Schmidt Consulting

External battery DC output UPS



Tom Schmidt

Revised 12/12/2021

Originated 7/26/2018



I want to maintain internet access during power outages even when our backup generator is not running. To do this we need to keep several network devices powered. Luckily they are all use DC wall warts so the obvious solution is a DC, rather than AC, output UPS. I built a DC UPS that uses an automotive jump power pack as the battery. The UPS has a high current charger/tender to charge the battery quickly and keep it fully charged over time. A separate 12 volt supply powers the LAN when mains power is available.  With this setup the LAN remains operational for hours without mains power. If mains power is available the jump pack can be removed and used as needed without affecting network operation.  We recently upgraded from DSL to high speed fiber internet requiring a few minor changes to the UPS.



Table of Contents


Overview.. 2

Network equipment power requirements. 2

Design considerations. 3

Automotive power pack. 4

Monitoring battery capacity. 5

UPS. 6

Low battery threshold. 7

Switches and indicators. 7

External connections. 8

Schematic. 9

Power consumption. 9

Cost rollup. 12

Operation. 13

Battery run and charge time. 13

Battery upgrade. 14




Several years ago I installed a portable generator to provide power during utility outages. Our intent is to run the generator a few hours in the morning and evening. This means we lose internet access when the generator is not running. Our critical networking devices are powered from DC wall warts. It is inefficient to utilize a traditional UPS to convert battery power to AC and then back to DC to operate the LAN. I wanted to build a UPS that outputs DC directly to keep our LAN operational during a power outage. 

I could have used any appropriately sized SLA battery to provide backup power but opted to use an automotive jump power pack. This is basically an SLA battery in a nice portable carrying case. With this arrangement we have the best of both worlds. The power pack keeps the LAN running when mains power fail. The UPS keeps the power pack topped up and ready for use if needed. The built-in jump pack charger is not suitable for UPS usage as is takes a long time to recharge the battery and it does not have a maintainer mode. For UPS operation we need a better charger with a high current rating for fast recharge and a maintainer mode to keep the battery topped up without risk of overcharge.  

The UPS is built into an old TiVo Series II chassis. I’ve used these for other projects as it is a nice sturdy metal chassis. Removing the front plastic trim panel reveals a solid metal panel except for a few small holes. To keep the battery charged I use a Deltran 5A battery tender. The high current capability is able to recharge the battery in three to four hours. This is important as we only intend to use the generator a few hours a day.  Once the battery is charged it enters maintainer mode, keeping the battery topped up while not over charging it. A separate 12 volt 5A supply powers network equipment when mains power is available. This way there is no load on the battery minimalizing recharge time. When AC fails the system switches to battery power and a low voltage monitor function protects the battery from being over discharged.

The UPS is connected to the jump pack through one of its accessary power (cigarette lighter) sockets.  This keeps the battery fully charged and the power pack can be easily removed if needed, without affecting LAN operation.

Network equipment power requirements

There are three devices that need power during an outage. The ONT, WiFi router and Ethernet switch. The Ethernet switch is not completely necessary but keeping it powered enables access to the server. The server is a laptop with its own battery.  The 5V buck converter needed for DSL is not required for the current fiber network. I left the module in the chassis but disconnected its input.

Typical 12 volt current draw of each device. Total is about 50% higher than for the DSL network.


Wall Wart

Typical current

Fiber ONT

12V @ 1A

 .170A (.210 POTS off hook)

WiFi router

12V @ 2A

 .640A (2 active enet ports)

Ethernet switch

12V @ 1A

 .282A (10 active ports






1.092A total

Design considerations

When using the UPS all network devices share a common ground, this is different than when each device is powered by a dedicated wall wart. I verified each chassis is tied to the negative DC power rail. If some devices tie the positive rail and others negative touching them together will short out the UPS. It is common engineering practice to tie negative to the chassis and that is the case for all our gear. Ethernet data lines are transformer coupled so it is not affected by how the device is powered.  The PoE specification uses an isolated power converter in the powered device; however there are many different implementations so it is good to double check. In our case none of our gear uses PoE.  

The power pack battery is rated at 17AH at a 20 hour rate so should have enough capacity to operate the LAN for about 10 hours without discharging it too deeply. The Battery tender is rated at 5 Amps; this is the maximum recommended charging current for the battery.  As such it should be able to recharge a fully depleted battery in 4-5 hours.

Figure 1 DC UPS system

Automotive power pack

I purchased a Harbor Freight model 62306 power pack. It is normally $58 ($70 2021) but went on sale for $40 prompting me to get one to build this UPS.  It consists of a 17 AH SLA battery, jumper clamps controlled by a switch, work light, battery charge indicator, USB power port, two accessary power sockets and a built in AC charger. The built in charger is not used in UPS mode as it is too slow and will over charge and damage the battery is left powered continuously. The battery is charged by the power tender in the UPS. This insures it is fully charged at all times without risk of damaging the battery. I did not remove the internal charger as this allows the power pack to be recharged when remote from the UPS.

The two accessary sockets are always live, protected by a circuit breaker. This provides convenient access to the battery. An accessary plug (cigarette lighter plug) connects the UPS to the power pack.

If I need to use the power pack elsewhere just remove the UPS accessory plug. The battery charger is smart and automatically removes power from the plug when it detects the battery is not connected. Removing the power pack has no effect on the LAN as long as AC power is available.


Figure 2 Automotive power pack


Monitoring battery capacity

The smart charger/tender does a good job maximizing battery life but lead acid batteries are the Achilles heel of any electronic project as they have relatively short life expectancy, under ideal conditions typically five years or so. I wanted to be able to periodically test the battery and replace it before it has lost so much capacity to be useless in a backup situation.

There are lots of fancy battery load testers but I went with a simply old school unit with an analog volt meter and low value high power resistor that loads the battery at approximately 100 Amps. The 100 Amp specification is rather loose because the resistive load only draws 100 Amps at 12 volts. These testers have been around for years. I purchased mine from Harbor Freight along with the automotive jump power pack.

Figure 3 Battery load tester

Using the tester is easy. Connect the test clips to the battery terminals. The meter will indicate no load battery voltage. This should be between 12.8-13.2 volts for a fully charged battery. The open circuit voltage is affected by surface charge and varies some depending on how recently the battery was charged and to what voltage. Pressing the test switch connects a low value high wattage resistor to the battery (.120 ohms). The meter displays the resulting battery voltage. The higher the cold cranking amp (CCA) rating the less voltage should decrease under load.

I connected the tester to the power pack battery clamps and activated power to the clamps. The load tester displayed open circuit voltage of 13 volts.  Voltage decreased to 10 volts when the load button was pressed. I used a clamp-on DC amp meter and measured a load current of 82 amps. Testing the power pack battery this way somewhat understates its capacity since the load is being drawn though the power pack switch and cables. But for our purposes that is fine. I’m not that concerned with the absolute value.  What I want to do is establish a base line and then every year or so recheck battery capacity. If the voltage under load begins to fall below 10 volts I know the battery is losing capacity and should be replaced.

Given the low price of the Harbor Freight power pack it will be interesting to see how long the battery lasts. I assume it is by far the most expensive component of the unit. Curiously the jump pack battery is rated at 17AH. I noticed other same sized SLA batteries are rated at 18AH so at least when it comes time to replace it the new battery will have a little more capacity increasing backup time.


As mentioned I mounted the other components in an old TiVo Series II chassis. It houses everything except the battery power pack. The main UPS components are:

·         12 volt 5 Amp power supply

·         Deltran 5 Amp power tender battery maintainer

·         ATC style 6 position fuse block

·         DC to DC buck converter (provides 5 V for DSL router, no longer needed)

·         Digital volt meter

·         Logic board

There are three devices we need to power to maintain LAN/internet/Wi-Fi functionality: ONT, WiFi router and Ethernet switch. I designed the UPS to be able to supply a maximum of 5 amps at 12 volts.  A DC to DC switch mode buck converter module provide 5 volts (no longer needed).

When AC power is available the power pack is disconnected from the load and charged by the battery tender.  This is a 5 Amp unit so it is able to recharge the battery in 4-5 hours as well as maintaining the battery at full charge without risk of over or under charging. For a 100 year old technology the optimum charging requirements of lead acid batteries is pretty complex. Power Sonic has a nice whitepaper about absorbed glass mat (AGM) sealed lead acid (SLA) batteries if you are interesting in the gory details.

A separate 12 volt 5 amp switch mode power supply provides network power when AC is available. This allows the battery to be recharged as quickly as possible as there is no load on it and the power pack can be removed and used elsewhere if needed without affecting the LAN. This also isolated the network gear from the higher battery charging voltage.

A simply voltage monitor activates a power transfer relay if the AC derived 12 volt supply drops by a volt. The LAN is then powered by the power pack battery. Lead acid batteries are sensitive to over discharge and will be permanently damaged if deeply discharged. Another voltage monitor disconnects the load if battery voltage drops too low. Once the relay is released the load is removed from the battery.

In addition to the automatic low voltage disconnect there is a battery disconnect switch on the controller. This is used at night time when everyone is in bed and there is no need to keep the LAN functioning. When the battery disconnect switch is turned back on the transfer relay is momentarily activated. If there is enough power remaining in the battery and AC power has not been restored the LAN is powered from the battery. The switch only affects the battery load; the battery is always connected to the charger/maintainer.  Regardless of battery switch position if AC power is available the LAN is powered. When we upgraded to Fiber internet I modified the switch so now it is not in the battery circuit but instead inhibits the transfer relay. This eliminates the slight voltage drop across the switch.

Low battery threshold

The open circuit voltage of lead acid batteries varies due to state of charge, as a result of chemical changes within the battery. A freshly charged 12 V battery will have an open circuit voltage of about 13.1 volts. This varies a little depending on the voltage used to charge the battery and temperature.  11.6 volts is considered fully discharged for a 12 volt battery.

The fully discharged voltage under load is dependent on the discharge current.  The higher the current the lower the terminal voltage and the battery will not be capable of delivering the full rated amphour capacity. In our case we are discharging the battery at approximately the .05C rate.  Theoretically the battery can power the LAN for up to 25 hours. However fully discharging lead acid batteries significantly shortens their life.

A complicating factor is that we are not measuring voltage at the battery directly but through cables, connectors and circuit breaker. As a result there is about a 200 millivolt drop between the battery and controller at a 1 ampere load. This results in the voltage at the battery being .2 volts higher than at the monitoring circuit when the battery is under load.

Using the PowerSonic document as a guide battery voltage at 100% of capacity is 12.6V, at 50% remaining capacity voltage is 12.0V, dropping down to 11.5V at 25% when discharged at a .05C rate. I set the disconnect level at 11.6V, equating to 11.8V at the battery.

I set the discharge limit fairly aggressively to maximize battery run time at the expense of battery life. We only expect a week long power outage every couple of years so a slight reduction in battery life due to deep discharge is acceptable.  The deleterious effects of deep discharge are somewhat minimized as the battery is quickly recharged once AC power is available and not left in the discharge state to sulfate.

Switches and indicators

The UPS front panel has a number of switches and indicators. The jump power pack is treated as a black box, all controls and indicators are on the UPS. The battery maintainer has two status LEDs that are visible through the USB front panel.  They are not used for normal operation but proved handy during testing.

Figure 4 UPS Front panel


Battery disconnect switch –  this is used when the LAN can be shutdown to minimize unnecessary battery drain. This disconnects battery from the load, the battery tender remains connected so the battery is recharged when AC is available.  When the battery is switched back on the transfer relay is momentarily energized. If the battery is not fully discharged and AC power has not been restored the LAN is powered up. If AC power is restored the LAN is powered from the AC 12V supply regardless of battery switch position.

DC voltmeter – small motorcycle digital voltmeter displays battery voltage. When AC power is available a MOSFET provides a low resistance ground enabling the meter to display battery voltage.  When AC power is lost it disconnects the ground turning the meter off to reduce battery drain.

Voltmeter pushbutton – if the meter is off pressing this switch grounds the low side of the meter activating the display.

AC status indicator – green LED powered by the 12 volt power supply. It is illuminated when AC power is present.

Battery status indicator – green indicates nominal battery tender voltage, red if outside this range. The range is set fairly narrow to monitor battery tender operation. Red if the voltage is too high or the tender fails and battery voltage falls due to self-discharge.

Battery alarm switch and sounder – pulsating audible alert if battery voltage is too high or low when AC power is available. Same thresholds as the red battery LED. The alarm should be turned off during AC fail events when battery is being charged as the voltage exceeds normal maintainer range.

External connections

The USP is located in the same closet as the Ethernet switch. The ONT and WiFI router are located some distance away from the UPS.  I strung 2-conductor 18 AWG cable to a terminal block located near the ONT and router. All devices use barrel jacks for power so I connected a short pigtail allowing the barrel jack to be plugged into the respective device. The Ethernet switch is wired directly to the UPS. Annoying each device uses a different size barrel connector.

Had a nice heavy gauge cigarette lighter cord set laying around so used it to connect the UPS to the power pack. Plug has a protective plastic cap that can be snapped on when not in use. For safety the battery tender will not energize the plug if it detects the battery is disconnected, but it is nice to have mechanical protection when the jumper pack is disconnected.

The UPS uses an IEC C14 power inlet and C13 detachable 3-wire AC line cord.  This is plugged into a high end surge protector.


AC power is supplied via a detachable power cord and fused IEC receptacle. Power is feed to battery tender and 12 volt SMPS.  The  12 volt supply output is protected by 7.5 amp ATC fuse then feed to the logic board transfer relay. The output of the relay feeds 2 amp ATC fuses on each load; only three are used in the current implementation. All three network devices require 12V DC.

The heart of the logic board is an LM336 2.5 volt reference and LM339 quad comparator. Two comparator sections act as a wired OR to control the transfer relay. One section monitors the 12 volt supply the other battery voltage. If the AC supply drops below 11.6 volts and battery voltage is above cutout value the transfer relay is energized powering the LAN from the battery. When AC power is present or battery voltage is too low the relay is deenergized disconnecting the battery and powering the LAN from the AC supply. A couple of 1000uf 25V caps provide bulk storage so voltage does not droop too much while the relay is switching between AC and battery.

The AC 12 volt supply will increase slightly when the load is removed so the comparator uses a couple hundred millivolts of hysteresis to prevent false triggering. When the battery load is disconnected battery voltage will increase substantially due to its internal resistance. I used capacitive feedback to temporally drive down the battery sense voltage. When battery voltage reaches disconnect threshold the transfer relay is de-energize causing Vcc to decay, the capacitive feedback prevents the relay from being turned on while Vcc is falling.

Two more comparator sections form a window comparator driving the red/green battery status indicator and the audible alarm.  The thresholds are set around maintainer mode voltage. This way it will alert if the maintainer fails and either over charges or allows battery voltage to fall due to self-discharge.

Another comparator section controls the fan. The TiVo chassis has a nice little 12V DC fan. I clipped a thermistor to the 12 volt supply heatsink feeding a simple bridge circuit. When temperature exceeds 93 F the fan turns on.

An ULN2803A 8 port Darlington array drives the relay, fan and indicators. The output drives different voltage rails so the built in snubber diodes cannot be used.

Power consumption

The 12 volt supply is rated at 84% efficiency so it dissipates 11.5Watts at full load. Deltran does not specify power tender efficiency. I assumed 75% (14V 5A) so it dissipates 23 watts. The UPS logic and display consume less than a watt. Network devices are external to the UPS so they do not contribute to UPS chassis heating.  Worst case power dissipation of the chassis is 35 watts.

Worst case AC power consumption is 165 W, this occurs under full LAN loading (5 Amp) and high current battery recharge (5 Amp).  AC power consumption with LAN operating and battery being charged is 99 watts. Once battery is charged power drops to about  16 watts.

Figure 5 UPS adjustments and fuses



Figure 6 Logic board


Figure 7 Power wiring

Cost rollup

I had most of the small components in my parts stock so do not have an accurate accounting of total cost.



5 Amp Battery Tender


3 in 1 Power Pack


100 Amp battery load tester


12V 5A power supply


TiVo Series II chassis


Motorcycle DC voltmeter


2A ATC fuses


6 position ATC fuse holder


Audible Alarm


12V SPDT 10A relay


DC/DC buck converter


Misc. components (Est)






The UPS and power pack are located near the Ethernet switch. During normal operation the Battery and alarm switches are in the on position.  As long as AC power is available network gear is powered by the 12 volt 5 amp supply and the battery tender keeps the battery toped up.

If the battery tender fails it will either over charge the battery or more likely the battery will self-discharge enough to turn the battery LED red and set off the audible alarm indicating the need for corrective action.  It is a good practice to periodically observe battery voltmeter. It should be between 13.0-13.2 volts. This is the safe maintainer mode voltage, high enough to keep the battery fully charged but low enough to prevent overcharge. The maintainer pulses the charging voltage, but the meter display average voltage.

During an outage the 12 volt power supply output will quickly decay. When it falls by a volt the UPS switches to battery.  The LAN remains powered from the battery until its voltage drops too much or AC power is restored.

When AC power is restored the network load is switched back to the 12V power supply and the battery tender cycles though battery charging mode. This voltage is high enough to activate the audible alarm.  During an outage turn it off to prevent nuisance alarms. The audible alarm is only active when AC power is available.

During an outage when the LAN isn’t needed flip the battery disconnect switch off minimizing unnecessary battery drain. Don’t forget to turn it back on as needed. Even with the battery switch off when AC power is restored the LAN is powered back on and battery charged.

Once power has been restored and the battery charged flip the audible alarm switch back on.

Every year of so perform a battery load test and replace the battery as needed.

Battery run and charge time

To verify run and charge time I used a 12V bulb to simulate the network gear. It draws 920ma - 900ma depending on battery voltage. This is not a perfect analog to networking gear as the bulb draws less current as voltage goes down whereas a switch mode power supply draws more. To bracket the test I ran it again using a 20 ohm resistor resulting in a 620ma load.

At 910ma system ran for 12 hours consuming 11AH or 65% of total battery capacity.  At the end the open circuit battery voltage was 11.9 indicating about 30% of remaining capacity left. At 620ma system ran for 17.25 hours consuming 10.5AH or 61% of total battery capacity. The open circuit battery voltage was 12.0V indicating about 35% of remaining capacity left.   Run time is compatible with how we intend to use the backup generator, so no sense opting for deeper level of discharge.

Recharge took 45 minutes to hit 80% state of charge and a little less than 3 hours for full recharge. I was a little worried the additional cable resistance would increase charge time but the charger has enough voltage compliance to compensate. Charge current maxed out at 4.88A right where is should be for a 5A charger. 

Battery upgrade

The HF jump pack came with a 17AH SLA battery. While searching on line I found a 22AH AGM SLA battery the same size a Mighty Max ML22-12. So when it is time to replace the battery I will opt for the 22AH battery hopefully yielding a 30% increase in run time for a few bucks more than a generic 18AH SLA.



It is comforting to know in the event of a power outage we will be able to maintain internet access indefinitely as long as we have gas to occasionally run the generator. Our cell phones preferentially use Wi-Fi so having the LAN operational is important here in terrain challenged NH and good insurance during an extended outage where not all cell sites have adequate backup provisions. We have a couple of laptops so will be able to use and recharge them during an outage. All in all an interesting and rewarding project.