Calculating energy draw for POE devices and LTO battery size

Uncle Sayit

New Member
Joined
Apr 11, 2021
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5
I'm trying to size a battery for a rural solar PoE security camera setup.

The battery will be lithium titanate (LTO) because I will need to solar charge it at -20C (-4F). Due to the high cost of the battery I want to size it correctly.

I'm a bit confused how to calculate energy usage for PoE devices. The specs sheets for each product states wattage and voltages but I am not understanding something about POE voltages. All my devices seem to be 802.3af POE, the standard supports up to around 13 watts per port. My understanding is that power is injected onto the cable at a voltage between 44 and 57 volts DC, typically 48 volts is used, but many devices use only 24V. Maybe the reality is that POE switches can output either 48V or 24V?

I'll list my gear here.
3 x Dahua 5442 cameras, each one uses 7.2W maximum PoE so 21.6W total
1 x Ubiquiti Gigabeam Plus, 13W maximum, 24V DC PoE input
1 x POE switch, 5W maximum, 24V DC POE input
1 x Alpha Network 8-port POE injector, 12VDC - 48VDC input
1 MPPT solar charge controller that supports LTO batteries
1 or 2 solar panels, size TBD
1 LTO battery with BMS

I need to size the battery. I'd like to have one capable of running all my equipment for 2 or 3 days to address cloudy / snowy days that provide very little solar power.

Do I simply add up the wattage (39.6W) x 24 hours = 950.4Wh per day? Then divide by the battery voltage to determine the battery size needed per day in Ah?

Since Amps = Watts / Volts, a LTO battery at 23V would require 41.32h per day for my application, right? But maybe the average voltage would be closer to 18V due to the battery chemistry slowly draining from 23V to, hmm... maybe 12V, thus requiring more like 52.8Ah of battery power per day?

LTO cells commonly output 2.3V and I see a couple BMS for a 10S battery which is how I got to 23V. If I need 106Ah to 158Ah of LTO battery power that's not exactly cheap. Hopefully I am not calculating something correctly.
 
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Samsonite801

Photon Sorcerer
Joined
Oct 15, 2020
Messages
1,672
I'm trying to size a battery for a rural solar PoE security camera setup.

The battery will be lithium titanate (LTO) because I will need to solar charge it at -20C (-4F). Due to the high cost of the battery I want to size it correctly.

I'm a bit confused how to calculate energy usage for PoE devices. The specs sheets for each product states wattage and voltages but I am not understanding something about POE voltages. All my devices seem to be 802.3af POE, the standard supports up to around 13 watts per port. My understanding is that power is injected onto the cable at a voltage between 44 and 57 volts DC, typically 48 volts is used, but many devices use only 24V. Maybe the reality is that POE switches can output either 48V or 24V?
Not all your devices are 802.3af... With Ubiquiti, you have to look at the datasheets to be sure.. In your case, your Gigabeam Plus is 24v 'passive POE' (do not plug it into a 48v passive POE PSE supply or it will fry, only plug it into a 24v passive POE port)...


You have to be sure your POE switch for Ubiquiti supports Passive POE output, because an 802.3af PSE port relies on testing the circuit for a correct PD device resistance value and if present, it will turn on the power to the port. Passive POE devices might not present this correct resistance value on the circuit, so the power may never turn on to the device. Passive POE PSE device will always apply power to the port (could fry PD devices if not correct voltage). 802.3af will detect proper resistance, and ramp up power on the POE if it is the compliant 802.3af PD device.

There are also differences between 24v and 48v POE, it will only output what voltage you are putting into it. If the PSE device can support 48v POE, and you plug 48-56v power supply input to it, it will output whatever the input voltage is, or if the PSE device is also the power supply for the POE side circuit, it will only put out the one voltage it was designed to put out.

I have a system I am putting up for an AP repeater site, where I have both 24v and 48v POE devices, so I basically built 2 different systems to supply POE power for those different devices. I have a 24v boost/buck supply and a 48v boost/buck supply, to feed different POE injectors... I also have a Ubiquiti POE router/switch for some of the 24v stuff (it also powers itself from a POE input port), and it has 'Passive POE' output ports on it, specifically, so those ports don't detect the PD device, they just put out power passively, all the time. There is a setting in the UI to turn off power manually on the individual Passive POE switch ports.


And on your other questions, I agree with NVS that I would more focus on actual battery load draw than rated specs for devices... The rated specs for the devices may not always indicate what the input draw will be, although they can be used as a rough number in basic calculations.

An amp-clamp is a good tool to measure actual current draw off the battery load side.
 
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Uncle Sayit

New Member
Joined
Apr 11, 2021
Messages
5
What you could do instead of trying to calculate it, you bench test it all hooked up to a watt meter.

Upgraded Watt Meter Power Meter Plug Home Electricity Usage Monitor, Electrical Usage Monitor Consumption, Energy Voltage Amps Kill Meter Tester with Backlight, Overload Protection, 7 Modes Display https://smile.amazon.com/dp/B09BQNYMMM/ref=cm_sw_r_apan_i_FEZ53YMYV2A4R5QJ7K3E?_encoding=UTF8&psc=1

Then you would know for sure.

That's a good idea, thank you. While everything will be in DC I can hook up my DC power supply to the meter and see what I get.
 

Uncle Sayit

New Member
Joined
Apr 11, 2021
Messages
5
Not all your devices are 802.3af... With Ubiquiti, you have to look at the datasheets to be sure.. In your case, your Gigabeam Plus is 24v 'passive POE' (do not plug it into a 48v passive POE PSE supply or it will fry, only plug it into a 24v passive POE port)...


You have to be sure your POE switch for Ubiquiti supports Passive POE output, because an 802.3af PSE port relies on testing the circuit for a correct PD device resistance value and if present, it will turn on the power to the port. Passive POE devices might not present this correct resistance value on the circuit, so the power may never turn on to the device. Passive POE PSE device will always apply power to the port (could fry PD devices if not correct voltage). 802.3af will detect proper resistance, and ramp up power on the POE if it is the compliant 802.3af PD device.

There are also differences between 24v and 48v POE, it will only output what voltage you are putting into it. If the PSE device can support 48v POE, and you plug 48-56v power supply input to it, it will output whatever the input voltage is, or if the PSE device is also the power supply for the POE side circuit, it will only put out the one voltage it was designed to put out.

I have a system I am putting up for an AP repeater site, where I have both 24v and 48v POE devices, so I basically built 2 different systems to supply POE power for those different devices. I have a 24v boost/buck supply and a 48v boost/buck supply, to feed different POE injectors... I also have a Ubiquiti POE router/switch for some of the 24v stuff (it also powers itself from a POE input port), and it has 'Passive POE' output ports on it, specifically, so those ports don't detect the PD device, they just put out power passively, all the time. There is a setting in the UI to turn off power manually on the individual Passive POE switch ports.


And on your other questions, I agree with NVS that I would more focus on actual battery load draw than rated specs for devices... The rated specs for the devices may not always indicate what the input draw will be, although they can be used as a rough number in basic calculations.

An amp-clamp is a good tool to measure actual current draw off the battery load side.

This is really good information. You are right, I just read the spec sheet and the Ubiquiti Gigabeam uses passive POE, thanks for catching that. So I will need to either get a different switch than I was planning, one that supports passive POE, or wire the Gigabeam with its own passive POE injector. I did not understand the difference between 802.3af POE and passive POE until now. At least all my devices will use 24V.

I found some DC to DC power converters from Tycon Power Systems, different models that support either 24V passive POE or 802.3af POE. With a wide range of DC input voltages and 24V output they'd work well with LTO batteries. But they each only output 19W. Maybe there are bigger ones available like 60W.

I will get all the gear together, purchased and plugged into a DC power supply and see what kind of load it generates. The spec sheets mention only the maximum wattage of each device so I'm curious how real life will look.
 
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