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BZ 500MPPT assembly issues

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dnix71:
I have a BZ MPPT 250 and an MPPT 500. The 250 was assembled with too-short circuit board standoffs and failed after 3 days. BZ promptly replaced the unit and the retailer was notified and pulled his stock and sent it back for rework.

My 500 is a couple of years old now. It has always reported the battery voltage on the display as being much higher than it really is when the unit is charging, but is accurate at rest. That forced me to set the float cutoff near the max to avoid the unit cutting back on charging when the battery was nowhere near full. This behavior has gotten worse with age.

The 250 is on a panel mount so it's easy to see the front and back of the board. The 500 is in a box with the FETs heat sinked to the back of the box. Since I figure I got my money's worth from the 500 and had a week off from work, I took the 500 apart to see if I could tell what was causing the voltage reading discrepancy.

The caps look okay but I don't have an ESR meter to check them.

The panel negative and battery negative are bridged on the back side of the board by two short wires that are soldered to the same spot. Thinking that the wires might simply be too thin I added soldered an extra wire over each and reassmbled the 500.

This "fix" made no improvement and actually seemed to make it worse in bright sun. So I took the 500 apart again and removed the wires.

Looking over the board carefully I noticed the ring toroid with 3 separate wires (which is visible when you remove the enclosure cover) is in the main positive output trace. It's probably an RF choke. Earlier BZ models were noted for interfering with CB radio. I have portable radios very near this unit and have never heard any noise from it.

Looking at the back of the board where the toroid was mounted I noticed the three wires were stripped of varnish at the end and tinned before inserting them through the board and soldered. The cut ends, however stuck out in the air because there wasn't enough solder on that pad to submerge the ends. I added a pool of solder over the pad until the ends were covered and reassmbled the 500. It still reads a bit high under bright sun but the difference is much smaller.

I also added clip on CB ferrites to the positive wires (panel and battery).

If you have a BZ 500 and it reads high when charging it might be worth the time to tae it apart and add some solder to the back side of that choke coil.

boB:
If it really did read better after disassembling and reassembling,  maybe it could also have
been something else in there that you jarred while moving its insides around ?

Kind of like when you smack the side of a TV set to get it to work right.

Or maybe it was just what you did by resoldering that wire.

You might try smacking it lightly to see if the voltage reading moves around while
doing that just to make sure.

boB

dnix71:
The voltage reads right when there is no incoming current. I bright sun it would jump up 2 volts. Nothing is loose. The controller is 2 stage. There is a relay to switch 2 FETs, but the voltage jump was occurring without the second stage in play.

ChrisOlson:
My MPPT500 did that too, to a certain extent.  But never 2 volts.  Maybe about .3 volts high on an average day and up to .4 or .5 volts on a good day.  But my Classics do that too.  They read the right voltage at rest and a couple tenths high during full power charging.  But with 60-70 amps flowing thru #4 THHN that's to be expected.
--
Chris

boB:

--- Quote from: dnix71 on January 29, 2013, 09:36:14 PM ---The voltage reads right when there is no incoming current. I bright sun it would jump up 2 volts. Nothing is loose. The controller is 2 stage. There is a relay to switch 2 FETs, but the voltage jump was occurring without the second stage in play.

--- End quote ---

Where is your external meter measuring that voltage ?   At the battery terminals ?  Terminals of the BZ controller ?

Sounds like the BZ's voltage sample point is not quite at the BZ's terminals but somewhat inside
the controller.  That makes sense then that your soldering job helped it out.  2.0 volts is a LOT
though !  For any decent amount of battery current, that would make one hot connection at
the weak point !

boB

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