Simple MPPT solar panel charge controllers

[Rick Willoughby]

Originally Posted by power

Hello BillyDoc,

Thanks for all that information on solar recharging. A friend of mine and I are working on a school project on designing a solar powered recharger to recharge lithium ion batteries efficiently. We are looking for a low cost solution and were wondering if the voltage regulation is necessary. Can't we simply use a diode in series with the panel and the secondary battery to prevent current flow from battery to panel? The battery voltage can be regulated by a comparator as you suggested, and whenever it is low, the panel can be charging the battery up. Since the battery voltage cuts off the power supply beyond a certain voltage, why do we need a voltage regulator at all?

Thanks

At the stage of development the life of most lithium cells is highly dependent on not taking them outside their voltage range. You can find references to Battery Management Systems (BMS) for charging and ballancing lithium batteries. They monitor each cell individually.

You can get economically priced BMS with buck and boost circuitry from here:
http://www.hobbycity.com/hobbycity/s...6&ParentCat=85

In answer to your question the life of the battery will depend on it not being overcharged or or fully drained.

The advantage of a MPPT is that it will get the maximum energy from a given solar cell. This is quite different to a charger that will guarantee good battery life.

If you use a simple regulator even with tight limit on peak voltage then you run the risk of cell imbalance and overcharging some cells while others are completely drained on discharge. Lead acid batteries are not as sensitive to cell differential in the battery.

Rick W

[Prajna]

Wow Bill, exactly what I have been looking for!

I am hoping to build 500w of solar panels for my motorhome and was researching charge controllers. Strange, I had naively imagined that all charge controllers would do mppt and was horrified to find that a. the idea is quite new and b. they are so damned expensive.

Batteries are complex beasts and rather esoteric. I have bought 300ah of elecsol, carbon fiber, batteries. The first thing to know about lead-acid batteries is that there are two distinct types: flooded and sealed, and they both require different charging regimes. Actually, we also need to distinguish between deep cycle (traction) batteries and starter batteries because deep cycle batteries are designed with thick plates so that they can be discharged often, at the expense of being slower to deliver power. Starter batteries have lots of thin plates, so they can deliver a high current, but they really suffer when deeply discharged.

Lead-acid battery charging is generally split into at least 3 stages. The first stage, bulk, delivers as much current as possible (up to the C20 rate generally) until the voltage reaches the 'charged' voltage. In the case of my Elecsol's this is 14.4v. Then there is an absorption stage, where voltage is held constant while the current the battery accepts drops until it reaches a specified level (usually about 1amp). After that there is a float stage where the voltage is dropped. Some chargers also have a pre-charge phase where the battery is pulsed to desulfate the plates and often the float cycle is pulsed slightly too. In addition to that flooded batteries can be 'equalized' to make sure all the cells are producing the same voltage.

For sealed batteries it is bad news to have them gassing because if you lose water from a sealed battery there is no way to replace it. For flooded batteries it is good to overcharge (equalize) them periodically (usually about once a month) which causes lots of gassing for an hour or two and that desulfates the battery and makes sure all the cells have the same voltage. Obviously one doesn't want to do that to a sealed battery.

There is a nice chip that is designed specifically for multi-stage lead-acid battery charging by Microchip. Here is the app note: http://ww1.microchip.com/downloads/e...tes/01015a.pdf

Maxim also have an app note for battery charging here http://www.maxim-ic.com/appnotes.cfm/an_pk/680

So not only is the mppt a concern if the design is to be of general use different battery parameters (including the Ah total of the bank being charged) needs to be configurable too.

Oh, and another thing: the charging voltages change with battery temperature too. Ideal is if a battery temperature sensor is included and voltages adjusted depending on the temperature. Also there is the problem that the voltage reading is the 'surface charge' on the plates, which is why a definitive state of charge can only be read after the battery has rested (after either charging or discharging).

Not sure if this helps but I hope so. I don't know so much about electronics but I have put a lot of effort into researching batteries and would be happy to answer, or find an answer, for anything that would help.

Oh, I am happy with programming too, so if it comes to writing firmware for a microcontroller I'd be happy to help with that.

regards, Prajna