Help understanding how long a battery will last.

[USMA36]

Why do you need your heater? I lost power January 22, for 8 Hours 50 minutes. The 65 Gallon Aquarium temperatures went from 78 F to 71.8 F during that time. I do insulate the Back Glass, Underneath and Back and Sides of the Sump, during the Winter. This is to save on electric bills. I had a quicker temperature drop on my 30 Gallons Quarantine Aquarium, used bottles of hot water and blankets on that Aquarium, this wasn’t difficult. I use a 12 Vdc Battery to Auto run two 12 Vdc Pumps from my Sump to the Aquarium.

Because I during outages I lose about 2° per hour.

 

[USMA36]

This is all great info and I thank everyone for the help, but jumping back to the beginning if I used the setup posted above with the 24 volt mpp inverter with the 24v 200ah battery how long could I possibly go with a 700w 7a draw? What is the likelihood I can get 4-6 hours of run time?

 

[Bret]

This is all great info and I thank everyone for the help, but jumping back to the beginning if I used the setup posted above with the 24 volt mpp inverter with the 24v 200ah battery how long could I possibly go with a 700w 7a draw? What is the likelihood I can get 4-6 hours of run time?

With that battery, assuming you get full (200ah) capacity, your 700w will last approx. 6.9 hours. This is ignoring a few variables, but that would be about the best you could expect.

 

[BeanAnimal]

add in your own variables to make the math work out easier...

This is all great info and I thank everyone for the help, but jumping back to the beginning if I used the setup posted above with the 24 volt mpp inverter with the 24v 200ah battery how long could I possibly go with a 700w 7a draw? What is the likelihood I can get 4-6 hours of run time?

7A @ 120V = 840W
7A @ 110V = 770W

Let's call it 115V and 800 Watts
800 Watts fed from a 24V source = 33 Amp load being fed to the inverter from the batteries

Assume inverter losses (heat, power factor) to be 30%

200Ah / 33 A = 6 hours

6 hours * .7 = 4.2 hours for fully charged brand new batteries

I would call that 3.5 hours avg as the batteries will age. Anything more is bonus runtime.


Why the significant loss calc?
Better inverters have less loss. Higher voltage inputs have less loss. Things like motors, coil and cap ballasts and poorly designed switch mode power supplies skew power factor. Lagging power factor also heats transformers (more loss).

That is why DC pumps are so good for battery backups... DC to DC drive incurs very little loss (let's not get bogged down in the fact the the DC motor can incur losses depending on how it is built).

 

[Amphibious Wallet]

Forgive my ignorance or illiteracy, but are we factoring in the 50% max DoD here?

 

[Bret]

Forgive my ignorance or illiteracy, but are we factoring in the 50% max DoD here?

No. The 50% max DoD is a rule of thumb generally applied to lead acid batteries to preserve their life.

Lithium batteries can typically be cycled 100% several thousand times before losing significant capacity

There are so many other unknow variables in this discussion that its easier to ignore them, figure best case theoretical scenario, then subtract for your inefficiencies and rules of thumb.

 

[Spicy Reef]

7A @ 120V = 840W
7A @ 110V = 770W

Let's call it 115V and 800 Watts
800 Watts fed from a 24V source = 33 Amp load being fed to the inverter from the batteries

Assume inverter losses (heat, power factor) to be 30%

200Ah / 33 A = 6 hours

6 hours * .7 = 4.2 hours for fully charged brand new batteries

I would call that 3.5 hours avg as the batteries will age. Anything more is bonus runtime.


Why the significant loss calc?
Better inverters have less loss. Higher voltage inputs have less loss. Things like motors, coil and cap ballasts and poorly designed switch mode power supplies skew power factor. Lagging power factor also heats transformers (more loss).

That is why DC pumps are so good for battery backups... DC to DC drive incurs very little loss (let's not get bogged down in the fact the the DC motor can incur losses depending on how it is built).

You're right, I made a huge miscalculation assumption. My brain went straight from 7amp draw on 12v dc and did not think for a moment he was talking about 7amp draw on AC.... remember the battery I posted was 24v 100Ah so... a bit more - double on the inverter I posted, I believe it's 95% efficient so even more.

 

[Pistondog]

maybe you can do the maths for us 12v 100ah system vs 24v 100ah system?
I still say the difference for what we are talking about here is negligible.

Watt-hour = volts * amps-hour
The 24v system has 2400 watt hours
The 12v system has 1200 watt hours.
2 agm batteries in series can run your 24v dc pumps for a day, longer at reduced speed. Tunze sells cheap 12v swichovers to run powerheads from batteries if power fails.
Lights and heaters are the big power sucks, lights probably not a necessity in emergency. Heater depends.
Careful with big batteries, shorting the terminals by accident is catastrophic.

 

[Reefering1]

4 agm(100ah) batteries, 2 banks of paralleled 12v connected in series for 24v on apc 1500 wil run my tank for about 16 hours. That's with lights

 

[Bret]

For anybody trying to design their own battery backup:
1. Start with your known loads. How many average watts, and for how long do you want to run them. Don't consider amps as amps are voltage dependent. For example: return pump, powerhead, and heater, average 500 watts/hour.
2. Determine how many hours you need to run these loads. Example 6 hours
3. Multiply your watts by your hours. 500 watts x 6 hours = 3000 watt hours
4. Select a battery with at least 1.5-2x your desired watt hours. Multiply the batteries advertised nominal voltage by the amp hours. Example 24v 200AH would have a capacity of about 4800 watt hours.
5. Consider other factors that will affect your capacity such as efficiency of your inverter, type of battery and temperature
6. Increase your battery size or quantity as needed

 

[Flreefjunky]

I personally love agm batteries however there is a down side to them. they aren't the best for a system that needs a battery to go from fully charged to dead and then recharge. yes they can do this but every time an agm goes dead or close to dead it roughly cuts its life span in half. This is why lithium batteries excel in this scenario. High quality agm batteries will have a higher ah rating than slabs. Odyssey batteries are probably at the top of that list. Slabs do take being depleted better but nothing compares to lithium. The other thing to consider is the charge rates of batteries. Quality agm and lithium batteries are much higher than slab batteries. I don't know what the chargers in the apc style back up systems are rated for but I would guess only the batteries they have inside of them. The charge rate on a quality agm is around 14.5 volts. Slab batteries are around 13.5 to 14 depending on quality. Lithium batteries are closer to agms but still there own rates. This is just something else to consider when choosing a battery system that fits your needs. If you can charge a battery to its specification the max output it can offer will be diminished.

 

[BeanAnimal]

You're right, I made a huge miscalculation assumption. My brain went straight from 7amp draw on 12v dc and did not think for a moment he was talking about 7amp draw on AC.... remember the battery I posted was 24v 100Ah so... a bit more - double on the inverter I posted, I believe it's 95% efficient so even more.

Yes - the current draw is the final LOAD - everything else in between just transforms the energy. The Source is 24V

so 24V ---> black box ---> 800 Watts

We don't care what is in between... we need to provide 800 Watts with 24V and that is 33A of current from the batteries.

The inverter itself may be 95% efficient at unity power factor. At anything other (lagging in this case) it will be less and the PF itself is robbing energy as heat as well because current and voltage are out of phase by some degree due to the inductive loads.

 

[BeanAnimal]

Watt-hour = volts * amps-hour
The 24v system has 2400 watt hours
The 12v system has 1200 watt hours.
2 agm batteries in series can run your 24v dc pumps for a day, longer at reduced speed. Tunze sells cheap 12v swichovers to run powerheads from batteries if power fails.
Lights and heaters are the big power sucks, lights probably not a necessity in emergency. Heater depends.
Careful with big batteries, shorting the terminals by accident is catastrophic.

I think you may have missed the question that he was trying to ask (it was not clear)

(2) 12V 100Ah batteries in parallel provide 100Ah at 12 volts.
(2) 12V 50Ah batteries in series provide 50Ah at 24 volts.

Spicy was asking what the difference was in terms of load runtime.

Let's take 1000 Watts of load.

The 12V system will have an 83.33 Amp draw on the batteries or an ideal runtime of 1.2 hours.
The 24V system will have an 41.6 Amp draw on the batteries or an ideal runtime of 1.2 hours.

Both system in theory are the same. In reality the 24V system at half the current draw will be more efficient due to Ohm's law (less resistance) and the inverter will be more efficient with less step-up.

He wanted to see the math to quantify the efficiency. We can't do that without wire lengths, transmission line losses, etc. But suffice to say, that the difference is real world and significant with regard to cost of materials to make the 12V system less lossy compared to the 24V system.

 

[Reefering1]

My apc until shuts down around 10. Something volts. Thus protecting the batteries. But the apc does take like a day to get them back bto 13.8v. And the stated 16 hours is because I got tired of the non stop beeping it does, after the 4th or fifth hour, and plugged it back in. It doesn't know what it has for capacity, but it slowly learns. Then gryes and fans will run on 12v direct until three battery is gone.(never got this far, I drag the generator out) I don't like lipo because of the expensive chargers and increased odds of itexploding. Especially high discharge rates.

 

[Reefering1]

I think you may have missed the question that he was trying to ask (it was not clear)

(2) 12V 100Ah batteries in parallel provide 100Ah at 12 volts.
(2) 12V 25Ah batteries in series provide 50Ah at 24 volts.

Spicy was asking what the difference was in terms of load runtime.

Let's take 1000 Watts of load.

The 12V system will have an 83.33 Amp draw on the batteries or an ideal runtime of 1.2 hours.
The 24V system will have an 41.6 Amp draw on the batteries or an ideal runtime of 1.2 hours.

Both system in theory are the same. In reality the 24V system at half the current draw will be more efficient due to Ohm's law (less resistance) and the inverter will be more efficient with less step-up.

He wanted to see the math to quantify the efficiency. We can't do that without wire lengths, transmission line losses, etc. But suffice to say, that the difference is real world and significant with regard to cost of materials to make the 12V system less lossy compared to the 24V system.

I thought 12v 25ah parallel would equal 12v 50ah, and 12v 25ah series would be 24v 25ah. It can't gain voltage and capacity, it's a trade off

 

[Pistondog]

I think you may have missed the question that he was trying to ask (it was not clear)

(2) 12V 100Ah batteries in parallel provide 100Ah at 12 volts.
(2) 12V 25Ah batteries in series provide 50Ah at 24 volts.

Yep, i missed his question.
Neither of those calcs above are correct.
1st will provide 200ah @ 12v
2nd will provide 25ah @24v

 

[Reefering1]

"The 12V system will have an 83.33 Amp draw on the batteries or an ideal runtime of 1.2 hours.
The 24V system will have an 41.6 Amp draw on the batteries or an ideal runtime of 1.2 hours."
The v/a concept is correct here, I think the other part was obviously a typo

 

[Reefering1]

But the 12v system will run at a higher amperage which equals higher voltage drop, which means lower efficiency/runtime

 

[BeanAnimal]

I thought 12v 25ah parallel would equal 12v 50ah, and 12v 25ah series would be 24v 25ah. It can't gain voltage and capacity, it's a trade off

I mistyped the second item
(2) 12V 25Ah batteries in series provide 50Ah at 24 volts.

should have been
(2) 12V 50Ah batteries in series provide 50Ah at 24 volts.

I edited the numbers several times and was typing on an iPad... my apologies for not proofreading. Also more than a bit under the weather right now unfortunately.

 

[Reefering1]

I mistyped the second item
(2) 12V 25Ah batteries in series provide 50Ah at 24 volts.

should have been
(2) 12V 50Ah batteries in series provide 50Ah at 24 volts.

I edited the numbers several times and was typing on an iPad... my apologies for not proofreading. Also more than a bit under the weather right now unfortunately.

The first part was incorrect also. (2) 12v 100ah in parallel would be 12v@ 200ah. You had the concept perfectly backwards, a obvious accident.