Battery Draw Bogus Math? Peukert UPDATE 2

I'm thinking your logic is Biased

Lithium 100ah use 80ampHrs reach 20% SOC
Recharge do it again tomorrow

Agm or FLA 200 amp hrs, use 80 amp hrs
Draw down to 60% SOC

Recharge do it again tomorrow

Which battery/bank is going to suffer the most degradation, which is going to give more hours, days, years of service,
Which will need the lesser amount of generator run time to recharge the same 80 amp hrs

Your statements above are only correct, If you plan draw them to zero SOC full discharge total output, and throw them away,

You need to compare the actual peukert function for each battery. As the internal resistance drops there is much lower effect due to high load. Also peukert is an exponential curve. Much lower effect at anything less than the 20 hour rate.

MrWizard wrote:
I'm thinking your logic is Biased

Lithium 100ah use 80ampHrs reach 20% SOC
Recharge do it again tomorrow

Agm or FLA 200 amp hrs, use 80 amp hrs
Draw down to 60% SOC

Recharge do it again tomorrow

Which battery/bank is going to suffer the most degradation, which is going to give more hours, days, years of service,
Which will need the lesser amount of generator run time to recharge the same 80 amp hrs

Your statements above are only correct, If you plan draw them to zero SOC full discharge total output, and throw them away,

My only thought was that the 80AHs are not the same running time at 5 amps. At 5 amps from the 100AH batt, that is 16 hours time.

5 amps from a 200AH bank would be a longer running time.

If your "use" is a 5 amp draw, then your "usable time" (vs a "usable AH") is more with the two batts.

IMO you have to do the same activity or "use" , which is the same amps draw, to make the "usable" comparison.

Useable aka Safe for the battery in question,
Is not the same as Total Storage drain it empty

OK he specifically ask what about Peukert.

The Peukert effect is something I do not see commented on enough. What is it?

Well take that 100 AH battery. Put a 5 amp load on it and it in 10 hours it is down to 50%.... (100/20=)

But what happens if we put a 10 amp load on it? Does it hit 50% in 5 hours? (Double the amps half the time?) NO it hits 50% in closer to 3 hours.. Exactly how fast I do not know.

This is what the Peukert effect is. The faster you draw amps the faster^2 the battery runs down. And no I do not know how much faster it runs down.. May vary with battery type.

One catch with the Peukert effect is it assumes a constant draw.

If you remove the draw, the battery will recover and you will have those "lost" amp-hr back.

So if it's a cyclic draw (say the furnace runs 15 minutes out of each hour over night), it won't have much effect on how many amp-hr you can pull out of the battery.

time2roll wrote:
You need to compare the actual peukert function for each battery. As the internal resistance drops there is much lower effect due to high load. Also peukert is an exponential curve. Much lower effect at anything less than the 20 hour rate.

Came across this table in an AGM battery spec;

1 amp rate -125AH
100 hr -----118AH
20 hr-------105AH
10 hr-------100AH

so the rates for that are 1, 1.18, 5.25, 10

It looks like if you use half the 20 hr rate you would get maybe thrice the time? That's a lot.

Is that right?

Close enough for discussion.

So you gain 5% by having 2 AGM to reduce the load from the 10 hr to the 20 hr.

I believe when you compare that to a single lithium of similar capacity you will not lose that 5% for staying with the single battery and discharging at the 10 hr rate and just going deeper to yield the same energy. Internal resistance and peukert effect is very close to zero with lithium.

And a flooded cell battery will be worse than the AGM.

I don't follow at all. My claim is that the "same" 80AH actually is about three times more time at 5 amps when the 20 hr rate is 10 amps.

EDIT--have to correct from 16 to 8 hours at 5 amps for the 200AH bank

With the one 100AH batt down to 20 you got 16 hours at 5 amps.

With the two 100AH in parallel down to 60 each, you get 8 hrs (not 16!)--but at 10 amps,

Apparently you would get 3 x 8 = 24 (corrected from 48) hrs if you ran them down to 60 each at 5 amps on the bank. But that is not 80AH anymore either!

I was looking at how my one SiO2 would do almost as much low amp running time as the two 27s do, based on AH as they advertize. But then my BS warning light came on. Hmmmm.

I just think the way they play with "usable AH" amounts to false advertizing they way I look at "use". "Use" to me is a draw for a certain time. They should use the same "use" for "usable" in each case.

A Carny Barker must have thought up "usable AH" :)

The 100 hour rate how I chose 3300 amp hours for my main bank. Lights fans freezers.