MPPT and PWM on same battery bank?

7 Posts
Feb 24, 2009 10:59 pm
MPPT and PWM on same battery bank?

I have a C40 charge controller (which I understand is PWM) charging 4 L16HC batteries from 4 Siemens Panels.  I also have 2 Mitsubishi 185w panels that came with a BZ MPPT 500, originally to go on a 12v system.  Since the nice Trace system is 24v, I want to get 4 more batteries to series/parallel together, and let all panels charge the 8 batteries, and have a simplified 24v system.
* Will the 2 different type controllers work with each other?
* If I need to get another controller, should I get the MPPT?
* When I combine them,whatever they are, is it OK to let them wire together on the bars inside of the Trace DC 250 Disconnect, or can the two leads not 'touch' each other?  If not, how then?

220 Posts
Feb 25, 2009 03:00 am
Re: MPPT and PWM on same battery bank?

 gee rhett,

 you have been holding out on us! what other good stuff have you got tucked away over there?

 as you know those Mitsubishi modules are what we are calling an 18 volt nominal panel ( a half step between nominal voltages?). lucky for us that we also have an mppt controller to go with it. now before we get too excited, a word^ about those bz 500's. i use the plural because that same controller came/comes in two different flavours and that may make a difference in our application here. first off these are all around great products for the price and i cut my teeth into mppt with these controllers, however they are not true open format mppt controllers like the mx, fx or apollo products and are "optimized" for their expected use. i have found that the 500 is good across limited ratio of mppt voltage conversion. example: on a 12 volt system we can run a 12 volt nominal module, an 18 volt or a 24 volt nominal array with good results. and any array with a volts open circuit (voc) above 50voc makes the bz 500 unpredictable. now thats the first flavour...the same looking model the bz500hv seems to like a voc between 30 and 80 voc and still has that same voltage conversion ability. ie across one full step in voltage. example: 24 volt system using 24 volt nominal, 36 volt and 48 volt nominal array.

 if you take the four screws off the front cover you will find a paper label giving the firmware version and the 500 or 500hv designation. i could never get a series pair of 18 volt nominal modules (36 volt nominal for the string) to run with the 500. but had no trouble at all running them with the 500hv.

 so i would have a look at the label and see what we have. if it's the 500hv i don't see a problem if it's the bz500 i would still try it and see if it might work. it would do no harm as they are both rated with a 100 voc limit. worse case would be to send it in for the upgrade service they offer. that company is very customer friendly, don't be surprised if you call and get frank himself on the line.

or via email:

Frank Lewon BZ Products  bzp @ "

 see a full discussion of topic on this thread:,676.10.html

"* Will the 2 different type controllers work with each other?"
 they should play well together, folks do this all the time.

"* If I need to get another controller, should I get the MPPT?"
 these oddball modules are most efficient with mppt but will work with standard pwm with lots of lost potential.

"* When I combine them,whatever they are, is it OK to let them wire together on the bars inside of the Trace DC 250 Disconnect, or can the two leads not 'touch' each other?  If not, how then?"

 yep, right to the same busbars.

 rhett, about adding onto that bank. for the purposes of this public forum lets assume that the existing battery bank is less than a year old and is in good shape. what i would do is double my bank with the exact same batteries using identical cable, lugs and wire lengths. an exact copy of what you have. then run that second series string up to the dc250 power center and parallel them there at the dc buss.

 best regards, dave

^ comments about the bz controllers are based on my experience only..your mileage may vary.
7 Posts
Feb 25, 2009 07:54 am
Re: MPPT and PWM on same battery bank?


I looked inside the box, and there is no indication that it is the 'HZ', it only says 'MPPT 500 Vo'.  The paperwork on the panels says that they have a Voc of 30.6. Wired together they are 61.2, right?  I am still going to try it, keeping in mind that there is an upgrade available.

The Mitsu panels came in a package system from another company, and included a Freedom inverter/charger, that I will be making obsolete by combining.  The Trace system came from a guy here locally who foreclosed on his house,selling his Y2K stuff (sad).  I plan to get another DR inverter to go with the one from that system.

The batteries are only a month old, and I will be getting 4 more in the next weeks.  Do I understand correctly that I am to 'mirror' the cabling of the first set, and run the new 10' cables from the DC250 (it has extra lugs) to the new batteries?  Basically the batteries are wired for series, and the parallel connection is made in the disconnect box?  I know thats what you said, but I like to double check, especially with the potential to zap a few hundred (or thousand) dollars!

When I do get the additional DR inverter, will it connect directly from the battery bank, maybe on the ends of the 4 new batteries?

I just want to say, and I know that all will agreed that this forum has been a tremendous help to all of us trying to make a difference with solar, wind, etc.  Thanks to AltE for providing it.
220 Posts
Feb 25, 2009 02:29 pm
Re: MPPT and PWM on same battery bank?

hi rett,

 we are getting into a controversial subject here on this wiring method question.

 i am always willing to reevaluate my thinking on a subject when more information becomes available. and believe there are really no set absolute "facts" just informed opinions that we arrive at when considering a topic.

 have a look at this diagrams:

 see figure 4, 5 and these are "official" diagrams from the company.

figure 4- is a classic example of a "daisychain" wiring method..i would avoid this one at all cost.

figure 5- is much better but we are still sharing the negative conductor. not too bad.

figure 6- nice dedicated inverter feeds and disconnects. this, in my view is the way to go.

 some good discussion on the topic of inverter stacking on this previous thread.,1232.0.html

 oops gotta go for now, we are sure to get more opinions on the subject..and thats a good thing when trying to weigh our options.


 back again. having another glance at those diagrams. number 5 is not that bad after all-and number 6 is a cleaner version of number 5 with a nice enclosure (like yours) with the ground block and grounding conductor shown. also they show the battery bank strings tied together at the bank, not such a bad thing with just two strings. an alternate way, especially when we start getting into many parallel strings, is to run them (all the strings) to a pair of busbars then sink and source from there. otherwise the standard advice of keeping the parallel strings to two or three max is important. anyway that is not an issue here and it looks like we won't have room.

 more sure would be nice if there was a product that would tie those breaker inputs together with a good sized block that we could land those big lugs on that are comming up from the bank. effectively creating a positive buss in a small space (like the midnight solar combiner fingers do in their PV combiner box). somewhere along the line we have become used to using our battery interconnects and battery posts as busbars...

 so i cast my vote on the method shown in figure 6 of the xantrex installation manual.

have fun. dave

"what one man can do another can do"
sir hopkins in the edge
« Last Edit: Feb 25, 2009 05:11 pm by david ames »

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