SBMS0 features

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Dave Festing

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Apr 26, 2021, 5:29:28 PMApr 26
to electrodacus
Hi Dacian,

I have been using the Dual PV array functionality for the last 6 months,
a nice feature.

When I changed-over to LiFePO4 about 4 years ago you might remember some
discussion about "micro-cycles".  I had built the hardware that if the
batteries were charged and one had a big load come on that I would ensure
the panels were switched on.  With the change to LiFePO4 I never
employed this "feature".

Having the panels supply the bulk of the current for hi-demand loads has
got to reduce the stress on the battery, but I have no idea of how much
benefit this would be.

What I have observed is that with a 50Amp load on my 220AH battery is
that over a period of 2 minutes the terminal drops about 0.4 - 0.5Volts
and recovers over the next 1-2 minutes.

As you have current measurement and the Dual PV array functionality in the
SBMS0 it might be pretty straight-forward to roughly match supply and demand.

Just a thought.

Regards,
Dave

Dacian Todea

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Apr 26, 2021, 5:53:33 PMApr 26
to electrodacus
Dave,

The typical solar systems are designed for about 0.2C charge rate and max 0.6C discharge rate (continues not peak) and that is absolutely nothing for any half decent LiFePO4 thus there is no stress as the discharge rates are fairly low compared to what LiFePO4 can do.
50A is very insignificant for a 220Ah battery and the 0.4V to 0.5V for the entire battery also is nothing and not fully related to internal resistance so not all that is heat.
There is a 3% hysteresis so worst case say the load turns ON just when solar stopped the charging then that 50A load will not work for more than 2 and a half minutes before SOC drops by 3% and charging will resume.
There will be an absolute max 20 of this 3% cycles per day likely much less in typical systems and that just means nothing for LiFePO4.  There will be no advantage in trying to force charging just because there is a load change and it may be worse than just the 3% hysteresis best case will be the same in terms of battery impact.
Also is very likely you have less than 50A from the PV array so battery will still continue to discharge even with PV charging enabled and that will happen anyway just with that max 2.5 minutes delay.
As you have a dual PV array it may be possible that you have over 50A available from both PV array but if say you limited the max current to 40A the SBMS0 will not connect more than one of the two arrays not to exceed that limit you set.
Many large loads are intermittent as they have a thermostat or in some cases they use a slow PWM as it is the case for my induction cooktop that at lower power levels it will just switch full ON for a few seconds then OFF for very few seconds and since the SBMS0 will react in order of seconds to current change there may be fairly bad mismatch meaning there will not only be high current on battery discharging but then also high currents on battery charging as it may be 2 or 3 seconds delay in response so they will always be out of sync especial with this worse case induction cooktop I have.
As there is no benefit to battery I prefer to keep things simpler as complicating things increases the chances of problems.


Dave Festing

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Apr 26, 2021, 6:26:00 PMApr 26
to electrodacus
Thank you for the detailed response.

Likewise I am going through a total system "simplification process" in order to make things easier when we have to sell this "off-grid" house.

Cheers,
Dave
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