Lithium leisure Batterys and PWM solar controllers - Not Advised. (But fine on any of the SLA chemistry battery types) - IMHO
Im doing some testing and come across some interesting data that leads me to advise that PWM solar controllers may not be the best thing to use with a Lifepo4 battery charging.
My advice would be to upgrade to a MPPT solar controller that has a proper Lifepo4 battery charge Profile.
Why?
we as we may know a PWM (Pulse Width Modulation) solar controller is just a fancy switch (in its basic form)
it will switch the panel voltage over to the battery on/off/on/off/on with varying pulse widths (all very fast, multiple times a second).
the issue seems to be that the solar panels can generate high voltages like 16,17,18,19,20,22v - which is too high for the standard Lifepo4 batty internal BMS.
these high voltage spikes can trigger the BMS to shut down or trigger Over Voltage alarms.
lets look at todays test . .
here is a 50w foldable panel connected to a PWM controller (set to Lifepo4 profile) .
the setup is connected to the Poweroad Lifepo4 battery box for some testing. (which has a Victron smart shunt and various other monitors)
being only 50W, we could expect a max of 3A charge current ( and a OCV of 18.5v - this is the important bit) . . (i got about 2.7A which was good)
but take a look at these 18.5v voltage spikes on the scope. . . . measured direct at the battery terminals !!!!
moving over to the Victron Smart Shunt we can see what's going on.
we can see that the battery is full and the internal BMS has shut-off the charge current going into the battery (the RED trace on the left of the right tablet).
now the PWM controller has no effective load or battery to sink the power into we can see the high voltage spikes. (the BLUE trace on the right of the right tablet)
The chart is showing 18.5v spikes across the battery as the PWM controller is trying to charge the battery. -this is the direct panel voltage being switch to the battery terminals.
here you can see the PWM controller showing a warning as its detecting the blocked current flow to the battery . . b01 error (check battery)
this has also triggered the "cell overvoltage" warning from the battery BMS.
the BMS is dealing with the situation by blocking any incoming charge current,
but it cant do anything about the high voltage spikes coming from the solar controller.
we can see the spikes as a ripple in the Bm2 monitor . .
but the spikes / pulses are to fast to be plotted on the 24hr plot.
so it seems that PWM may not be best suited for Lifepo4 batterys that have internal BMS.?
in comparison look at yesterdays charging trace . . when a MPPT controller was used.
the charge plot is flat and smooth.
MPPT solar controllers use the same charging stage / profile as a regular mains charger.
The output is controlled and regulated.
there are no high voltage spikes and the charge current is delivered in a controlled manor.
all the way up to the point when the battery 100% fully charged . .
So my advice to myself and anyone interested is to use MPPT solar controllers for Lifepo4 battery's.
Notes:
This effect will be seen when the battery is 100% fully charged and the internal BMS shuts off the incoming charge current. (revealing the voltage spikes from the controller)
these spikes are absorbed by the battery 0-99% SOC and are not seen as the current flow and low internal battery resistance cause a volt drop - masking the spikes.
The Victron Smart shunt has higher resolution so the spike can be seen and plotted on the graph.
The BM2 has a lower resolution so the spikes are not so evident.
the spikes can be verified with a scope or potting multimeter.
A Renogy Voyager 20A PWM was used in this test.
A Victron 12/30 240v charger was used in this test.
A Victron 75/15 MPPT was used in this test.
A Poweroad 100Ah Lifepo4 was used in this test.
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Edit:
it seems that a major battery manufacture agrees with these findings, i received this the following day . .
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Hi Dellmassive,
Thanks for the effort.
You are correct, the MPPT is more suitable for our batteries.
The interval and peak current of PWM charger is hard to control.
In addition, the big peak charging current is also not good for our battery.
Best regards,
Poweroad Tech
POWEROAD (XIAMEN) RENEWABLE ENERGY TECHNOLOGY CO., LTD.
7/F Haiyun Building, No. 16, Haishan Road, Xiamen 361013, Fujian, China
Lithium-ion Battery: www.poweroad.com
Energy Storage System: www.poweroad-ess.com
Advanced Lead-acid Battery: www.yucell.com
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