two charge controlers

scottie

would it be dangerous to use two charge controllers to charge one battery bank. ie. a 20 amp mppt renogy controller connected to two 100 watt panels and one 40 amp mppt controller connected to four 100 watt panels. this would allow me to build one pole mount rack and initially purchase 4 100 watt panels and a 40 amp controller and in the future add two more panels and a 20 amp controller. this would also give me some degree of redundancy.

Estragon

Multiple controllers on a bank is fine. I have two controllers charging my 48v bank and will soon be adding a third.

It can be useful (but not essential) if the controllers can sync stages, so when one goes to float the others follow. Also helpful if you can adjust for actual voltage so both agree.

scottie

thanks so much, it seems my learning curve is larger than it should be. I couldnt see why it would be a problem however the  product information on the renogy website seemed to advise against it. do you know if the renogy controllers will sync? Also I noticed that the input voltage on the mppt controlers allow higher voltage than the charging voltage, does that mean that I could put my panels in series and raise the voltage allowing me to use smaller guage wire for my supply leads? If so will the controller automatically charge the bank with the correct voltage? Sorry for so many questions. thanks

Estragon

I don't know if those controllers sync. If they advise against using more than one CC on a bank, I doubt it.

For most MPPT CCs you have to use higher voltage array than charging voltage. Assuming this is a 12v bank with 12v panels - charging voltage will be around 14.5v. A 12v panel will typically have a voltage at max power (Vmp) of around 18v. Although this would be enough to charge, putting panels in parallel increases current, which means more wire losses and/or requiring heavier wire on longer distances from panel to CC. Putting panels in series increases voltage while leaving current the same, so smaller wire can be used. Off-grid controllers can generally handle 100-150v, some even higher. In your example, a string of 4 12v panels might have a Vmp of ~72v and ~5a current. If the wire run is short, you could wire it as two strings for 36v and 10a. MPPT CC turns this into around 14.5v and 25a current in either case. Going from 36v to 14.5 is a bit more efficient than from 72v in the controller, but the higher current means more wire losses. Unless the wire run is short, 72v is likely to be better.

mcgivor

scottie said:

would it be dangerous to use two charge controllers to charge one battery bank. ie. a 20 amp mppt renogy controller connected to two 100 watt panels and one 40 amp mppt controller connected to four 100 watt panels. this would allow me to build one pole mount rack and initially purchase 4 100 watt panels and a 40 amp controller and in the future add two more panels and a 20 amp controller. this would also give me some degree of redundancy.

As @Estragon said it shouldn't be a problem, as long as each CC has a separate array and both have the same charging algorithm, the one with the largest array will lead, as soon as the voltage requirements are met the one with the smaller array will follow, at least that is the explanation Morningstar gives for their controllers. The one thing they caution is that they cannot share an array, so if as you state they are divorced as far as array is concerned and both are programed identically, there should be no issue.

littleharbor2

And as a small bonus you have the redundancy factor. If, for some reason, one of your controllers ceases to function you will still have one working while you find a replacement. 

Estragon

@mcgivor - good point- definitely separate arrays. On the same array the controllers would mess with each others heads during MPPT sweeps. Depending on location there may also be some benefit to orienting each array differently.

Not sure about the larger array leading though. Maybe assuming higher voltage drop in cable from CC to bank and no voltage sense on bank?

scottie

Thanks for the information. I have had a conversation with Renogy and they have also been a big help. I am about ready to take the plunge when I get it completed I will post photos so you all can critique.

Johann

Most controllers will not sync/follow me  and if you have an MPPT then you may make the situation worse.

mike95490

Johann said:

Most controllers will not sync/follow me  and if you have an MPPT then you may make the situation worse.

Maybe.   I have 2 different brands, each with it's own PV array, and with careful manual software adjustments, they now share the load (charging batteries) pretty equally.

mcgivor

Here is some information
http://www.morningstarcorp.com/parallel-charging-using-multiple-controllers-separate-pv-arrays/

HumMichael

Don't mean to hijack but I just added a second mppt cc, Tristar 45 , for a small array facing West.
My question is whether it is ok to connect the negative battery cable to my whizbang jr shunt? or just connect to battery bank sepatate.
I don't want to confuse my Midnite200 by doing this but it seems like it would be ok.
Thanks for your advice.
Michael

mike95490

If you want it counted by the shunt, add to same place as other charge controller minus connects to.

HumMichael

> @mike95490 said:
> If you want it counted by the shunt, add to same place as other charge controller minus connects to.

That makes sense. Thanks