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bms questions

ElectricDanimal

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Sep 21, 2020
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I have a 48v inverter, charger, (mpp LV5048) but i have to use separate solar charge controllers at 24v. I will link 2 -24v LifePo4 batteries in series. (I have a maximum of 60vdc from my panels and they can not be used in series)
1. can i use 1-48v bms even tho the solar chargers are at 24v?
2a. is there any advantage to using 2- 24v bms on each battery ?
2b. what happens when the inverter charger tries to charge at 48v using the on board charger if the bms are 24v each?
3. can anyone help or recommend which bms to use? (i would like bluetooth compatible)
I am new at this and would appreciate any help.
 
1. Absolutely, positively a horrifying idea. Get 48V charge controller(s). You're asking for trouble. You WILL apply different input to two halves of the battery.
 
1. Absolutely, positively a horrifying idea. Get 48V charge controller(s). You're asking for trouble. You WILL apply different input to two halves of the battery.
thanks for your response.. unfortunately, I can not use a 48v charge controller. I only have 59v to play with and this will not give me much solar pv wattage because of the lack of voltage differential.. the panels can not be seriesed. for a higher voltage.
 
Are you running into the PV input limit on the MPP, ie you have high voltage panels, or is there another factor at work here? The 5048 has a PV input maximum of 145V.
 
"can't be used in series." Why? Do they not have wires that come out that could be cut and splice together?

You have three choices:

1. Do something that's a hardcore bad idea.
2. Adapt your components to work in a sensible manner.
3. Replace the components that aren't working for you.
 
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"can't be used in series." Why? Do they not have wires that come out that could be cut and spliced together?

You have three choices:

1. Do something that's a hardcore bad idea.
2. Adapt your components to work in a sensible manner.
3. Replace the components that aren't working for you.
@ElectricDanimal option #1 is to be avoided.
I'm gonna guess that you can make series strings.
Maybe even all your panels in one string.
 
"can't be used in series." Why? Do they not have wires that come out that could be cut and spliced together?

You have three choices:

1. Do something that's a hardcore bad idea.
2. Adapt your components to work in a sensible manner.
3. Replace the components that aren't working for you.
The panels i am using are 1. on grid 2. TenK APEX-500 modules using LeadSolar inverters..
I am working on adding battery for stand by power and would like to get the pv power to my battery backup..
the apex-500 panels from TenK are very special.. they were built with an on board MPPT module on each panel and as such, they can not be wired in series (easily, but that is a whole different rabbit trail) the on board module will not allow it to produce any voltage for safety reasons until it sends a pulse to charge capacitors in the LeadSolar Inverters. once they are communicating with the inverters i have 59 vdc available with the grid off, and 52vdc while the grid is on . The system uses the voltage differental from 59-52 for producing grid power.
 
Ah, so, the BMS issue is secondary. Do you have a workable backup solution?

Have you investigated AC coupled solutions where you add another "backup" inverter with batteries. When your grid goes down, you switch over to the backup inverter, and the grid-tie system synchronizes with your backup inverter. Your backup inverter is charged from your existing solar by the grid-tie system.

This depends on the backup inverter you select and how well the grid-tie system behaves with an inverter vs. the grid.
 
Are you running into the PV input limit on the MPP, ie you have high voltage panels, or is there another factor at work here? The 5048 has a PV input maximum of 145V.
these panels can not be wired in series.. i only have 59vdc max
 
Ah, so, the BMS issue is secondary. Do you have a workable backup solution?

Have you investigated AC coupled solutions where you add another "backup" inverter with batteries. When your grid goes down, you switch over to the backup inverter, and the grid-tie system synchronizes with your backup inverter. Your backup inverter is charged from your existing solar by the grid-tie system.

This depends on the backup inverter you select and how well the grid-tie system behaves with an inverter vs. the grid.
this is basically what i am doing.. the LeadSolar microinverters on only for production when the grid is connected... i have an mpp lv5048 with 2-24v forktruck batteries wired in series to get my 48vdc for backup power... I am adding solar charge controllers to capture the power during power outages to charge my batteries. I want to upgrade my 24v forklift batteries to LifePo4, but i have to charge them at 24v either way because of my 59vdc max from the panels
 
It doesn't sound like what I described, or you wouldn't be trying to do what you're doing. If you get an inverter capable of AC coupled operation, and your existing grid-tie inverter will work with an attached inverter instead of the grid, you're done. The backup inverter itself uses power from the grid-tie system to charge the batteries.

Will the MPP Solar work in AC coupled mode?


Also, https://www.altestore.com/store/info/ac-coupling/
 
"can't be used in series." Why? Do they not have wires that come out that could be cut and splice together?

You have three choices:

1. Do something that's a hardcore bad idea.
2. Adapt your components to work in a sensible manner.
3. Replace the components that aren't working for you.
I have done his exact idea using 2 12v controllers on a 24 volt battery with a 24 volt inverter without any problems at all.
 
It doesn't sound like what I described, or you wouldn't be trying to do what you're doing. If you get an inverter capable of AC coupled operation, and your existing grid-tie inverter will work with an attached inverter instead of the grid, you're done. The backup inverter itself uses power from the grid-tie system to charge the batteries.

Will the MPP Solar work in AC coupled mode?


Also, https://www.altestore.com/store/info/ac-coupling/
the mpp solar lv5048 has an on board charger to keep the batteries charged at 48vdc, but the high voltag output is connected to a critical load panel with an automatic transfer switch.. therefore it will not produce ac voltage for use when the grid is on.. when the grid goes off, the transfer switch activates it to draw from the batteries to produce power for critical loads.
 
Putting aside the bad practice of mid-point charging, anyone that understands this stuff will appreciate why it's a bad idea, what else is there to take note of.

The intent is to tap a common ~55VDC supply to charge the battery with two 24V chargers. Typically SCCs are not DC isolated devices. There is a DC path through either - or +. Put 2 24V common - chargers fed from the same DC input source, the panels, and you place a short across the first 24V half of the battery. There are ways to prevent this, but ... you don't know what you don't know, so you can't defend against it.
 
I have done his exact idea using 2 12v controllers on a 24 volt battery with a 24 volt inverter without any problems at all.
thanks, this is essentially my big question.. I will have 4 -24v batteries (byd-24v used modules) that i need to charge at 24v then pull power from them in series at 48v.. I will be using 8-makeskyblue charge controllers from the dc breakers on my pv array to charge the batteries .. I saw on a different thread that I can set the parameters on the charge controller to protect the cells on the high side with these, then protect the low side with the bms.. I am planning on using 1 bms for 2 modules at 48v so it can shut off power to the inverter if the voltage is to low. Unless anyone on here can tell me a better way... also, does anyone have a recommended bms that is bluetooth and 16s with low temp cut off?
 
Putting aside the bad practice of mid-point charging, anyone that understands this stuff will appreciate why it's a bad idea, what else is there to take note of.

The intent is to tap a common ~55VDC supply to charge the battery with two 24V chargers. Typically SCCs are not DC isolated devices. There is a DC path through either - or +. Put 2 24V common - chargers fed from the same DC input source, the panels, and you place a short across the first 24V half of the battery. There are ways to prevent this, but ... you don't know what you don't know, so you can't defend against it.
I am not sure what you mean by mid-point charging.. I am not sure if my charge controllers are dc isolated.. I have not tested using 2 of them to charge the batteries at 24v each while the batteries are wired in series at 48v.. soo I am definitely open to trying a different approach.. when I use either the built in charge controller on the lv5048, I can only get about 200 watts of power from 3000watts of pv array.. when i use a makeskyblue charge controller and charge just one 24v battery I can get over 1000watts easily.. (i used a load tester to draw the battery down a little)
 
With the propensity for MakeSkyBlue controllers to randomly get stuck on knee voltages rather than Vmp and their also random habit of simply staying in mode 7 charging rather than going to MPPT it is a 100% guarantee that you will be doing asymmetric charging.

Sorry, but IMO you are just digging the hole deeper going down this path.

*edit*
in case it wasn't able to be determined this situation would be a problem if the array is split up to avoid the DC path mentioned previously
 
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Mid point charging means attempting to charge a 48V battery (in this case two 24V batteries in series) by placing chargers across each 24V battery.

MakeSkyBlue chargers are not DC isolated. My crystal ball shows sparks in your future. ;)
 
@ElectricDanimal option #1 is to be avoided.
I'm gonna guess that you can make series strings.
Maybe even all your panels in one string.
the apex-500 panels from TenK are very special.. they were built with an on board MPPT module on each panel and as such, they can not be wired in series (easily, but that is a whole different rabbit trail) the on board module will not allow it to produce any voltage for safety reasons until it sends a pulse to charge capacitors in the LeadSolar Inverters. once they are communicating with the inverters i have 59 vdc available with the grid off, and 52vdc while the grid is on .
With the propensity for MakeSkyBlue controllers to randomly get stuck on knee voltages rather than Vmp and their also random habit of simply staying in mode 7 charging rather than going to MPPT it is a 100% guarantee that you will be doing asymmetric charging.

Sorry, but IMO you are just digging the hole deeper going down this path.
Its beginning to sound like i should sell the lv5048 and invest in lv2424's It might be the simplest way to solve a bunch of problems.
 
update... so I decided to sell my lv5048 and purchase at least 2 of lv2424. keeping the entire system at 24v seems like an advantage to avoid mid point charging and unequal charging.. sooo my next question, if I have 4 of the used byd-24v batteries, what do i need for bms, ? do i need 1 bms for each battery? or even if they are used, can I parallel all 4 batteries and run one 8s bms ? can anyone direct me to the proper bms? I want bluetooth capable, low temp cut off and low volt cut off. thanks in advance for all your input. https://www.ebay.com/itm/Single-BYD...230104?hash=item5b74995658:g:ByYAAOSwnNdeHUbu
 
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