Battery ripple current

Hedges

I See Electromagnetic Fields!
After reading of someone tripping battery breakers at well below rating, I realized that could be due to pulses of current drawn by the inverter. Ideally capacitors would smooth out current draw so high frequency switching pulses and 60 Hz draw all came from the caps, and current from batteries was DC. But current drawn from a capacitor reduces its voltage and battery will respond to that, so a "low frequency" inverter can't help causing ripple.

The idea is to determine if fuses and BMS need to be oversized to account for extra I^2 x R power dissipation due to ripple current.

My system has four Sunny Island SI-6048US wired 2s2p. I considered reconfiguring as 2s for half as much capacitance, but didn't do that.
Using the oscilloscope and current transformers described in another thread, I instrumented it by putting one CT on the negative battery cable of one Sunny Island, an one around the AC output wires of two Sunny Islands on a single phase:


I don't have a DC clamp ammeter, so I relied on Sunny Island to report watts and battery voltage, and calculated average current. The current transformer measured just the AC ripple component. It is rated for 100A rms, and used on just one inverter's cable kept DC + AC component within that limit.

offgridPV050821.jpg

PV charging battery, 7.2A rms at 240V for 1728W (probably aborption.)
The two channels have 300:1 scale factor entered, an what it shows as "V" is amps.

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Battery powering A/C (PV offline), 11.4A rms for 2736A delivered.
Strange triangular waveform of AC current, wasn't expecting that.
CT on battery shows 2.85A rms ripple in draw from battery.

offgridpool050821.jpg

Battery powering Pool pump. This is a VFD driving 2hp 3-phase motor. VFD has diode/capacitor front end, so current draw from AC just clips tops of sine wave (poor PF).
Note 5.08A rms ripple in battery current, a spike each time AC draws current.
Hmm, Guess I should have oriented CT the other way so more current would be up not down.

housePVpool050821.jpg

On grid, with PV producing and pool pump running.
Note how the sine wave current from PV has notches taken out of the sine wave peaks by VFD's diode/capacitor front end.
I didn't run this pair together off-grid because the transformerless Sunny Boy don't like that. But tied to the bottomless grid, no problem.


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Battery powers 10kW electric heater. Now we're talking!
Inverter reported 10.2kW output, 47V battery which means 217A average draw from battery (just over 0.5C for 48V, 405 Ah battery)
CT measures 41.9V rms on AC line, x 240V = 10,056W
CT on battery cable shows 16.24A rms. Note that is for a single inverter; for all four, 65A rms.

Instead of a steady 217A, superimposed on that is a sine wave +/- sqrt(2) x 65A. (125A minimum to 309A maximum)
Rather than 217A rms, by numerical methods I came up with 227.6A rms, about 5% more heating of fuses and wires.

This doesn't seem like enough to be a big deal. Would probably be higher if only a pair of inverters was used (half as much capacitance available), or if load was near 100% of inverter capability rather than about 40%.

If an inverter had less capacitance than these, ripple would be higher. Although,
With LiFePO4 rather than AGM, lower internal resistance (possibly 1/5th as much) would mean capacitor voltage wouldn't draw down as much and more current would come from battery.

I'd like to see battery ripple current waveforms or clamp AC ammeter readings for other inverters.
 

Hedges

I See Electromagnetic Fields!
That is the main reason i use a supercap in parallel with my battery.

Got a clamp AC ammeter you can take readings with? (each branch of the path you can fit it around.)

Sunny Island has something between 0.01F and 0.10F of electrolytics.
Your supercaps are probably multiple farads.
A capacitor intended to provide power over extended time could be different from one intended to smooth out ripple. A few supercaps I see specs for show 10's of milliohms series resistance at 10's of millifarads. Those don't seem like they would hold up a load well enough to keep voltage from dropping and AC current being drawn from battery.


What are the specs of your supercaps?
 

toms

Solar Addict
I use 3x500F(16V) Maxwell Supercaps in series, works out to 166F

I don’t have a scope, i’m not the only one with this setup though - i’ll try to find the video of the scope traces that convinced me to use the supercaps and post it here.
 

Hedges

I See Electromagnetic Fields!
3 x 500F, 16V. 2.1 milliohm each so 0.0063 ohm total.


Using the 65A RMS I saw with my setup,
65A x 0.0063 = 0.410Vrms

Compare to 16x LiFePO4 cells with 0.25 milliohm spec, 0.17 milliohm reported, 0.0027 ohms total.

I would expect your caps to deliver 1/3 of the ripple current and LiFePO4 battery to deliver 2/3.
If AGM, other way around.
I'm ignoring resistance of any long cables here.

We won't know your ripple current, but you could measure AC volts. With and without caps connected?
If you have a shunt (or other reasonably well known resistance like a length of cable), can measure that voltage to get ripple current.

Trying to see if some inverters draw battery current in a way trips breakers not sufficiently oversized.
 

Hedges

I See Electromagnetic Fields!
Here's my analysis to show how much larger a fuse is required to handle ripple current.
After seeing how large a ripple I measured with 40% of full load in my inverters, I'll assume that at full load 100% comes from battery and not from capacitors in the inverter.

Consider delivering 4800W from a 48V battery to an inverter driving resistive load. Current draw will be a rectified sine wave with average (mean) current of 100A. Power delivered is the average of 48V x current at each point in time, or 48V x average current, 48V x 100A = 4800W.

Wires, fuses, breakers are heated by current through their resistance. The equation is W = I^2 x R. Computing RMS current gave the value 111 Amps RMS.

What this means is you need to size your fuse to carry 111A rather than 100A. (11% larger.)

When starting with power of AC load, we also need to include inverter efficiency. This won't be the peak efficiency given on data sheet; hopefully it provides a graph. For my favorite inverter SI-6048US, the graph shows efficiency at full load of 91% (for 63V battery) or 92% (for 41V battery).

We also oversize fuses by 25% to avoid nuisance trips.

If driving 6000W AC from 41V battery, fuse should be at least:

6000W / 41V / 92% x (1 + 11%) x (1 + 25%) = 220.7 Amps
Compare that to the nominal value we first think of, 6000W / 48V = 125A

Battery ripple current.jpg
 
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Hedges

I See Electromagnetic Fields!
220A or larger fuse needed for one 6kW inverter - I realized I'm using one 350A fuse per two inverters.
2 x 220 / 350 = 1.26, I'm missing the 25% extra to avoid nuisance blowing.
For a moment I thought that meant I might blow it operating full load. But looking at the curve, I see that a 350A fuse can carry 350A for more than 1000 seconds (15 minutes). In fact, a 225A fuse can carry 350A for more than 15 minutes.
My battery bank isn't big enough to deliver full load 24kW for more than 20 minutes (Peukert and all), so nothing to worry about.

 
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