sthawk11 avatar image
sthawk11 asked

MPPT 250/85 Battery Current limit with load

I have an MPPT 250/85 wired directly to 2x 12V 100AH “Deep discharge” batteries (i.e.) a 24V system. There are two 2400W inverters connected directly to the batteries.

The solar array is 6x 24V panels in series, two strings in parallel, giving me around 230V.

In order to limit the battery charge current to 0.1C, I selected battery “Max charge current” 10A. (float 27V, Absorbtion 28.8V).

How can I get the full power out of the system (via the inverters) without risking too much current into the batteries during bulk charging.? Do I need a BMV or 8x as many batteries in order to safely set the current to 85A? My purpose is not so much to charge the batteries as to make full use of the solar array by powering the inverters.

I may have already damaged the batteries as they struggle to deliver power for more than a few hours after dark, or I am suffering the problem others have mentioned about short absorbtion times.

MPPT Controllers
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chris-677 avatar image chris-677 commented ·

Same problem here. No solution

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boekel avatar image boekel ♦ chris-677 commented ·

There are all kinds of solutions, but it starts with configuring a correctly sized system.

2kWh (1kWh usable) of Lead Acid batteries for 4.8kW of inverters is never going to work well.

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4 Answers
Mark avatar image
Mark answered ·

Yes you have nowhere near enough battery capacity for your loads or in relation to the avaliable PV.

I would also recheck the manufacturer's recommendation for max charge current. Normally 20% of C20 rating is fine. Some are even OK with 30% if you use temperature compensation.

You are probably doing more damage running them empty all the time as compared to a higher current recharge.

For lead acid deep cycle batteries you should really size your bank to avoid ever discharging below 50% SOC if you want to get any reasonable life from them.

Also, NO a BMV alone won't help as it can't control MPPT. But in theory it is possible & would help if the MPPT current was controlled in relation to the BMV reading.

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sthawk11 avatar image sthawk11 commented ·

Thanks, yes, the battery is nowhere near the capacity or the array or load, they are only there to act as a buffer for brief periods. The manufacturer (Century N70T) recommends a maximum charge current of 10A, but I will try 20A.

My point is still, is it possible to configure the SmartSolar to deliver a lower current to the battery while making available the full 85A to the inverters, or perhaps by somehow (in software on a device on VE direct) dynamically changing the current limit after the battery goes on float.

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Mark avatar image Mark ♦♦ sthawk11 commented ·

I know exactly what you want to do. I'm not totally sure but if you add a BMV & also a Venus device (like a CCGX) then enabling DVCC with a lower current limit (say 10 or 20A) than you should still be able to specify the full 85A in the MPPT settings & it MAY work as you would like... but I'm not totally sure as it's normally used to limit a combined total when you have multiple charge sources & I haven't tried this myself. The documentation also doesn't mention this use.

Someone else may be able to confirm 100% - possibly Victron themselves.

Then again are you willing to purchase all that equipment anyway?

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sthawk11 avatar image sthawk11 Mark ♦♦ commented ·

Thank you, I’m going to try with a Raspberry Pi runing Venus and will get a BMV. I’ll report back how it goes.

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robbo avatar image
robbo answered ·

Batteries will only take what they require so I would knock it up to 85amp. The batteries are likely to only get a surface charge but as you only use it as a brief buffer it's nowlt to really worry about if they hardly get used.

Note deep cycle batteries are not really best if you hardly use them and when do only breifly as they won't be able to provide the amps the inverters need. Leisure batterys sound more appropriate for you.

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Mark avatar image Mark ♦♦ commented ·

This is not correct for any time/phase when the battery voltage drops or starts under the voltage set-point. Particularly during bulk phase. The battery will take whatever current is available & the MPPT will provide whatever it can.

This would only work when your already at the voltage set-point. Then the MPPT will automatically limit it's output to maintain the voltage & run the load (within the limits of available PV).

Another scenario is if your sure that the load will be constant & take all of the 'unwanted' charge current at any particular point in time - a very risky configuration/assumption & not practical to execute/control manually in reality.

Please don't set a 85A charge limit with an 100Ah battery. It may not end well.

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robbo avatar image robbo Mark ♦♦ commented ·

If the batteries are mostly at a high SoC then it won't be a issue as you will be rarely in bulk - here is more info for surface charge; Think more starter batteries rather than deep cycle. My alternator is 85amp, starter battery is below 100ah, that's a large charger for a small battery, but as the battery hardly gets below 90% SoC it's not an issue.

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boekel avatar image
boekel answered ·

This sounds like a job for a high power lithium battery.

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sthawk11 avatar image sthawk11 commented ·

That’s the plan once I’m prepared to run off the inverters through the night.

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red-dirt avatar image
red-dirt answered ·

You need a shunt and venus device so that the system can measure the loads and ramp up solar to suit whilst ensuring actual charge into batteries stays within limits

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