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Forum Index : Electronics : small scale plug in solar/mains UPS
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| greybeard Senior Member  Joined: 04/01/2010 Location: AustraliaPosts: 161 |
I've done a search and haven't found much on this topic. At this stage it's an idea I wouldn't mind trying and it seems like something that most of the components should be allready out there and just need connecting together or it may allready exist. Basically it consists of a battery charger with solar input, a mains backup input ( mains comes on when battery is low ), a battery and a DC invertor to provide a small mains power source. The system would be scaled to normally use solar to recharge the battery and provide enough energy to run 24*7 but if the solar output was insufficient to keep the battery charged then mains would kick in to recharge the battery. It would be a simple way for someone to dip their toes into a solar powered system without having to rewire their homes. The component I can't seem to find at the moment is a dual input battery charger that will give priority to solar and only use mains if the battery is low. I can find chargers that will switch to mains if the solar fails but that would just mean the mains would start charging the battery every night and not use the stored capacity of the battery. My initial application would be to power my home servers/firewalls as they only draw ~100-150W and this is something I could just 'plug and play'. Has anyone done this before and/or seen such a system? |
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| greybeard Senior Member Joined: 04/01/2010 Location: AustraliaPosts: 161 |
And a pic to show what I mean. At #1 the solar starts charging the battery. At #3 the battery is charged At #4 the sun goes down Invertor runs on battery until #6 when the sun comes up and the solar starts charging again. At #8 it gets cloudy and the solar stops charging and the invertor is on battery. At #9 the battery voltage is low so the mains charger starts to charge the battery. At #11/12 the sun comes up and the solar takes over charging again. Pretty much a mains charger that only starts when the battery is below a preset value and then stops charging when the battery voltage is above a preset value and/or there is enough solar power to take over charging. |
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Wombat Regular Member  Joined: 27/05/2011 Location: AustraliaPosts: 72 |
Sounds like you need a UPS style inverter with gen start. Just hook the gen start to your mains switching and your done. 
Look on Ebay and you can find some cheap units. You get what you pay for.  find a local suppler.
I run them at half power max. 
Regards Russ.  |
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| greybeard Senior Member Joined: 04/01/2010 Location: AustraliaPosts: 161 |
Wouldn't a gen start try to go active as soon as the solar input drops off? The auto start/failovers I've worked with in data rooms will start the generator whithin a few seconds of a mains fail. I'm trying to defer the mains connection as long as possible and run on the batteries until either solar is restored to recharge or mains has to be used to recharge the batteries. I know it isn't really the usual purpose of a UPS but I'm looking at it as a possible way of reducing mains use and replacing it with solar as much as possible. |
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| Privatteer Newbie  Joined: 09/06/2012 Location: AustraliaPosts: 39 |
Basically what I have with my system. I looked quite hard for a system that does what you want and did not find much so built my own. Normal solar regulator. Lets assume 24v. Find a current limited DC power supply that you can set to about 25.8V. Install a diode of appropriate current rating on the positive terminal to prevent backfeed into the DC power supply. Personally I used 2 in parallel at twice the current rating as they do get quite hot and are cheap. Diode creates 0.6v voltdrop so 25.2v seen at the battery. If sun/solar is available battery voltage rises to float of around 27.6v. With no sun the battery slowly discharges to it reaches 25.2v. At that point the DC power supply starts supplying current. Obviously you have to size the DC supply to your load plus bit of headroom. Since you not discharging batteries fully they should last longer as well. Example powersupply |
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| greybeard Senior Member Joined: 04/01/2010 Location: AustraliaPosts: 161 |
Cheers for that. I'm just waiting on some 12V power supplies for my servers to arrive to confirm power draw before I scale the system. |
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Downwind Guru  Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Its not a good practice to parallel diodes, as no 2 diodes are exactly the same and the weaker one will do most of the work. Better to use just 1 diode of a suitable rating, or even 1/2 a bridge rectifier as these are cheap with good current ratings and also able to be heat sunk. Pete. Sometimes it just works |
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| Privatteer Newbie Joined: 09/06/2012 Location: AustraliaPosts: 39 |
Agreed a diode with heatsink would be better. Its surprising how hot a 10A diode (without heatsink) gets caring just 4Amps continuously. I mainly added the 2nded diode for fault protection but the few I tested carried the load fairly evenly. A bridge rectifier is really 4 diodes in a block... |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Well thats a supprise? otherwise it would not be a full bridge rectifier. 
I was suggesting to use just 1 of the 4 bridge diodes and a suitable heatsink. Sometimes it just works |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I am thinking that at 25.2 volts the batteries will not discharge at all, 25 - 25.2 is the resting volts for most batteries anything higher than that is usually residual surface charge on the plates. The batteries will only start to cycle when the mains goes down. Would an alternative be to add a voltage switch ? This would give good control of the power supply's on voltage and let you decide how much the mains can add back to the batteries with the hysteresis adjustment before it switches off. example switch on at 24.68 volts (calculate using resting volts state of charge table) switch off at 27 volts (calculate using charging volts state of charge table) I'm confused, no wait... maybe I'm not... |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I thought the same, till i worked out it was never meant to charge from the mains and the mains was only a backup supply when there was no solar or battery power. The 25.2v was just to supply the inverter via the battery, and hence the diodes were to prevent the solar back feeding to the power supply. Sometimes it just works |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
I don't dispute the reason for the diodes. I am saying that it wont perform the task that greybeard is thinking of, specifically I can see that the inverter is being supplied from the mains at night. Doesn't the voltage need to be set lower than 25.2V for the battery to go below 100% SOC? Then it would certainly provide a state of charge floor that the batteries would not go below and the solar would recharge them the next day. That is still not the same as lifting the charge level from 55% SOC to 75% SOC from the mains during the night, after the odd very cloudy day. After thinking about it a bit more, the switch on voltage in the example I gave in the last post should be lower than that. It should take into account the load from the servers on the battery drawing the voltage down. I'm confused, no wait... maybe I'm not... |
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| Privatteer Newbie Joined: 09/06/2012 Location: AustraliaPosts: 39 |
The exact levels are going to depend on your battery but the idea is an UPS setup where you are not discharging the batteries to any significant amount. Greybeard will have to decide how much reserve capacity he wants to leave for a mains failure event and pick the dc voltage based on that. |
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| greybeard Senior Member Joined: 04/01/2010 Location: AustraliaPosts: 161 |
I seem to have created a monster 
Starting again. Ignore the UPS words. I've got a low power server and a firewall, some batteries, dc-ac invertor, solar panels, solar regulator and a mains charger. Normal ops: the server/firewall run 24x7 DC-ac invertor powers server/firewall Batteries power invertor. Batteries get charged via solar during day. System is good and no mains is used. Abnormal ops: Batteries have discharged due to insufficient solar. Then and only then, mains charger starts to recharge batteries ( and power invertor ). If the batteries are low and the mains has failed then the system stops. So if I get a couple of days of cloud and rain and then the mains fails, so do the computers. The mains charger should never normally operate. Solar should provide day to day power. I am waiting on some more equipment / components to arrive so I can calculate actual power requirements and then do the relevant calculations to scale the power usage and storage needs. The concept of monitoring the battery voltage and the mains charger supplying power only on low battery until such time as the solar charging system can take over operation again is the key as I see it. Being able to use off the shelf components is also important. The same concept could ( if it's cost effective ) be used on other household systems to allow a 'plug and play' conversion to solar for people to experiment with. Thanks for the ideas guys. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
A "normal" UPS should do the job, but you will need to be very careful about checking the exact specifications of the system, and the exact mode of operation. One thing you don't want is a fully charged battery first thing in the morning. Normal mode of UPS operation is to always keep the battery topped up at full charge. If you held the battery at around 50% charge on the charger, there will be both some reserve capacity if the mains fails, and also extra capacity to absorb additional solar charging. This is going to take some data logging and experimentation to get the system working as intended, but it should be possible. All things considered, it may be better to use an independent battery charger, battery, and inverter than a self contained modular UPS. That way, you have more control over sizing and adjusting how the various parts work together. Cheers, Tony. |
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jebz Regular Member Joined: 13/06/2011 Location: AustraliaPosts: 79 |
I think you're describing something like a Jaycar SuperCombi Power Management System |
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| greybeard Senior Member Joined: 04/01/2010 Location: AustraliaPosts: 161 |
I'll give it a go if you pay for it
I've a suspicion that the $ cost of the panels and storage requirements may just be a show stopper. I'll crunch the numbers again when I've got some better data from the servers. And/or it'll inspire me into another round of consolidation and power reduction on my system setup. |
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