Re: (ET) Ooops, timer died

["David Roden" <etpost drmm net>]

Here is a simple but effective charger controller.  It was used in the old 
CVI Comuta-Car.  I found an almost identical reference design in an 
engineering manual.  

http://www.evdl.org/docs/c_car_charger.jpg

This one is designed for a 48 volt battery, but it shouldn't be difficult 
to 
modify for 36 volts.  You could try swapping CR6 for a 9.4 volt zener, or 
adjusting the values of R4 and R6, which with R5 act as a voltage divider 
into CR6.  R5 provides fine trimming of the finishing voltage. 

L1 is a small 12 volt indicator lamp.  In the C-car, it was one of the 
instrument panel indicators.  If anyone wants to try building this, I'll 
see 
if I can find one in my old parts box to check its current.  However, you 
should be able to determine and appropriate value experimentally.  I'm not 
sure it's all that criical anyway.  I actually used a resistor and LED in 
my 
C-car, but I don't recall the value of resistor I used.

SCR is a small low-current SCR.  When mine failed, I used a generic one 
from 
Radio Shack (2 amp?) and it worked fine.  CR3 is an SCR with ratings 
appropriate for the charging voltage and current you're regulating.

When the battery reaches the preset target voltage, 2.4 to 2.5 volts per 
cell, the charger begins to cycle on and off.  The closer the battery gets 
to full, the shorter the on periods and the longer the off periods (lower 
duty cycle).  This is a coarse PWM which reduces the average charging 
current. When the "ready" light is blinking off briefly every 10-20 
seconds, 
the battery may be considered full.  Pull the plug.  This charger should 
not 
be left connected indefinitely.

If you want a faster charge with a higher average finishing voltage and 
current, turn up R5, and turn off the charger when the ready light is 
blinking off every 5-10 seconds.

This controller has no temperature compensation, so a bit of voltage 
trimming summer and winter isn't a bad idea.  You'll also want to adjust 
it 
down a bit as the batteries age.

This circuit shouldn't be used as a battery maintainer.  But then, I've 
found that battery maintainers generally do more harm than good.


David Roden - Akron, Ohio, USA

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