Battery capacity 101

[Tundrawalker]

I’m sure it’s been covered many times before but I’m just trying to understand the math. 12V 100Ah marine/deep cycle battery = 1200Wh. If Im drawing 100 watts does that mean I can expect 12 hours of use until dead >11.8v or is the actual amp/watt capacity significantly less than rated due to practical vs theoretical SLA capacity?

 

[E. Cho]

Your math is okay if it is a LiFePO4 battery but not applicable if an AGM or Lead Acid deep cycle battery. Go to Batteryuniversity.com to read about battery materials?

 

[mikefitz]

If its a lead acid battery the capacity is calculated on the 20 hour rate. For a 100Ah battery that's 5 amps for 20 hours from full to completly empty, around 10.5 volts.
A higher current results in a lower effective capacity and a lower current a higher effective capacity. Peukerts law.

 

[Tundrawalker]

If its a lead acid battery the capacity is calculated on the 20 hour rate. For a 100Ah battery that's 5 amps for 20 hours from full to completly empty, around 10.5 volts.
A higher current results in a lower effective capacity and a lower current a higher effective capacity. Peukerts law.
View attachment 160762

Thank you. That makes sense. So if I had 200 amp hours capacity, would that effectively mean I could draw 10 A for 20 hours or 5 A for 40 hours?

 

[mikefitz]

Not quite, the effective capacity changes with current. If you have a 200 Ah battery rated at the 20 hour rate, then yes, loading with a 10 amp load will run the battery down in 20 hours.
If the load is reduced to 5 amps the exact same battery is now , in effect a bigger battery, say 220 Ah, so the battery will power a 5 amp load for 44 hours.
The reverse is true if we Increase the load to 20 amps. At this current the battery will have an effective capacity of ,say 180 Ah.
So we do do not get the expected 10 hours , the battery runs down in 9 hours.
( note I have used example values to make the sums easy)
Some lead acid battery makers produce data for different discharge rates, many don't.
The data I have for a deep cycle Trojan battery has the following specification, sold as a 225 Ah battery.
5 hour rate, 185 Ah, 10 hour rate 207 Ah, 20 hour rate 225Ah, 100 hour rate 250 Ah.
So with this battery, a current of 11.25 amps will drain the battery in 20 hours. A lower current of 2.5 amps will run the battery down in 100 hours. If we used the nominal capacity of 225 Ah one would expect only 225/2.5 = 90 hours.

It's all down to the efficiency of the chemical process being related to current flow, the higher the current, the lower the efficiency.

The effect can be computed using Peukerts constant. Very few battery monitors incorporate this in computing battery ' time to go to empty'.

Mike

 

[Tundrawalker]

Wow. That’s a lot to think about but it helps. Thank you.

 

[Rednecktek]

It also depends on the manufacturer. Everready (WallyWorld deep cycle marine dc29's for example) are rated at a 1a rate for their capacities.

 

[Tundrawalker]

My main use case is running a security camera and NVR recorder plus a modem in a remote location for construction site monitoring. I have been running 2 parallel connected Group 31 100Ah deep cycle batteries connected to a 300w inverter. I believe the draw is about 50w. The batteries were connected with 2x100w solar panels and I have now added a third 100w panel. Plus as I discovered higher capacity awg wiring. Still having troubles maintaining a full charge. Are there online calculators to determine a proper configuration based on draw?

 

[Sabre36]

Thank you. That makes sense. So if I had 200 amp hours capacity, would that effectively mean I could draw 10 A for 20 hours or 5 A for 40 hours?

We would need to know the Peukert value for that battery to calculate that..

 

[Sabre36]

It also depends on the manufacturer. Everready (WallyWorld deep cycle marine dc29's for example) are rated at a 1a rate for their capacities.

that’s a misprint by Walmart's Marketing Department. I got a good chuckle out of it when I saw it…

 

[Rednecktek]

that’s a misprint by Walmart's Marketing Department. I got a good chuckle out of it when I saw it…

Really? It's right on the label of most of my DC29's. ?

 

[mikefitz]

Still having troubles maintaining a full charge

It's probable the batteries have degraded if the system has been running for some time without the batteries being fully charged.
Solar yield has so many variables its difficult to estimate the power generated. 300 watts of solar could generate 1500 watt hours under ideal conditions with a quality solar controller, it could/ will be, considerably lower. With a 50 watt estimated load that's 1200 watt hours a day.
A Victron Smart controller would record the daily yield Ah for the previous 30 days with maximum and minuimum battery volts.
This date would be useful to determine what's wrong with your system.

Lead acid batteries take a long time to fully charge, often there is not enough solar hours to completly charge the battery. The charge controler you have may not be set up idealy to charge the batteries.

My guess is that 300 watts of solar and your charger, is not, on average, keeping up with the demand.

Full system equipment details and an accurate estimate of load power would be useful.

Mike