Battery capacity checker

Get a 10 ohm power resistor (25W) and a digital voltmeter. If you
don't have a digital voltmeter - BUY ONE; they are a must have tool!
Connect the resistor across the battery and start recording the
battery voltage. The discharge current is V/R (Ohm's law), and is
about 1.2 amp. Your typical battery is 6 A-hr, so your test will be
over in about 5 hours (less if your battery is tired). In any case
stop the test when the battery voltage drops below 11V. Your
incremental value per reading is:
(Time interval [min] / 60) * (Voltage / Resistance)
Add these incremental values together to get the A-Hr capacity of the
battery.

If you don't want to spend $100 on the ELK battery tester (or have 6V
batteries like I do) buy a 2nd resistor. Attach the first resistor and
measure the battery voltage. Do the same thing for the second
resistor. You will have to compute the following:

I1 = V1/R1, I2 = V2/R2

deltaI = I2 - I1
deltaV = V2 - V1

mhos = deltaI / deltaV

Mhos are the inverse of ohms, so more is better as far as a battery is
concerned (more than 100). Check a new battery for a base-line value.
The resistors should load the battery close to its operating point.
The resistors could be a single power FET with a variable voltage
control. Suggested resistor values: 10 and 20 ohm (10W min):

http://www.digikey.com/scripts/us/dk...534&Row=282775
http://www.digikey.com/scripts/us/dk...090&Row=279318

You can also parallel 10 100 ohm 1W resistors to get a 10 ohm 10W
resistor.

Tom

Another question on Peter Nyffeler's circuit:

I think the part number for the dual operational amplifier
is wrong. It should read "LM358" NOT "LM385". This last
item is a precision voltage reference, not an amplifier.

Cheers, John G.

"John Giddy" wrote in message
...
| Hi Peter,
| Good circuit !
| However I have a couple of questions:
|
|
| 1.
| I presume the FET (Q2) will need a heatsink, as it will
| dissipate approximately 12W when discharging at 1 Amp ?
|
| 2.
| R9 should be 470 ohm, not 470 K
|
| Cheers, John G.
|
| "Peter Nyffeler" wrote in
| message ...
| | In article
| , "Roger
| Druce" wrote:
| | Perhaps someone out in r.a.s land may be able to point
me
| in the direction
| | of a commercial piece of equipment which would perform
| the following task.
| |
| | I would like to be able to charge up a 12 volt
(nominal)
| gell cell battery
| | and then plug it into a piece of equipment to check its
| total AmpHour
| | capacity during discharge at some constant rate current
| (say the "10 hour"
| | rate or the "20 hour" rate). All this towards
assessing
| whether the battery
| | has proven useful capacity and life left in it, or has
it
| reached its
| | discard date?
| |
| | The equipment would show the delivered amp-hours up to
| the point at which
| | the voltage has fallen to a nominated end voltage.
| |
| | I realise that you can do it in a way with a light
globe
| to discharge the
| | battery and then time the length the light holds up
| glowing brightly. This
| | is all too primative for this day and age, and there
| ought to be some piece
| | of equipment available for the task.
| |
| | Or some circuit from an electronics hobby magazine??
| |
| | Thanks in antcipation.
| | Roger Druce
| |
| | I built one myself.
| | You find the schematic at
| http://www.ppc.ethz.ch/~peny/sf/ah_test.html
| |
| | Peter Nyffeler
| | Tel P 01 363 62 42, Tel G 01 632 43 60, Fax G 01 632 10
21
|

In article , "Chris" wrote:
Looks like a good project, what should the value of R7 be ?

Sorry I for the missing value
R7 = 10 k ohm
R9 should be 470 ohm
and don't forget a heatsink with more than 5 Kelvin/Watt
Unfortunately I don't have access to the webpages at the moment to replace the
schematic diagram.

Let me warn you to be carful with Chargers for RC modelers.
Most of them are not well designed to charge lead acid batteries.

A good reference for charging characteristics of lead acid batteries can be
found at
http://www.panasonic.com/industrial/...eal/index.html
in the pdf document "VRLA Charge Methods - 508KB".

I use a charger with current limited "two-step constant voltage charge control
method" and temperature coefficient compensation, published some years ago in
Elektor.

Peter Nyffeler
Tel P 01 363 62 42, Tel G 01 632 43 60, Fax G 01 632 10 21