question

Lothar Ledgerd avatar image

Charge rate limited by Victron charge algorithm on flow / high impedance batteries?

Hi folks,

Does anyone out there know the voodoo behind the charging algorithm or what, other than temperature, could be causing Quattros and MultiPlus's to not reach their maximum charging currents?


Problem

The inverter/chargers are not reaching their maximum charge rate. A cluster of 6 x Quattro 15000 can theoretically push 1200A. There are 45 flow batteries which can each accept up to 50A during charge.

However, test results on this system showed a maximum of 1050A reached which after 5 minutes reduces to 900A which is then sustained. This is only 150A per Quattro. This results in 9 hours to charge rather than 6 hours which should be possible. This impacts the business case for some clients. This was the same on both a 35 degree day and an 18 degree day (Celcius).

The system is running the ESS module. The tester commented that charging fewer batteries and/or with ESS not running he has observed the 200A charging on other systems, but for some reason, it has not been able to be reached in this system.


Ideas

The comparatively high internal impedance of the flow battery being 80 to 100 milliohms (all connected in parallel) may be causing the algorithm to not adjust the voltage high enough to push enough current into the batteries.

What does the "lithium batteries" checkbox in the charger page actually do/change? Again, I imagine this would change some characteristics of the charging algorithm, even if lithium battery is not chosen from the list of battery defaults. Li


Questions

How does the charging algorithm work for the charger in Multiplus II and Quattros?

Is it possible that the charging algorithm would not raise voltage enough either due to high impedance?

or the ESS module somehow altering behaviour?

charging batterychargervoltagecharge current limitRedFlow ZCell Flow Battery
2 comments
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As far as I can tell, from limited experience, it will NOT increase the voltage over the Absorbtion setting, to meet the max. amperage specification.

So the max charge current is also limited by how much current the battery wants to "suck" at the Absorbtion voltage.

It's basically a constant voltage power supply (the Absorbtion voltage) with a configurable limiter of the max current. But it does not take "active measures" (i.e. increase the open-circuit voltage) to keep the current at that max setpoint.

The system's absorption voltage is set to 57 V and there is a MaxiGX running CANbus DVCC to the Battery's BMS. This should be updating both the voltage and max current limits in the Quattro cluster. 57 V is enough to push over 1800 A into these batteries, so why is the Quattro cluster not pushing through the full 200 A each (1200 A)?

Hence the question about the algorithm. We are telling it to push hard in both voltage and current, but it just won't do it. All I can think is that the charging algorithm assumes for x increase in voltage it will cause y increase in current.

Of course, the other option could be a temperature limit, but as I said, the charging was the same on both hot and cold days. The discharge test results changed with temperature inline with expectations, but no change in charge. So to me this is all pointing to software.


If you are telling me that it has no dynamic charge voltage control and operates purely via current limiting at constant voltage, then why is the current being limited well below what the BMS is asking for despite the voltage being requested is high enough?


2 Answers
MrT avatar image

By default Multis/Quatro's have their 'Charging Current Limit' set to 75%.

Excerpt from manual ….

5.5.1.2 Charge current limitation (default setting 75%) For maximum battery life, a charge current of 10% to 20% of the capacity in Ah should be applied. Example: optimal charge current of a 24V/500Ah battery bank: 50A to 100A. The temperature sensor supplied automatically adjusts the charging voltage to the battery temperature. If faster charging – and a subsequent higher current – is required: - the temperature sensor supplied should be fitted to the battery, since fast charging can lead to a considerable temperature rise of the battery bank. The charging voltage is adapted to the higher temperature (i.e. lowered) by means of the temperature sensor. - the bulk charging time will sometimes be so short that a fixed absorption time would be more satisfactory (‘fixed’ absorption time, see ds5, step 2).
Procedure The battery charging current can be set in four steps, using DIP switches ds4 and ds3 (default setting: 75%). ds4 ds3 off off = 25% off on = 50% on off = 75% on on = 100%
Note: when WeakAC is on, the maximum charge current is reduced from 100% to approximately 80%.

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MihaiR avatar image

Doing some mats it seems to be 25% lower than specs, exactly the same like in my system but with different Multi:

https://community.victronenergy.com/questions/33089/what-is-the-max-real-life-dc-charging-current-for.html

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