Basic 12V DC Power Backup Circuit

[pwnell]

Hi,

I want to build a simple 12V DC power backup circuit that will provide battery power in case of AC power failure, to a 12V DC load.  To make this efficient, I do not want to use a UPS with its double conversion (AC - DC - AC).  The idea is to be able to power a DC pump for an extended period (for a reef aquarium).

I was contemplating building the circuit below (pardon the childish use of pictures and scribbles as opposed to a proper circuit diagram).



The components are: 12.8V 30Ah LiFePO4 battery, Haibro Car Battery Charger (Lithium LiFePO4 Lead-Acid Smart Trickle Charger), this relay from amazon (https://a.co/d/0hfdXAnM).

To make the transition from AC to DC uninterrupted I was thinking of adding a large capacitor across the output terminals of the relay.

Would this circuit work? 

[coromonadalix]

with the 12vdc supply you need to keep a relay opened at all time

when the 12vdc drop or get cut the relay close and the battery voltage will be applied

for sure you may have some delays / lattency .. it depends on what you're powering up and how much it consumes ...

sure a big cap may help   but there is always a limit on how big you have, at some point it could create problems too

[Phil1977]

I would recommend to make it much simpler:

Connect your aquarium to the LiFePo-Battery. Connect the battery to a 12.8V-13V power supply with a current limit of a little more than the aquarium needs.

That way the battery will stay in an upper state of charge, but not 100%. Keeping a lithium battery charged to 100% shortens its lifespan. If AC power is interrupted the battery supplies everything without any delay. As soon as AC returns it supplies the aquarium and recharges the battery.

[calzap]

You don’t state how many amps your pump uses.  But 12 VDC battery-backed-up PSUs that plug into mains are plentiful.  Cost is largely a function of amperage but may be less than cobbling something together like you are proposing.  Many of these have nice features like an input fuse, output short protection and indicator lights for things like mains power, battery operation, charge level, bad battery, etc.  Some have 5 VDC USB output too.

Mike

[pwnell]

I intend to run three devices each rated up to 2.5A at 24V. These devices run at half speed when connected to 12V, saving capacity on the battery.

When active the devices consume about 7W, 7W and 25W, respectively. I need several hours of runtime so a 30Ah battery should suffice. I could not quickly find a 12V DC UPS with high enough capacity.

[pwnell]

I would recommend to make it much simpler:

Connect your aquarium to the LiFePo-Battery. Connect the battery to a 12.8V-13V power supply with a current limit of a little more than the aquarium needs.

So trickle charger to battery, battery to load?

[Phil1977]

So trickle charger to battery, battery to load?

Lithium batteries do not need a dedicated trickle charger. They feel most comfortable just with a CC/CV DC supply. Please do not connect a LiFePo-battery to a lead-acid trickle charger.

But if you want to drive the pumps with 24V nominal and 12V for backup then you need some kind of relais or at least a diode. The direct connection - as I proposed - only works if the voltage level stays the same.

[mariush]

You could use switch chips like TPS2121 or TPS2120  to automatically switch to the highest voltage of two with very low losses. Can be multiple of these chips (ex one per device).

TPS2121 : 2.8V TO 22V IN, max. 4.5A current out : https://www.digikey.com/short/5vpfdbd8

TPS2120 : same, but max. 3A current out : https://www.digikey.com/short/hd052frr

TPS2120 switches between inputs in 100us , TPS2121 is faster at 5us.

TPS2120 has a SELECT pin : Active low Enable for IN1. Allows GPIO to override priority operation and manually select IN2. TPS2120 only.

So if you don't want auto switch , double check it, but I think you could place the battery on IN1, adapter on IN2 and when put voltage (but paying attention to respect maximum voltages tolerated on SEL pin, I think it's 6v max) from adapter on SEL pin to force the chip to switch to IN2 (the adapter)

TPS2121 has a comparator pin CP : Enables Comparator Operation and is compared to PR1 to set switchover voltage.
Connect to GND if not required. TPS2121 only.

There's a whole list of behaviors on page 18 in datasheet which you need to be aware of if you use this pin.

There's also the option of using some high current Schottky diodes to combine the voltage from two sources, even common cathode packages like

VS-40L15CTS-M3  max 15v 20A per diode, 0.4v drop at 20A : https://www.digikey.com/en/products/detail/vishay-general-semiconductor-diodes-division/VS-40L15CTS-M3/5426200

STPS40L15CT  same specs : https://www.digikey.com/en/products/detail/stmicroelectronics/STPS40L15CT/1039687

MBR3030 30v 30A  <0.5v drop : https://www.digikey.com/en/products/detail/onsemi/MBRB3030CTLG/1477215

SMB2045  45v 10A : https://www.digikey.com/en/products/detail/panjit-international-inc/SBM2045VDC-R2-00001/15800912

So you could use an adapter that outputs 13v-ish (tweak the voltage up a bit if the psu has a TRIM potentiometer) and use a diode or something to drop the voltage going to battery so the voltage will always be lower than adapter voltage and the battery doesn't get too high voltage. Then the two diodes on output will allow the higher voltage to go through.

[J-R]

Most modern consumer UPS units already by default operate in the high-efficiency bypass mode until AC failure.  They don't even have a double-conversion feature and that is why they can take half a day to recharge.  Double conversion units are typically found in larger units where the AC-DC charger is able to fully support the UPS unit's rated load and recharging the batteries might only take a couple hours.

Also, a lot of newer UPS units are very efficient at lower load percentages, whereas older units were very inefficient at low loads.  So the old units were 1 hour at 50W, but also only 1 hour at no load at all, vs. a new unit where it might go 12 hours with no load.  We also can get pure sine wave units now at the consumer level.


More lithium ion battery damage occurs the higher the charge level you store it at.  So lithium ion should not be float charged, because it reduces the life of the battery significantly.  If you really must, float the battery well below its resting voltage, so perhaps 4V or even 3.8V.  These voltages are for standard individual lithium ion cells.  A ~12V LiFePO4 pack will need different numbers. 

For stationary applications, I think sealed lead acid is still the best option.  Inexpensive, lowest risk of burning your house down, little need for individual cell balancing, likes being stored at 100% charge plus a float charge...

[pwnell]

Would your comments regarding li-ion also apply to lifepo4? I was hoping to use the float charger I listed in my original post to charge it, and have it on standby the rest of the time.

[J-R]

Yes, this applies to lithium ion and LiFePO4.  One thing that clouds the issue a little is that it seems some of the battery manufacturers would like to sell more lithium ion & LiFePO4 replacements for lead acid, so they skip over some of the finer details.

Everything I've read points to the fact that lithium ion and LiFePO4 suffer higher capacity loss over time when stored at a higher state of charge.  Up to 6% per year at 100% SOC is a number I've seen quite often, whereas 70% SOC might only be 3% degradation per year.  This is less of an issue for your cell phone where it might spend quite a bit of time at a lower SOC than 100%, but it's definitely a problem for backup applications.  Floating at say 4.2V per cell is going to be particularly bad.

But now you have a problem for backup because you don't want to kill your batteries but you also want them to be fully charged because that is their job!  So the compromise that all the companies seem to go with is charge the batteries to 100% SOC then stop charging and let them sit.  They have a very low self discharge rate compared to lead acid.  Run yearly discharge tests and recharge at that time.  Worse case they might be at 95% capacity at the wrong moment.

From what I've seen, most of the pricing for quality lithium ion and LiFePO4 is between 5-10X the cost of lead acid, although of course there are plenty of no-name brands out there advertising with various YouTube content creators.  One of the things the lithium ion and LiFePO4 sellers like to point out is how many more cycles you can get out of them, but due to the cost difference you can just build a larger lead acid battery bank and cycle it less.  Then you'll also have that "reserve" capacity if necessary.  In other words, I'd rather have a lead acid battery bank that has 5 times the capacity in a backup application for the same amount of money.  Even then, modern deep cycle AGM has a pretty decent cycle life anyway.

And to clarify, this is for backup applications.

Lithium ion and LiFePO4 obviously still have advantages over lead acid, especially with mobile and high-rate applications and certain off-grid solar setups.  If you cycle your lithium ion or LiFePO4 battery bank off-grid on a regular basis from say 30% to 80%, you will get thousands of cycles out of it no problem.  Lead acid in the same situation might be dead in a year.

I have a 4 year old ~15kWHr battery bank for my backup system and it typically only sees a dozen events a year and most are only a few percentage of discharge.  A handful have been around 70% and only once was 50%.  They are AGM and are sold as high-rate, long-life (10 years).

I also have a ~3.5kWHr AGM battery bank at a remote seasonal cabin and it's over 10 years old with very little capacity loss.  It gets cycled a few times a year to maybe 50% and otherwise is float charged at 100% via solar.

[Phil1977]

That´s right, if you want a very robust, maintenance-friendly and well-tried system then (sealed) lead acid still has advantages.

But regarding the use of LFP for buffer application: The trick is to stay outside of the high-voltage part of the charge curve. If e.g. the end-of-charge voltage for 100%SOC is 3,6V per cell, then you can set your charger to 3.4V. That way only 60-80% of SOC can be reached, and charging may take a very long time because currents are very low, but the crucial point is: The high cell voltage is one of the main reasons why such cells age quickly when always fully charged. If you just manage to stay out of the last 0.2V of the voltage range, the lifetime gets much better.

But for someone who just wants a working system without thinking to much about everything I´d also propose to either buy an off-the-shelf system or take SLA.

[pwnell]

Thanks for all the input.