Discharge constant current from battery
Discharge constant current from battery
(OP)
Currently, i am making a simple discharge circut to discharge batteries at a constant current. Using a resistor won't work because the close circuit voltage will be different depending on the health of the battery. The question is how can i achieve this constant current draw from the battery for the whole lifespan. If i need to, i might have to do it using microcontrollers to control the load resistance while observing the voltage of the battery. but really don't want to go there. Is there a simpler solution to this? Thanks in advance.





RE: Discharge constant current from battery
RE: Discharge constant current from battery
The simplest way of doing ot to use a transistor with an emitter resistor and then feed the base of the transistor with a voltage that corresponds to voltage drop over resistor plus Vbe.
An example: Say that you want to discharge with 2 A constant. Select a transistor that can dissipate current times voltage, say 12 V. That will be 24 W. Use a heat sink that corresponds to that. The base resistor can be something like 1 ohm. That will give you a 2 V voltage at the emitter. Then feed the base with 2.8 (approximately) V to get the constant discharge current. Use a power rheostat (more than a few hundred mW) to adjust base voltage to get the current you want.
Gunnar Englund
www.gke.org
RE: Discharge constant current from battery
But the big question is still; What size battery. ?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Discharge constant current from battery
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Discharge constant current from battery
RE: Discharge constant current from battery
Desired current: 50mA to 200mA
cutoff: 2 volts
Thanks for all the response
RE: Discharge constant current from battery
http://cache.national.com/ds/LM/LM117.pdf
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Discharge constant current from battery
If you use low power parts (example, the LT6000 can be powered by 1.8 volts at 13 uA with rail to rail input and output), you can have the whole thing powered by the battery itself.
Add an opamp to sense the voltage and a reference and you shouls be able to do the whole thing under about 50 uA.
Just adjust the current to account for the circuit drain, and be aware that the small current will still be loading the battery once you've reached the cutoff voltage. But even that should drop to even closer to zero below 1.4 volts or so.
Of course, heat sink the transistor adequately for the watt or so you're dissipating.
RE: Discharge constant current from battery
RE: Discharge constant current from battery
A bandgap refernce will get you 1.24 volts with 10 uA or so of bias. Use a couple high value resistors to divide this down to 100 mV or so as the reference to the current sink and comparator to do your end of life voltage shut-down. That will leave plenty of voltage to drive the MOSFET to conduct 200 mA or more.
With this, you won't need any external source.
As OperaHouse says, if you go the bipolar junction transistor route, you'll need to account for the base current. Probably about 1% or so with a decent transistor. A Darlington might help, and you'll probably have enough voltage to drive the two Vbe drops even at end of life.
RE: Discharge constant current from battery
100 at medium voltage i.e 25 ohm == 60 mA @ 2 V and 114 mA @ 3.6 V.
If you want, for 5 mA more you can add a comparator+LED
to indicate low voltage.
If you want to disconnect, use a saturated NPN
and recalculate the R -- if you are fussy -- to make
up \for the Vsat.
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Discharge constant current from battery
Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>
RE: Discharge constant current from battery
RE: Discharge constant current from battery