Help, designing a simple solar charger (controller) - Page 1
Forum section: electronics / general

author: rr100
author_rank: Frequent Contributor
timestamp: February 03, 2012, 08:51:36 pm
content: I try to come up with a solution for a friend - basically there's a 50W solar panel that can give about 3A at about 15-19V. This charges a deep cycle Pb battery , we're talking about 3+A. The controller just needs to stop the charge when the voltage on the battery reaches some level (like 14V or something similar, we don't care about more complicate algorithms or temperature, etc). And preferably charging should resume when the voltage drops under some level (it can be the same threshold if the controlling element can handle it).Now the challenge is to do this about as efficiently as just looking at the voltmeter and pulling the plug by hand (this is in fact easier than one can imagine as the energy balance is usually negative when charging via solar panel, we eat with other loads more than the sun gives us).Anyway as I said I want this to be basically as efficient as the "manual" option. I'm not talking about fancy MPPT, just something that kills the charge while wasting almost no energy while charging. It is ok to waste it once the battery is full. I started thinking about a Low-dropout regulator but I'm having trouble finding one that meets the specs (probably I can do the same just with FET+opamp). Any elegant and obvious solution I'm missing?

author: kripton2035
author_rank: Super Contributor
timestamp: February 03, 2012, 09:19:45 pm
content: you can get free ideas here :http://openenergymonitor.org/emon/emontx/solarpvor here :http://www.timnolan.com/index.php?page=arduino-ppt-solar-chargeror here :http://www.solorb.com/elect/solarcirc/scc3/or here :http://www.solorb.com/elect/solarcirc/spc3/index.htmlalso siliconchip articles #112335 or #103353also this simple design (attached)hope this helps.

author: rr100
author_rank: Frequent Contributor
timestamp: February 04, 2012, 01:40:33 pm
content: Thanks for the answer, yes the diode is a big part of the fight towards "no loss". In fact current setup has a switch that shorts out the diode in case the panel is connected only during the day (and anyway it depends on the panel and the distribution of enough light/not enough light, it might be better to just forget about the diode and live with the small overnight discharge).I don't particularly like to drop the input voltage just by shorting the panel to the ground but in the last link also it's mentioned that the FET acting as a diode might for a short time actually short out the battery over the other FET that's shorting out the panel! And the protection for this transient event is an inductance in between. Frankly I don't like the sound of that!Another thing: can somebody share some experience with MPPT and such low power setups as mine, is it really worth it (in terms of efficiency, cost up to a point is secondary)? Now if the voltage generated by the solar panel is about the same (or even lower) than battery voltage I can understand a DC-DC converter would push much more current (as opposed to very little or none) and a 5-20% loss in the converter would be irrelevant. However when the panel at some 15-19V it seems that not much could be gained by playing with voltage changes.Anyway, while I was wrapping up this post I think I've got the magical solution I wanted based on a LATCHING RELAY! "OFF" position will take out the battery completely, "ON" will connect the panel directly. The logic works like:(1) solar panel voltage under battery voltage => OFF(2) battery over 14V => OFF(3) battery under 13.5V and NOT (1) => Pulse ONIt is "Pulse ON" because we don't want to waste current feeding the relay all the time. In OFF, we don't really care - battery is anyway out (the electronics is fed from the solar panel) so even if we waste something it's not something we would've used anyway.I kind of like it, it's close to manual operation. Any thoughts?