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[on:]

Hi,

I'm trying to build a battery charger for a 7.2V backup battery for powering my Arduino project.
I've build a simple backup circuit using diodes, and now I'm researching the charger part.

The project I'm working on is meant to be plugged into a powersource 24/7, and Ideally the battery will never be used - but it still needs to be there just in case.

So I was wondering if it would be possible to constantly charge the battery with 7.2V and C/some-low-number amps?
I know the battery wouldn't reach max capacity and that it might take a long time, but would it be possible to charge it to 7.2V like this? Even if the battery started out flat?

Any help would be greatly appreciated.

[Reply #1 on:]

Yes what you want to do is / was commonly done. I would suggest that you do the research first. You have a good idea of what the numbers should be and Wiki should do nicely. Since you have some part of a controller available you could also monitor battery condition by discharging the Ni-Cd battery at a set rate/time and noting the voltage at the end of the test period. While not terribly accurate the method can be interpolated to give a rough estimate of charge/condition. Charge the battery properly (I like to sense the voltage rise at EOC) but follow the MFR's rec's and then discharge it at a known rate for a fixed time. The End of test voltage will decrease as the batteries age, more or less in a linear manner until nearly end of life, a rapidly decreasing step in the change of the voltage indicates battery issues. I did this with SLA batteries primarily but it worked well with a couple of NI-Cd powered devices I built several years ago as well. I stored the values in EEprom when the device was powered up along with the time/date from a DS3231 clock and graphed it with custom chars onthe new  a 4 X 20 LCD display against the new values.

Bob

[Reply #2 on:]

Slow charge is usually defined as a charging current that can be applied to the battery
indefinitely without damaging the cell (this method is sometimes referred to as a trickle
charging).

The maximum rate of trickle charging which is safe for a given cell type is dependent on
both the battery chemistry and cell construction.  When the cell is fully charged, contin-
ued charging causes gas to form within the cell.  All of the gas formed must be able to
recombine internally, or pressure will build up within the cell eventually leading to gas
release through opening of the internal vent (which reduces the life of the cell).

This means that the maximum safe trickle charge rate is dependent on battery chemis-
try, but also on the construction of the internal electrodes.  This has been improved in
newer cells, allowing higher rates of trickle charging.

The big advantage of slow charging is that (by definition) it is the charge rate that
requires no end-of-charge detection circuitry, since it can not damage the battery
regardless of how long it is used.  This means the charger is simple (and very cheap).

The big disadvantage of slow charge is that it takes a long time to recharge the battery,
which is a negative marketing feature for a consumer product.

Slow Charge Rates
NI-CD:  most Ni-Cd cells will easily tolerate a sustained charging current of c/10 (1/10 of
the cell's A-hr rating) indefinitely with no damage to the cell.  At this rate, a typical
recharge time would be about 12 hours.

Some high-rate Ni-Cd cells (which are optimized for very fast charging) can tolerate
continuous trickle charge currents as high as c/3.  Applying c/3 would allow fully charg-
ing the battery in about 4 hours.

The ability to easily charge a Ni-Cd battery in less than 6 hours without any end-of-
charge detection method is the primary reason they dominate cheap consumer products
(such as toys, flashlights, soldering irons).

A trickle charge circuit can be made using a cheap wall cube as the DC source, and a
single power resistor to limit the current.

NI-MH:  Ni-MH cells are not as tolerant of sustained charging:  the maximum safe trickle
charge rate will be specified by the manufacturer, and will probably be somewhere
between c/40 and c/10.

If continuous charging is to be used with Ni-MH (without end-of-charge termination),
care must be taken not to exceed the maximum specified trickle charge rate.

Are you using Arduino to control the charger? this are some reference info
http://www.atmel.com/images/doc1659.pdf
This are some circuit for the charger
http://www.ti.com/lit/an/snva557/snva557.pdf

[Reply #3 on:]

@ Billho.. thank you for the really outstanding references they outlined what I did exactly.. Really good stuff.     i didn't try to get quite as precise though. I just wanted the batteries out as they were no longer reliable... I I later changed to eeproms for better version control.

Bob

[Reply #4 on:]

Thanks for all your answers :-)

@Docedison, cool to store battery info in the EEprom, but I'm actually trying to avoid having to sense the end of charge, and instead just use a fixed voltage (equal to the batteries full charge voltage) and a fixed amp.

@BillHo, I've read this document prior to posting and was thrilled that I could trickle charge the battery indefinitely, but when I researched it further every other source I found said a battery should never be trickle chargede for prolonged times.

A floatcharge (same voltage as the battery instead of a much higher) should be able to run indefinitely, but I am unsure if it can ever reach the final voltage.

For example, I have a 7.2V 1000Mah battery which full charge voltage is about 7.8V.
If the battery gets drained to a low voltage e.g. 6.4V and I apply 7.8V external power at 10Mah – Will the battery ever actually reach 7.8V? And will this be safe?

(Sorry if this is a ridicules question, I'm still trying to learn)