MPPT input voltage comparison

[BruceS]

Looks like the "A" series?
http://www.epsolarpv.com/en/index.php/P ... /am_id/134

[Noggins]

Or like this one
https://www.ebay.com.au/itm/Solar-Contr ... xywFhTcxC1

Ron

[jon_d]

Bruce.

If you look at the specs, it's MPPT with a PWM output.

(I assume no DC-DC convertor)



Features:
• Advanced MPPT technology
• High tracking efficiency no less than 99.5%
• Peak conversion efficiency of 98%
• Ultra-fast tracking speed
• Accurately recognizing and tracking of multiple power point
• Multi-function LCD displays system information intuitively
• User programmable for battery types, load control etc.
• 3-Stage charge with PWM output

[T1 Terry]

All controllers both MPPT and PWM use PWM output control because there is no other way yet to control battery output voltage, even the dump load type use the same control method, just some switch faster than others. This means the only time an MPPT controller actually acts as a device designed to extract the max solar output is during the bulk charging phase, after that the output is limited by the PWM function so no gain is actually possible. During the absorption and float stage the only advantage an MPPT controller can offer is converting a high voltage input to a suitable battery voltage output, say 24v panels charging a 12v battery.
There is no advantage using a 24v panel to charge a 12v battery, the price per watt is the same for 12v and 24v and even grid tie 36v or 45v panels, but the grid tie panels are way too big for RV roof installation. You can buy second hand low efficiency grid tie panels for around 50 cents per watt, but these are generally 190w panels when they were new and they take up the same area as 250w or 270w or even 290w panels on the market now for $1 per watt, I just can't see the logic in using these where space is a premium.
MPPT controllers will always use more of the solar energy harvested than a PWM controller because they have a lot more work to do converting the high voltage/low current to low voltage/higher current and no electronic equipment is anything like 100% efficient. This means some of the precious harvested solar energy is converted to heat and wasted, what is left goes to charging the battery.
A PWM controller simply lets all the current from the solar flow through to the battery until the battery reaches the desired voltage, then the same PWM control as an MPPT controller uses takes over.
Don't be fooled by the 98% conversion efficiency into believing it will converter 98% of the rated panel output into battery charging current, it really says at least 2% of what comes in will be used by the controller and the best you will get out as battery charging current is 98% of what came in and that is not even close to the advertised panel output.

T1 Terry

[bagmaker]

T1 snip- of what came in and that is not even close to the advertised panel output. -snip
which we have shown on other threads to be about 80% of the panel rating.

Ok a little more from todays tinkering. I pulled a few quick amps out of the battery at high noon to ensure the controllers went into bulk mode.
I am not positive of the actual unit specs, sorry all, they came with the rig and no literature. For some reason I believe them to be 30a rated, there are no model numbers on the front or sides. Perhaps the previous owner told me they were 30a.

Under full sun they both dropped to bulk mode within 5 seconds of each other, outputting between 25-28 amps ( 12.4v). In bulk output they both read a similar 2- 3 amp fluctuation. I would say the series wired controller pushed out slightly more at times but its a line call.

So, musing, does this mean

a- T1 is right (horror - we cannot have that here)
b- if I connected BOTH in parrallel there would be a shorter charge time at the higher battery voltages -providing the battery could accept it?
NB, the battery cannot accept it so the point is moot. But for discussion.................?

[T1 Terry]

This is something I have never actually tested but a theory based on what I know about lead acid batteries and MPPT controller actions V PWM controller actions.

The MPPT controller takes in the panel output at what it believes is the voltage the array produces the maximum amps. It then determines the best voltage to apply to the battery to get it to charge at the highest rate and the lowest voltage so there is a gain between the voltage x amps coming in compared to the voltage x amps going out.
To do this a good quality controller will continually try a higher input voltage and measure the amps coming in to see if it drops, then a lower input voltage and see if the amps improve. A cheap unit has a look up table, picks the mix of input voltage compared to output voltage someone at factory decided was the ideal combination and never tests it these figures are right or not. A medium level controller does a test at set intervals, uses the results for a set period and then retest. A premium level controller does this constantly.
The same test needs to be conducted on the battery, a premium level controller measures the voltage on separate wires at the battery terminals, the others measure it at the controller output inside the controller itself. The same goes for PWM controllers, ones that work well have a separate connection point to monitor battery voltage, the not so good measure internally.
For an MPPT controller to accurately measure the battery voltage it must stop charging the battery because that will affect the battery terminal voltage, once this figure is recorded it will recalculate the best mix between incoming voltage and out going voltage to achieve the best results. How often and how long the charging stops depends on the quality of the controller and if it senses the voltage separately to the charging cables or if it senses it internally.
If this charging voltage was plotted on a graph there would be a fairly straight line that climbed as the battery approached the point where the amps were coming in faster than the battery could accept but dropped every time battery voltage was tested and then resume if the controller sensed the battery still needed charging. This voltage is held at each point by the battery, not the controller, as the battery reaches the point where it can not accept all the amps the controller is trying to push in without the voltage climbing higher than the set point the operation changes to PWM mode with a voltage determined suitable for absorption mode.

The PWM controller simply connects the solar output to the battery terminals and switches this connection off if the battery rises above the required voltage and back on if it drops below the required voltage. It does not alter the voltage coming from the solar at all, it is a direct connection or no connection, it just does this on/off very quickly.
If it senses the voltage is above the set point it turns off, the effect is immediately sensed and if the voltage is lower than the set point it turns on again. While the solar is disconnected the input voltage increases until it reaches open circuit voltage, when reconnected the battery pulls that voltage down until it can not accept all the amps the solar can supply and forces the voltage to climb until the off set point is reached and the solar is again disconnected and the cycle repeats.
If this charging voltage was plotted on a graph the line would slowly climb as the battery reached the point where it could not accept all the amps the solar can supply, drops for a microsecond and then jumps higher than the point when charging stopped because the solar had climbed to a higher voltage but drops back quickly as the battery absorbs the amps and pulls the voltage back down. The voltage again climbs and the cycle repeats. So the chart would now look like saw teeth, getting smaller and smaller as the battery approaches the point where it can not accept very much current until the voltage climbs above the off point.
This effect is similar to pulse charging rather than flat line voltage charging and it is well documented that pulse charging not only cleans sulphates from the battery plates but also charges faster with reduced gassing compared to flat line charging.

T1 Terry

[jon_d]

Terry,

Mine is different to the description below.

"The MPPT controller takes in the panel output at what it believes is the voltage the array produces the maximum amps. It then determines the best voltage to apply to the battery to get it to charge at the highest rate and the lowest voltage so there is a gain between the voltage x amps coming in compared to the voltage x amps going out.
To do this a good quality controller will continually try a higher input voltage and measure the amps coming in to see if it drops, then a lower input voltage and see if the amps improve"


It sweeps the panel array from a high voltage to a low voltage.

While it does this, it tracks the OUTPUT current and selects the input parameters that gives the highest output current.


And, it doesn't connect straight to the batteries, it uses an adjustable output DC-DC converter to set the output voltage or current (constant current mode) depending if its in bulk, absorb or float.


Here's a video of it this morning in low light and one side in shade.

JUST NOTE: The right hand side amp meter is the MPPT Output Battery Current. NOT the panel "combined input" current as 'alluded to' in the video.

https://youtu.be/13BsrbtXfPk

[native pepper]

LIfepo4 love mppt chargers, they love bulk charge and in my experience, are perfect for the job. PWM, come a distant last when charging lifepo4. With an mppt bulk charger, they provide the maximum input possible for the entire charge. My old mate once told me, mppt was designed for lifepo4, not lead acid and using pwm chargers on lifepo4 is a waste of money and energy.

[SteveW]

It had to come!

[Busman]

Gotta disagree, been using my 3 PWM chargers on my 800 Ah bank for a few years now, no problems. Fill the bank up and then stop, so what is the issue ? Using basically good quality LA charger on my 200 Ah 12 pack as well, still no issues.
Too much male bovine excreta about this whole topic in my opinion.
I would like to see some scientific studies from LiFePo4 battery makers (or others) doing a comparison of PWM charging and MPPT charging on these cells, not just reference to "what my old mate felt was best" Possible ?

Cheers
William