Which CC for small home-made LiFePo4 camping battery?

the_snark

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My first post here. I am building a 4S 12V LiFePo4 portable battery to power our camping escapades. It's 70 Ah and the BMS will be a JK B2A8S20P (which has bluetooth). The process of building my own setup is intended to be a bit of fun while also giving us a practical system that's good to use. I have an EE degree from some time back in the dark ages, but am very much learning about batteries and solar.

I plan to buy a cheap solar blanket not intended for Lithium, and replace its controller with something else. Which "something else"... is the question.

Firstly, the cheap solar blanket costs about $150 Australian and has specs that look like this.
Maximum Power Output (Pmax): 120 Watts
Maximum Power Voltage (Vmp): 17.8 Volts
Maximum Power Current (Imp): 6.75 Amps
Short Circuit Current (Isc): 7.54 Amps
Open Circuit Voltage (Voc): 20.8 Volts

The controller needs to be fairly small and light.

The 2 controllers I'm considering are:

1. Renogy Wanderer 10A PWM (AUD $26)
Why: Because it's cheap, small and light, and PWM should be nearly as efficient as MPPT in this application. I won't be over thinking it by getting this. (...or will I?)
But: Will it damage my cells? Will my BMS protect them if it tries? I've seen threads (https://diysolarforum.com/threads/renogy-wanderer-10a-lifepo4-excess-production.30593/) which might be a concern (but was it configured for lithium - maybe it was trying to equalise?). And reviews elsewhere saying it may not last very long - but if any product breaks, people are more likely to leave a bad review.

2. Victron SmartSolar MPPT 75/15 (AUD $151)
Why: Because MPPT is more efficient, and allows me flexibility to upgrade the blanket or put 2 in series. Future proof. Easy configuration via bluetooth to make sure it's treating my battery well. Has good reviews. Buy once, buy right.
But: 6 times the price of option 1. And the user manual tells me that it needs the PV voltage to be at least 5V above battery voltage before it will start charging (1V above to continue charging). If this makes me miss charging opportunities, I might as well go PWM.

Guidance please.
Have I missed any great options?
Am I overthinking? I already know the answer.
 

DThames

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MPPT, hands down. Be sure to get one large enough to cover the power needs when you build more battery or add more solar. That amp rating is what goes to the battery, so you can quickly max it out if you want to go bigger someday.
 

Tim Tim

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I live in Australia but visitIng the UK at the moment.

Ok which cheap solar blanket?

Rather than buy some things which you will later put into a spares bin which you probably won‘t use anymore have a good think about your future power usage and what you will need. Yes buy the PWM if just a mess about but your can also buy a larger Renogy Rover for future use rather than that small Victron.

For the price you are paying even a Kings solar blanket may be the best option for you even if it isn’t a ‘true’ MPPT controller.

Kings 200 watt solar blanket
 
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the_snark

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which cheap solar blanket?
Yep, it's the Kings 120W, usually $159 but yesterday was $143.
Today the Kings 200W is reduced to $149. It's probably a bit higher powered than I need, and physically a bit bigger than I want.
The Kings MPPTs are apparently not suitable for lithium.
Renogy Rover is bigger and heavier than I like for portable purposes.

MPPT, hands down.
What about the Victron requiring 5 volts more than battery before it starts up? Is this going to be a problem when light is marginal? Is it normal for MPPT controllers?

I'm looking for the sensible middle ground, saving costs where appropriate given this is not going to be a heavily used system, but happy to spend more if it's worthwhile.
 

DThames

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Yep, it's the Kings 120W, usually $159 but yesterday was $143.
Today the Kings 200W is reduced to $149. It's probably a bit higher powered than I need, and physically a bit bigger than I want.
The Kings MPPTs are apparently not suitable for lithium.
Renogy Rover is bigger and heavier than I like for portable purposes.


What about the Victron requiring 5 volts more than battery before it starts up? Is this going to be a problem when light is marginal? Is it normal for MPPT controllers?

I'm looking for the sensible middle ground, saving costs where appropriate given this is not going to be a heavily used system, but happy to spend more if it's worthwhile.
Normally with MPPT you start off with series panel string and the voltage is not an issue. For example for my backup system I have 3, 24v panels in series (120+ volts open circuit0 to charge a 24v battery. So being 24v+5v on the solar happens at first light. You should start with 2 series panels if using MPPT. (my opinion)
 

Tim Tim

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Yep, it's the Kings 120W, usually $159 but yesterday was $143.
Today the Kings 200W is reduced to $149. It's probably a bit higher powered than I need, and physically a bit bigger than I want.
The Kings MPPTs are apparently not suitable for lithium.
Renogy Rover is bigger and heavier than I like for portable purposes.


What about the Victron requiring 5 volts more than battery before it starts up? Is this going to be a problem when light is marginal? Is it normal for MPPT controllers?

I'm looking for the sensible middle ground, saving costs where appropriate given this is not going to be a heavily used system, but happy to spend more if it's worthwhile.
Consider that solar blankets laid flat will produce about 50% of the rated power. Even tilted then you will struggle to get 80%.

Will take a look at the Victron for you.
 

Rednecktek

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The link says it comes with a MPPT controller which is usually about $100 on its own... if it's legit.

As for the 5v difference, that's pretty normal as there has to be a voltage differential to make charging happen in the first place. Most units need 4-5v so don't stress on that.

I'd say since it says it comes with a controller anyways, start with what they give you and then go from there. Unless you're last name is Bezos or Musk it's not normally worth spending hundreds of dollars for what could be a 1-2% gain.

If in the future you decide the controller just isn't cutting it, there are many, many other brands of controller that will turn solar DC into battery DC for much less money.
 

the_snark

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solar blankets laid flat will produce about 50% of the rated power. Even tilted then you will struggle to get 80%.
Yeah. This is all a bit of an experiment until I work out what I really need.

I'd say since it says it comes with a controller anyways, start with what they give you and then go from there. Unless you're last name is Bezos or Musk it's not normally worth spending hundreds of dollars for what could be a 1-2% gain.
Problem is the controller it comes with is not intended for lithium. That's why I was considering the $26 Renogy as a cheap lithium-compatible replacement for it.

I'd just go for the cheap Renogy if I was sure it wouldn't damage the cells (see link in my first post).

The Renogy Wanderer PWM 10A manual (page 14 or 15) says when it's configured for lithium, you can choose the "charge voltage". That's the only parameter you can set.

Thanks all for the input.
 

Tim Tim

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Yeah. This is all a bit of an experiment until I work out what I really need.


Problem is the controller it comes with is not intended for lithium. That's why I was considering the $26 Renogy as a cheap lithium-compatible replacement for it.

I'd just go for the cheap Renogy if I was sure it wouldn't damage the cells (see link in my first post).

The Renogy Wanderer PWM 10A manual (page 14 or 15) says when it's configured for lithium, you can choose the "charge voltage". That's the only parameter you can set.

Thanks all for the input.

Sounds like that Renogy will be a good start for you. Just remember to make sure you choose 12v when programming for LiFePO4. Would appreciate how your Kings blanket performs as I test lost of panels and recently did a test on the Kings small semi flexible panel.

Good luck.
 

DThames

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Yeah. This is all a bit of an experiment until I work out what I really need.


Problem is the controller it comes with is not intended for lithium. That's why I was considering the $26 Renogy as a cheap lithium-compatible replacement for it.

I'd just go for the cheap Renogy if I was sure it wouldn't damage the cells (see link in my first post).

The Renogy Wanderer PWM 10A manual (page 14 or 15) says when it's configured for lithium, you can choose the "charge voltage". That's the only parameter you can set.

Thanks all for the input.
Have you considered the EPEver MPPT chargers? They can be fully configured if you connect them to a PC, get the Bluetooth adapter, or the little push button interface.
 

pollenface

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The victron smartsolar 75/15 is a great little unit. Built in bluetooth, easy to configure and small form factor to fit into tight places.

it needs the PV voltage to be at least 5V above battery voltage

You might want to be more concerned about killing your lifepo4 battery with the PWM.
 

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