2 MPPT on a battery

Mirkus

Hello guys

For first, I am pretty noob with photovoltaic system.. 
For second, I am not English and my English isn't good 

Now I can explain my situation and my idea

Well I have 4x105w solar panels on one mppt, with 2 array, and 2 150a lifepo4 in parallel.. Now I want do a upgrade.. Unfortunately my mppt support only 450w,well I need a second mppt..

Well my idea is:

I buy 2 solar panels (idk right now how many watt) and the renogy dc-dc mppt charger.. And to connect the 2 mppt I use a bus bar.. Like so:

Positive cables of the 2 mppt on and the positive cable of the battery on a bus bar.. The negative cables oft the 2 mppt to the shunt, together with all the other negative..

Now.. I contacted renogy and liontron (the producer of the battery) and both told me, that this is not a good idea .. I asked to some friends, not noobs like me, and they told me, that my idea works good.. I trust more my friends than renogy and liontron, but I want other feedbacks from people with high skills on DIY..

Well the question is.. Does my idea work?

I am sorry for my English and I hope you can understand what I wrote

mike95490

Each PV array has its own MPPT charge controller , and the output of each controller goes to the battery bank,
the 2 controllers in parallel at the battery or the battery common bus.

You need to know the overcharge current where the battery BMS shuts off the battery.   You never want the battery to disconnect from the MPPT controller output, the MPPT relies on the battery for it's internal DC power and load management.  if the battery disconnects and the controller has no load / battery, its a good chance it will fry itself.   Good systems use the BMS to trigger a relay that disconnects the PV panels from the controller, or place the controller into standby

BB.

Here is a good explanation of how best to connect batteries in parallel to ensure they share current properly during charging and discharging:

http://www.smartgauge.co.uk/batt_con.html

In general, you can connect two or more solar chargers in parallel to a single battery bank just fine. Some controllers can synchronize charging states (bulk, absorb, float)--While two controllers just paralleled to a common battery bank (and no sync) will not always "match charging states"...

But that does not matter. Each controller will make its decision as to battery charging just fine and if one changes state before the other--No issues. In the end, the charge controller with the highest voltage set points "will win" as the battery finishes charging and floats.

As long as your maximum charging current and charging voltages do not exceed the battery requirements/limits, you should be OK.

As Mike says--You don't want the Battery Management System on the Lithium bank to cut off/turn off the batteries in "normal operations". That should only happen if something goes terribly wrong--Batteries being taken dead (more loads, less charging), or you have a charger failure (over voltage).

-Bill

Mirkus

BB. said:

Here is a good explanation of how best to connect batteries in parallel to ensure they share current properly during charging and discharging:

http://www.smartgauge.co.uk/batt_con.html

In general, you can connect two or more solar chargers in parallel to a single battery bank just fine. Some controllers can synchronize charging states (bulk, absorb, float)--While two controllers just paralleled to a common battery bank (and no sync) will not always "match charging states"...

But that does not matter. Each controller will make its decision as to battery charging just fine and if one changes state before the other--No issues. In the end, the charge controller with the highest voltage set points "will win" as the battery finishes charging and floats.

As long as your maximum charging current and charging voltages do not exceed the battery requirements/limits, you should be OK.

As Mike says--You don't want the Battery Management System on the Lithium bank to cut off/turn off the batteries in "normal operations". That should only happen if something goes terribly wrong--Batteries being taken dead (more loads, less charging), or you have a charger failure (over voltage).

-Bill

Wow.. I read the link.. This is incredible.. And ofc I connected the batterys with the first method  

Anyway thanks bb and Mike.. Well the mppt, which I have now, I can set easy the charging voltage.. Right now is set to 14.3v..I tried the 14.4v but the bms of my battery show me that the battery was on overvoltage.. So.. For don't have problem I just set, if possible, the second mppt to max voltage 14.2v or 14.1,right? 

mike95490 said:

Each PV array has its own MPPT charge controller , and the output of each controller goes to the battery bank,
the 2 controllers in parallel at the battery or the battery common bus.

You need to know the overcharge current where the battery BMS shuts off the battery.   You never want the battery to disconnect from the MPPT controller output, the MPPT relies on the battery for it's internal DC power and load management.  if the battery disconnects and the controller has no load / battery, its a good chance it will fry itself.   Good systems use the BMS to trigger a relay that disconnects the PV panels from the controller, or place the controller into standby

I ll connect a relay to place the controller into standby  

mike95490

Mirkus said:

I ll connect a relay to place the controller into standby  

Thats the best way, you are not switching HV DC from the array, and if your gear supports it, that's the best way

Dave Angelini

The very best way and easiest, the most costly, the most accurate, is a system that all devices are networked to communicate with the BMS. No relays to fail!  The system below is showing an alarm and can also be monitored on a smart phone from anywhere there is internet. The weather at the site is there for someone to monitor from far away.