Microcontrollers > Microcontrollers/General
LITHIUM BATTERY TESTING WITH A UNO
OperaHouse:
I've done a little more work with the battery tester. Another battery pair tests at about
30 amp minutes. With just a few minutes rest, it can just about do that a second time without
recharging. This same thing happened with the prior battery pair. Testing was done at only
a half amp load. Higher currents greatly shorten the total amp minutes. While these batteries
are junk for any reasonable load, they are still quite good for low current LED lighting.
I changed the menu selection of currents. 1-6 now represent 100ma to 600ma which is more
applicable for these end of life batteries. I use a cheap cell phone charger for the 5V to
supply power to the micro and the charger board. The regulation is rather poor and voltage
readings vary significantly when charging That can change the stop point when in stand alone
operation. To solve this I changed the voltage divider resistors and used the internal 1.1V
reference instead of the 5V power. A single line in the program changes this.
analogReference (INTERNAL); // use with (62K, 5K pot), 15K voltage divider, for .7V input
I usually avoid this for a couple reasons. 1V is really down in the soup. Any noise that will
be picked up will be significant. Circuits like a charger that have currents of an amp or more.
That can induce small voltages in the common ground circuit. These could greatly affect the
measured voltage if care isn't taken in the wiring. Finally, most sensors operate on 5V making
the voltage range incomparable without adding circuitry.
analogReference can be dangerous if EXTERNAL mode is used. It would seem convenient to just
tie the AREF to some other source like 3.3V or other power source that may be used elsewhere
in the circuit. Should you forget to include this EXTERNAL statement in the code, both power
supplies will be connected. This short will damage the micro and/or one of the power sources.
In fact, if this was hard wired on a circuit board the micro would have to be programed before
inserting it into the board.
There is a work around for this. Connect a 5K or higher resistor from the external reference
to the AREF pin. The AREF pin is not a high impedance input. There is an internal 32K resistor
to common. The input resistor will will then form a voltage divider making the reference voltage
lower. An advantage to this is the reference voltage can be changed on the fly without damage.
However, the A/D readings will not be stable for a couple readings. Never use a reference higher
than 5V and DO NOT USE THE THE ON BOARD 3V3 as a reference unless that has at least a 2K load
resistor on it. With insufficient load on the 3V3 power line, the 5K resistor can supply enough
current to raise the 3V3 line to to 5V.
OperaHouse:
ZB2L3 Battery Tester
This is a nice little battery tester I received today. Just wanted to show you I'm not
married to the UNO. Every thing you want to build can be bought cheaper in CHINA. Features
are limited to 15V maximum and 3A. 3A is really pushing it for that little FET, limit it to
half that current. Current is selected by external resistor, it comes with two 7.5 ohm 5W
resistors. One gives you about a half amp with a lithium cell, with two in parallel being
about 1A. On a 12V SLA battery the two resistors in series (15 ohm) would be .83A and dissipate
a hair over 10W. I'd add a little extra resistance to it to be safe. Board is about $4 shipped
and of course there were no instructions. End voltage can be set from, .5V to 12V. The
unit was tested with a 121 ohm 1% resistor and it correctly indicated a current of 32ma.
The board is powered by a mini USB connector. If a 5V wall wart is not available with that
connector, it can be wired directly. On the back of the board just under the USB connector
there is a square solder pad which is the +5V. The IN- pad is the negative. Suggest you make
a solder connection here rather than use the screw terminal. A loose wire could damage the
board. Load resistor goes to the two R pins, test battery to the + IN - terminals. You will
want to have an easy way to disconnect power to the board. Install a switch in the +5V line
if a pull out connector is not used.
This board does not have reverse battery protection. Connect battery before you program or
error message will be given when button is pressed.
1) When first powered the battery voltage will be given on the display. It will stay in VOLT
mode till + or - is pressed to change the end voltage. It will then display P 3.0 the default end voltage setting. I do not like to bring Li below 3.4V because it can alter the chemistry and shorten life.
2) Press + till the display indicates P 3.4 for the end voltage. It can only be adjusted in
.1V steps.
3) Press OK. This starts the current flowing and the display will keep sequencing through AH,
A, and V.
4) When the battery has reached the end voltage the display will rapidly flash the ending amp
hour. It will remain on AH till display power (USB) is removed or OK is pressed again starting
the test over. Note the last ending voltage will not be kept and the standard setting P 3.0
will be used.
Bruce S:
How well did this do against your UNO tester?
OperaHouse:
I think it is a nice all in one board limited to about 1 1/2 amp and current is selected by resistor. If it ever breaks it can never be fixed. It seems like a fully acceptable option and quite accurate at low currents. It can be used up to 15V, cut out at 12V. Mine has adjustable current by PWM and pretty much has to be run off a laptop. Calculated current in software from a fixed voltage. It could be expanded to any voltage and any current. It was just to demonstrate how an immediate need could be solved. I have a later version of software that improves it a little, not something I am going to support. It could be made to look just like the other one with additional parts. I will likely turn this into a solar panel tester.
Gin83:
Good topic. The only question is...can UNO be replaced by NANO?
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