NiMH Battery Question

[ABLE1]

Hello all,

Quick question. What would the expected voltage be
for a new VARTA 3.6vdc 70mAH NiMH rechargeable battery??

Or maybe the better question would be:
If the voltage is at 1.76vdc would the battery be good??

Thanks in advance for all insight,

Les

 

[Dave Platt]

Hello all,

Quick question. What would the expected voltage be
for a new VARTA 3.6vdc 70mAH NiMH rechargeable battery??

That depends on its level of charge.

Or maybe the better question would be:
If the voltage is at 1.76vdc would the battery be good??

Possibly, if it's newly manufactured, and had been built and shipped
in a discharged state.

NiMH cells have a nominal voltage of around 1.2 volts per cell, so a
"3.6-volt" nominal battery consists of three cells in series.

They're often rather higher when freshly/fully charged... I see around
1.45 volts when they're being charged.

A NiMH or NiCd battery should not be discharged to below around 1 volt
per cell. Doing so risks fully discharging one cell before the
others; as the others continue to discharge the weaker cell will be
"reverse charged" and this can permanently damage it.

So, 1.76 volts in a three-cell battery would probably indicate one of
two things:

(1) Newly made, and the cells were never charged (or were test-charged
and then individually discharged, or were charged and then self-
discharged over time), or

(2) A battery which has been discharged to below the safety limit, and
might have been damaged.

Manufacturers often ship NiMH and NiCd cells and batteries in a
discharged condition. Safer that way - less risk of fire if something
accidentally shorts one or more cells. Older-style NiMH cells have a
high self-discharge rate - they'll lose 10% or more of their charge
every month even if not being used at all.

You'd need to actually test it to determine. Charge it fully
(according to the manufacturer's recommendations). It should rise to
at least 3.6 volts very quickly, then probably to 4 or 4.2 volts as it
continues to charge. If it doesn't behave like this, it may well be
damaged or defective.

When fully charged, discharge it at a safe rate (5-10 mA in
this case) until it drops to 3.0 volts, then disconnect. Multiply the
discharge time by the discharge current, to get the capacity. If it's
not in the 50-to-70 mAh range, it may be bad.

Expect that you may not get the full rated capacity until the battery
has been charged and discharged several times - NiMH cells have a
distinct "break-in" effect.

 

[ehsjr]

On 5/30/2018 9:08 PM, ABLE1 wrote:

Hello all,

Quick question.  What would the expected voltage be
for a new VARTA 3.6vdc 70mAH NiMH rechargeable battery??

Or maybe the better question would be:
If the voltage is at 1.76vdc would the battery be good??

Thanks in advance for all insight,

Les

Bad battery. It should read 1.43 volts per cell
immediately after it is fully charged. Yours is
a 3 cell battery, nominal voltage 3.6 V, voltage
immediately after a full charge should be 4.29 V.

Ed

 

[ABLE1]

On 5/31/2018 1:43 AM, Dave Platt wrote:

Hello all,

Quick question. What would the expected voltage be
for a new VARTA 3.6vdc 70mAH NiMH rechargeable battery??

That depends on its level of charge.

Or maybe the better question would be:
If the voltage is at 1.76vdc would the battery be good??

Possibly, if it's newly manufactured, and had been built and shipped
in a discharged state.

NiMH cells have a nominal voltage of around 1.2 volts per cell, so a
"3.6-volt" nominal battery consists of three cells in series.

They're often rather higher when freshly/fully charged... I see around
1.45 volts when they're being charged.

A NiMH or NiCd battery should not be discharged to below around 1 volt
per cell. Doing so risks fully discharging one cell before the
others; as the others continue to discharge the weaker cell will be
"reverse charged" and this can permanently damage it.

So, 1.76 volts in a three-cell battery would probably indicate one of
two things:

(1) Newly made, and the cells were never charged (or were test-charged
and then individually discharged, or were charged and then self-
discharged over time), or

(2) A battery which has been discharged to below the safety limit, and
might have been damaged.

Manufacturers often ship NiMH and NiCd cells and batteries in a
discharged condition. Safer that way - less risk of fire if something
accidentally shorts one or more cells. Older-style NiMH cells have a
high self-discharge rate - they'll lose 10% or more of their charge
every month even if not being used at all.

You'd need to actually test it to determine. Charge it fully
(according to the manufacturer's recommendations). It should rise to
at least 3.6 volts very quickly, then probably to 4 or 4.2 volts as it
continues to charge. If it doesn't behave like this, it may well be
damaged or defective.

When fully charged, discharge it at a safe rate (5-10 mA in
this case) until it drops to 3.0 volts, then disconnect. Multiply the
discharge time by the discharge current, to get the capacity. If it's
not in the 50-to-70 mAh range, it may be bad.

Expect that you may not get the full rated capacity until the battery
has been charged and discharged several times - NiMH cells have a
distinct "break-in" effect.

Dave & ehsjr,

Thanks for the quick response. What I suspected was a bad battery.
The detail you provided was an extra bonus. I purchased from
a local "Battery" store for a replacement backup battery that needs
to be soldered onto the motherboard.

The guy at the store said it just needs to be charged and shrugged his
shoulders. The others that were checked in the bin all had the same
voltage. I ended up taking one of the others and only checked the
voltage when I returned to by office. This one was for what ever reason
at 3.47vdc. Which makes more sense.

What frustrates me is that the "Battery Store Expert" doesn't know
batteries.

BTW as a side question. Are the individual cell welded together in the
shrink tubing or are they just held together by the shrink tubing??

Thanks,

Les

 

[Jon Elson]

On Thu, 31 May 2018 08:04:42 -0400, ABLE1 wrote:

This one was for what ever reason

at 3.47vdc. Which makes more sense.
Well, that has a much better chance to work OK.

What frustrates me is that the "Battery Store Expert" doesn't know
batteries.

No great surprise, unfortunately.
BTW as a side question. Are the individual cell welded together in the
shrink tubing or are they just held together by the shrink tubing??

They would most likely be tabbed, with a single tab folded between cells.
If mass-produced, it is possible they are welded together at the factory.
You can easily tell by gently flexing the stick. If it is totally rigid,
they are welded. If there is a little flex to it, they are tabbed.

Jon

 

[ABLE1]

On 6/4/2018 6:36 PM, Jon Elson wrote:

BTW as a side question. Are the individual cell welded together in the
shrink tubing or are they just held together by the shrink tubing??

They would most likely be tabbed, with a single tab folded between cells.
If mass-produced, it is possible they are welded together at the factory.
You can easily tell by gently flexing the stick. If it is totally rigid,
they are welded. If there is a little flex to it, they are tabbed.

Jon

Jon,

I replaced the battery on Friday and all is good.
I did a autopsy on the removed battery and they are tabbed
together.

I was going to post then but got distracted.
(which happens often) Phone calls, and other stuff.

Another interesting side note. The battery was replaced
primarily due to age and the suspected system trouble
due to a power outage. The system was new when I installed
in 2000. That is 18 years.

I just checked the battery and the voltage is 3.826vdc
I realize that it has been sitting on my desk with no
actual load attached.

But still, I guess it was not a weak battery after all.

Les

 

[Ian Field]

"Dave Platt" <dplatt@coop.radagast.org> wrote in message
news:6le4ue-fb4.ln1@coop.radagast.org...

Hello all,

Quick question. What would the expected voltage be
for a new VARTA 3.6vdc 70mAH NiMH rechargeable battery??

That depends on its level of charge.

Or maybe the better question would be:
If the voltage is at 1.76vdc would the battery be good??

Possibly, if it's newly manufactured, and had been built and shipped
in a discharged state.

NiMH cells have a nominal voltage of around 1.2 volts per cell, so a
"3.6-volt" nominal battery consists of three cells in series.

They're often rather higher when freshly/fully charged... I see around
1.45 volts when they're being charged.

Those figures sound about right - but don't forget the temperature
coeficient that makes the terminal voltage to drop slightly at full charge.

They're slightly endothermic while charging - at full charge the energy put
in starts producing heat.

 

[Brian Gregory]

On 31/05/2018 06:45, ehsjr wrote:

On 5/30/2018 9:08 PM, ABLE1 wrote:
Hello all,

Quick question.  What would the expected voltage be
for a new VARTA 3.6vdc 70mAH NiMH rechargeable battery??

Or maybe the better question would be:
If the voltage is at 1.76vdc would the battery be good??

Thanks in advance for all insight,

Les



Bad battery. It should read 1.43 volts per cell
immediately after it is fully charged.  Yours is
a 3 cell battery, nominal voltage 3.6 V, voltage
immediately after a full charge should be 4.29 V.

Ed

But he hadn't charged it then!

--

Brian Gregory (in England).

 

[Ian Field]

"ehsjr" <ehsjr@verizon.net> wrote in message
newseo26b$7ar$1@news.eternal-september.org...

On 5/30/2018 9:08 PM, ABLE1 wrote:
Hello all,

Quick question. What would the expected voltage be
for a new VARTA 3.6vdc 70mAH NiMH rechargeable battery??

Or maybe the better question would be:
If the voltage is at 1.76vdc would the battery be good??

Thanks in advance for all insight,

Les



Bad battery. It should read 1.43 volts per cell
immediately after it is fully charged. Yours is
a 3 cell battery, nominal voltage 3.6 V, voltage
immediately after a full charge should be 4.29 V.

An enthusiastically charged cell could read as high as 1.47V - settling to
the nominal 1.2V should take QUITE a while.