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Lead acid or LifPo decision time

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Shunt Czar

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Joined
Aug 31, 2021
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35
Replaced 12 flooded lead acid golf cart batteries today.
Total capacity 620 A/H at 24 V = 14880 kWh
Using cheap garbage Interstates from Cosco total cost $1300
Batteries were 5 years old.

Did a quote for a LifPo4 battery. Of course you only need 7 kWh
from LifPo for comparable performance to lead acid.

LifPo quoted at $7000. Lifetime=? whatever (most LifPo advocates
make me think it is for all eternity). That LifPo purchase could
support 29 proven years of lead acid purchases.

Can't find a way for LifPo to make sense for me.

On another unrelated note. I've got regular charge controllers
and MPPT charge controllers. My experience for over 10 years is
that the MPPT isn't worth it, at least not here in the sunny
Arizona desert.
 
24V 5120kWh is around 2K in server rack batteries… less if built with sok, or chins batteries…
 
Ha ha!
We need to stop meeting this way.

If LiFePO4 then I think MPPT would be the way to go.

Server rack batteries are around $1500 or so for 100 Ah, 5000 Wh.
Assuming you need double that, $3000 maybe $3500.
Claim is 6000 cycles (that would be 16 years every night), but I don't know I can believe it.
If so, would be a fraction the cost of lead acid over that period. At least 10x the cycle life of your cheap ones?

I use SunXtender AGM, because grid-backup, only expect a couple hundred cycles at most over 10 years.
 
If the lead acids lasted 5 years in your use, you’re probably right to keep on going with them.

However, unless you did a capacity test before decommissioning them, you don’t really know how little you were truly getting away with. As in, if those things were down to 50% of their original ratings, the. you’d only need 3.5kwh of lifepo4, which changes the math.
 
I mean it’s clear from this post and the other one you made around the same time, the attitude you came in with. But here we are, playing ball with you. ?‍♂️

Enjoy your day. ?
 
One more lick... sorry couldn't resist.

Actually I'm not LiFeP04 fan boy at all costs either but one thing I've not seen mentioned is that if one is starting from scratch and carefully designing a system you can use a smaller solar array on LiFePO4 batteries than with FLA due to the increased charge/discharge efficiency and lower self discharge.

Shunt Czar said:
It's clear that everyone here is an expert. Thanks all!
Click to expand...
There's a bunch here for for sure. Thank you for sharing your experience.
 
Last edited:
Vigo said:
I mean it’s clear from this post and the other one you made around the same time, the attitude you came in with. But here we are, playing ball with you. ?‍♂️

Enjoy your day. ?
Click to expand...
I probably should get professional help to quit posting on message boards. I really need to quit.
 
Or you could listen and learn.
I have over 40 years industry experience (mostly in different areas from yours), and I've learned a lot in the past two years both from people here and in my garage plus on the bench at work.

Ever work with transformers? Do you think you understand them?
They are far from "ideal", and there is a lot of engineering that goes into optimizing transformers and chokes for any given application.
Those things matter more with off-grid, not having unlimited power to waste in them.
 
Hedges said:
Or you could listen and learn.
I have over 40 years industry experience (mostly in different areas from yours), and I've learned a lot in the past two years both from people here and in my garage plus on the bench at work.

Ever work with transformers? Do you think you understand them?
They are far from "ideal", and there is a lot of engineering that goes into optimizing transformers and chokes for any given application.
Those things matter more with off-grid, not having unlimited power to waste in them.
Click to expand...
 
When I did they were audio transformers, some mic input but mostly tube output stages. The toughest problem was leakage inductance that was dealt with by progressive winding interleaving until the limit was reached when a/3N^2<c where a is total winding thickness, N is the number of leakage flux areas, and c is the thickness of each section. Insulation between sections is the limiting factor so as voltages rose, both static and dynamic (DC plate voltage and signal swing) it became more difficult.
 
Audio may be particularly sensitive to distortions some people can hear.

As a kid I played with transformers salvaged out of old tube gear and used filament winding for 5V TTL supply.

A few years ago I designed RF board to drive a quadrupole atomic mass spectrometer (RGA, like a smog-check machine.) That was 11 MHz 700 Vpp. I used a powdered iron toroid a bit larger than a lifesaver. After finding too many turns of magnet wire resulted in self-resonant frequency below operating frequency, I tried 4 turns primary, 40 turns secondary (inductance and voltage ratio according to turns^2 and turns ratio.) But coupling was so weak, so much leakage inductance, I ended up with about 20:20 turns. Driving a resonant load that gave 10x boost in voltage and clean signal, harmonics < -60 dB. SPICE model had k = 0.05 due to leakage. The model came from measuring inductance with other winding open and shorted. I hand wound those, then transferred to a tech and finally a manufacturer.

I tried interleaved windings like you mention, but felt the parasitics were too variable. Because it operated as a tank circuit resonating with capacitive load (Q around 10), I needed more consistency between parts. I used Kynar wire-wrap wire for low capacitance, and primary and secondary were both single layer, and spaced apart from each other.

Chokes, I found when modeled that way (just R and coupled L), SPICE worked too perfectly, didn't match measurement of effectiveness. Learned how to measure & model saturation and hysteresis from EEVblog. I use a variac and scope with voltage & current measurements, plot V^2 vs. current.

More recently, looking at inrush current. Manufacturers say much higher inrush when driving a coil against the core vs. outer winding which has more leakage to air. So when partially saturated, the inner winding becomes much lower inductance. I did some measurements of inrush (which is only an issue at power-up.) I've wanted to boost 120/208Y to 480, but some step-down transformers perform badly when driven in reverse.

diysolarforum.com

3-Phase SMA Sunny Island (15kW) + Sunny TriPower (30kW)

Showing a work-in-progress assembling 3-phase SMA system. 3x SI-5048-US are configured for 3-phase and produce 120/208Y, rated 15 kW continuous. Sunny TriPower 30000TL-US-10 is 480V delta (or 277/480Y), rated 30 kW. TriPower combiner box will be used, but for now just one string being connected...
diysolarforum.com diysolarforum.com

I have to configure step-up with a transformer that doesn't put too much load on my battery inverters.


I wonder, when grid drives a 480D to 120/208Y step-down, does a similar issue occur when GT PV inverters backfeed the transformer? Or is the problem not direction of power flow (phase of current vs. voltage) but just who provides the exiting voltage? Can the transformer even know the difference?
 
You may want to consider Lead Carbon. They are right on the price point for cycles, etc. What you have not considered as well is that you need twice the amount of lead acid batteries than LiFePO4 as LA only takes you down to 50% before a cycle and they have way less cycles.
 
You didn't see this in my post:
Replaced 12 flooded lead acid golf cart batteries today.
Total capacity 620 A/H at 24 V = 14880 kWh
Using cheap garbage Interstates from Cosco total cost $1300
Batteries were 5 years old.

Did a quote for a LifPo4 battery. Of course you only need 7 kWh
from LifPo for comparable performance to lead acid.
 
I had 8 group 8D 225Ah lead acid batteries in 4S2P for about 22,000 Wh (11,000 Wh usable) that finally gave up after a hard life. I bought 36 230 Ah LFP cells and made two 16S batteries and one 4S battery.

The two big batteries will be in 16S2P for about 22,000 Wh (18,000 Wh usable). I have not tallied up the total cost, but I think I'm below $5,000 for the three batteries with BMSs and compression cases for all three. The concept of DIY is what made LFP attractive to me.

My biggest disappointment with lead acid was that they get too soft as they age, and the voltage drops too low during high current events. I expect much better performance from LFP.
 
Costco GC2 served me well. And so glad to to be rid of them. See you again in five years. All the best.
 
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