Converting an LA UPS to LiFePO4 -- Failure before launch :-(

svetz

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The battery inside a particular UPS is a 12V & 5 Ah with a constant charge voltage of 13.5-13.8 in standby and cycle of 14.4-15.0. The manufacturer says it's not replaceable. I can find the battery on AliExpress for $30 or someplace I've never heard of (eBuy7 for $22). The old battery died after 20 months of service.

Question: Amazon has something close with a constant charge voltage of 13.6-13.8 in standby and cycle of 14.4-14.9. How important is that .1V?

Option 2: Replace the LA battery with an 18650 LiFePO4 assembly

From the dimensions it seems a 4s5p would fit. Using the 3.2 Ah NCR18650B, then 10-14.6V and 16 Ah (tripling the capacity) and cost ~$100.

Or use some $2 mystery cell to get the price (and Ah) down.
1599924289866.png

  • BMS that fits and can handle the current ✅
    ... the 40 amp 4S Daly is 48x75x9 mm.
  • Thermal Problems ✅
    I'm not shy, I could always ventilate the case or leave the battery external
  • Why don't I read about people doing this? ❌
    3.2 Ah x 5 = 15 Ah... but the inverter draws 390 watts, 390/12V = 32.5 Amps ... I'd have to go to 10p :cry:
 

Ampster

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To me the biggest issue is finding a work around for the UPS constantly trickle charging the pack. If that Float voltage and current can be adjusted then you could match it to the overhead of the UPS and be good.
On the other hand if I understand correctly, some UPSs operate in a mode where the internal inverter is actually powering the protected devices so there is no delay when power is lost. In that case the charging from the grid should be equal to the power consumed by the protected devices plus any overhead.
 
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gnubie

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Rather than worry about the 0.1V different charge spec for the battery, what does the UPS charger output? If it aims for 14.7V, or a lower point, then it doesn't matter.
 

gnubie

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The original battery isn't lithium. The possible replacements listed, ebay etc, are also lead acid. If the UPS charger targets voltages inside the ranges mentioned, and it probably does, the 0.1V difference doesn't matter, in the context of the question asked.

Obviously if the option to move to lithium is taken 0.1V matters, but it all comes down to what voltages the charger actually targets. I'm guessing that being a lead acid charger it will not be kind to lifepo4.
 

Ampster

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In that case the charging from the grid should be equal to the power consumed by the protected devices plus any overhead.
To clarify my earlier post I should have also added the voltage to that statement. Assuming that voltage is at 3.3 or 3.4 per cell then that would work. If it is higher then the cells are going to be maintained at possibly 3.65 volts each and that will not contribute to long life.
 

svetz

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The "charging voltage" is fine, it's the "cycle voltage" of 15V that was a little surprising, they must have done something in the battery to suppress electrolysis (that's off the battery label, didn't measure the UPS's actual voltage). Considering the battery saw at most 4 discharges greater than 40%, it had a fairly short life (wonder if a battery with more Ah would have a greater lifespan, possibly it's C-Rate death?). It might also be a bad use case for LiFePO4 as it's normal use case would be fully charged.

There are some LiFePO4 drop-in 12V replacements, but I didn't see any sub $100 that could handle over 40+ amps.

@GXMnow's Headways are huge... but I wonder... would 18 LTOs work:
max continuous: 13A draw, 1.3 Ah,​
18.7 x 65.3 mm​
2.4V nominal, 2.85V cutoff​

13A x 3p = 39 amps continuous ✅

The cut-off voltage is 2.85V, so 6x2.85 = 17.1V ... plenty high ✅
Nominal is 2.4V, so 6x2.4=14.4V. So that's a bit low. :cry:
If the cycle V is over 14.4V it should be okay
1.3 Ah x 3p = 3.9 Ah, so less run time than the 5 Ah battery. ❌

No room for a BMS and rather pricey at $230 😲 ❌
Arrangement to keep IR the same on all Ps
1599943536648.png

Guess I need to wait for Battery Day.
 
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GXMnow

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The 8 Headway's would be external. Gives 16 AH and can take over 200 amps. They are beasts. Jehu Garcia used 12 in parallel to melt a wrench.
 
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