Electronics > Power/Renewable Energy/EV's

Efficiency comparison question between PWM and MPPT and a simple relay LifePO4

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bigfoot22:
Hi,
I'm enquiring about charging efficiencies specifically relating to charging LifePO4 batteries. I have a simple 555 solar charge controller designed by Michael Davis of mdpub.com and was wondering how efficient it would be compared to MPPT and PWM methods.

The scc version that I'm working on charges from the solar panels via a pair of relays (one relay for negative from solar panels and one relay for positive). MD's design is that it simply uses a single relay.

Just how efficient is simply connecting the solar panel directly to the battery then interrupting it once the voltage goes above a threshold?

Assuming all solar panels are either 12v nominal or 24v nominal panels, not the higher voltage ones.

Here is the thread related to the scc that I'm redesigning (2nd attempt)
https://www.eevblog.com/forum/projects/please-improve-and-critique-this-design/

Here is the link to the webpage:
http://mdpub.com/555Controller/index.html

The goal here is to produce a dead simple reliable and easy to diagnose solar charge controller for remote regions where society (Jaycar) and ebay aren't available or even possible to contact. Something that can be repaired in the field from a box of spare parts if needed.

Yes I do realize the irony of using a LifePO4 battery bank with a BMS and how the BMS superseeds the complexity of this 555 circuit, however this idea should theoretically reduce the amount of possible failures. And I do understand that just simply having a second solar charge controller to replace the existing one and swapping it out would be easier but this is a design challenge. And this is for a hypothetical special use case, eg remote africa or poor countries.

I want to know if I'm on the right track here or should I go and design something using PWM or MPPT. (I dont like the idea of using some charge controller chip that will become obsolete and unavailable in 10 years either)

bigfoot22:
How would you even calculate such a thing?

Ignoring the losses of the circuit itself for the time being (555 chip, LEDs and 7805 powering a meager circuit) and the power used to drive the coils. (I'm thinking maybe solid state relays would be a possibility in the future)

Seekonk:
Personal Disclosure: I was around when the 555 came out and the initial ones had problems. I've never gotten over that and consider any 555 design a joke. There are far better chips which are decades old which will give a much better design.

A constant voltage supply is all you need. Chances are you will never get near top charging current with your source. An occasional manual top balance is all that is needed.  MPPT indicates solar panels.  What do you have.  I modified a standard 120V to 12V LED switching power supply to work at 60V for my array.  These can be set up to maintain a minimum panel voltage as well as lower voltage buck converters for near MPPY performance.

bigfoot22:
Interesting so what you are saying is that I can simply use a DC-DC coverter to covert the higher panel voltage down to a battery voltage. A neat approach especially considering that LifePO4 doesn't require trickle charging. Thanks for your suggestion!

I have 2x 200w 12v nominal solar panels that I could wire in series and 1x 100w 12v solar panel. I have considered selling the 12v 200w solar panels and getting something of a higher voltage.

What I could do with is a suggestion for an IC which will put out 500w at 12v or 24v into a battery and not require firmware or other such nonsense.

Failing that having a ceiling of 150w would be better than nothing.

Geoff-AU:

--- Quote from: bigfoot22 on December 01, 2022, 10:13:20 am ---Just how efficient is simply connecting the solar panel directly to the battery then interrupting it once the voltage goes above a threshold?

--- End quote ---

Direct connect from a 12V panel (max power at 17V) to a 12.8V LiFePO4 costs you around 25-30% efficiency, but if ultimate reliability is your goal then take the money for your fancy MPPT charger, buy 25% bigger panels instead, and don't worry about it.  Charge termination is the only thing to consider, for intermittent use I am relying on the BMS in my pack and the wide safety margin of LiFePO4 chemistry.  But I would not do that for continual use, I only solar charge when camping.  For continual use I'd probably stick a buck converter (or PWM charger) on it.  MPPT is only worth it if the difference between panel voltage and battery voltage is large.

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