Michael Ross wrote:

> I am investigating how to test cells and packs - I will need to charge and
> discharge.
> 
> If I have a programmable electronic load, rather than purely resistive
> loads, is this an unrealistic way to discharge batteries?

Thought I'd mention that while a programmable load is an *excellent* way to 
discharge when testing batteries, it can be pricey (typically about $2/W on the 
used market).  If you have a load bank, or need one for other reasons, then by 
all means use it.  If you are considering buying one just for this purpose, 
then carefully consider what other equipment you have or will need to acquire 
to go with it.

If you get a load large enough to handle your needs on its own (i.e. it can 
handle the desired discharge current even with a fully-charge battery), and you 
get one equipped with a communications interface (GPIB/IEEE-488 or RS232, 
typically), then you may be able to use its own voltage and current measurement 
capabilities to avoid the need to purchase a standalone data logger or USB 
DAC/digital I/O device to take measurements and control things.

If you already have a data logger or other means of measuring/controlling a 
load and charger, then perhaps consider using a passive load in parallel with a 
smaller electronic load, so that you can save money on the electronic load 
purchase.

Or, you may be able to go in entirely the opposite direction and use a 
completely passive load bank.  I started out with a battery cycler setup 
consisting of a bank of 12VDC Edison-base (household screw-type) light bulbs 
switched by a continuous duty Ford starter solenoid-type RV battery switch.  A 
simple QBasic program on an old laptop controlled the relay for the load and 
another for the charger via the parallel port.  An E-meter with the RS232 comms 
option provided voltage, current, etc. measurements to the QBasic program.

Incandescent bulbs have the nice property that the current they draw varies 
less with voltage than would a purely resistive load, so you can get a 
reasonably constant discharge current simply.  Unless your battery voltage is 
significantly below the rated voltage of the bulbs, they also provide a nice 
visual indication that they are on and hot. ;^>

Cheers,

Roger.

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