Inverter Maximum Input Voltage

[Dave3011]

I have silver calcium batteries in my off grid PV system and as such need the absorbtion voltage set to 29.8 volts. Monthly equalisation charge is set at 31.2 volts.
Question is can the inverter handle this?
The inverter spec sheet does not give a maximum DC input voltage and only specifies 24 volts as the nominal voltage.
I've once had it running with the voltage reading 29.2 volts and it did sound like it was under a bit of distress but no fault alarms were sounding yet.

Any ideas on what I should do if I'm worried? Would putting a diode or two in line from the battery to the inverter help as I expect it would reduce the voltage when it's running that high? I would then I guess have to put a bypass switch in as well so that I can bypass the diode(s) when running off the batteries at night so the voltage doesn't go too low. I'm assuming the diode idea won't result in any significant net-loss in power - right or wrong?

Any advice please?

[daz]

Question is can the inverter handle this?
The inverter spec sheet does not give a maximum DC input voltage and only specifies 24 volts as the nominal voltage.

Perhaps have a look online to see if you can find more information on your inverter. Alternatively get hold of the supplier/retailer that you bought the inverter from to try get some more information.

Would putting a diode or two in line from the battery to the inverter help as I expect it would reduce the voltage when it's running that high?

Diodes have a forward voltage drop, which as you refer to, drops the voltage by a small amount. However this is not practical, as you would need significantly large diodes to be able to handle large currents that the inverter draws...so using diodes would not be recommended...and.......

I'm assuming the diode idea won't result in any significant net-loss in power - right or wrong?

Any "reduced" or "lost" voltage is a loss of power (W = I x V). So if your diode had a forward voltage drop of 0.7V (aka a generic silicone diode), and your inverter was drawing 15A through the diode(s), you would have lost: 0.7V x 10A = 7W (per diode used), or roughly 4% power loss if on a 12V system.


It would be far better to find out as much as you can about your inverter instead of trying to place diodes in the system as first choice, as that will cause unnecessary loss and complications in the system. Perhaps go onto your inverter's manufacturer's site and see if they have datasheets available there.

From personal experience, I have found that UPS inverters have a much lower input voltage tolerance compared to off-grid/stand-alone inverters. However it would still be better to find out what the maximum input voltage is (to avoid any headaches in the future!).

Does your inverter have an alarm or protection of some kind for over-voltage on the input?

[Dave3011]

Hi Daz,

I see you're in South Africa too so perhaps you'll know better about the type of inverter I have. It's one of those UPS type inverters I got at Ellies (2000VA/1200W). It has no specific brand but from what I've managed to find out is that it is probably a chinese make that is sold all over the world under different brand names. Inverex is the closest I have come to finding out who makes them. Anyway finding reliable information on the inverter is very difficult, even Ellies seem a bit stumped when approached on the matter.

Obviously a true sine wave solar inverter would be the best option for upgrade but at this stage my budget MSW UPS inverter is doing the job just fine.

Yes I agree on the problems of using diodes, and also prefer not to because it will prevent using the UPS to aid charging the batteries on cloudy days.

D

[daz]

I see you're in South Africa too...

Nice to meet another SA chap!
Whereabouts in SA are you? Im actually in nice sunny Durbs!

so perhaps you'll know better about the type of inverter I have. It's one of those UPS type inverters I got at Ellies (2000VA/1200W). It has no specific brand but from what I've managed to find out is that it is probably a chinese make that is sold all over the world under different brand names. Inverex is the closest I have come to finding out who makes them. Anyway finding reliable information on the inverter is very difficult, even Ellies seem a bit stumped when approached on the matter.

We have not had very good reports about Ellies solar products. Most of those products that are brought to us, or that people mention they have, is not very good. The quality of product itself is the short-coming...especially the little solar kits they sell...not very good.

Obviously a true sine wave solar inverter would be the best option for upgrade but at this stage my budget MSW UPS inverter is doing the job just fine.

Deciding whether to go with TSW or MSW depends on what it is that you want to power. If it is just general electronics like LCDs, computers, lighting, etc, MSW is fine. For motors, CRTs, microwaves, etc it is better to go with TSW as it is far better. This is especially true with regards to motors.

What are you powering with your inverter? What size solar array do you have? What type of charge controller are you using?[/QUOTE]

Yes I agree on the problems of using diodes, and also prefer not to because it will prevent using the UPS to aid charging the batteries on cloudy days.

You say that you are charging the batteries via the UPS on cloudy days...how are you isolating the charge controller/solar side of things when charging via the UPS?

[Dave3011]

Hi Daz,

Nice to meet you! I'm in PE, sort of sunny only half the time.............

Yes I'm busy grappling with what started off as just a small UPS system (No Solar panels yet) for lights and a TV and has subsequently grown into a medium sized solar PV system. The growing pains have been many - especially with the batteries...!

I'm currently running an 880 watt panel array running at 36volts through a 30Amp charge controller (also Ellies unfortunately). The battery bank is 24volt/306AH (6x102AH 12v in 3 parallel strings of 24v). The charge controller and inverter are by far the cheapest parts of the system and could easilly be upgraded in future fortunately!

Right now I'm running 3 LCD TV's, a Hi-Fi, Fan and several energy saving lights off the system. The power demand/supply balance is very good in sunny weather but falls short in cloudy weather. The quality of the modified sine wave is actually very good compared to the inverters they sell in car shops like Midas, there is no noticible buzzing on any of the appliances except the fan which only makes a very soft high-frequency buzzing noise if you listen very closely.

I am looking at upgrading to a true sinewave inverter by December and an MPPT charge controller, but for now am just looking at getting the most out of what I have.

Yes again - the growing pains of expanding from something very small are well known.........

Regards,

D

[daz]

Nice to meet you! I'm in PE, sort of sunny only half the time.............

Ah! Just down the road hey!

The growing pains have been many - especially with the batteries...!

Batteries batteries!! The achilles heel of any system!
Most people get caught, especially in SA, of using the wrong type of battery for solar systems. Most end up buying high-cycle/marine/leisure batteries...which do not last in a solar system. We have even had a few people wanting to use their automotive batteries... ...

Most of the guys looking at solar look at the cost of the batteries, and end up going for the cheaper batteries, which are not really suitable for solar. For solar you need a proper deep-cycle battery with thick plates...otherwise you will end up replacing dead/damaged batteries periodically!

The battery bank is 24volt/306AH (6x102AH 12v in 3 parallel strings of 24v). The charge controller and inverter are by far the cheapest parts of the system and could easilly be upgraded in future fortunately!

See the above in bold...
You should not be paralleling batteries up as far as possible, as it affects their charge/discharge and life. Generally the outside banks work harder than the inside bank, which causes them to fail earlier, leaving the last remaining good bank taking a lot of strain...causing your batteries to fail earlier.
If you have to go with parallel strings, you need to connect the charge/discharge leads on opposite ends, as well as ensuring that the battery interconnects are identical (in resistance). It is far batter to use a higher voltage battery bank in series as opposed to paralleled batteries.

Having a look at your batteries, it would be better to go with a 48V battery bank, as you will have a much more balanced system, as well as less losses! Also you are looking at going for an MPPT controller, so you need to aim for a more efficient system (so that you benefit from the MPPT).

NB the 102AH batteries are high-cycle batteries intended for UPS applications. Due to their very very low cycle life, they are not suitable for an off-grid solar system. Those 102AH batteries used in an off-grid application every day to 50% DOD, would yield an expected lifespan of 10 -15 months......
They will however last a bit longer if discharged to a lower DOD. The 102AH batteries are really cheap, and retail for R1,300 (they are cheap because they are intended for UPS applications).

Have you replaced the batteries yet? If you have, what was their lifespan?

I'm currently running an 880 watt panel array running at 36volts through a 30Amp charge controller (also Ellies unfortunately).

880W @ 24V is 36.6A......so you would need to go with a minimum of a 40A MPPT controller @ 24V, or you have to rewire your battery bank for 48V, in which case you can use a cheaper 20A or 30A (better) controller plus you will have less system losses!

880W @ 36V is 24.5A, what size cabling are you using? What is the distance the cabling is running to get to the charge controller?

Right now I'm running 3 LCD TV's, a Hi-Fi, Fan and several energy saving lights off the system. The power demand/supply balance is very good in sunny weather but falls short in cloudy weather.

To be able to calculate what size system you need, as well as how long your batteries will last, you need to list the appliance's wattage as well as hours used. Then we can actually work out what size system you should be aiming for, as well as how long your batteries would last supplying that load. Do you know the wattage rating of the appliances above?

The quality of the modified sine wave is actually very good compared to the inverters they sell in car shops like Midas, there is no noticible buzzing on any of the appliances except the fan which only makes a very soft high-frequency buzzing noise if you listen very closely.

I am looking at upgrading to a true sinewave inverter by December and an MPPT charge controller, but for now am just looking at getting the most out of what I have.

The ones sold in Midas are automotive inverters with a very low duty cycle...aka they are really cheap Chinese! They work, but dont expect long life or miracles from them!!! A UPS inverter is far better than these cheap units!

True sinewave inverters are very pricey! For example, a Cotek 2KW 48V true sinewave inverter retails for about R12,000...so you will need to see what it is that you want to power with it, and see if it is worth it. For most, the MSW are fine (with a few exceptions such as motors).

Yes again - the growing pains of expanding from something very small are well known..........

Eish! Thats all we can say!
Where did you purchase your equipment from? Did they advise you with regards to what you would require? Or how to go about installing?

[Dave3011]

Daz,

Thank you for all your input - there is so much to learn!!!

My whole system has been a DIY job and I've used my background knowledge of electronics to help me through so far.

You asked about wattage and power consumption - well I've got one of those power consumption meters that I plugged into the inverter AC output to monitor the usage. It hardly measures more than 2 - 2.5kw/h overnight which is when the batteries are being drawn upon. So taking into account all losses I could assume that it is not cycling the batteries more than 50% and is probably closer to 30 or 40%. I have a habit of switching off the inverter during the day when I'm not at home so that the batteries can take 100% of the PV power when I'm not at home and at the office. If the PV production is low like on a cloudy day then I switch on the UPS in mains mode to help charge the batteries. The UPS is connected directly to the batteries and not through the "load" terminals on the PV charge controller.

Yes I do know that using multiple parallel strings is not the best way of doing things but in this case I have to as these batteries were purchased in ignorance of what I know now. They are only a month old. Started with only 2, then added another 2 and then another 2 all in the space of a month!!! I have wired them with double 10AWG tinned copper solar cable and fused with 80AMP fuses. The charge and load are connected to the middle string. I've rotated and tested the batteries with a voltage and curent load tester twice in the last month and all seem reasonalbly eqaul in ability. I plan to rotate them once every 2 to 3 months from now on.

I have noted that I would have paid nearly R20k for similar capacity if I went along with the best batteries on the market. You are right these batteries (FNB SMF100 batteries) are cheap - I got them all for R7500. But I'd be really stoked if I can get a few years out of them before replacing with something more suited. I've no doubt that by then my system would be bigger and would need a significant upgrade anyway.

I'm not getting the full 880 watts from my PV's right now as the PWM controller simply takes whatever amps the modules offer and just regulates the voltage. On a clear winters day I'm seeing about 24 amps at noon. On a cloudy day I've seen the amps surge to nearly 30 amps after the clouds pass but this only lasts about 30 seconds before settling at 24 amps.

Accepting inevitable losses in the wiring (15 meters from modules to batteries) I opted for a 36v array to give me the marigin needed to ensure the batteries get the required voltage. Future upgrades will see me changing the whole system to a higher voltage as you have advised. I could configure the panel voltage to 72 volts and the battery vontage to 48 with a new inverter and charge controller.

D

[daz]

Daz,
Thank you for all your input - there is so much to learn!!!
My whole system has been a DIY job and I've used my background knowledge of electronics to help me through so far.

The learning never stops! At least you have a background in electronics, so that will help when it comes to understanding some of the terms and calculations!

You asked about wattage and power consumption - well I've got one of those power consumption meters that I plugged into the inverter AC output to monitor the usage. It hardly measures more than 2 - 2.5kw/h overnight which is when the batteries are being drawn upon. So taking into account all losses I could assume that it is not cycling the batteries more than 50% and is probably closer to 30 or 40%. I have a habit of switching off the inverter during the day when I'm not at home so that the batteries can take 100% of the PV power when I'm not at home and at the office. If the PV production is low like on a cloudy day then I switch on the UPS in mains mode to help charge the batteries. The UPS is connected directly to the batteries and not through the "load" terminals on the PV charge controller.

Your batteries can supply 7,344WH of which the maximum you should use is 3,672WH (50% DOD). At 2500WH you are running at about 34% DOD. If I remember correctly on those 1250's, that means you should get about 450 cycles, or roughly 1 - 1.5 years worth of life out of them.

The charge and load are connected to the middle string.

You need to move the charge/discharge cabling to opposite sides of you battery bank, to help try balance the resistances. Having it connected in the middle means you are using the centre two batteries more than you are using the outer two batteries (due to the resistance in the connecting wires). By having the charge/discharge connectors on opposite sides of the battery, it helps balance it a bit more.

I have noted that I would have paid nearly R20k for similar capacity if I went along with the best batteries on the market. You are right thes batteries (FNB SMF100 batteries) are cheap - I got them all for R7500. But I'd be really stoked if I can get a few years out of them before replacing with something more suited. I've no doubt that by then my system would be bigger and would need a significant upgrade anyway.

Yes, the proper batteries are a bit expensive...but they last a lot longer! In any case, you have your training batteries....so when you get the good ones...you will be up to spec on what needs to be done!

I'm not getting the full 880 watts from my PV's right now as the PWM controller simply takes whatever amps the modules offer and just regulates the voltage. On a clear winters day I'm seeing about 24 amps at noon. On a cloudy day I've seen the amps surge to nearly 30 amps after the clouds pass but this only lasts about 30 seconds before settling at 24 amps.

PWM controllers : current in = current out. So with a PWM controller you are operating at roughly 65% efficiency. With MPPT you get up to roughly 95% efficiency.

So with PWM: 880W / 36Vmp = 24.44A x 24V = 586.67W is what you are actually going to get out of your system max.
With an MPPT controller: 880W / 24 = 36.66A....or 880W (minus losses)....minus roughly 5% for controller inefficiency, you are looking at 836W out of your array.

Accepting inevitable losses in the wiring (15 meters from modules to batteries) I opted for a 36v array to give me the marigin needed to ensure the batteries get the required voltage. Future upgrades will see me changing the whole system to a higher voltage as you have advised.

You should be seeing at least a minimum of 32V in at the controller to ensure that the batteries are charged properly.
You mentioned something about 10AWG cable being used (was that just for the batteries or for the array as well?). 10AWG is roughly 2.5mm (if I remember correctly), which has a maximum current carrying ability of 27A @ 20'C. With 2 x 2.5mm together, you are looking at roughly a 5V - 6V drop in line losses on a 24A current (over the distance from the array to controller).

What size cabling did you use for the solar array to controller? What is your input voltage to the controller?

I could configure the panel voltage to 72 volts and the battery vontage to 48 with a new inverter and charge controller.

Just a word of caution, a lot of the cheaper MPPT units (especially the Chinese ones) have a maximum voltage input of about 50V, so you will need to select your controller carefully. You will need to go for the controllers with a 150V input here.

[Dave3011]

Thanks Daz, very informative stuff!

I'm using 10AWG (2.5mm core diameter correct!) double insulation all around. I doubled them up on the batteries to be safer. Yes I did the ohm's law calculation and came up with a 5v drop from the panels to the batteries which was just about ok. The cables become ever so slightly warm when the panels are producing near full capacity.

I am definitely going to follow your advice on connecting the load and charge at opposite ends of the strings as you say - however I'm quite amazed that it would actually make such a difference as the resistance on the connecting wires is so small - they are only about 20cm between strings and are doubled up - but if in your experience it does make a difference then I wont argue the point

Tell me something - you mention multiplying the amps by 24 to get the watts (24.44A x 24V = 586.67W ) however what I'd like to know is that when the charger is feeding say 24 amps at 29volts into the batteries (near the completion of the boost phase), what is the wattage banked? I meen 29vx24A=696watts right? There is a significant amount of time where the voltage is above 24volts during charging so is this excess voltage useful of is it simply the amper-hours that matter?

Again thanks so much for your input!

[daz]

Thanks Daz, very informative stuff!

No problem. Glad to be able to help!

I'm using 10AWG (2.5mm core diameter correct!) double insulation all around. I doubled them up on the batteries to be safer.

2.5 can only handle 27A max @ 20'C, so I suppose theoretically 2 lines could give you a max of 49A, although you will have to derate for heat. Even at 49A though, that only gives you 1.17KW peak power, and I wouldnt operate at anywhere near the theoretical limits of the cabling. So lets say we knock off 30% for heat/losses/etc, that leaves you with a max peak power of 800W of watts.


The actual cable size that you require will depend on your inverters rating.What is the wattage of your inverter?

It may also be better to go with either 6mm or 10mm cable (I am not sure what that is in AWG!), as then you would have a 47A (6mm) or 64A (10mm) to work with. As you say that you can feel the cable heat up, it would be advised to replace the cable with a bigger gauge, as warm cables are never a good sign (rather go with something a bit bigger than right at the limit)! As cables warm up, they carry less and less current, and exceeding the maximum current level (at that specific temp range) raises the potential of an electrical fire. Eish! Most of the cabling sold here is rated at 20'C conductor temp, and 30'C ambient temp. As SA is a sub-tropical country (on the coast), we generally have fairly warm temperatures, so you have to be very careful about the temp when calculating for cable sizing!

Yes I did the ohm's law calculation and came up with a 5v drop from the panels to the batteries which was just about ok. The cables become ever so slightly warm when the panels are producing near full capacity.

The 5V drop brings you very close to the minimum threshold, as you will also lose some forward voltage through the charge controller as well. Try aim for a minimum of 32V into the controller, that way you know you are safe. With that said, a 24V battery bank should charge up to approx 29V for cycle use (although the exact voltage will depend on battery type).

I am definitely going to follow your advice on connecting the load and charge at opposite ends of the strings as you say - however I'm quite amazed that it would actually make such a difference as the resistance on the connecting wires is so small - they are only about 20cm between strings and are doubled up - but if in your experience it does make a difference then I wont argue the point

Well, there is a bit of math behind the idea! Although the resistances are small, you need to factor in current draw. The higher the currents, the more noticeable the difference will be.
Also bear in mind that batteries are chemical in nature, so heat also affects them! Being chemical, each battery may have a slightly different chemistry from the battery next to it. Add the chemical difference, variance in heat between each battery, resistance of the battery interconnects, and the current being drawn....it all adds up. So we try do as much as we can to make sure everything is balanced...it just (hopefully!) means less problems!!

Tell me something - you mention multiplying the amps by 24 to get the watts (24.44A x 24V = 586.67W ) however what I'd like to know is that when the charger is feeding say 24 amps at 29volts into the batteries (near the completion of the boost phase), what is the wattage banked? I meen 29vx24A=696watts right?

Power (aka watts) is equal to voltage multiplied by current. P=IV.
If you have a set current source (like a solar panel), adjusting the voltage adjusts the wattage. As you have shown above, you have 586W @ 24V but 696W @ 29V. However, there is a catch....resistance (R).

As a battery is charged up, its resistance increases, thereby allowing less current into it. The only way to pump more current in would be to increase the voltage (which can only be done to a certain level). That is why when a battery is at a low SOC it will take whatever you give it, and as it charges up, you will notice that it takes less and less current. So, although in your absorb phase you are seeing 29V, your current would have tapered off by then. If your controller has a current meter on it (or if you have a separate ammeter), you will be able to monitor this. When the battery is low, it accepts a larger current, and as it charges up, it accepts less and less current.

There is a significant amount of time where the voltage is above 24volts during charging so is this excess voltage useful of is it simply the amper-hours that matter?

The 12V, 24V, 48V, etc are used in calculations because they are the "nominal" voltage of your battery bank. It just makes everything simpler!

What I think you are referring to here is "opportunity" loads. IE when you your batteries are near full charge or fully charged, you can use the excess energy available from the solar panel array to power something.

Lets hypothetically assume that at the absorb level of 29V your battery bank draws 5A to finish its charge cycle. Because you are on PWM you know that you have a max available 24A, so 24-5=19A that you could potentially harvest (depending on time of day, weather, etc). Which means you can use that "extra" available power to run an appliances, charge something else etc.

[Dave3011]

Ah ok yes its all making a lot more sense now!

I did a calculation on voltage gradients in the battery bank under full load (60 Amps more or less) and I can see your point! Could be looking at 0.1 volts difference between individual batteries which is not good in a system where things are so sensitive. I've got some aluminium L-Bars left over from when I was putting up the panels and I'm thinking of using them to make the battery bank connections. Can drill holes and fit them to the batteries' bolt-studs. I'm sure they can handle a huge amount of current! Just need to go and work out how to make it safe with the required fuses and insulation....