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Charge 24V LiFePo4 with BMS, direct with parallel 60Cell PV Modules

ThaiKai

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Sep 14, 2021
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Hello Forum Members,

I´m Kai, living in the Western Area of Germany.

i just design a Garden Shed PV, i have an East/West Roof, 20 Degress Azimut, und 2 x 3 PV-Modules ( 345 Watts ) with 60Cells each.
I plan to use an 4p8s Setup with LiFePo4, so a 24 Volt Setup, with a 150 amp BMS.

Now i have 2 Options:

- Connect 3 Modules in Serial and use a MPPT 150/45 Volt Charger to Charge the Battery ( $$$ )
- Just connect all Modules in Parallel ( 32 V MPPTS, 40 Volt OC ). and let the BMS protect the Cells as it should anyways .... and invest the Money in a bigger Battery Bank :cool:

Are there Experience with Direct-Changing 24V with 60 Cell PV Modules ?

Best Regards, Schöne Grüße,

Kai
 
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Hello Forum Members,

I´m Kai, living in the Western Area of Germany.

i just design a Garden Shed PV, i have an East/West Roof, 20 Degress Azimut, und 2 x 3 PV-Modules with 60Cells each.
I plan to use an 4p8s Setup with LiFePo4, so a 24 Volt Setup, with a 150 amp BMS.

Now i have 2 Options:

- Use an MPPT 150/45 Volt Charger to Charge the Battery ( $$$ )
- Just connect all Modules in Parallel ( 32 V MPPTS, 40 Volt OC ). and let the BMS protect the Cells as it should anyways .... and invest the Money in a bigger Battery Bank :cool:

Are there Experience with Direct-Changing 24V with 60 Cell PV Modules ?

Best Regards, Schöne Grüße,

Kai

BMS is a safety system. Relying on one for routine control is undesirable. The more one is utilized, the more likely it is to fail.

Furthermore, if you want to get the most out of your array, your idea will max out at about 85% rated power due to forcing your 32V panels to work at battery voltage.

Here's what your situation looks like:
1) panels dump max power to battery
2) BMS cuts charge (and discharge if it's not a separate port BMS, i.e., all battery output is cut off as well)
3) Cell voltages drop to reconnect
4) GOTO 1)

It will likely do this dozens/hundreds of times per day.

Short answer: Get an MPPT or get an Electrodacus BMS w/solar relays. I generally don't recommend those, but your 32V panels are a good match for it. The MPPT would get the max output potential of your array. The Electrodacus would be cheaper and get the same power as your direct-connect approach.
 
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Cheap MPPT if you're on a budget is getting an Epever. They have become pretty nicely priced, the Tracer 4210AN is now 115 euro at https://solarv.de/product/mppt-laderegler-tracer-4210an/

you need 2 pieces, and have to connect the panels in parallel due to the max input voltage of 100V, but that's probably the cheapest option
With 345W x 3 @ 24V each string provides around 43A, thus a 40A charger would fit perfect. (Those 3A more the panels could provide isn't archived much of the time, so not worth a more expensive controller imho)

As said, I won't directly connect. It will keep the cells at near 3.65V all the time (keeps looping as explained), which is definitely not a recommended charge profile, unlike using a MPPT which just will stop charging once the voltage is reached.

What are you going to power with the setup? If you're planning on using an inverter you can use an all-in-one as well.
The Growatt SPF 3000TL LVM-24P for example would match your panels, max PV input is 145V DC and includes a 80A MPPT

An MPP Solar is slightly cheaper. Also allows higher PV voltages, but with different orientations I would recommend 2 strings (1 east and 1 west) for optimal performance. However, 3 panels might not be sufficient for all inverters, some have a higher mppt working range.
 
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Thanks for your answers.

Just a few Words to the UseCase of this Setup.

I will run a small 300W GridTie Inverter like a Hoymiles, to support the primary Energie Storage System of the Main Building ( 48V, 12 KW Battery, 5 KW Peak 230V Output ). This huge System has an awfull Amount of Energy wasted just for Idle Standby, so i want to provide some extra Power, to keep the big System on Standby.

The Battery ( 24V, 300-400 AH ) will be drained every Night by the GTI, with BMS and GTI in parallel. I already have a MOSFET BMS, so that it can Charge as soon as there is some Output of the PV, when the voltage gets high enough for the GTI to start up and the Overflow goes into the Battery.
BMS Dischange Enable Voltage can be Set to 3,3 Volts per Cell, to prevent heavy looping.

In Case of a Power Failure, i can still use another Standalone-Inverter, without needing a Grid.

Thanks for your Answers.
 
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