Charging batteries with Solar ...

[CaTcH21]

Can anyone tell me if this is possible and if yes, how do I go about ?

 

I have a UPS that has a built in 15A charger. Now if my UPS shuts off due to low battery voltage, it would be great if I can get Solar to charge the batteries while AC is out.

 

The question is can this be done ? Meaning, obviously when AC power is online , that would mean the UPS charging the batteries together with the Solar.

 

My concern is overcharging of batteries.

 

Any input is appreciated.

[Clint]

This a difficult one.

I would get a charge controller for the solar and connect it to the batteries via a switch.

You will have to switch off the mains otherwise it will fail

 

Or you could use a 220 V relay switch that connects the solar when the mains are off.

 

My 2 c

[CaTcH21]

Thanks Cint for the information.

 

I wonder if anyone has done this before ... hmmm ...

[Wetkit]

Your biggest problem is the battery voltage.

Solar charge controllers only go up to 48V DC, where UPS battery voltage can go up to 300V DC!!!

 

If you UPS battery voltage is 48V or lower, an MPPT solar controller would work well.

It should work together with the built-in charger.

Thing is, the solar will only work if Eksdom power is off.

When normal power is back, solar will stop charging.

[CaTcH21]

Hi Wetkit,

 

I actually don't mind it right now where the Solar charges only when City power is cut off. My UPS is a 48VDC system.

 

So how I connect this. I have the Solar panel wired to the controller, and then wired to the batteries ?

 

Also the battery maximum charge current is 30A , so would this mean I need a 30A controller or can I buy a bigger one ?

 

Thanks again for the input/advice.

[Wetkit]

Connection is from PV to MPPT and then to the battries. I would also install an extra fuse between the MPPT and the battries, just to be safe.

 

If you need 30Amp at 48V then you going to need about 1500W of PV.

Now PV efficiency is very poor, so you need about 3000W PV to ensure that 30Amp constantly!!!

Yes, you can get bigger MPPT than the 30Amp. You should be able to limit the charge current by software settings or by your PV size.

 

In the end it depends on how much money you will to spend and if you doing this for future expansion or just for the UPS.

[CaTcH21]

Thanks for the information Wetkit.

 

I decided to buy a new 5kw Hybrid inverter. The only problem with this specific inverter is that it can only operate in Hybrid mode ( meaning auto switching between PV-DC-Grid) is if I have the controller built-in . The controller is not a MPPT controller but PWM.  Now I have heard that MPPT is far better than PWM , but now in this case would it be a train smash if I end up with a PWM ?

[SolarShack]

Just to put my 2 bits in, there are more variables just to throw a spanner in the works.

you get wet Cell batteries, you get Gel batteries, Deep cycle and so on. these all have their ( Preferred ) way of being charged.

 

best is to " know" your batteries.

[SilverNodashi]

Thanks for the information Wetkit.

 

I decided to buy a new 5kw Hybrid inverter. The only problem with this specific inverter is that it can only operate in Hybrid mode ( meaning auto switching between PV-DC-Grid) is if I have the controller built-in . The controller is not a MPPT controller but PWM.  Now I have heard that MPPT is far better than PWM , but now in this case would it be a train smash if I end up with a PWM ?

The PWM charge controller will work, but not as efficient as the MPPT charge controller. i.e. you may not necessarily get 30A, but perhaps more like 26A. 

 

I can't see why what you want todo won't work. The only problem / caveat is, how big is the UPS battery bank, and is it really worth-it?

[Guest]

Note: UPS inverters are not designed to run extended periods.

 

If you invest in solar panels, you need to use as much of what they produce, as you can.

 

See here what I did, for I also started off with the idea of charging a UPS battery bank: http://powerforum.co.za/topic/281-off-grid-eskom-as-backup-and-batteries-as-last-resort/ 

[superdiy]

Note: UPS inverters are not designed to run extended periods.

 

It depends if it is a line interactive or online design. Online UPS inverters run all the time. Line interactive only kick in when required.

[Guest]

 It depends if it is a line interactive or online design. Online UPS inverters run all the time. Line interactive only kick in when required. [/quote]

 

Quite right.

 

My experiences to date dictate that most online UPS'es cost a arm and a leg and most, if not all, can only handle limited periods of time like 10-15min at full power. You can extend their run-time by adding external battery packs, but at -+R5k per pack, it quickly becomes very expensive.

 

Most use 7-9ah batteries (and lots of them) so a APC online UPS (800w) @ +-R8k plus extension pack @ +-R5k per pack = +-R13k for 800w to run maybe 1-2 hours for the first +-10 times before the batts go.

Eaton 3000w range, you are looking at +-R30k with extended battery pack ... not very cost effective solutions.

 

Nor are they very efficient inverters either. Solar inverters are 95%+ efficient versus online inverters being +-85% efficient.

 

BUT!!! If you want to protect equipment for a few minutes whilst the gennie is started and settled, they are the best solution.

 

 

Because of users having to replace 8-18 small 9ah batteries, I got a few of them in that people chucked, APC and Eaton. Was very excited for I can use the inverter!!! Problem was not one could charge large ah deep cycle batteries.

 

Having said that, if you have a 24 or 48v one, as a start (before you buy a real inverter) you can use the online inverter by disconnecting the unit from Eskom and charging the batteries from solar only, but not if it needs a 96v battery pack. That is very expensive ito batteries and solar panels.

Yes, I am sure you can find better ones today, but at those prices, it is more cost effective to go solar inverter.

 

As you can gather, I tried very hard to make them work for me. 

[Guest]

The reference I made to UPS'e not being able to run extended times, are because I tried very hard to use a UPS inverter, instead of a solar inverter.

I mean, UPS'es have been around for a very long time.

 

Every single UPS company I spoke to at the time, when asked if I can run their inverters for +-8-10 hours every day, said no, don't do it. They are designed for intermittent use, not extended uses.

 

A simple test a wise old toppie once suggested I do is to feel the weight difference between say a 1200w UPS inverter (like Inverex, Ellies etc) versus a 1200w solar inverter (like i.e. a Victron). The difference in weight is massive.

The heavier the unit, the more copper tends to be inside, the longer it is designed to operate under load.

 

Most UPS'es are designed to run +-2-4 hours today, maybe next week or next month another few hours, and so on. Not 8-10 hours every single day. And the small little UPS you get for under R1000 ... they are designed to last the length of the little 7ah battery, few minutes. Waste of money.

 

Rule of thumb: UPS'es are designed to run as long as their batteries hold. If you can add extra batteries, that is fine. If you cannot, there is a reason for it.

[superdiy]

A simple test a wise old toppie once suggested I do is to feel the weight difference between say a 1200w UPS inverter (like Inverex, Ellies etc) versus a 1200w solar inverter (like i.e. a Victron). The difference in weight is massive.

The heavier the unit, the more copper tends to be inside, the longer it is designed to operate under load.

 

Not entirely true. Some inverters use high frequency switching (e.g. 100KHz), as in a switch mode power supply and others use low frequency (typically 50/60Hz). The transformers used in HF inverters are small and very light compared to the transformers used in LF inverters of the same rating. These inverters using HF are also sometimes referred to as "transformerless", becauae of the small HF transformers used. Obviously the bigger transformers contain more metal (iron and copper) than the small transformers used in HF inverters and that will influence the weight. The HF inverters are newer technology, which was perhaps not around in the "old toppie's" younger years, but it is not a true measurement of how "long it can operate under load".

[Guest]

Your make a very good distinction: Do not compare old and new tech.

 

But the old toppie was en is nog steeds reg, if you compare LF transformer with another LF transformers weight. :-) 

[___]

I would think most or all of the new hybrid inverters would be high frequency designs. That is, they have an input stage using smaller ferrite transformers running at a higher frequency, then rectify it back to DC (probably 325VDC and more), then you can switch that onto an existing waveform in much the same manner that a GTI does it... or run independently using your own reference frequency.

 

With HF designs, you look at the quality of the mosfets, the size/number of the transformers, and whether the input stage mosfets are tied to a proper heatsink, or where the case serves as the heatsink, whether they even make proper contact with the case, does the fan suck out or blow in (iirc blowing into the case is better, because it causes more air turbulence), and does the case have proper airflow. So the old toppie is probably still right on some level: You can judge it on the amount of aluminium tied to your switching transistors :-)

[superdiy]

 You can judge it on the amount of aluminium tied to your switching transistors :-)

 

The more the losses in the mosfets (R_on), the more heat they generate and the bigger the heatsink(s) required to keep them cool. 

[___]

Indeed, another test is to look at the MOSFETS. If it has that nice backwards IR on them (International Rectifier) you're in good company. Infineon is also good. If you look it up online and you can't find the exact number, just one that's vaguely similar to it, it's likely a chinese knockoff.

 

MOSFETs are interesting buggers. Ever wonder why the price of a charge controller doubles as your open circuit voltage goes over 50V? Because have you seen what a MOSFET with a high reverse voltage and low R_on costs? Have you seen what Schottky diodes of that type costs? You can get it cheap as long as you're willing to give up on one of those things... :-)