12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

westbranch · June 2007

Hello All, new to the board. I currently have a 12 v Mitsubishi 120 W panel and a 1055 ah Absolyte gell cell with BlueSky 2000E solar boost charge controller at our cabin in Central british columbia 53* N lat. We use the cabin all year round with fewer trips in the winter due to snow, but in most cases 3 days at a time

All use is in 12 v, primarily track lights using 12v LEDs (MR 16 mounts) and some quartz halogen bedside reading lights.

I am considering adding a second 12 v battery, for more storage capacity as in winter we get only about 3 hrs direct sun (in summer we are ok).

I can get, locally, a 'Sun' 24 v 170W panel for $1055 Cdn, or another 12v Mitsubishi 120 w panel for $899 Cdn ( PS this is a major price break for us up here for the Sun panel).

I am at a cross roads as to whether to change over to 24 v or stay 12 v. converting would mean that I could have the panel further away from the batteries and a better placement for solar interception as there are Aspen trees shading during the peak hours summer and winter... but I would need a 24 to 12 transformer to be able to use the current lights, a new charge controller and probably some other parts I havent thought of as yet.

Finally, just to make this a bit morre perplexing,, we will be replacing the current cabin iwth a new all season house which will need a much larger solar array... and the orientation envisioned does not lend itself to solar panel mounting as we presently have, so a post mounted array will be needed.

comments please...

Eric

mike95490 · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

disclaimer: I are not an expert, I have a grid tie system, not an off grid.

Thats out of the way. With 5 minutes of reflective thought, this is what I would do.

1) erect the pole mount system (in it's final location, got to put it up anyway!)

2) ascertain if the mx60 charge controller, can be fed 48V from PV, to charge 12v cells
appears so: http://store.solar-electric.com/outpowmxmp.html

3) will your household loads be 12V dc or 115V ac ?
if 12v, use panels as needed to have enough capacity to keep batteries charged.
if 115Vac chose an pure sine inverter w 48V input, to supply power. 48V battery bank requires smaller gauge wire, and has less loss.

4) switch over when new house is built, from the 12V system to the new scheme, with suitable panels.

5) have a small emergency generator for battery recharges, if something goes wrong, you don't want to sulphate your battery bank.

6) be sure to work with the installer to UNDERSTAND all aspects of the system, the installer will often be glad to install and walk off with your $$, but cannot be found when problems arise in 6 months.

System2 · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

Well said mike90045.
Those are my thoughts as well.

Also, the panels must, if at all possible, be out of the shadows of those aspen and all other trees.

Wayne

westbranch · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

thanks for the comments , a few things to determine/investigate .

Wayne, Yes, the current panel location is not the best but a compromise of a buch of factors. I have removed other trees to get to the present config. and SWMBO comes into the picture as to removal of more shade trees.

thanks again

Eric

niel · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

be sure to have enough charge power for your battery(s) as you are currently feeding less than a 1% charge. without any loads we recommend no less than a 3% charge. with loads it should be at least 5% with 10% being great, but expensive as you'd have more than 10x the number of pv watts as you do now. expanding the battery capacity is ok if you can keep the proper charge going to them and your present battery is in excellent shape so as to not degrade the newer battery.
as to using a 24v or higher system it would yield some benefits, but you'd have the expence of getting another controller capable of downconverter to 12v and get one that will handle you future expansion needs so as to not having to buy another controller again. those controllers that do downconvert will be of the mppt variety, as i would expect you'd want anyway, but you will need about 200-300w plus in pvs(depending on the make and model of controller) to be somewhat efficient in their usage. judging by what i read you should have more pv power anyway just for the present battery to charge propery, so expand your pvs and get an mppt controller that can downconvert to 12v along with those pvs of your choice.

westbranch · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

thanks for the added detail Niel. from other strings here, I have seen the recommendations you have stated as to getting enough input to avoid sulfation.

The Solar Boost controller is an MPPT so I have that covered, limitation is that it is (only) 12 v 25 amp, the Mitsubishi panel is 6.5 - 7 amps so I could add 2 more panels and still be within limits AND almost reach the 2% mark. Wish I had known this small detail last year when I started. the battery must be very near full charge as the readout on the controller lately has been showing 'full charge' and occasionally 'equalize' when there is a clear sunny day... do you feel that it would benefit from a 'smart charger' boost that can put out 15/10/2 amps? I do have a small genset also.

I can see that I need to investigate controllers more and at least get one that will have capacity at or over 60 amps to satisfy the ' min 5 % battery capacity' rule. as a reference point I see that the Outback MX 60 ( seems to be frequently mentioned in other strings) can work with up to 8 other MX 60's. IS there a logical limit of most controllers at 60 amps? Most brands seem to have this as their max. (quick search)
thanks
Eric.

crewzer · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

as a reference point I see that the Outback MX 60 (seems to be frequently mentioned in other strings) can work with up to 8 other MX 60's.

Eric,

Can you explain what you mean by your comment?

Thanks,
Jim / crewzer

westbranch · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

http://www.wholesalesolar.com/products.folder/controller-folder/outbackmx-60.html you can use a 'MATE' to give monitoring data for up to 8 MX 60's

"An optional Outback Mate can be connected to the MX60 to allow monitoring and control of up to eight MX60's "

from this I took that at least 8 MX60's can be in use in one installation (in parallel?) to give one the capability of applying , say, 180 amps to your battery bank.
Is this a correct interpretation?

Eric

niel · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

for the record, the mx60 can do 70amps with a small change to it (the last i checked) and it may be more now. yes ,they can be paralleled as most controllers can be, but i doubt you'll ever need 8 of them and do note that using the mx60 controller at 48v or 24v will up the overall watts that can be utilized.
as to the automotive charger, you can use that to supplement the bulk charging. you must be aware when using those types of chargers that they are NOT regulated and can overcharge a battery, plus they sometimes go defective and can damage batteries and controllers as i had one feed 110vac when the transformer malfunctioned. i recommend that you watch when using them that they aren't unattended and the use of fuses or circuit breakers between the charger and the battery is highly recommended.

mike95490 · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

I believe the statement is "The Mate can MONITOR up to n MX60's" That may not mean that they can be connected in parallel. Maybe it can, but I don't have two to play with.

As to Bulk, Absorb & Equalize : Perhaps, you can charge from the generator in the AM, before solar noon, and let the genset do the Bulk, and then the solar can take over the final top-off and/or Equalize cycle.
If you get to running a 48V system, 60 amps is a respectable amount of power (2.880KW)

crewzer · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

Is this a correct interpretation?

It’s true that a bunch of MX60’s can be “networked” together via a Hub. However, it just means that they can share a single BTS, and they can perhaps respond to manual universal commands such as “EQ”. Otherwise, their individual operations will not be synchronized.

Like many other charge controllers, several MX60’s can be connected in parallel to charge the same battery bank from independent PV arrays.

However, the implication that connecting several controllers in parallel to control a very large 12 V battery bank probably needs some reconsideration. The mix of 12 V batteries in parallel, relatively low system voltage, relatively high current, big cables, and various system inefficiencies can often lead to all sorts of problems.

The generally more effective solution for high power installations is a higher battery bank voltage, i.e., 24 V or 48 V. The MX60 can be used to illustrate a practical example of the benefit of higher voltages: A single MX60 can handle a 3,200 W STC PV array for a 48 V battery bank, but it would take two MX60’s to control a 3,200 W STC PV array connected to a 24 V battery bank, and it would require four MX60’s to control a 3,200 W STC PV array connected to a 12 V battery bank.

HTH,
Jim / crewzer

westbranch · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

OK. Went to the cabin this weekend and found that the batt. is 'fully charged' according to my smart charger when set to AGM setting.
All this from a single 120 w 6.5 a 12 v panel with MPPT controller..
The batt, was checked last year when I put the panel up and it read 88% at that time. The charger is not the same exact one. Number 1 failed a few months ago when charging up my deep cycle electric fishing motor battery. It is the same model but replaced, so..... there could be some variance in readings.
Am I missing something ?

PS Decision made to add a second panel to get 240 w for the interim until a pole can be erected, then go to 24 v arrangement due to distance etc with a 'kick down ' MPPT controller, eg MX60, 24 v to 12 v as the battery (1055 ah) should meet all our demands for the next year or 2...

Eric

mike95490 · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

westbranch wrote:

do you feel that it would benefit from a 'smart charger' boost that can put out 15/10/2 amps?
I do have a small genset also.

By all means, fire up the genset once in a while, in the AM, and use it to pump as many amps in the batteries as they can take, after half an hour, you can shut it down, and let the solar top off and equalize the rest of the day.
(use the genset to get the "bulk" charge into the batteries with any charger that can crank the amps out, let your solar controller do the final charge steps.)

niel · June 2007

Re: 12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

it will take more than a half hour to bulk charge with the genny at the charger's 15amps max listed if the battery is down to 50% and you probably already know that, but just making sure.

12 v VS 24 v : TO SWITCH OR NOT TO SWITCH?

Comments

Categories