backchannel: AGM @ 80% depth of discharge

I’ve seen similar claims before, but I haven’t done the homework until I was inspired by this one:

>they are design such that you can use more of it without damaging the battery's lifespan. So comparatively, AGM compared to FLA batteries with the same rated capacity (the same amount of lead in them) will have more USABLE capacity.

I could think of no particular reason that vanilla AGM should enjoy more cycles at very deep depth of discharge (DoD) compared to flooded, excepting maybe the exotic carbon-foam variant.

So I started doing the homework.  It took less than a day because so little information is published.  Specifically, AGM-only makers trumpet how many more cycles their batts get than “regular” batteries (comparing them to SLI?), but battery manufacturers that make both types tend to publish publish duty cycle data only for the flooded batts.  It makes apples-to-apples comparisons difficult, which a skeptic might say is the point.

To further complicate matters, some just publish cycle life at a given DoD, typically 20% or 50%.  For that reason, batteries appear/disappear on some of the lists below.

some baseline info

  • Deep cycle batteries typically have their greatest cycle life at shallow DoD like 20%.

  • They typically give the most Ah/$ at 30% DoD

  • Vandwellers and offgrid folk usually discharge to 50% DoD as a compromise between “dead lead” and cycle life.  There is nothing magic about the 50% DoD!

  • our discussion here is about discharging very deeply to 80% DoD, and whether AGM has any advantage way down there.

number of cycles at 80% DoD

  1. Trojan T-105RE (carbon-doped) 6v flooded, 1050 cycles

  2. Trojan Solar  6v AGM, 1000 cycles

  3. tie, 750 cycles

1. Trojan T-105 6v flooded

 	
2. Rolls-Surrette S6-275 6v AGM

 	
3. Crown CR-205, 220, 235, 260 6v flooded

  1. Rolls S6-245AGM-RE 6v AGM, 650 cycles

  2. tie, 550 cycles

1. Trojan type 31 AGM 12v

 	
2. Trojan type 27 AGM 12v

 	
3. Lifeline GPL-4CT AGM

lifetime cost per KAh @ 80% DoD

Lowest price per 1000Ah delivered to 80% DoD over entire lifetime (KAh hereafter)

  1. $1.79 per KAh, Crown CR205 6v flooded

  2. $1.85 per KAh, Trojan T-105 6v flooded

  3. $1.97 per KAh, Crown CR220 6v flooded

  4. $1.99 per KAh, Crown CR235 6v flooded

  5. $2.17 per KAh, Crown CR260 6v flooded.

  6. $2.19 per KAh, Trojan T-105RE 6v flooded

  7. {$2.60 per KAh, Battle Born LiFePO4 10yrs@ 100% DoD}

  8. $3.82 per KAh, Trojan Solar  6v AGM

  9. $4.77 per Kah, Rolls-Surrette S6-275 6v AGM

  10. {$5.13 per KAh, Firefly carbon-foam}

  11. $6.43 per KAh, Trojan type 31 12v AGM

  12. $6.71 per KAh, Lifeline GPL-4CT 6v AGM

  13. $6.80 per KAh, Trojan type 27 12v AGM 

lifetime cost per KAh @ 50% DoD

  1. $1.79 per KAh, Crown CR205 6v flooded

  2. $1.85 per KAh, Trojan T-105 6v flooded

  3. $1.97 per KAh, Crown CR220 6v flooded

  4. $1.99 per KAh, Crown CR235 6v flooded

  5. $2.17 per KAh, Crown CR260 6v flooded.

  6. $2.30 per KAh, Trojan T-105RE 6v flooded

  7. {$2.42 per KAh, Firefly carbon-foam}

  8. {$2.60 per KAh, Battle Born LiFePO4 10yrs@ 100% DoD}

  9. $3.47 per KAh, Trojan Solar  6v AGM

  10. $4.02 per Kah, Rolls-Surrette S6-275 6v AGM

  11. $5.39 per KAh, Trojan type 31 12v AGM

  12. $5.70 per KAh, Trojan type 27 12v AGM 

  13. $5.91 per KAh, Lifeline GPL-4CT 6v AGM

  14. $6.65 per KAh, Renogy 200A 12v AGM

  15. $7.80 per KAh, Renogy 100A 12v AGM

  16. $9.00 per KaH, Universal Battery 100Ah 12v AGM

lifetime cost per KAh @ 20% DoD

  1. $1.79 per KAh, Crown CR205 6v flooded

  2. $1.85 per KAh, Trojan T-105 6v flooded

  3. $1.97 per KAh, Crown CR220 6v flooded

  4. $1.99 per KAh, Crown CR235 6v flooded

  5. $2.17 per KAh, Crown CR260 6v flooded

  6. $2.30 per KAh, Trojan T-105RE 6v flooded

  7. $3.32 per KAh, Trojan Solar  6v AGM

  8. $3.58 per Kah, Rolls-Surrette S6-275 6v AGM

  9. $4.85 per KAh, Duracell GC2 /Deka GC10 6v flooded

  10. $4.91 per KAh, Duracell ECC2/Deca GC15 6v flooded

  11. $5.44 per KAh, Trojan type 31 12v AGM

  12. $5.75 per KAh, Trojan type 27 12v AGM 

  13. $6.42 per KAh, Lifeline GPL-4CT 6v AGM

Conclusions

  1.  there are no apparent differences between flooded and AGM duty cycle life at any DoD.

  2. 6v batteries dominate the class of batteries with longer lives at 80% DoD

  3. 6v flooded batteries dominate the lowest cost per KAh rankings.   The most cost-effective AGM cost 2.13x as much as the lowest cost KAh flooded (Crown CR205), and 1.51x as much as the highest cost KAh flooded (Trojan T105 flooded).

  4. There are reasons to buy AGM, but cycle life and cost per KAh aren’t among them.

Updated:

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