DIY Lithium Ion Charger

[paulca]

Hi Guys, Fairly new to lithium, I've built an AC side microcontroller based charge controller for lead acid, been running it for 2 years (lockdown drove me crazy , had to invent something)

Now I've bought a 4.8kwh lithium battery and factory charge controller, However , after monitoring the response times of the algorithm used by the controller it responds to change like a snail on tranquillisers. Whether discharging or charging it seems to change the power delivered at a rate of ~350W per MINUTE.

Massively inefficient when clouds are flying by every minute, PV power goes from 3kw to 500w in four seconds, But charge power is still at 3kw.

Most commercial systems seem to report stats at 5min intervals , so it hides all this inefficiency.

Now my DIY system would do a 'sort of' binary search to find the correct power quickly, if a cloud came over for 10 seconds, it'd react accordingly and within a second the charger power would be enough to keep the grid usage between -100W and 0W.

So my question is.... Is the factory unit deliberately designed that way to avoid rapid changes to current in/out of the battery because lithium likes CC, or, is it just a simple poor linear algorithm that they came up with.

If I plugged my system into it , it reacts much faster, I'd waste less energy in the 'cloud transitions' but would I really wreck the batteries by changing the charge current at the same rate the solar grows and falls when clouds pass? (which isn't really that high a frequency change)

Please don't just chime in saying CC means constant current yada yada , I know all that , but these factory systems vary the charge into the battery based on power available from PV , so it's not CC , it's very very slowly changing current from what I've observed. Is it deliberate? or just a bad algorithm ? Is there a 'recommended acceptable change of current per second' ?

https://liionbms.com/pdf/bostonpower/swing5300.pdf

 

[Bud Martin]

The MPPT will try to extract the most power from the PV but it does take time for it to perform tracking, some brand works faster than other brand I guess it depend how who design the circuits. When you have less Sun, the CC output will be less than what you set the CC to since the PV can no longer supply the demanded current due to clouds in the sky, but when the cloud goes away if you have good design MPPT then it will recover faster. So you can design good MPPT then you may have something good in your hand.

"So my question is.... Is the factory unit deliberately designed that way to avoid rapid changes to current in/out of the battery because lithium likes CC," Charger does not control the output current of the battery, the load does.

 

[paulca]

The MPPT will try to extract the most power from the PV but it does take time for it to perform tracking, some brand works faster than other brand I guess it depend how who design the circuits. When you have less Sun, the CC output will be less than what you set the CC to since the PV can no longer supply the demanded current due to clouds in the sky, but when the cloud goes away if you have good design MPPT then it will recover faster. So you can design good MPPT then you may have something good in your hand.

"So my question is.... Is the factory unit deliberately designed that way to avoid rapid changes to current in/out of the battery because lithium likes CC," Charger does not control the output current of the battery, the load does.

Thanks Bud , don't think I explained this properly , these SP units only put excess power into the battery , so if your solar's producing 500W more than the house is using , it pumps 500W into the battery. So 500W at 50V is 10A . Then the sky clears, full sun, and you're suddenly producing 2KW more than the house is using. The SP unit then changes to push 2KW into the battery....2KW at 50V = 40A . Now this change doesn't happen quickly , the rate of change is about 350W per minute, so going from 500W to 2000W takes 4-5 minutes. So I'm wondering if its deliberately slow to avoid rapidly changing the charge current. Has anyone got a system that charges lithium that reacts super quickly to the available PV power ?

 

[paulca]

A picture says a thousand words, so here's a discharge example. Green = power from/to the grid, Yellow= power from the battery , sudden rise in grid usage is because some heater turned on, for 10 mins . The SP battery controller spots this, so starts adjusting it's discharge rate to avoid drawing power from the grid, it has a 2kw limit and it takes 5 mins to get up to that... then when the load turns off , the battery discharge is too high, so for a couple of minutes it's actually exporting it's power to the grid.



The charging rate of change is the same....slow , So i could build a system that reacts much faster , I'd always be charging at under 0.5C , even at maximum rate , but would it damage the cells if I quickly changed from 0.5C to 0.1C for 30 seconds , then back to 0.5C ?

 

[Pim57]

Think of an electric car where you accelerate (discharge 1-3C) and then decelerate (charge up to 0.5C).

Lithium is very good at these rapid changes in current.

 

[paulca]

Think of an electric car where you accelerate (discharge 1-3C) and then decelerate (charge up to 0.5C).

Lithium is very good at these rapid changes in current.

Brilliant example, thanks @Pim57 , it's quite obvious when you think about it like that. I just got concerned reading the datasheets for these batteries when it shows the CC charge phase followed by the CV phase, was just worried I'd be reducing the lifespan of them if I'm ramping current up and down too quickly. So these factory SP units must just have a pretty bad algorithm for adjusting the power then.

 

[Bud Martin]

Thanks Bud , don't think I explained this properly , these SP units only put excess power into the battery , so if your solar's producing 500W more than the house is using , it pumps 500W into the battery. So 500W at 50V is 10A . Then the sky clears, full sun, and you're suddenly producing 2KW more than the house is using. The SP unit then changes to push 2KW into the battery....2KW at 50V = 40A . Now this change doesn't happen quickly , the rate of change is about 350W per minute, so going from 500W to 2000W takes 4-5 minutes. So I'm wondering if its deliberately slow to avoid rapidly changing the charge current. Has anyone got a system that charges lithium that reacts super quickly to the available PV power ?

I do not know what SP unit you have, my SCC will charge up the battery using battery charging profile, once the battery is full then the SCC will no longer pull the current from the panels, and if my battery is full and I have loads running then the SCC will basically supply the power to my loads and the no current is drawn from battery.

 

[sunshine_eggo]

IMHO, I think you have a junk charger. My Victron MPPT reacts without delay.

 

[paulca]

IMHO, I think you have a junk charger. My Victron MPPT reacts without delay.

I'm testing out the Growatt SP 2000 , and the SP 3000 , DC side units that sit between the PV and your existing inverter and distribute the PV power between battery and inverter. Both seem equally as crap. Had a mixed bag of weather today sun / cloud / sun and I've been trying out an SP 3000 . The charging algorithm is pathetic


So in the above example, green=grid , yellow=inverter power ..... 13:26:00 Full sunshine, and I'm exporting 1.5kw to the grid. It gradually starts increasing the charge to the battery and decreasing the amount it sends to the inverter. By 13:31:30 , 5 and a half minutes later , it's putting 1000W into the battery, but I'm still exporting about 500W to the grid. Then there's a brief burst of cloud for a couple of seconds , and back to full sun at 13:32. But because it got close to drawing from the grid, the SP 3000 has reset to zero charge, then starts gradually ramping back up at a snails pace. I'd be really interested to see some charts from systems that do react immediately. My friend @biggogibbo has a brand new hybrid system but unfortunately his monitoring system only shows 5 min averages, so you can't really see how quickly it's reacting.

 

[paulca]

If everything remains constant , sun, house load etc , it works OK, it ramps the battery charge up until the grid is close to 0 , but as soon as you add an extra load or a cloud passes, it resets to 0 and starts the painfully slow ramp again.



and If there's one thing we get a lot of in the UK , it's passing clouds !

 

[sunshine_eggo]

Wait a minute... I didn't realize you're grid interactive. Reading fail.

I suspect that has nothing to do with the MPPT and is more a function of the INVERTER gradually increasing output as PV is available while staying synced to the grid. This may be a feature or a requirement of the grid interaction.

Have you tried disconnecting from the grid and seeing how it reacts? I suspect you'll see nearly instantaneous PV reaction to inverter or battery (charging) loads.

Or... Perhaps there is a hidden "solar reluctance" setting that is automatically enabled when it realizes it's operating in the UK...

 

[paulca]

Wait a minute... I didn't realize you're grid interactive. Reading fail.

I suspect that has nothing to do with the MPPT and is more a function of the INVERTER gradually increasing output as PV is available while staying synced to the grid. This may be a feature or a requirement of the grid interaction.

Have you tried disconnecting from the grid and seeing how it reacts? I suspect you'll see nearly instantaneous PV reaction to inverter or battery (charging) loads.

Or... Perhaps there is a hidden "solar reluctance" setting that is automatically enabled when it realizes it's operating in the UK...

TBH guys I was just ignoring the MPPT comments and trying to give more info . Big fail on my part was not explaining what the purpose of these SP units was. SO , grid tied means when you produce more power than the house consumes it exports it to the grid. These SP units plug in between the PV array and your inverter and have a CT clamp on the hot cable coming into the property. It controls hhow much power goes to the battery and how much goes to the inverter .... It knows when you're exporting power and increases charge to the battery until you're not exporting, all excess power goes into the battery. Well that's the idea , but , reacting so slowly makes it really really inefficient when clouds pass by, or someone boils a kettle.

 

[Goboatingnow]

I suspect it’s just a design decision to vary power availability slowly on the assumption that the solar change is passing . It’s also preventing rapid load changes on the grid

Review G98 and G99 U.K./ European specs that this equipment must meet

 

[paulca]

I suspect it’s just a design decision to vary power availability slowly on the assumption that the solar change is passing . It’s also preventing rapid load changes on the grid

Review G98 and G99 U.K./ European specs that this equipment must meet

Unfortunately that's not true , it reacts so slowly that it puts huge rapid changes onto the grid whenever a variable (solar / load) changes suddenly. E.G.... If it's charging and holding the grid at -50W , then you turn on a 100W lamp for 10 seconds , it resets charge power to zero resulting in a sudden huge push to the grid.

 

[shavermcspud]

Unfortunately that's not true , it reacts so slowly that it puts huge rapid changes onto the grid whenever a variable (solar / load) changes suddenly. E.G.... If it's charging and holding the grid at -50W , then you turn on a 100W lamp for 10 seconds , it resets charge power to zero resulting in a sudden huge push to the grid.

Whatt Growatt inverter do you have, SPH series?

 

[paulca]

Whatt Growatt inverter do you have, SPH series?

No it's the older SP2000 and SP3000 I've been testing.