Re: (ET) Pulsing

["David Roden" <etpost drmm net>]

I've seen similar behavior in road EV controllers.  Something similar also 
 
happens in some power inverters (12/24/36/48 volts to 120/240 volts).  

What I think may be going on: many EV controllers limit battery current to 
a low value - typically 30-50 amps - when the battery voltage falls to 
around 1.75 volts per cell (31.5 volts on a 36 volt nominal battery).  

In a healthy battery, 1.75 vpc indicates ~20% state of charge.  At this 
point you're in danger of reversing a cell if you carry on with drawing 
high current, so the controller cuts back the current to protect the 
battery.  

It sounds like your battery may have one or more cells with somewhat high 
internal resistance.  Effective internal resistance also increases as the 
battery discharges.  

Under heavy load at lower SOC, the battery voltage falls.  The controller 
sees low voltage and throttles the current.  Under the reduced load, the 
battery voltage bounces back up, above the controller's threshold, and the 
controller lets you have more current for a moment  Under the new higher 
load the battery voltage falls again, so another controller cut.  Rinse 
and 
repeat.  

I suggest putting an analog voltmeter on the battery (it's easier to see 
this with a moving needle than with constantly flickering numbers on a 
digital display). If this is the problem, you'll see the battery voltage 
falling and rising in time with the pulsing.  


David Roden - 25 Years with ETs

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