Replace PWM solar controllers fixed voltage buck DC-DC for extra efficiency? - Page 1
Forum section: electronics / general

author: Freesurfer
author_rank: Regular Contributor
timestamp: August 04, 2020, 01:50:28 am
content: I have a solar system made up of 28x 12V/40Ah VRLA batteries (7 in parallel, by 4 in series)They are charged with four 60W polycrystalline panels, each panel charging each group of 7 batteries. I am using 4 simple PWM charge controllers for this, one for each. So they can sort of be considered 4 individual 12V systems.The only issue is that the 60W panels are 36-cell ones, with an 18V nominal output voltage (can probably go up to 22V+ open circuit) and they deliver around 3A short circuited. So the most I can hope to get out of them is in theory 36W because there is no DC-DC / MPPT. MPPT trackers are very expensive and not really worth it for my setup.But then I thought of replacing the PWM charge controllers with some cheap DC-DC buck converters from ebay, set to output a fixed voltage at 13.5-13.8V, hoping to be able to get maybe 4-5 amps of current on sunny days. Does anyone have any experience with this? How will the batteries last if they are only ever "float charged" at 13.8V and never topped up with 14.4V like the PWM controllers do? I understand that the charge may slow down towards the end, but I am mostly concerned about topping up the batteries quickly from the lowest state-of-charge and don't mind if topping up takes longer than usual.

author: tom66
author_rank: Super Contributor
timestamp: August 04, 2020, 07:29:48 am
content: You'll likely find that if the buck converters aren't MPPT, as soon as you go into shade, they'll drop into an operating point where they draw a large amount of current at a low voltage, which will prevent the efficient use of your cells.There are some open-source MPPT controllers and the concept of operation is fairly simple. But I think a regular buck converter would perform poorly. Worse than a PWM controller? I don't know for sure.

author: butech
author_rank: Contributor
timestamp: August 04, 2020, 08:24:12 am
content: Quote from: Freesurfer on August 04, 2020, 01:50:28 amI actually did something similar, although at much lower power. I build a very cheap USB solar charger using a 10W pv panel and a buck converter with constant output voltage of 5V. But as already suggested, this works only when there is plenty of sun available, otherwise the input voltage will drop and you will not get much power out of it. Therefore this solution could be worse than a simple PWM charge controller (I just used a dc/dc buck converter as I had the spare parts available at home and now I have at least an emergency/outdoor USB charger).

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 04, 2020, 12:38:03 pm
content: Quote from: Freesurfer on August 04, 2020, 01:50:28 amBuild your own based on a microcontroller.

author: rhodges
author_rank: Frequent Contributor
timestamp: August 04, 2020, 01:37:52 pm
content: Quote from: NiHaoMike on August 04, 2020, 12:38:03 pmHere is one from 2013, using a PIC16F1503 (AN1521):https://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en561417

author: ahbushnell
author_rank: Frequent Contributor
timestamp: August 04, 2020, 04:18:27 pm
content: Quote from: tom66 on August 04, 2020, 07:29:48 amI'm confused when people talk about buck converters, PWM controllers and MPPT. A buck converter is a PWM converter. MPPT converters use PWM power supplies and hase a computer controlling them so they stay on the MPP.A buck converter can act as a MPPT converter if it has proper controls.

author: Seekonk
author_rank: Super Contributor
timestamp: August 04, 2020, 06:04:29 pm
content: I've done a lot of this. Any buck converter can be faked into following the panel voltage and acting as a liner current booster with an upper voltage limit. I have a 60V array and have used $5 72 to 12V buck converters which then feed into a cheap $5 PWM regulator. Set the upper voltage to 14.5V. When array voltage drops below 56V, it throttles down the buck converter by faking the feedback pin into thinking the output voltage is too high. Done this also with simple LM2956 boards on 12V panels. It doesn't take much, TL431 or even a FET. Constant voltage gives you 90% of the MPPT performance benefits. Simple temperature tracking gets you even further. I don't see why more of this isn't done except nobody can build anything anymore.https://www.eevblog.com/forum/projects/solar-panel-regulator-design-question/msg2525382/#msg2525382

author: fourfathom
author_rank: Super Contributor
timestamp: August 04, 2020, 06:20:18 pm
content: Quote from: ahbushnell on August 04, 2020, 04:18:27 pmWhen talking about solar panel controllers, "PWM" means connecting and disconnecting the panel to the battery, using a relay or transistor as a switch. These "PWM" controllers rely on the constant-current nature of the panels. They turn on when the battery voltage is below a given setpoint. and turn off when the battery voltage exceeds that setpoint. Some PWM controllers do multi-stage charging and will use a faster PWM to control the average current into the battery as it goes through the charging cycles (bulk, absorption, float) but this is still connecting and disconnecting the panel, so you won't get the optimum power transfer of a regulator that uses buck/boost switching to operate the panel at MPP (Maximum Power Point).

author: trobbins
author_rank: Frequent Contributor
timestamp: August 04, 2020, 11:25:12 pm
content: Freesurfer, each 40Ah 12V monobloc may have a maximum charge current spec of around 10A (you would need to confirm for your specific batts), in which case you would not exceed that spec with your PV module. As such, the appropriate charge voltage to use is the maximum temperature-compensated 'equalise' voltage, or whatever the battery manufacturer allows, compared to just using the recommended 'continuous charge' float voltage. That is typical of PV systems which have daily intermittent charging - especially if there is some low continuous loading (like the converter voltage monitoring, and self-discharge).

author: ahbushnell
author_rank: Frequent Contributor
timestamp: August 04, 2020, 11:27:35 pm
content: Quote from: fourfathom on August 04, 2020, 06:20:18 pmI guess that would be okay with lead acid. But sounds like a bad idea for Lithium ion.

author: trobbins
author_rank: Frequent Contributor
timestamp: August 05, 2020, 02:23:36 am
content: Ahbushnell, the OP specifically identifies VRLA. Why bring up Li-Ion or any other chemistry battery (ie. start another thread for other battery types)?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: August 05, 2020, 04:21:00 am
content: Sorry I made an error. No need to jump on my ass.