Help with lower than expected charging rate for battery bank

[Ample]

I have 3 x 100 Ah LiFePO4 batteries in parallel.

I get inconsistent or unexpectedly lower charging rates from both my Renogy 60A solar charge controller and my 40A Renogy DC-DC charger.

For example, this is happening right now:

Clear day, Pacific Northwest, 2:30 in the afternoon, my 600W array of panels and the Renogy 60A MPPT is putting out 18A = 250 W into my battery bank which is at 50% capacity. Each battery is at approx same level of charge (about 50%) and their voltage as measured by a shunt meter at each battery is 13.3 V. The Renogy is trying to charge them at a claimed 14.4 V but only 18 A is going into the batteries.

Here's the weird part: when I fire up a big load, such as a toaster, the Renogy immediately bumps up the power it produces to 25 A or 350W. The additional current is goes to the toaster.

When I turn off the toaster, and after a short while, I can see that the output of the MPPT goes down to 18A 250W again.

It looks like when the load on the charge controller is only the batteries, the Renogy MPPT isn't putting out as much power. It's almost like the BMSes of each battery is limiting the current into each battery. I can understand this if the batteries are close to max capacity but they are only at about 50%. The 3-battery bank should be able to suck power as fast as the Renogy can provide it (theoretically, it should be able to draw at 3 x 100 A = 300 amps.

Now, if it were only the Renogy MPPT charge controller that's doing this, I might think the problem is with the MPPT. But the 40A Renogy DC-DC charger is also doing this odd thing.

For example, I started my vehicle and the DC-DC charger immediately starts to pump about 40A into the battery bank. Good.

But after a while, it decreases to 20A and sometimes to 10A. And this is while the bank is nowhere near capacity and should be able to draw as much as the DC-DC charger can deliver.

Both the MPPT and the DC-DC is set to lithium chemistry.

I'm confused why both are having trouble pumping out their max charging capabilities into a battery bank that should be hungry for power.

The configuration is simple: the MPPT, DC-DC, and the batteries are all hooked up to the bus bars in parallel.

Anybody else seen this inconsistent charging and could help me out?

 

[wpns]

Either the charge controllers are heating up and derating themselves, or the wiring is small enough that the wire resistance is causing issues. Measure your DC voltage at the SCC terminal and batteries, and see if there's any difference.

And pictures. Pictures always help, someone can sometimes see something that's 'obviously right' to you.

 

[HarryN]

So this is the nature of that family of solar charge controllers. In addition, they have fairly high standby power use, which is not a huge problem during the daytime, but kind of an issue at night.

All solar controllers to some extent will do that - as the batteries fill, they charge slower, but the renogy are particularly bad at it, and changing the settings is not completely obvious that it actually works.

 

[hardtop]

Probably from heat although the scenario where if you draw 15A 120V then you get what you expect out of the solar proves that wrong. Does that occur with the DC-to-DC charger too? If the DC-DC charger throttles down to 20A and you draw with the toaster does it increase to 40A?

It could be multiple issues, but for my case I get a similar issue where my 60A charger will put out 60A, but then after x time it dwindles down to 40A in some cases. The manual states that when it gets too hot it wont charge at full capacity. When it gets over 90F ambient or so it starts to throttle down.

 

[Suijkerbuijk]

This the reason.

All those controller have on big problem.
It do not know the battery real volts.
The problem lead vs lifepo4 is this .
Lead battery is always pulling on the controller and if thare a load the the battery volts go down.
The lead battery is pulling on the mppt .
The different in volts from the lead and the mppt is so high that the mppt push more .



Lifepo4 do not do this .
Is almost 80% of the time even with a pull 13.2 volts.
It do not pull or the volts do not go down.
So the charge will not do the max output .
The inverter kick in and the inverter pull out of the battery and the mppt and that is the reason the mppt will do more amps .
All of them do the same .
Than we stap over to a AIO systeem.
That systeem do know the real volts of the battery
It know that the battery is use and that is reason that those unit do more amps .
Oke now the victron simpel solution and Victron eco systeem (not Victron venus os pi )

Victron mppt can be used with a victron battery sensors.

That controller sensor tell to the mppt 2 things .

Volts of the battery
The temperature of the battery
All in real life use monitor.
This unit let the mppt charge more amps .

If you use a victron gx or you use a Smart shunt that do the same and you do not need a Victron battery sensor.

And use the Bluetooth v-e direct protocol. (Smart shunt and mppt no Victron gx)
And tell the mppt the real use of the battery.

A Raspberry Pi with venus os do not know the real volts of the battery .
Its use a 5volts systeem and do not know .

Reason that it need a Victron inverter or shunt or a battery sensor .

To tell the real volts of the battery.
I use a victron 100/50 with 570watt solar.

And max charge is 16a (250watts)
The epever's i had do the same a max of 16a and never see the 20/25a that is beult for
See this

Smart Battery Sense - Victron Energy

Improve the charging-efficiency and prolong the life of your batteries with the Smart Battery Sense. Find a dealer near you.

www.victronenergy.com

Parts of the link

Smart Battery Sense is a wireless battery voltage and temperature sensor for Victron MPPT Solar Chargers.

With voltage and temperature sense in place, batteries will be better charged; improving charging-efficiency and prolonging battery life.

 

[mikefitz]

seen this inconsistent charging and could help me

Measure the voltage along the charge path on both positive and negative cable runs. It could be there are voltage drops in the system. Chargers should be as close to the batteries as possible .
Any breakers in the charge path can Introduce unwanted volt drops. Renogy chargers can have voltage calibration errors.
Initially measuring the volts at the charger output terminals and comparing to the volts on the battery terminals would be useful.

One advantage of using Victron chargers and shunt, is that the shunt reports actual battery volts and current to the charger, thus volt drops are ignored.

It's also possible there are other issues.
The Renogy chargers have non ideal charge profiles.
The chargers are overheating and derating.
The capacity meter is incorrect and the batteries are nearer full charge than the meter indicates
The Renogy equipment is behaving as often suggested. ( badly)