Loads in Series

Why Electronic Loads Can't Be Connected in Series

A fully-charged ePure battery produces in excess of 58 volts.  Unfortunately, my active loads are only specified to 55 VDC.  The load's user manual talks about connecting the loads in parallel (to double the current/power capability) but does not mention operating them in series (to double the voltage/power capability).  So there must be some potential (pun intended) issues to overcome.

An active load operates in constant-current mode by changing its internal resistance in response to a change in the applied voltage. 

By placing two loads in series, we intend to drop half the battery voltage across each load (two things in series having equal resistance and passing the same current would each produce a voltage drop of one-half the total voltage).

Unfortunately, it's not possible to independently set two active loads for exactly the same current - they will always be slightly different.  Let's say one load is programmed for 10.01 amps and the other load is programmed for 9.99 amps.  The 9.99 amp load will control the current and operate in constant-current mode.  However, the 10.01 amp load will try to pull more and more current by lowering its internal resistance - but it can never draw more than the 9.99 amps.  Soon, the 10.01 amp load's resistance will be very nearly zero ohms (like just a piece of wire).  Consequently, the 9.99 amp load is forced to drop the entire battery voltage.  If that voltage exceeds the load's rating, bad things could happen.

Although I think it would be possible to modify both loads to run inside a single feedback loop (with a single commanded current), it probably would not be worth the effort for my use. 

Luckily, my active loads will operate safely slightly above their 55 VDC rating.  In fact, the operating manual talks about testing the over-voltage protection circuit to make sure it kicks in between 57 and 70 volts (a pretty wide margin).  I was able to run the load at 60 V (the endpoint of the analog meter on the front panel) just fine.  That was sufficient to test the ePure's battery.