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Building DIY double-conversion UPS - How to handle battery charging?

torveo

Sep 28, 2021
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After really bad experiences with off the shelf UPS units, I have decided to build my own double-conversion UPS. I was planning on using the almost new 12v 9AH battery I already have. Its kinda small but I don't need long runtimes, I just want my PC to not shut off when the power goes out, or reboot like it does with these crap UPS units.
I have an inverter picked out already. A 600W pure-sine-wave 12VDC - 120VAC converter with good reviews. What I'm stuck on, is how to charge the battery. My setup peaks at about 370w. Normal usage is much much lower. <100w. So I'm thinking 35-40 amps on the charger should be able to handle the inverter load and charge the battery at the same time. The problem is, I really don't know how the charger will interact with the battery while they're both connected to the inverter. For example, how do I ensure the charger won't charge this small battery with too much amperage while still allowing the charger to power up to 370w on the inverter? Will it be a problem?
 

kellys_eye

Jun 25, 2010
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Wrong thinking. The charger should only be used to charge the battery, NOT to power the UPS (which it won't be able to do so anyway of the power is out....).

The usual method is to have a charge rate of around 10C (i.e. 1/10th the capacity of the battery) - they sometimes use different 'C' values, check with the battery manufacturer for their recommended value. In other words for a 9Ahr battery you would charge it at 0.9A (call it 1 amp) for 9 or 10 hours (from flat so the usual recharge time will be a lot shorter).

A standard run-of-the-mill lead-acid (presuming it's a sealed lead acid cell?) charger suitably rated could be connected permanently to maintain the battery. As said, in the event of mains failure the charger 'stops' and the UPS runs solely off the battery for however long it can sustain it.

If you want to run the PC via an inverter permanently (called an 'online' setup) you will, as you say, need the charger to deliver the full amps (and using say 480W as a guide - to account for losses etc - you will need 40A!!!). Seems to me to be overkill. Perhaps you need to rethink your purchase methodology and get a UPS that has guaranteed change-over (less than or around 20mS) which should deliver an uninterruptible service.

A lead-acid charger with a 40A rating won't be easy nor cheap to source and a DIY version no better - that said you can get ex-server power supplies that deliver 12V @ 50A and upwards for 'pennies' and there are Youtubes describing their modification for 13.8V output (necessary to charge a battery).

Another 'bodge' would be to fit a set of large smoothing capacitors across the PC PSU main power lines (+12V, +5V and +3.3V) to give a better 'buffer' to supply glitches.
 
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torveo

Sep 28, 2021
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I have a few questions.

The charger should only be used to charge the battery, NOT to power the UPS
So how would I power the inverter and charge the battery at the same time? Would I need two separate 12VDC circuits? One for charging the battery, and the other a high amperage 12V source to power the inverter when AC is available. Then separate the two with diodes and/or a relay which closes when the high power 12V circuit voltage drops to ~0v?

(which it won't be able to do so anyway of the power is out....)
That's why there is a battery there which will supply 12v DC to the inverter when the charger loses AC power.

The usual method is to have a charge rate of around 10C (i.e. 1/10th the capacity of the battery)
Did you mean 0.1C? From what I understand, 10C, assuming 1C means 1x capacity, would be 10x the batteries capacity in charging amps.

If you want to run the PC via an inverter permanently (called an 'online' setup) you will, as you say, need the charger to deliver the full amps (and using say 480W as a guide - to account for losses etc - you will need 40A!!!).
Yes, this is exactly what I am aiming to do. I was looking at something like this for the charger. But going back to my original question, would this damage the battery? In your first paragraph you stated that I should not do this. But here it seems like you're saying it would be OK, just expensive to source? Could you please clarify?

A lead-acid charger with a 40A rating won't be easy nor cheap to source and a DIY version no better - that said you can get ex-server power supplies that deliver 12V @ 50A and upwards for 'pennies' and there are Youtubes describing their modification for 13.8V output (necessary to charge a battery).
Personally I am okay with a high initial cost since the line-interactive UPS units I have used in the past were 300+ Canadian dollars and were absolute trash. Advertised 8ms switchover times, but still caused PC's, and even a TV I was testing to reboot during power outages and even during their automated self tests with no AC utility fault! (super infuriating). And this is with multiple different units from different brands and RMA's.

On-line double-conversion units from triplite and competitors are priced at just under 1,000+ Canadian dollars for the same 600W I would be getting with my DIY setup. So even if I have to pay more than the cost of the line-interactive units for my setup, I would still be happy since I'm getting the quality and reliability of zero-switchover time and paying less than the off the shelf on-line units cost.

Another 'bodge' would be to fit a set of large smoothing capacitors across the PC PSU main power lines (+12V, +5V and +3.3V) to give a better 'buffer' to supply glitches.
This is interesting... Though, I would rather avoid modifying my PSU unless absolutely necessary. If my DIY plan fails, I will definitely consider this, however.
 

kellys_eye

Jun 25, 2010
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Did you mean 0.1C?
Yep, my bad.

Regardless of the PSU used to run the inverter, the battery will only take what it needs. Just coz the charger is rated at 40A doesn't mean it constantly outputs 40A. Think of it in the same way your car alternator (150A) charges the battery and runs the car electrics - it's not charging the battery at 150A!

Of course, there's nothing wrong with using two PSU's, a charger and a heavy-duty one for running the inverter but that's what an on-line unit does anyway - well, the decent ones that is...... there are loads of quality UPS devices on eBay (used) that inevitably have dud battery banks (I use a couple of APC 750W jobbies in my workshop, got for free due to dud batteries and now running on external car batteries). Have a look locally as sellers will often only do 'buyer collects' as they cost a fortune to ship.

Also, have you looked at any internal UPS cards?? These were commonly used to deliver just enough power to (correctly) shut down a PC (saving open files etc) in the event of power failure. PCI-sized plug-in card with appropriate software to 'do the business'.
 
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