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Old 21-02-2014, 10:02   #1
KTP
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My proposed LiFePO4 experiment setup

I am planning a 24V system with 1000 watts of solar, a Midnite Classic 150 MPPT solar controller, and a 24V input Magnum 4000 watt hybrid inverter.

Those are the main components. I also need 12V to power various items such as water heater controls, some electronics, led lights, and the furnace fan.

I initially was going to go with just lead acid, but am entertaining the idea of LiFePO4. I want to have at least 3.5kwh of useable battery capacity and so was considering four Lifeline 6V 300ah AGM (so they can mount on their side in the space available).

I am not comfortable right now with the state of LiFePO4 to sink $3000 into it as the main battery bank, but I am considering using the LiFePO4 as the 12V house battery so I can experiment with it. If I have great results, I would eventually switch out the AGMs for 8 big LiFePO4 cells.

The current plan is complicated, but I think it has some potential. I would utilize the high current from the solar during the peak sun hours to bulk charge the lead acid, then when they needed to be in absorption charge state, I would switch to charging the LiFePO4 house battery. In this way I would be able to utilize the remaining peak sun amps instead of letting them go to waste.

I am thinking of programming a microcontroller (just an Atmel 8 bit) to monitor the four 3.2V 100 ah LiFePO4 cells of the house battery and also control the charger for them, which would be powered off of the main house batteries/Midnite MPPT solar charger. I may need a sensor to determine the level of insolation available at the solar panels and I need to research what the Midnite controller would do if it was charging the 24V lead acid in absorption and suddenly a 15 amp load on those batteries kicked on (the 24V to 14V LiFePO4 charger).

Even though this all sounds complicated, it does sound doable, and doesn't really use that many more parts than other methods since I need 12V and you can't directly tap from two of the cells in the 24V lead acid pack without causing imbalance (unless you use special devices). It may be a fun way to dip one's toe into LiFePO4 without breaking the bank, as a 12V 100AH pack should cost me under $600 from CALB.

Opinions?
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Old 21-02-2014, 11:57   #2
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Re: My proposed LiFePO4 experiment setup

Im confused , how do you intend to switch the thresholds on the Midnite when you switch it over to the Lifepo4 bank.

Or maybe I, reading it wrong , the easiest way would be to have a dedicated battery to battery charger like the Sterkling power unit , and set that up for Li charging, leave the solar charging the AGMS all the time and just steal power across as needed.

fusing an Atmel, make sure the ADC has enough resolution across the voltage range. ( you'll need probably 10-12 bit) don't forget LVC and HVC cutoff relays.

Dave
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Old 21-02-2014, 12:05   #3
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Re: My proposed LiFePO4 experiment setup

How about a 24 to 12 volt converter?

New Pyle PSWNV720 24 Volt DC to 12 Volt DC Power Step Down 720 Watt Converter | eBay
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Old 21-02-2014, 12:07   #4
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Re: My proposed LiFePO4 experiment setup

Quote:
Originally Posted by senormechanico View Post
if it was adjustable voltage , yes, also it needs to handle over current by foldback, whereas lots just crowbar.


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Old 21-02-2014, 12:17   #5
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Re: My proposed LiFePO4 experiment setup

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Originally Posted by goboatingnow View Post
Im confused , how do you intend to switch the thresholds on the Midnite when you switch it over to the Lifepo4 bank.

Or maybe I, reading it wrong , the easiest way would be to have a dedicated battery to battery charger like the Sterkling power unit , and set that up for Li charging, leave the solar charging the AGMS all the time and just steal power across as needed.

fusing an Atmel, make sure the ADC has enough resolution across the voltage range. ( you'll need probably 10-12 bit) don't forget LVC and HVC cutoff relays.

Dave
That is what I meant. Have a dedicated battery charger for the Lithium which is powered off of the lead acid (which is in turn charged by either the solar or the generator). Thus to the lead acid the Lithium battery just looks like another load.

Doing this would have the advantage of giving me more storage capacity overall, more capability of using the maximum amps available in a short window on the solar, and would allow me to play with a smaller Lithium that wouldn't kill the wallet if I killed it.

Since I need 12V anyway, I would be buying something to convert the 24V to 12V, which means the main extra costs in my version are the actual battery and the Atmel BMS.

Say your 1000W solar profile looked like this over a ten hour period (making this up)

100W, 200W, 400W, 700W, 800W, 800W, 700W, 400W, 200W, 100W.

Your lead acid is at 70% from overnight, your lithium is at 40%, so you start charging in the morning taking the maximum amps from the MPPT and bulk charging the lead acid, taking no power to charge the lithium. About noon the batteries have reached 80% and the current is folded back from the MPPT controller into absorption charging, but you are still getting peak power from your panels. Your Lithium charger kicks on and uses this excess amperage to charge the Lithium house battery from 40% to 100%, while keeping the lead acid charging in absorption. Any time there is excess load that would prevent the lead acid from continuing to fill up, the Lithium charger cuts out.
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