Battery Basics Question

I plan to use two 12v Deep Cycle AGM batteries in series for 24v system.

If the batteries are rated for 30Ahrs each, I believe that means that I can still get 30Ahrs at 24v, yes?

Also, what will be the typical state of charge (SOC?) when/if I hit the 30Ahr? 50% charge?

I "assume" - dangerous, I know - that, since the batteries are "rated" by the manufacturer for 30Ahrs, that they will not be damaged by using 30Ahrs from them on a regular basis, as long as I fully recharge them. Correct?

Many Thanks and God Bless!

Kugel

Kugel8x57 said:

I plan to use two 12v Deep Cycle AGM batteries in series for 24v system.

(1) If the batteries are rated for 30Ahrs each, I believe that means that I can still get 30Ahrs at 24v, yes?

(2) Also, what will be the typical state of charge (SOC?) when/if I hit the 30Ahr? 50% charge?

(3) I "assume" - dangerous, I know - that, since the batteries are "rated" by the manufacturer for 30Ahrs, that they will not be damaged by using 30Ahrs from them on a regular basis, as long as I fully recharge them. Correct?

Many Thanks and God Bless!

Kugel

Click to expand...

(1) Yes. Batteries in series: Voltages add, capacity stays the same. Parallel: voltages same, capacities add.
(2) impossible to say because battery voltage depends on state of charge AND current flow. The exact SAME SoC could have notably different voltage readings depending on if it was being charged, discharged lightly, discharged heavily or at rest.
(3) No. They will be damaged. All batteries are damaged when you cycle them. AGM batteries typically have a 90% DoD (90% Depth of Discharge) life of 300-500 cycles. Limiting them to 50% can push that to 2000+.

Also, it's probably worth pointing out that "life" means they will retain 80% of their capacity or better after the cycle life, so it's not like an off switch; however, the rate of degradation usually accelerates because 1) they're just older/weaker and 2) the same daily discharge represents an ever-increasing share of the total capacity, e.g., 15Ah discharge (50%) is not 50% anymore. It's 15/(30*.8) = 62.5% of the total capacity.

Aaaahhh... Thank you Snoobler! So, if I understand you correctly... If I want 30Ah of usable charge but only want a 50% discharge, then I need to go with about 65Ah batteries. Correct?

Yes. Decide how much capacity you need... then double it.

Thank you, Sir. I'm learning more every day!

Or bite the bullet and go LiFePO4. Get 80% or better DOD and much longer lifespan.

“Buy once, cry once” as they say ?.

Because of the differences in LiFePO4 and Deep Cycle AGM batteries, do I still need to double the capacity? I.E. Do I still need a 60Ah LiFePO4 battery "bank" for a 30Ah application?

If not, the LiFePO4 Batteries are looking much more attractive, cost wise.

Many Thanks and God Bless!

Kugel

No, but dividing it by 0.8 is a good idea to get even more life out of them (utilizing only 80% of them instead of 100%).

Thanks Snoobler! I definitely understand Fs (Factor of Safety)!

Many Thanks and God Bless!

Kugel