Use grid tie inveter to tie Lead acid bank to Lithium bank ?

[ghatikar]

I was wondering if my thinking is correct or I may end up with a big paperweight on my hand. 

Recently I came across this product - https://www.aliexpress.com/item/1000W-Battery-Discharge-Power-Mode-MPPT-Solar-Grid-Tie-Inverter-with-Limiter-Sensor-DC22-65V-45/32762489820.html

It is essentially a 1000W  Grid-tie inverter with built-in export current limiter which can work with lead acid battery bank instead of PV panels as DC input 

I was thinking to use it on the output of 5 KW  Quattro with Current limiter CT inserted between Grid-tie   and Quattro ( see crude sketch attached)

So in normal working,  the Grid tie will supply the house power essentially stoping Quattro from discharging its battery( BYD lithium in my case)  or drawing from the grid . ( using islanded mode) The grid tie  inverter can be set to stop working on any voltage setpoint so as an example set it to use  lead acid upto 48V or even insert a power contactor driven by BMV to disconnect SOC below 50 %)

The grid tie maker claims 3 second reaction time on the power control  ( from Full 1000 watt to Zero ) .

 

If the combination work  can give many advantages 

1. Use Large /OLD lead acid bank fed from solar charge controller - remotely located ( SAY in a different shade 50 meters away) and tie to the main inverter

2. Reuse the old lead acid bank instead of discarding when changing to lithium  using an el Cheapo charge controller 

3.  tie two different banks together of different battery types /chemistries 

4. Increase the total load capacity e.g. 5KW Victron can handle 6 KW load at 40 DEG ambient with one 2000 watt grid tie attached for the temporary peaks of washing machines or vacuum cleaners 

 

I am aware that the power conversions from DC to AC do not make it the most efficient  systems

In real life case 

My lead acid of 400 AH is being fed from 5.5 KW solar panels through 2 No.s of Axperts ( total 10 KVA) and gets used only on weekends, public holidays as it supplies only airconditioning and washing machines, water heating etc. So essentially the 5.5. KW panels can feed the Quatro through the grid tie but also keep the lead acid bank topped up for nighttime AC operation in case of power failures. The Axeprets are set to charge bank only using AC and set at default solar mode so use grid only in the night 

 

T

 I have successfully setup a APS systems microgrid tie to work with Quattro AC/OUT 2   for my office. as the only power required is during weekends to keep batteries charged when generators are not running and the grid is not available - see attached logic and scheme  ( I am on ground floor of 4 story building and the 4 mm2 cables would have been costlier than the whole setup) 

I am using PV inverter assistant and generator Logic to disconnect PV inverter when the grid is available in addition Quattro ACout will switch off using the assistant with battery voltage higher than 57V to avoid feedback into the main battery  - see similar scheme in operation in my office ( using panels on 500-watt grid tie inverter) 

 

 

 

 

assitants summary.docx

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The Quattro can take a grid-tied inverter on its output and is not going to care whether the source of energy is PV or a battery. So it should work fine, with some caveats.

The first is that the GTI must preferably support GFPR, grid frequency dependent power reduction, and then you must install the relevant assistant on the Quattro. When in off-grid mode, the Quattro can then shift the frequency to signal the GTI to back off production. That way you don't need the limiter CT.

The second caveat is that this mode of operation is usually not very stable. Sudden load changes on the Quattro usually leads to momentary voltage drops and or vector shifts which will trigger the anti-islanding on the GTI, leaving the Quattro to supply the load until the GTI reconnects. So dreams of getting 6kw capacity out of it is likely ill-conceived. The Quattro must be able to carry the whole load if you don't want random overloads to shut it down.

Third, when the lithium battery is very close to full, with the suggested CT solution and a 3-second back-off, I can guarantee you're going to overvolt the battery and cause the BMS to disconnect the charger. If there is no grid to push into, the energy provided by the PV-inverter has to go into the battery. If the bank is small or very highly charged, even a single ampere is enough to cause the voltage on the battery to rise sharply. For example, a fully charged BYD rack can spike very quickly from 55V to 58V in about ten seconds with single-digit currents. If it gets to 58.5V... click, darkness for you. Personal experience. I would be careful about putting a slow-adjusting GTI on the output with nowhere to put the power other than a very full lithium battery :-)

[ghatikar]

1 hour ago, plonkster said:

 

The first is that the GTI must preferably support GFPR, grid frequency dependent power reduction, and then you must install the relevant assistant on the Quattro. When in off-grid mode, the Quattro can then shift the frequency to signal the GTI to back off production. That way you don't need the limiter CT. - 

 

1

GFPR specially tested one like Fronius is expensive and I am sure the Chinese have no product with that. it does disconnect at 51 HZ  So my Generator logic of SOC based contractor does that - the GTI only gets connected if the SOC is below 95% . or there is enough load on output ( was 100 watt for 500-watt GTI  for 1000 watt GTI I can even set 500-watt minimum load needed before GTI kicks in 

 

I agree with the second caveat - but a man can dream 

 

to prevent battery voltI have a contactor disconnecting GTI ( SOC above 99 %) plus AC OUT2 disconnects at 57V as a backup. ( can be set lower but then AC out2 flip flops during normal charge cycles )