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diy solar

Another BMS question

MhoytTC

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Sep 16, 2022
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I am building two 24v batteries with 304AH 3.2v cells. I have 1200w of solar and want a larger 3000w or 4000w inverter. What size BMS should I use? Is this overkill?
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Time for some math.

If you have a 4000W inverter and it runs at 90% efficiency it will draw 4000W/.9 = 4444.4W from the battery.
The highest current will be when the battery is at a low SOC and the voltage is low, so lets use 24V. That means the current will be
4444.4/24=185A. Brief surge currents could be much higher. ( Note: It works out to 139A for a 3000W inverter)

Edit: Corrected the voltages in the following sentence.
I would say a 200A BMS would be the minimum size for a 12V battery with a 4000W inverter and 150A for a 24V battery with a 3000W inverter.

Note: I am not a fan of the Daly BMSs. My current favorite is the JK BMS line. Some of the newer JBD BMSs look very good, but I do not have enough info on them to be able to recommend them yet.
 
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Thank you FilterGuy, I currently have 12 volt solar panels, can they be used to charge a 24 volt system?
 
Thank you FilterGuy, I currently have 12 volt solar panels, can they be used to charge a 24 volt system?
Yes, but you will probably need at least two in series to make it work. (Depending on your charge controller, it may be best to have even more in series.)

The 12V and 24V specification for solar panels are a bit old. It comes from a time when most of the charge controllers were PWM and you needed to match the input and output voltage to get the best performance. Most people now use MPPT controllers. With MPPT controllers, the input PV voltage needs to be higher than the max output voltage, but it can be as high as the controller can tolerate on its input and the controller will convert it down to what the battery needs. As an example, you could have a string of panels that have a combined voltage of 100V but a 150/xx mppt controller can take that down to the voltages needed for a 12V battery.
 
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