Selecting the right BMS and MPPT

Roventae

New Member
Joined
Apr 17, 2022
Messages
1
Hey!
I have learned so much from this forum and the videos, this community is amazing!

I'm planning an off grid system powered with six 540w half/split cell panels in 3s 2p format. The panels are 41.4v and 12.9A each.

Regarding the MPPT solar controller: I can only find solar chargers which have an MPPT range @ operating voltage of up to 115v, or greater than 120v. My issue is that I expect the voltage of the array to be around 100-124v due to some shading in the afternoon. If I take the 120-180v MPPT, will it not charge when there is some shade?

As for the BMS: I'm buying two 24v LifePo4 battery packs with 200 Ah each. The plan is to wire them in series to make a 48 volt battery. Even though I think each of these battery packs has a BMS built into it, I understand that I will need to put a BMS across them. Do I need a 24 volt or 48 volt BMS? (It's the voltage of the BMS for the cells or for the entire pack?)

Thank you so much for your help!
 

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
4,038
Location
USA
Hey!
I have learned so much from this forum and the videos, this community is amazing!

I'm planning an off grid system powered with six 540w half/split cell panels in 3s 2p format. The panels are 41.4v and 12.9A each.

Assuming that's Vmp and Imp. Guessing Voc is 48-50V?

Regarding the MPPT solar controller: I can only find solar chargers which have an MPPT range @ operating voltage of up to 115v, or greater than 120v. My issue is that I expect the voltage of the array to be around 100-124v due to some shading in the afternoon. If I take the 120-180v MPPT, will it not charge when there is some shade?

You're going to need a 145-150Voc max MPPT to handle 2S. 80Vmp with 2S panels is more than adequate to deal with shading. More voltage isn't going to do any measurable good. The only benefit you'll get from 3S is reduced wiring losses if you have long PV wire runs; however, you're going to need to upgrade to a 200V MPPT.

As for the BMS: I'm buying two 24v LifePo4 battery packs with 200 Ah each. The plan is to wire them in series to make a 48 volt battery. Even though I think each of these battery packs has a BMS built into it, I understand that I will need to put a BMS across them. Do I need a 24 volt or 48 volt BMS? (It's the voltage of the BMS for the cells or for the entire pack?)

Confirm that the 24V batteries can be wired in series. Some can't.

Ensure that they are both truly fully charged at 100% SoC as 24V batteries (preferably in parallel) before you put them in series.

There's no such thing as you describe. The best you can do is to use a set of balancers that monitor the two 24V batteries and work to keep their voltages equal by passing charge from the higher voltage battery to the lower voltage battery. These are prolific for 12V batteries in series, but I'm not aware of anything that works with 2X 24V in a 48V system.

Another option would be to install a Victron Smartshunt or similar that has mid-point monitoring. That would report the total voltage, and the voltage of one of the 24V batteries.

Even Battleborn recommends you periodically break down a 24, 36V or 48V bank made up of its 12V batteries and individually charge each 12V to full.

Here is an example of a 48V balancer for 12V batteries:


I use a similar product on my 4S2P bank of 12V Trojan lead-acids.
 
Top