pretty typical for a 9V NiMH) will
have about 0.07mA self discharge current. Leaving some margin,
something like 0.3 to 0.5 mAh trickle charging current should keep the
battery nicely topped off.

There are two currents of thought. Some say that if you use trickle
charging currents between C/20 and C/40 you are in the clear; most
manufacturers declare this kind of trickle currents in their datasheets,
for example:

http://www.varta-microbattery.com/en/mb_data/documents/data_sheets/DS55615.PDF

In this case the capacity is 150 mAh and the trickle current is 4.2 mA,
i.e. about C/36.

Some others (few) say that a continuous charge of C/20 could lead to a
long-term degrade of the battery, and advise to use a much lower trickle
current, between C/100 and C/500. I don't have enough long-term data to
categorically disprove these claims, but they don't look very convincing to
me.

 

[linnix]

Un bel giorno Alexander Avtanski digitò:


There are two currents of thought. Some say that if you use trickle
charging currents between C/20 and C/40 you are in the clear; most
manufacturers declare this kind of trickle currents in their datasheets,
for example:

http://www.varta-microbattery.com/en/mb_data/documents/data_sheets/DS...

In this case the capacity is 150 mAh and the trickle current is 4.2 mA,
i.e. about C/36.

Some others (few) say that a continuous charge of C/20 could lead to a
long-term degrade of the battery

Suggestion is to pulse (boost) charge it, not continuously (cook)
charge it.

 

[Mike V]

Un bel giorno Alexander Avtanski digitò:


There are two currents of thought. Some say that if you use trickle
charging currents between C/20 and C/40 you are in the clear; most
manufacturers declare this kind of trickle currents in their datasheets,
for example:

http://www.varta-microbattery.com/en/mb_data/documents/data_sheets/DS55615.PDF

In this case the capacity is 150 mAh and the trickle current is 4.2 mA,
i.e. about C/36.

Some others (few) say that a continuous charge of C/20 could lead to a
long-term degrade of the battery, and advise to use a much lower trickle
current, between C/100 and C/500. I don't have enough long-term data to
categorically disprove these claims, but they don't look very convincing to
me.

My understanding is that long term damage is caused by overheating. I
wouldn't go over C/40 for trickle charge. As self discharge depends
on temperature I probably wouldn't go much less than this either.

We service stuff we made 10 to 15 years ago which uses C/40. I change
the back up battery if it's been installed for 5 years or more. I've
yet to see premature battery failure.

 

[Alexander Avtanski]

I'm working on a hobby project (something like a really fancy timer).
The thing will be powered mostly from the grid, but when necessary I
should be able to unplug it and use it on internal power.  My plan is
to use a rechargeable NiMH 9V battery, that is being kept topped off
by a trickle charger when the thingy is plugged in (that would be,
like, over 95% of the time).

[...]

Hello again and thanks for the answers.

So, there is no major issue charging with the diode/resistor charger.
Anything below C/40 should be pretty much safe and wouldn't damage the
battery, at least not too soon and not catastrophically.

Pulse charging also sounds very attractive and may turn out to be
better. It is also easy for me to implement, since I have a PIC
microcontroller already in the device and I can ask it to control the
pulses for me, so almost no extra parts. What is stopping me from
using pulse charging, however, is that it seems to be a bit more
dangerous (what if the microcontroller hangs and leaves the charger in
a high current state, or if something else goes wrong?). The resistor
charger looks foolproof to me, and if there is nothing I miss I would
go with it, even if it might not be the best for the battery - so
what, a 9V NiMH is not that expensive, big deal if I have to change it
once every few years.

Any other considerations?

Thanks,

- Alex

 

[linnix]

I'm working on a hobby project (something like a really fancy timer).
The thing will be powered mostly from the grid, but when necessary I
should be able to unplug it and use it on internal power. My plan is
to use a rechargeable NiMH 9V battery, that is being kept topped off
by a trickle charger when the thingy is plugged in (that would be,
like, over 95% of the time).

[...]

Hello again and thanks for the answers.

So, there is no major issue charging with the diode/resistor charger.
Anything below C/40 should be pretty much safe and wouldn't damage the
battery, at least not too soon and not catastrophically.

Pulse charging also sounds very attractive and may turn out to be
better. It is also easy for me to implement, since I have a PIC
microcontroller already in the device and I can ask it to control the
pulses for me, so almost no extra parts. What is stopping me from
using pulse charging, however, is that it seems to be a bit more
dangerous (what if the microcontroller hangs and leaves the charger in
a high current state, or if something else goes wrong?). The resistor
charger looks foolproof to me, and if there is nothing I miss I would
go with it, even if it might not be the best for the battery - so
what, a 9V NiMH is not that expensive, big deal if I have to change it
once every few years.

Any other considerations?

Just add temperature monitoring and you are almost doing a real
charger.
A 11 cents themoresistor will work with any uC for temperature. I am
about to buy a thousands 100K themoresistors.

 

[linnix]

If it self-discharges at 30% per month, you should be able to keep it
happy at a charge rate of 100% per month. That's equivalent to C/744.
So something in the C/500 range should be fine.

Some of the manufacturer data sheets I've read for NiMH cells say that
this chemistry's efficiency of charge acceptance is very poor at these
low charge rates. Almost all of the energy tricked in at such low
rates is wasted (ultimately as heat, which probably isn't an issue at
such low currents) and very little ends up rebuilding the
electrochemistry. A trickle at C/500 might not be enough to overcome
self-discharge.

One data sheet suggested that if maintenance charging is to be
performed, it's preferable to use a low-duty-cycle pulse charging
method at a higher current level. As an example (from memory, not
from the data sheet) - try a one-second pulse of current at the C/5 to
C/10 rate, once a minute or so.
[/QUOTE]

As long as you monitor the voltage and temperature, you can pulse
charge it at C or even higher. Just use long enough delays between
pulses.

 

[Alexander Avtanski]

[ ... ]

Admittedly NiCd is a different chemistry from the long-term charging POV,but I
have used C/100 as a charge rate for the 9V backup batteries in reticulation
controllers for over a  decade without any issues.  For your NiMH I couldn't see
any reason to go higher than C/100 unless their charge acceptance is problematic
at these rates.

Well, I said NiMH just because that is what I saw in the local
RadioShack the last time I looked. It doesn't have to be NiMH, if you
think NiCd might turn out to be better.

What do you think?

- Alex

 

[linnix]

[ ... ]

Admittedly NiCd is a different chemistry from the long-term charging POV, but I
have used C/100 as a charge rate for the 9V backup batteries in reticulation
controllers for over a decade without any issues. For your NiMH I couldn't see
any reason to go higher than C/100 unless their charge acceptance is problematic
at these rates.

Well, I said NiMH just because that is what I saw in the local
RadioShack the last time I looked. It doesn't have to be NiMH, if you
think NiCd might turn out to be better.

What do you think?

- Alex

NiMH is friendlier to the environment. Cadmium in the Cd is extremely
toxic. If you already have the microcontroller, why not try a little
harder in using NiMH?

 

[Alexander Avtanski]

[ ... ]

NiMH is friendlier to the environment.  Cadmium in the Cd is extremely
toxic.  If you already have the microcontroller, why not try a little
harder in using NiMH?

So be it, then - NiMH.

About the charging mode: what worries me with the pulse charging is
the remote chance that the PIC might go haywire - power surges,
interference, who knows. I run it at 20MHz and I noticed that
sometimes I get resets if I touch the wrong pin with my hand (might be
something loose in my prototype board). And if it does fail, it might
just blow the battery - something I want to avoid at all cost, because
I plan to be using this as a clock at home too. It will stay powered
on all the time and unmonitored potentially for days. The simpler
charger looks to me completely foolproof - I cannot imagine a way for
it to fail catastrophically. What do you think?

On the other hand, if I was doing a dedicated battery charger, sure, I
would go with pulse charging and a PIC with simpler program, that is
dedicated only to monitoring the battery. Also, such a device would
not stay unattended for long periods of time.

I don't know... The pulse charging still looks tempting - it would
take me minimal effort to implement it too. Hmmm...

- Alex

 

[[email protected]]

I'm working on a hobby project (something like a really fancy timer).
The thing will be powered mostly from the grid, but when necessary I
should be able to unplug it and use it on internal power.  My plan is
to use a rechargeable NiMH 9V battery, that is being kept topped off
by a trickle charger when the thingy is plugged in (that would be,
like, over 95% of the time).

[...]

Hello again and thanks for the answers.

So, there is no major issue charging with the diode/resistor charger.
Anything below C/40 should be pretty much safe and wouldn't damage the
battery, at least not too soon and not catastrophically.

Pulse charging also sounds very attractive and may turn out to be
better.  It is also easy for me to implement, since I have a PIC
microcontroller already in the device and I can ask it to control the
pulses for me, so almost no extra parts.  What is stopping me from
using pulse charging, however, is that it seems to be a bit more
dangerous (what if the microcontroller hangs and leaves the charger in
a high current state, or if something else goes wrong?).  The resistor
charger looks foolproof to me, and if there is nothing I miss I would
go with it, even if it might not be the best for the battery - so
what, a 9V NiMH is not that expensive, big deal if I have to change it
once every few years.

Any other considerations?

Thanks,

- Alex

There really is no such thing as a 9V NiMH since the cells are
nominally 1.25V.

The self discharge nowadays is not as bad as 30% per month. That is
the number when they first came out. The chemistry is different these
days. If you used Sanyo ENELOOPS, the self discharge wouldn't be an
issue at all.

I really don't know if dendrites are an issue anymore, which is the
reason people didn't keep NiMH or even Nicads under constant charge.

Personally, since you have the grid, this sounds like an application
for a gel cell. They like constant charging. Further, they self
regulate their current, so all you really need to do if provide a
constant float voltage. Charge the battery through a Schotky diode,
and you have your power back up switch solved.

 

[Jasen Betts]

If you can handle the size and weight, lead-acid batteries are very
well suited for standby charging. You can get them in fully sealed
versions, so there's no worry about acid splashing about.

I have never seen them smaller than a few Ah, though, so they may not
be an acceptable replacement for a 9V battery.

The smallest I've seen was the size of 2 AA cells, a single cell,
it was used to power a walkman.