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Ariel Rocholl | 6 Sep 21:46

[EE] Trickle charge current on NiMh

I have a single-item application that requires battery backup. I am
planning, for sake of simplicity, to use 4 cell 650mhA NiMh and keep them in
trickle charge permanently. Simple limiter resistor will put the trickle
charge at 20mA max (<1/30C) which is far below the 1/10C most people comment
as acceptable for trickle charge. The NiMh are high quality Kan batteries.

I saw different recommendations but in most cases manufacturers do not
recommend trickle charging batteries permanently, although most admits that
shouldn't damage the batteries. Therefore I perfectly understand this is not
a recommended practice. Certainly a pulse trickle charge or a monitored
dedicated charge is a more elegant solution.

What I am looking for in the list is real-life experiences on permanent
trickle charging being a real problem for NiMh. I am not thinking on this
for large volume application which may require much better design (and time
dedication), but in a single-item application I am ok assuming the NiMh may
degrade to a point in which it has to be discarded in 2-3 years because of
trickle charge.

Thanks in advance

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Ariel Rocholl
Madrid, Spain
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Brendan Gillatt | 6 Sep 22:03

Re: [EE] Trickle charge current on NiMh


Ariel Rocholl wrote:
> I have a single-item application that requires battery backup. I am
> planning, for sake of simplicity, to use 4 cell 650mhA NiMh and keep them in
> trickle charge permanently. Simple limiter resistor will put the trickle
> charge at 20mA max (<1/30C) which is far below the 1/10C most people comment
> as acceptable for trickle charge. The NiMh are high quality Kan batteries.
> 
> I saw different recommendations but in most cases manufacturers do not
> recommend trickle charging batteries permanently, although most admits that
> shouldn't damage the batteries. Therefore I perfectly understand this is not
> a recommended practice. Certainly a pulse trickle charge or a monitored
> dedicated charge is a more elegant solution.
> 
> What I am looking for in the list is real-life experiences on permanent
> trickle charging being a real problem for NiMh. I am not thinking on this
> for large volume application which may require much better design (and time
> dedication), but in a single-item application I am ok assuming the NiMh may
> degrade to a point in which it has to be discarded in 2-3 years because of
> trickle charge.
> 
> Thanks in advance
> 

I think you should be fine. After all, many motherboards have NiMH
batteries for their BIOS retention power. I can't imagine they use a
complex charging solution.

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Bob Blick | 6 Sep 23:02
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Re: [EE] Trickle charge current on NiMh

Nimh and nicad are completely different with regards to trickle
charging. Unless the individual battery manufacturer specifically states
otherwise, you should assume that continuous trickle charging will kill
nimh cells within a few months.

Also, some of them may not take a predictable amount of charge at 1/30C.

Nimh is the most complicated battery to charge.

How about lithium-ion or variants?

Cheerful regards,

Bob

Ariel Rocholl wrote:
> I have a single-item application that requires battery backup. I am
> planning, for sake of simplicity, to use 4 cell 650mhA NiMh and keep them in
> trickle charge permanently. Simple limiter resistor will put the trickle
> charge at 20mA max (<1/30C) which is far below the 1/10C most people comment
> as acceptable for trickle charge. The NiMh are high quality Kan batteries.
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William "Chops" Westfield | 7 Sep 01:08

Re: [EE] Trickle charge current on NiMh


On Sep 6, 2008, at 2:02 PM, Bob Blick wrote:

> How about lithium-ion or variants?

Is there a reasonably safe way to "trickle charge" Li-ion (especially  
li-poly, since they come in such a variety of form factors)?  In  
theory, you can just apply a current-limited 4V (assuming for backup  
you don't need max capacity) and let the current drop to zero, but  
all the official charging info I've seen is pretty adamant about  
having a time limit.

Also, there's the rumored "2-year lifetime" for Li-ion.  (hey; I've  
actually got some data on that;  My old cell phone "died" in the  
sense that it began showing noticeably less battery lifetime,  
followed by complete battery failure.  It had been in continuous use  
(mostly "standby") for about 4 years.

BillW

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Apptech | 7 Sep 08:49

Re: [EE] Trickle charge current on NiMh

> Is there a reasonably safe way to "trickle charge" Li-ion (especially
> li-poly, since they come in such a variety of form factors)?  In
> theory, you can just apply a current-limited 4V (assuming for backup
> you don't need max capacity) and let the current drop to zero, but
> all the official charging info I've seen is pretty adamant about
> having a time limit.

AIUI it should be perfectly safe to hold most liIon chemistries at a 
terminal voltage slightly less than the maximum set by the temperature at 
the time. Holding them at somewhat less again is even safer. a few percent 
redution can make a significant difference to the fractyion of full charge 
stored. eg a cell rated at 4.3V at full charge will be at less than full 
capacity at 4.2V. Note that I suggested 4.1V in a previous response on this 
thread. By the time you get down to 4V you would be far far below full 
capacity. How much so i don't know.

Related semi random ramblings - feel free to improve where / if off course: 
Note that when charging LiIon from empty you charge at constant current 
until the desired terminal voltage is reached and then charge at constant 
voltage so that the current tapers to zero. For a 4.3V transition voltage 
you are at 2/3 capacity and are 1/3 of the time through the charge cycle. So 
you get 2/3 charge in 1/3 time and the final 1/3 charge in 2/3 time so the 
MEAN rate of energy transfer is 4 times higher in the constant current mode. 
Also, the current to voltage transition point is the best place at which to 
store the battery to get longest lifetime overall.

> Also, there's the rumored "2-year lifetime" for Li-ion.  (hey; I've
> actually got some data on that;  My old cell phone "died" in the
> sense that it began showing noticeably less battery lifetime,
> followed by complete battery failure.  It had been in continuous use
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Apptech | 6 Sep 23:26

Re: [EE] Trickle charge current on NiMh

>I have a single-item application that requires battery backup. I am
> planning, for sake of simplicity, to use 4 cell 650mhA NiMh and keep them 
> in
> trickle charge permanently. Simple limiter resistor will put the trickle
> charge at 20mA max (<1/30C) which is far below the 1/10C most people 
> comment
> as acceptable for trickle charge. The NiMh are high quality Kan batteries.

I have some experience in this - but the information from reputable 
manufacturers provides good guidelines.

You'll find that competent makers of larger capacity NimH strenuously note 
that NimH should NOT be trickle charged. Several degradation mechanisms are 
suggested but it seems that heating becomes more important with increasing 
density of reactants. It's interesting to note that some lesser know NimH 
makers (typically those from a very large country) still provide data sheets 
which seem just to be scaled up versions of older ones from other people. 
Given that this is not what the leading manufacturers do, you need to be 
careful which information sources you believe.

However, older NimH and low capacity NimH do not have the same restrictions 
and modern very low capacity NimH even have suggested allowable trickle 
charge rates equal to the fast charge rates. As a rule of thumb it seems 
that anything under about 1200 mAh is OK for trickle charging and at 650 mAh 
I'd expect that you SHOULD be OK. Using a quality name brand is, of course, 
essential. Previously rates of C/10 have been recommended but in more recent 
data sheets and at higher capacity C/30 seems more favoured. My reading 
suggests that energy density per volume seems to be an indicator for typical 
cell shapes.
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Sean Breheny | 6 Sep 23:40
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Re: [EE] Trickle charge current on NiMh

Russell,

When you say "trickle charging", do you mean continuous charge with no end?

As far as I know, it is always OK to charge a NiMH cell, even high
capacity ones, at 1/10th C, as long as you still terminate when
completely charged. Some datasheets still call this trickle charging,
meaning simply that the rate is very slow compared with the 1C rate or
greater which is usually used for NiMH.

It MAY be that charging at 1/10th C doesn't allow the typical "battery
full" indication (negative dV/dt). However, if you have other means to
determine when the battery is fully charged, I think it is OK.

The device I described in a previous email draws a predictable amount
of current during operation. The PIC is able to keep track of the
amp-hours drained from the battery. The charging circuit is a 250mA
constant current regulator which the PIC can shut on and off. Using
coulometry, the PIC decides when to switch from 1/10C to 1/40C. There
is also a voltage threshold below which the running sum of the battery
state of charge is set to near zero (to prevent extreme drift of the
SOC estimate from the actual) and if the voltage climbs over a certain
value during charging, charging is turned off to prevent damage. There
is no temperature monitoring because the max charge current is so low.

Do you agree that this scheme can be acceptable?

Sean

On Sat, Sep 6, 2008 at 5:26 PM, Apptech <apptech <at> paradise.net.nz> wrote:
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Apptech | 7 Sep 02:49

Re: [EE] Trickle charge current on NiMh

> When you say "trickle charging", do you mean continuous charge with no 
> end?
>
> As far as I know, it is always OK to charge a NiMH cell, even high
> capacity ones, at 1/10th C, as long as you still terminate when
> completely charged. Some datasheets still call this trickle charging,
> meaning simply that the rate is very slow compared with the 1C rate or
> greater which is usually used for NiMH.

Rushing out ...

I meant long term low level charge after after the battery is "full".
Some manufacturers say you should not do it at all for higher capacity 
cells.
Some say only for a brief period.

SOME manufacturers do say that low rates are not advisable. Bib B also 
mentioned this.

R 

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Ariel Rocholl | 7 Sep 21:07

Re: [EE] Trickle charge current on NiMh

Thanks all contributors to the topic. I had the batteries under test for
over 12hrs and they didn't warm at 1/30C. I agree with Sean comment in that
sense of hot temperature would indicate that is charging at a rate beyond
trickle charge level, so this was the first thing I checked. I am very
familiar with the differences between NiCd and NiMh, certainly 1/10C is too
much for permanent trickle charging NiMh.

Given the feedback received on real experiences here, I then plan to keep
the current approach and review the battery status in 2 months, just
applying some 2-3 cycles and observing whether the capacity was reduced or
not, that hopefully will be a good indicator of battery health before the
batteries are really damaged. In the worst case, I will learn real
experience on how feasible is this approach.
The TL431 with 2 resistors for a voltage clamp is a great and simple idea. I
will move to that if above approach shows inefficient and really degrades
these batteries.

I agree on Li-ion being a better solution for this but would not be cheaper
as I have the 4 Kan cells handy whereas I would have to buy a Li-ion cell.
Another approach could be to use some new Enerland NiMh that doesn't
self-discharge but again I would have to buy new ones.

I will report results here just if this is useful for anyone.

TTYL
2008/9/6 Apptech <apptech <at> paradise.net.nz>

> >I have a single-item application that requires battery backup. I am
> > planning, for sake of simplicity, to use 4 cell 650mhA NiMh and keep them
> > in
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Ariel Rocholl | 8 Sep 10:12

Re: [EE] Trickle charge current on NiMh

Where I wrote Enerland please read Eneloop.

2008/9/7 Ariel Rocholl <arocholl <at> gmail.com>

>  Thanks all contributors to the topic. I had the batteries under test for
> over 12hrs and they didn't warm at 1/30C. I agree with Sean comment in that
> sense of hot temperature would indicate that is charging at a rate beyond
> trickle charge level, so this was the first thing I checked. I am very
> familiar with the differences between NiCd and NiMh, certainly 1/10C is too
> much for permanent trickle charging NiMh.
>
> Given the feedback received on real experiences here, I then plan to keep
> the current approach and review the battery status in 2 months, just
> applying some 2-3 cycles and observing whether the capacity was reduced or
> not, that hopefully will be a good indicator of battery health before the
> batteries are really damaged. In the worst case, I will learn real
> experience on how feasible is this approach.
> The TL431 with 2 resistors for a voltage clamp is a great and simple idea.
> I will move to that if above approach shows inefficient and really degrades
> these batteries.
>
> I agree on Li-ion being a better solution for this but would not be cheaper
> as I have the 4 Kan cells handy whereas I would have to buy a Li-ion cell.
> Another approach could be to use some new Enerland NiMh that doesn't
> self-discharge but again I would have to buy new ones.
>
> I will report results here just if this is useful for anyone.
>
> TTYL
> 2008/9/6 Apptech <apptech <at> paradise.net.nz>
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Alexandre Guimarães | 14 Sep 02:45
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RES: [EE] Trickle charge current on NiMh

Hi, Russel

	At what voltage did you set the clamping ?? I used that technique a
long time ago with rechargable alkalines but the "top" voltage was well
defined within the temperature limits. On NiMh won't it be very dependent on
temperature ? Sounds like a great idea to make the batteries last longer.

>If absolute energy density is not required then clamping the battery
voltage at a level somewhat below full charge is liable to produce a safer
result than ongoing trickle
> charging. This can be achieved with as little as eg a
> TL431 and 2 resistors. I have taken that approach in a design which needs
to be as robust as possible while costing approximately $0.00 to implement.
> This also extends battery cycle life, although that's not liable to be an
issue in most backup applications.

Best Regards,
Alexandre Guimaraes

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Sean Breheny | 6 Sep 23:31
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Re: [EE] Trickle charge current on NiMh

Hi Ariel,

It is good that you are asking for advice and doing some research on this.

I designed a product two years ago which used a single NiMH AA cell as
its power source. The device was intended to sit in a charging cradle
until needed and then returned to the cradle.

Not knowing any better, I thought that the term "trickle charge
current" meant "acceptable continuous charge current." One of my
coworkers, who had designed various battery-powered devices in the
past even told me that this was the case. So, I designed the device to
continually charge at 1/10C.

The batteries died within about 2 months. Failure mode was that their
voltage would shoot up over 1.6V during charge and then they would
hold perhaps only 1/3rd of their rated capacity. These were Energizer
2500mAh AA NiMH cells.

It turns out that my coworker's experience was with NiCD and he
assumed that NiMH was the same. It's not.

After a little more research, I found that the manufacturer said that
these should not be continuously charged at more than 1/40C. Thank
God, my design allowed the PIC (of course there was a PIC in it :) )
to turn charging on and off, so I ended up doing some coulometry and
voltage measurement and roughly deciding when the battery was finished
charging and then I would switch to a 1/4 time pulsed charge
(basically on for 1 second, off for 3 seconds).
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