WbSr W/Sense Leads That Measure Battery V?

[Vic]

Have recently encountered a situation with cyclic Opportunity Loads disturbing Absorb Current due to Vdrop in Classic's battery connections.

Upgraded the A/C unit that cools the Power Room from 6 K to 10 K BTU -- a Window A/C.  Near the end of Absorb,  when this larger A/C runs it needs about 16 Amps at Vabs of about 58 V.  This additional current in the battery connections to the Classic 150 result in an additional several hundred millivolt voltage drop in the battery cables.

This added V drop causes the battery charge current to drop a bit,  and could cause the Absorption stage to be terminated,  as we always,  only,  use WB EA to end Asorb (have watched this process for some time,  but only once thought that the A/C load ended Absorb early).

Also noted that the added voltage drop in the battery cables  causes the battery to supply part of the added load,  thus reducing the WB measured charge current.  When the A/C cycles off,   the current removed from the battery to help run the cycling load is returned,  causing the measured charge current to increase.

This dithering of measured charge current,  at times seems to delay the exit from Absorb ...

So,  it seems increasingly important for the Classic to be able to measure actual battery voltage,  to help eliminate the effects of Opportunity or other heavy loads during the final stages of absorb,  if not at other times,  like during EQ,  when a fairly accurate EQ voltage is desirable.

One thought has been,  that perhaps an/the extension of the functionality of the WB,  to include the ability to measure and communicate battery voltage might not be such a huge Engineering task.

OR,  perhaps this is something that the Black Box Project might be able to do.  Seems to me that the measurement frequency would not need to be too high -- something like one sample per second might be perfectly adequate ...

With very inexpensive PVs,  these days,   seems that more and more users expect to run heavier loads,  even in the later part of Absorb,  where this effect could cause battery charge issues,  IMO.

Just a thought,   FWIW,    Thanks,    Vic

[Westbranch]

Hi Vic, based  on our experience with the fridge and now adding s much load as possible,Ii think that if the draw is big enough it could also impact on the final SoC  once float has been reached.

Today we were  into Float, just, and SWMBO wanted to test the electric broom, a SEBO (old upright Hoover style) vacuum.  Well it pulled 30+-A and we almost pulled the batt. out of Float when the fridge kicked in,   by my reckoning.  It took quite a while for the Amps to drop back to a float norm of ~3  to 5 A 

[Resthome]

+1 for the addition of the battery voltage sense leads.

[Kent0]

+2 for the voltage sense leads so that the charge controller can regulate the actual battery voltage.

Some good things happen from that. The charge controller can be located farther from the battery. Not a desirable thing to do, but sometimes it is necessary. I've had to used brand M controllers on a couple projects where the charge controller had to be quite a distance from the batteries. Also, coordination of multiple charge controllers would be improved if they all agree on the voltage measurement.

[australsolarier]

+3  yes i agree with the voltage sensing. it is not so much the wires but the fuses and breakers that cause a voltage drop of 0.2 volt/40amps whilst charging in my system. the midnite classic goes about 2,5 hours before the battery is fully charged into absorb mode.

[T]

Seems like this could be done without another lead to the batteries.

Battery voltage = Classic voltage - (Classic current x resistance from classic to terminal block) -(WBjr current x resistance from terminal block to batteries)

Terminal block is the point where the load wires connect to the system.

The Classic would need to input the two resistances from the user.

[boB]

+4 for another input to look at Vbatt.   You'd think that those guys would have done that to start with !

I've wanted to do that for a while now but after the WB Jr. was designed.  Just takes an extra A/D converter
on the same board and of course another word of data.  We wanted the WB Jr. to be as inexpensive as possible.


Well, it isn't that hard actually, but would need to be re-designed of course.  Just not that difficult though.

I'll as the others here if that might be something to do in the next year.

boB

[tecnodave]

+5 for the WBsr   With voltage sensing......I'd gladly upgrade my WBjr's

td

[Resthome]

+5 for the WBsr   With voltage sensing......I'd gladly upgrade my WBjr's

td

+6  The votes are mounting up boB.

[dgd]

...
Well, it isn't that hard actually, but would need to be re-designed of course.  Just not that difficult though.

I'll as the others here if that might be something to do in the next year.

boB

If it has to be redesigned then there is probably the opportunity to include a few more useful features 

Since there is a voltage sense wire now going to the battery +ve then this, or an additional wire, could be used to power the WB, a voltage regulator would likely be needed too. This removes the power dependancy on the AUX port.
The data could be sent via rs232 to one of the Classic serial ports, this would also likely need the follow-me loopback removed but this would be a good thing anyway as the present arrangement just wastes serial ports.
This would then free up the AUX2 port so that it could be used for more appropriate auxially purposes.

To complete the new WB as a battery data acquisition device the RTS could be connected to the WB and the RTS port on the Classic/KID repurposed for some other use.

Last, with an independantly powered WB and rs232 port for data then the progression to a stand-alone battery monitor looks very feasible.
Based on a WB and MNGP where the MNGP is powered via its rs232 (as exists now) from the WB.
Even nicer if it had an aux port or two - relays or PWM
A new product and revenue stream for MIdnite 

dgd

[zoneblue]

With only a few feet of cable to our banks, i dont see what the fuss is about cable drop. Who wants to drop volts there? Get a decent cable and call it good. However dgds entire line of reasoning in the post above feels pretty darn sound to me.

[Vic]

With only a few feet of cable to our banks, i dont see what the fuss is about cable drop. Who wants to drop volts there? Get a decent cable and call it good. However dgds entire line of reasoning in the post above feels pretty darn sound to me.

Hi zone..,

Not every system is petit enough to enable,  "only a few feet of cable".

Somewhat larger systems use inverters and attendant conduit boxes that are several meters in width.  Often, with larger inverters,  minimizing the cable length from the battery to the inverter is very important  --  the CC to DC breakers and to the battery are really secondary considerations.

Also,  some of us are in retrofit situations,  where CCs have "always" needed to be mounted where access to the front panel controls,  and the ability of reading the LCD display may dictate that the CCs need to be placed in an accessible spot that requires a relatively longer CC to battery cable.

Furthermore,  there are additional considerations for some of us,  like CC emission reduction filtering.   The Classic really wants significant RFI filter inductance in a Common-Mode choke on the CC-to-battery leads,  to try to reduce HF radio emissions.  Here,  this means about three,  or so meters of # 6 AWG bifilar cables,  wound on a stack of Hockey puck toroidal cores.  Larger cable means less inductance (fewer turns on a given core size,  due to a fixed core window area),   so the cores would need to be larger,  increasing the cable length for each turn on the core.  Winding these inductors with # 4,  or # 2 AWG is quite a challenge  --  I am simply not strong enough to use any cable larger than #6.

Often,  there is not a single solution for systems which need to accommodate several important variables.

When using Shunt EA,  the voltage drops resulting from fairly large Opportunity Loads during Absorb,  can have  a fairly large impact on the adequacy of the Absorb charge stage.

We are all trying to design systems that balance essentials with  what is available at the time,   IMO.

Just my read on things,   not to try to lecture too much.   FWIW,   Vic

[dgd]

...
Also,  some of us are in retrofit situations,  where CCs have "always" needed to be mounted where access to the front panel controls,  and the ability of reading the LCD display may dictate that the CCs need to be placed in an accessible spot that requires a relatively longer CC to battery cable.

thats a really good reason for the human interface, the Classic's relatively tiny lcd screen and fiddly rubber keys, NOT being mounted on the controller, instead being remotely mounted elsewhere more human convenient.
The proper solution to this would be access using a web browser - maybe the next generation Classic 

dgd

[Vic]

...
Also,  some of us are in retrofit situations,  where CCs have "always" needed to be mounted where access to the front panel controls,  and the ability of reading the LCD display may dictate that the CCs need to be placed in an accessible spot that requires a relatively longer CC to battery cable.

thats a really good reason for the human interface, the Classic's relatively tiny lcd screen and fiddly rubber keys, NOT being mounted on the controller, instead being remotely mounted elsewhere more human convenient.
The proper solution to this would be access using a web browser - maybe the next generation Classic 
dgd

Yes,  that has always been my plan -- to mount the Classics just above the DC Conduit box,  with MNGP/s mounted where the CCs are now,  but,  this is really an opportunity to re-do some other things,  and that makes this a bit of a project.  There are several other projects of much higher importance right now.

Vic

[australsolarier]

the lifepo4 batteries have a very small difference between fully charged and fully discharged. in my case with a 12v system when charging with 50a through the wizzbang the differential between midnite classic terminal and battery terminal is 0.36v. this difference is not so much caused by the wiring but the fuses and breakers. in my case it means the midnite goes into absorb something like 70% soc.
also it causes a very "soft" approach in recharging during float. it means it could charge much more much faster after a temporary discharge, when the sun goes behind a cloud or consumption is bigger than the solar array can provide.
to give you an idea:  at the moment the system is 66% soc, whizzbang shows 41.6a.  battery midnite says 13.8v.  the actual voltages at the battery is 13.469v, the midnite 13.763. so when the midnite battery hits 13.9v it hits absorption and starts reducing charging the battery. (actually 14.1v. for some reason when you program absorption 14v, the actual absorption kicks in earlier and then goes up to 14.1v, which in my case works out alright, with around 14v at the battery terminals).
yes with hindsight the whizzbang should have been equipped with a sensing wire.
for example the selectronic smpc pro 481 inverter  has such sensing wires.

another disadvantage is, i my case i have to walk to the shed to check on the real voltage of the lifepo4 battery, as the solar status panel is not reliable for the above reasons.

having said all that, naturally a 48v system is much easier in this voltage differential.
ok, i hope the midnite guys can understand what i mean.