Midnite Brat

[seneysolar]

Is anyone using a Brat PWM that can comment on the performance or efficiency?

I have heard stories about efficiency only being 60% for PWM but that is just generic pwm stuff.

Is there any data or tests on this unit in particular.

 

[chrisski]

I don't have a brat, but for $140 Midnite Solar Brat a 60 volt 30 amp PWM SCC, and the$130 for a Victron 100/30 100 volt 30 AMP 100 volt MPPT SCC, I would like to see any reason to get that PWM.

I have the Victron 100/30. These efficiencies are a bit hard for me to quantify. At max charge hours on end at 24 volts, that will be close to 700 watts, it gets warm but not hot, so I I believe the efficiency rating of 98%. If the PWM was 60% efficient, it would be too hot and untouchable.

I've seen Will's videos where he compares a PWM to MPPT on cloudy days and the PWM is more efficient, but by only a few watts, and so little power is produced on a cloudy day to start with.

 

[seneysolar]

I am considering the Brat for its outdoor rated enclosure.

Going to be in a rough environment. Single panel 24v, lead acid batt. Seems like the best candidate for the price. I see them for sale for about $88.

Yes i kinda doubted the 60% efficiency, had higher hopes for Midnite engineering.

 

[seneysolar]

Update: Due to lack of reviews on the unit and not much input here, I decided to install the Midnite Brat in my current setup as an experiment. I had to change the panels from series to parallel but otherwise everything else stayed the same. I removed the tried-and-true Victron 100/30 and swapped in the Brat....

I'm currently trying some Trina 335 that I had laying around as new surplus out on my 2-panel pole mount.

As a baseline reference, the Highest charge current I have ever seen from the Victron unit in this setup was 18.7A.

Panel STC (perfect lab conditions) Specs are:
Watts: 335
Vmpp - 34.0
Voc - 40.7
Also its important to note the NMOT specs:
Watts: 254
VMPP - 31.9
Voc - 38.4

Charging a 4 series Lead acid L-16 bank (24V) in full sun, the most current I have seen from the Brat has been 17A with my amp clamp meter while charging in bulk, battery voltage was at or passing through 26V on its way to 28.8. So 17A x 26V = 442W

So, NMOT rating @ 442w/508w = 87%
STC rating @ 442W/670w = 66%

17A/18.7A - Victron is doing about 10% better. (with panels in series)

Disclaimer!!!!! I have not done any temperature compensation calc or voltage drop calc on my 150ft run of PV wire. Victron scenario was set up in series and the Brat was parallel, so running a long distance at half the voltage is a handicap right off the bat. 10% perhaps??? To be fair I should hook up the Victron while panels are parallel.

Conclusion--- when it comes down to it, I'm fine with either one. Would I discard one and buy the other? Nope.

 

[Hedges]

PWM efficiency (the device itself) will be high. Just whatever voltage drop you measure between PV and Battery, divided by battery voltage.
The efficiency loss is PV panel operating at battery voltage, which is below Vmp.

You can study the I/V or W/V curve on panel data sheet, calculate what is being lost.
Problem is Vmp varies with temperature. If high enough to fully charge on a hot day, will be higher on a cold day, PWM pulls it further below Vmp.
However, battery when cold needs higher voltage.

Also note battery voltage vs. SoC (bigger range for lead-acid than LiFePO4), so charging when battery low operates panel less efficiently than supplying power to loads while SoC higher.

Besides tracking Vmp, MPPT allows much higher array voltage, series connected PV panels, less loss over long wire. Costs a couple percent efficiency loss in the SCC to do this.

Probably PWM great or small system, MPPT for larger.

 

[hwy17]

What's the load you're serving? Efficiency is a false god in some applications compared to simplicity and reliability.

My gate opener is the most reliable solar system I know, I'm not even sure it has a PWM. 12v solar panel and 12v lead acid. Runs unattended and unmonitored for at least 5 years, never runs out of power in any weather conditions. But the load is low, maybe 2-12 gate opens per day.

 

[seneysolar]

I think the term "efficiency" gets tossed around a lot and should be relative to the system as a whole...... Why does the MPPT get warm and require a large heatsink, whereas the PWM has no heatsink and does not get warm at all? Heat is wasted energy, right? I think this is what @Hedges is referring to as the "device itself" being very efficient.

Don't get me wrong, the MPPT controller solves a ton of industry problems being able to series connect panels and maximize available watts for a given array. Throw any size panels at it while staying under the max Voc and they work.

Victron, Morningstar and Midnite are all very smart companies and all still manufacturing PWM controllers for a reason. I just don't think they are as bad as everyone says.

FWIW - I paid $88 for the Brat
Current Price on the Victron MPPT is $128
At today's value that's a 45% price increase for the extra 1-2 amps of real world charging current in my setup.

 

[Hedges]

I took a class where we analyzed switchers - Buck, Boost, other architectures.

A MOSFET off burns no power, and saturated on, just milliohms resistance, little power except at very high current.
But during switching transitions, it has full voltage across it while current ramps back up through inductor. Most power dissipated by the FET is during that time. The higher the operating frequency, the more it dissipates. But inductors can be smaller, or power handling can be higher for a given size/weight/cost (except for thermal issues.)

Inductor has resistance, and at full power dissipates more.

The trade-off of component size and switching frequency, vs. cost, means there is some power dissipation.

But MPPT design is applicable across wide range of voltages. The different models with 100V, 150V, 250V, 450V, 600V capability are also a trade-off. Costs more and has greater losses to use the 600V models, but worthwhile for some applications.

MPPT is being used in 3kW to 30kW SCC and inverters. For HV DC used in GT PV inverters, pretty much the only way to go. Used to be just Buck and highest efficiency, but narrow range of PV voltage. Many today appear to be buck-boost and work from 100V to 600V PV. Better for wide range of roof configurations, keep microinverters from encroaching on the business too much.

20 years ago when PV panels cost $5/W, efficiency was everything. Today at $0.20, not so much.

 

[seneysolar]

I took a class where we analyzed switchers - Buck, Boost, other architectures.

A MOSFET off burns no power, and saturated on, just milliohms resistance, little power except at very high current.
But during switching transitions, it has full voltage across it while current ramps back up through inductor. Most power dissipated by the FET is during that time. The higher the operating frequency, the more it dissipates. But inductors can be smaller, or power handling can be higher for a given size/weight/cost (except for thermal issues.)

Inductor has resistance, and at full power dissipates more.

The trade-off of component size and switching frequency, vs. cost, means there is some power dissipation.

But MPPT design is applicable across wide range of voltages. The different models with 100V, 150V, 250V, 450V, 600V capability are also a trade-off. Costs more and has greater losses to use the 600V models, but worthwhile for some applications.

MPPT is being used in 3kW to 30kW SCC and inverters. For HV DC used in GT PV inverters, pretty much the only way to go. Used to be just Buck and highest efficiency, but narrow range of PV voltage. Many today appear to be buck-boost and work from 100V to 600V PV. Better for wide range of roof configurations, keep microinverters from encroaching on the business too much.

20 years ago when PV panels cost $5/W, efficiency was everything. Today at $0.20, not so much.

Thank you! Finally a simple explanation I am not educated in depth on this subject which is why im experimenting with both.

Have you by chance seen my post asking about SES Flexcharge? This one still baffles me. You seem like the one to ask.