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tamirlance avatar image
tamirlance asked

force MPPT 75/15 to stay in bulk

I'm trying to use the charge controllers to keep the PV panels at the max operating point. Is there a way to force the MPPT to always stay in bulk mode and not go into float or absorption modes? I have a power resistor that turns on at a high voltage and off at a low voltage. But I can't seem to find the right voltage limits to prevent absorption phase in all cases. Basically I don't want it to limit the current. I don't care about long term battery health (SLA batteries). All setups are using remote battery sense units

Some of my setups work and some don't:

MPPT Modules STC Vmp Battery Voltage Load Voltage Turn On Load Voltage Turn Off Behavior
1 60 24 (12x2) 27 22 bulk only/
no clipping
2 26 12 13 11 absorption/
clipping
3 30 12 13 11 absorption/
clipping
4 19.5 12 14 11.8 bulk only/
no clipping
5 14.5 12 13 11 absorption/
clipping
6 38 24 (12x2) 27 22 bulk only/
no clipping


MPPT Controllers
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2 Answers
ripper avatar image
ripper answered ·

Your resistor has to burn enough juice to never reach absorption voltage. Voila only ever bulk and bad for the battery. It has to burn 28.8V x 15A = 432W or more and can then be switched on and off. If it only burns, say, 350W it will hit first absorption voltage which doesnt change the output of the resistor, then it drops to float and due to the lower voltage the same resistor will burn only significantly less W which helps in staying in float longer.


For highest gain, use 24V setup since 14.4V x 15A is half of 28.8V x 15A. Since the amperage is set at 15A you can consider dialing the volts up even higher , definately cooking the batteries (or putting another couple of cells in series) to gain even higher wattages that also need to be able to be burned by the resistor.

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tamirlance avatar image
tamirlance answered ·

@ripper - the power resistors are plenty big. I can't understand why it's working in some cases bt not others. I thought I was setting the voltage at which the power esistor turns on lower than the absorption voltage. So I'm not sure why it's even getting into that mode

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ripper avatar image ripper commented ·

What are the specs of your power resistors? Maybe that gives a hint. I got some big as heating rods that are rated to do 3000W or 16A at 230V AC, so 17 Ohms ish. At 24V they barely do 50W. At 12V a quarter of that... Yuo probably have something similar that has big numbers but since its running at low voltage nothing is happening. I assume some of your tests were successful because the weather was just bad enough to not reach absorption.

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