Choice between Step-Down DC Transformer and Load in Series

[digimart]

Hi,
My PV panels ( 400 Watts ), and storage batteries ( 100 AH ) both are configured on 24V DC. Now I have to choose between 2 options with regards to the DC load to be connected. The load size would be about 200 Watts, and ALL the load appliances are at 12V DC.

1. One option is that I can purchase a Step-Down Transformer ( 24 to 12 V ), connect it in between the Load connectors of MPPT charger, and Load Junction Box. I presume that as my load be about 200 Watts, so a Transformer with capacity to handle upto 400 Watts should be sufficient.

2. Second option is that I connect every 2 appliances in series with a common On/Off switch. So in this way all the bulbs and fans be in pairs, and thus be at 24V, and this would match with batteries on 24V.

Now question is cost of a Transformer, as compared to lighting extra bulbs due to their connections in pairs. In case of transformer , some power would be used by the transformer itself too.

Any suggestions / advise that which of the above 2 options be better or any other different option for me.

Regards.

[billvon]

1. One option is that I can purchase a Step-Down Transformer ( 24 to 12 V ), connect it in between the Load connectors of MPPT charger, and Load Junction Box. I presume that as my load be about 200 Watts, so a Transformer with capacity to handle upto 400 Watts should be sufficient.

Not a bad option. (Note that it's really not a transformer but a buck converter.) One benefit is that you get very well regulated 12 volts (or 13.8 volts, which is commonly what people consider "12V.") Drawback is some efficiency loss; you'll lose 5-10%.

2. Second option is that I connect every 2 appliances in series with a common On/Off switch. So in this way all the bulbs and fans be in pairs, and thus be at 24V, and this would match with batteries on 24V.

Bad option. You'll tend to blow devices unless they are perfectly matched both during startup and operation and that is rare.

Let me offer a third option. Run three conductors - neutral, +12V and +24V. Protect both legs. Connect some loads neutral to +12, some +12 to +24. Try to balance loads on both legs. Use a battery balancer across the battery bank to prevent any spurious battery imbalances that might result.

[digimart]

Hi Billvon

In my second option ( connecting two loads of 12 V each in series ), as i be switching On/Off these with a single switch then will the imbalance occur of unmatached start-up ?


I do not understand the option you suggested ( Run three conductors - neutral, +12V and +24V. Protect both legs. Connect some loads neutral to +12, some +12 to +24. Try to balance loads on both legs. Use a battery balancer across the battery bank to prevent any spurious battery imbalances that might result.)
Can you give some more details about this ?

[billvon]

In my second option ( connecting two loads of 12 V each in series ), as i be switching On/Off these with a single switch then will the imbalance occur of unmatached start-up ?

Yes. Here are a few cases:

1) You connect two 12V compact fluorescent lamps in series. Both have soft start circuits. One starts up just a few milliseconds after the other. Thus the second one sees 24 volts across its input and burns out.

2) You have two fans in series. One has a breeze blowing the wrong way through it, the other has a breeze blowing the right way. The first fan takes longer to start up; the second fan starts quickly. One thus has significantly more voltage across it than the other one during startup.

Can you give some more details about this ?

I assume you can tap into your battery bank at the 12V point. Connect some loads between the 0V point (the - of the battery) and the 12V point. They will see 12 volts. Connect other loads between the 12V point and the 24V point. They will see 12 volts. They will not have problems during startup because your battery is large and can supply that load for a long time.

Ideally an equal amount of power should be drawn from both sides. In reality that never works 100% which is why you need a charge balancer. Since the charge balancer only works during imbalance, if your loads are reasonably well balanced you see very little in the way of losses.

[digimart]

Thanks
I understand your point about 2 loads on single connection, and i agree with it.

Regarding your idea of Three conductors; I be connecting 2 batteries of 12V in series, so do u mean that I connect equal number of loads on + and - terminals of battery 1 and battery 2 ?
For the Charge Balancer , does it have some trade name ? i mean how I search for this balancer on the net ?

[billvon]

I be connecting 2 batteries of 12V in series, so do u mean that I connect equal number of loads on + and - terminals of battery 1 and battery 2 ?

Equal energy loads, not equal number. Ideally you want to draw about the same energy from both sides. The balancer will make up the difference.

For the Charge Balancer , does it have some trade name ? i mean how I search for this balancer on the net ?

Google "EQ 12/24." They are made by a company called "Solar Converters Inc." They are available in a variety of powers.

[Sunny Solar]

May I ask one quick question.? where do you buy DC TRANSFORMERS??

[Sunking]

May I ask one quick question.? where do you buy DC TRANSFORMERS??

There is no such thing as a DC Transformer. What you are asking for is a 24 to 12 volt DC to DC Converter. There are many on the market. Just be aware most are inefficient. 50 to 75% efficient. The Switch mode ones are efficient but you will pay a premium price for them.

[axis11]

OP, is there any problem if you make a 12 volt system? 12v batteries for 12v loads?

[Sunny Solar]

I know there is no such thing as DC transformers but this was the heading "Choice between Step-Down DC Transformer and Load in Series"And none of the replies had said anything about it to the OP.

[billvon]

And none of the replies had said anything about it to the OP.[/COLOR]

Reply #2 explained that he really wanted a buck converter not a "DC transformer." It's like someone asking for a solar panel regulator. They really want a charge controller, but it's pretty clear what they mean.