question

skamp_x@hotmail.com avatar image
skamp_x@hotmail.com asked

Effect of low voltage under load on charging

Hi there,

i got a off-grid system i build but got to the point i need a bit of info.

Got a equivalent to 560watt of solar panels and might expand on that soon, on a 100/20 mmpt charging a 100A/24v lithium batteries pack.

Phoenix 1200/24v inverter in use for inhouse , tv , pc or other.

This setup been going good for a year but because of not wanting to risk of damaging the batteries i always only had it to deliver power in evenings and nights.

The reason is because i struggle with this:

When under load the voltage drops a good part, like from 28V to 26.5v for example.

Now i worry that if the panels during peak give more power then what im pulling from the batteries , the batteries would still be full but because of the lower volt the regulator would try charge them anyway and overcharge the batteries right?



battery charging
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1 Answer
wkirby avatar image
wkirby answered ·

If the battery is full and there is still plenty of sunshine, then the MPPT is throttled back and does not yield the full potential of the PV because the battery is full and there are not loads to take up the excess PV.

Let's set up a scenario. You have 560W of PV power and it is a gorgeous day outside.
The battery is charging, so it takes all of the 560W Then when the absorb Voltage is reached, the charge current tapers off to maintain the maximum Voltage (28V) for that particular charging stage. After a while, only 200W is going into the battery because it is nearly full, so you have 360W of potential PV power not being used.
If you switch on a 200W load then the PV can supply this directly. 200W continues to go into your battery to maintain the 28V and 200W goes from the MPPT directly to your load.
Some time goes past and the battery is full and hardly any current is flowing into it anymore, maybe 10W. Now is a very good time to switch things on. If you keep below 500W, then all of the PV power will go directly to your loads, super efficient because there are no losses involved with charging and then discharging the battery. The battery just sits there in the corner with a big smile. You are also smiling because you are powering loads without using the battery at all!

If a cloud passes overhead and drops the PV power down to 200W whilst you are using the 500W, then all 200W of PV will go to the load and the battery will do its job and step in to cover other 300W, you won't notice a thing unless you were looking out of the window.
Once the cloud has wandered off, the the full PV will come back. 500W of PV will go to the load and 60W will go to the battery to top it off again. The MPPT will not let the battery Voltage go any higher that the setting for the stage that it is in, so no problems.

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