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24v vs 12v for small off grid system
#1
I started building a small offgrid 12v system for my 180 gallons + acuaculture garden, to run the water pump and air pumps. So far I only have a gell based battery.

When it comes time to replace this I’d like to do so with the longer lasting lithium ion variety. Diy?

I read on here that 38120’s are better for 12v than 18650’s. So I ordered a couple new ones to start collecting such. More expensive.

But 24v systems are more efficient? Does that only apply if an inverter is in the equation? I wasn’t planning on using one at all. My water pump is 12v. The air pumps are being powered by USB plugged into a car cigarette lighter plug also on the charge controllers load.

And I also learned that there is devices that can step down the 24v to 12v.

Does it all come down to cost?

Trying to weigh the options. My 60 amp charge controller can do either 12 or 24v.

Thanks, Peace!
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#2
LiFePO4 for direct replacement of 12V Lead Acids. Which, I'm "guessing" you mean by the 38120's and being Headway's.

It's not the size that makes it better, it's the chemistry. LiFePO4 also comes in 18650 format.

LiFePO4 will be better 100% for direct replacement of Lead Acid setups because of it's voltage range. 4s, 8s, 12s, 16s all drop in perfectly for replacements. For the inverters, it just makes it a lot easier to match up. So at that point, it's not that the LiFePO4 is more efficient than LiCo's, just more suited.
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#3
With 12 volt and solar controllers, if the BMS on the battery disconnects the charging you will get voltage surges. I encounter this with both pwm and mppt controllers, the voltage surges will go above 15 volts destroying any pumps/lights/fans connected to the battery. This happened to me with both a 3s li-ion and 4s lifepo4.  

The reason it does this is because the BMS leaks voltage (when it stops accepting amps) which the controller can read as a low battery, and tries to force amps into the battery. The battery wont accept amps and is safe but anything connected to the battery gets surges. Once the BMS trips and stops charging, controller will cycle back and forth trying to charge battery until you disconnect the solar panel.

I recommend a 12 volt stabilizer, its what I use. I lost many fans and pumps on my swampcooler from these voltage surges, but stabilizer can handle the surges.  You can also try and connect your devices to the load out of the controller which will disconnect when surges occur but some load outs won't reconnect automatically. 

This is a diagram of how I charge lithium with solar, I use an overvoltage protection relay to disconnect panel at a voltage lower then when the the BMS will stop charging. But the overvoltage relay is not quick enough to disconnect in some instances, thats why I still use the voltage stabilizer. 





voltage stabilizer
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#4
Cost and availability...

Depending on your load requirement.... I would go 24v and get a DC-DC for your 12v loads using a Daygreen unit.

I have some LiFePO4 cells, but nothing substantial. For me it makes a lot more sense to use a 24v setup, find/buy 24v native things, and step down for the items that are 12v.

Even if you take into account the DC-DC conversion loss, which isn't that bad, I got loads of 18650 cells to just pad the bank. Trying to do the same with LiFePO4 would either cost me a lot more, or most likely end up with a physically larger battery too due to capacity.
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#5
@Korishan Can you just replace a lead acid battery from an UPS with LiFePO4 or does the charging system needs to be adapted too ?
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#6
Obviously check the spec sheet for the cells you are using, but generally speaking LiFePO4 cells charge to 3.6v max x 4 = 14.4vdc.

If your cells are balanced and your charging system does not exceed that voltage, then you will most likely ok with a “drop in” solution.

That said, I wouldn’t do it without testing, / monitoring voltage.

I don’t have enough experience on the lead acid charging profiles to comment / compared to CC/CV typically required for lithium.
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#7
Some LifePo4 are rated to be direct replacement of LA. Not all and not all devices will work over time so you need to check that out.
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#8
(03-11-2019, 07:24 AM)Overmind Wrote: @Korishan Can you just replace a lead acid battery from an UPS with LiFePO4  or does the charging system needs to be adapted too ?

There are a few problems with this. The first of which, is if you use a BMS that disconnects the battery in an under-voltage condition you may have to remove and charge it to power the UPS on again. Second, LiFePO4 has a very flat discharge curve, so the UPS will have no idea of the batteries state of charge. So it will think the battery is full, until either the BMS cuts the battery power, or the UPS will suddenly shut down due to low voltage. Last of all, lithium batteries do not like being stored fully charged for long periods of time. So I strongly doubt that the batteries will outlast lead acid anyway.

The only time I would use lithium batteries with a UPS would be if I were using it as an inverter. Then I would opt for Li-Ion and a 24v server grade unit.
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#9
Good point about the flat discharge curve… another reason why SoC monitors are generally a lot more useful with LiFePO4 than any other type of battery chemistry. Something programmed to read lead acid curves will rely heavily on voltage to determine SoC.

When LiFePO4 goes empty it pretty much drops off hard.
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#10
I agree, Geek/Crimp-Daddy.

The best type of UPS to use is one that is 48V, as it is generally more sophisticated and can usually be programmed (not all, but usually).

I would also recommend using a bms as an intermediate if the UPS is used to charge the battery pack. Just make sure that the BMS can handle what the UPS is capable of. If the UPS can do up to 60A surge, for example, the BMS needs to be at least 60A surge as well. Don't need the BMS triggering when there's no real reason to do so.

If it can be done (space barring), then use the BMS outside of the UPS and charge completely separately from the UPS. At this point, the UPS is strictly a glorified inverter.
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