How can I calculate maximum "healthy" amp draw on a battery bank?

[SolarWatt]

When under any load you reach 1.75 volts per cell or 10.5 volts for a 12 volt battery.

FWIW a rested 12 volt battery will measure 11.8 volts

Thanks

[Sunking]

When under any load you reach 1.75 volts per cell or 10.5 volts for a 12 volt battery.

FWIW a rested 12 volt battery will measure 11.8 volts

[SolarWatt]

I should explain myself better:
I want to determine the Peukert coefficient of my battery.
For that I need to know the time for 100% DoD in two different discharge rates.
One I know, 20 Hours at 3.5A (Because it is 70A-H battery)
Now I Discharge at rate of 7A and measure the time it takes to reach 100% DoD.
But how I know I reached 100% DoD ??

I am using the formulas at: http://www.smartgauge.co.uk/peukert_depth.html

Thanks
Elad.

[SolarWatt]

What is the definition to 100% DoD ??

[PNjunction]

.. 3 to 5 years is a good guideline on how long it will be between battery replacements, I think. My brother believes you can maximize this by using the battery as little as possible, even not using them as much as possible. I contend that using the batteries minimally, under the Peukert's law "sweet spot", will maximize battery longevity...

Depending on the battery, you are looking at a 3 - 5 year replacement from old age alone - regardless of how nice you are to it. Yes there are float applications that can go many more years if done well.

One other way to think about this is if I am looking at my batteries dying in 5 years anyway, why be super-nice to them if I only use them once a week and merely sip some capacity from them - leaving me with 1000's of cycles? The battery will die from old age first, wasting the power capabilities had you only used them. I suppose you could convince your brother to actually beef-up his consumption, and not worry about it so much.

The super conservative approach combined with old-age failure, may be a determining factor when deciding upon how much of a DOD you can withstand, if relatively few cycles are needed. Still, the best idea is to determine how much power he will be using, even if miniscule, to accurately size the battery required.

[Sunking]

Gotcha ... the question then being: when batteries sit "idle" like that (with 30A output and 30A input) is that better for the batteries than to have them idle with zero output and zero input, or a trickle sort of charge?

OK a fully charged battery takes no current so to speak. Assuming there is no load device demanding power, and full noon sun the panels are not generating any power because there is no place for it to go. Reality is all batteries have a self discharge rate and under the conditions I just listed the batteries would draw about .01% of the C/20 rating to overcome self discharge

The central question is about how many years a battery will remain functional. Is it better to use a battery within its "sweet spot" as determined by Peukert's law or is it better to not use them as much as possible?

Dave you are talking two different issues. Cycle life is dependent on the Depth of Discharge (DOD), and the amount of time you leave a battery less than 100% fully charged. To demonstrate this graphically look at the Trojan T-105RE spec sheet page 2 DOD vs Cycle graph. If you discharge only to 20% you can get up to 4000 cycles, to 50% around 1500 cycles, and 90% less than 800 cycles. Now with that said temperature has a large effect on cycle life so unless you can keep your batteries cold, you will never see those kinds of result in real life.

Peukert law is demonstrated on page 1 PRODUCT SPECIFICATION under the table with 2 to 100 hour discharge rates. If discharged at the 2 hour rate (112 amps) the battery is rated at 146 amp hours. If discharged at the 100 hour rate (2.25 amps) is rated at 250 amp hours.

So back to your question what is the optimal discharge rate is a bit confusing to people. True Deep Cycle flooded lead acid batteries have fairly high internal resistances, and as you increase current flow th evoltage drops on the battery terminals. So for True Deep cycle batteries C/8 discharge current is about as high as you want to take before voltage drop becomes a serious issue. Same for charging because when you are pumping in current into that resistance causes the battery to heat up and you start to waste energy by burning off as heat which causes boiling of the electrolyte.

Sure you can draw a C/2 discharge from the battery listed of 112 amps, but the battery terminal voltage will be down around 3 volts on a 6 volt battery. That is not useful unless we are talking about a DC electric series wound motor. Use two of them in series for 12 volts on an inverter, and that inverter will quit working when the voltage drops to about 10.5 to 11 volts. So in this example a pair of T-105RE can supply about a 500 watt inverter. You can certainly try to run say a 2000 watt inverter and it would work for a while at high power levels but would shut down long before you expect it too.

[billvon]

Gotcha ... the question then being: when batteries sit "idle" like that (with 30A output and 30A input) is that better for the batteries than to have them idle with zero output and zero input

The two cases are identical. (Google "kirchhoff circuit laws" for more info.)

or a trickle sort of charge?

You'll always need a small trickle charge to offset self discharge.

The central question is about how many years a battery will remain functional. Is it better to use a battery within its "sweet spot" as determined by Peukert's law or is it better to not use them as much as possible?

Things that will make a lead acid battery last as long as possible:

-minimal cycling
-cool temperatures
-maintenance at a fairly high state of charge (>75%)

[Dave_Rasch]

Gotcha ... the question then being: when batteries sit "idle" like that (with 30A output and 30A input) is that better for the batteries than to have them idle with zero output and zero input, or a trickle sort of charge?

The central question is about how many years a battery will remain functional. Is it better to use a battery within its "sweet spot" as determined by Peukert's law or is it better to not use them as much as possible?

3 to 5 years is a good guideline on how long it will be between battery replacements, I think. My brother believes you can maximize this by using the battery as little as possible, even not using them as much as possible. I contend that using the batteries minimally, under the Peukert's law "sweet spot", will maximize battery longevity.

Dave

[Sunking]

Hmmm,

Does healthy discharge rate mean the actual discharge rate or a calculated one? Meaning, does any charging offset the discharge rate?

100Ah AGM 12v battery has a healthy discharge rate of (C/4) 25A
If I'm drawing 40A and have 15A of charge going to the battery does that achieve the healthy 25A rate?

Won't last forever of course, but the question remains - is the "healthy" rate maintained while the capacity is available?

Dave

Let's see if I can get you on track. There are 3 possibilities.

1. If the batteries are fully charged up, there is a 40 amps load, and the panels have enough light and can actually supply 40 amps, then the panels will supply all the power and the batteries set idle regulate the voltage.

2. Same as above except the panels can only supply 30 amps, then 30 amps is supplied by the panels and the batteries make up for the shortage and are in discharge.

3. Same as above except it is dark and no panel power, the batteries supply all the power.

When you have a primary power source like the solar panels, and they are generating power, the energy has to go somewhere and equals out so that Power out = Power In

[Dave_Rasch]

Actual or Calculated discharge rate?

Hmmm,

Does healthy discharge rate mean the actual discharge rate or a calculated one? Meaning, does any charging offset the discharge rate?

100Ah AGM 12v battery has a healthy discharge rate of (C/4) 25A
If I'm drawing 40A and have 15A of charge going to the battery does that achieve the healthy 25A rate?

Won't last forever of course, but the question remains - is the "healthy" rate maintained while the capacity is available?

Dave

[Sunking]

So, the voltage at the connection to load in real life is significantly lower than voltage at battery terminal, is it what you are saying?

Yes.

Let's see what we have here in a real world aspplication: A pair of Trojan T-105 RE batteries wired in series using 6 inches of #6 AWG and compression connectors feeding a constant current load of 40 amps through 10-feet 1-way distance terminated with compression connectors. What is the voltage at the battery terminal and load?

Hint just the resistance of the #6 AWG is .001 Ohms which is 1/10th to 1/20th of the total resistance. Battery and connectors make up the rest.

[inetdog]

So, the voltage at the connection to load in real life is significantly lower than voltage at battery terminal, is it what you are saying?

Yup.

[Shmel]

The tests pretty universally measure the voltage at the battery terminals, and load is adjusted to keep the same current flowing independent of battery voltage and wire resistance. To make a reproducible standard measurement you eliminate all of the variables. In a practical application you do not have that luxury.

So, the voltage at the connection to load in real life is significantly lower than voltage at battery terminal, is it what you are saying?

[inetdog]

What resistance did the the wire and connection had during battery test performed to get that chart? Why did it work for them?

The tests pretty universally measure the voltage at the battery terminals, and load is adjusted to keep the same current flowing independent of battery voltage and wire resistance. To make a reproducible standard measurement you eliminate all of the variables. In a practical application you do not have that luxury.

[Shmel]

Very good academic theory. Now add in wire and connection resistance and it will not work.

What resistance did the the wire and connection had during battery test performed to get that chart? Why did it work for them?