Michael Ross wrote:

> I have heard of putting a roll of fencing in a tank to steam off heat.  Is
> this useful with a programmable load?  Or is this just a crude way to
> discharge.

Probably not, unless the electronic load you choose is designed to be used with 
an external load bank, or you plan to control the electronic load from a remote 
source rather than simply setting it to regulate the desired load current on 
its own.

An external load bank (coat hanger or other wire in a bucket of water or 
fan-cooled power resistors, etc.) can be useful if you want to increase the 
power capability of your electronic load (or conversely, to be able to use a 
smaller - and cheaper - electronic load than you could otherwise).

For instance, when testing a 12V battery you will typically discharge to about 
10V minimum; if your desired discharge rate is 75A, then your minimum load is 
about 750W.  If you want to be sure that you can pull the full 75A from a 
freshly charged battery, it needs to be able to handle the maximum load of 
about 15V x 75A = 1125W.  If you use a resistive load to handle most of the 
minimum load requirement, then your electronic load only needs to be sized to 
handle the difference (in this case 1125-750 = 375W) instead of the full amount.

> Someone mentioned needing a heat dispensing means would be needed with a
> programmable load.

Not unless you pick up a water-cooled load.  The air-cooled loads have built in 
fans and will cool themselves.  If you are discharging a large capacity battery 
in a small room, you may need to open a window or otherwise allow for some 
ventilation as the room will warm up.

> Any recommendation of actual units to look at?

I've mostly used DLP- and RBL-series units from TDI (Transistor Devices).  They 
are often available used, and are generally basic rugged units.  You don't need 
particularly fancy features for battery discharging as most often you will use 
the load in constant current or possibly constant power.  In constant current 
mode, the TDI loads can have the load current controlled remotely by a 0-5V 
signal, so you can use an external means to monitor the total load current 
drawn by the electronic load and a resistive load and then control the 
electronic load to regulate the total current at the desired level.  They also 
can be enabled/disabled remotely with a simple relay or opto so that your data 
logger can turn the load off when the battery voltage reaches the desired 
end-of-discharge voltage.  Some models support IEEE-488 (GPIB) communication, 
in which case you can achieve all of this controllability through software 
control, provided you spring for a GPIB interface for your PC.

I've also used loads from NH Research (NHR, this is what I have at home) and 
Chroma.

This site provides an idea of what sort of devices are available on the used 
market:

<http://www.alltest.net/s.nl/sc.2/category.16/.f>

Cheers,

Roger.

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